1002 Rev.0 - Product Technology - Note Book

PRODUCT TECHNOLOGY FOR ALL NDT METHODSNASA-ADMIN-1002 REV.0

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TABLE OF CONTENTS

INTRODUCTION TO METALS ............................................................................................... 2

WROUGHT PRODUCTS ...................................................................................................... 19

WELD TERMINOLOGY........................................................................................................ 31

WELDING PROCESSES........................................................................................................ 36

STEEL WELD METALLURGY................................................................................................ 57

WELD DEFECTS.................................................................................................................. 65

CRACKING.......................................................................................................................... 75

OTHER PROCESSES AND TECHNOLOGIES.......................................................................... 80

TERMINOLOGY .................................................................................................................. 83

NORMATIVE DOCUMENTS................................................................................................ 93

NON-DESTRUCTIVE TESTING............................................................................................. 94

SUMMARY OF DISCONTINUITIES ...................................................................................... 99

INTERPRETATION VS. EVALUATION ................................................................................ 100


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INTRODUCTION TO METALSWhat is a metal?

Metals may be described as substances having a 'metallic' lustre and are usually malleable,ductile, of high specific gravity and are good conductors of heat and electricity, although somematerials classified as metals may lack some of these properties.

Common metals are iron (steel), copper, lead, aluminium etc. Metals usually occur ascomponents of an 'ore' in the earth crust and need separation and refining to allow their use.Metals may be combined with other metals to form 'alloys'.

Pure materials are known as 'Elements' and consist of atoms that are a collection of particles heldtogether by various bonds. These particles are known as protons, neutrons and electrons.

The number of protons and electrons determine the type of element each having a differentnumber. An element is a pure material that cannot be separated into a simpler substance. Thereare 94 naturally occurring elements and around another 20 that can be made artificially.

Atomic Structure

Iron has an atomic number of 26 which means that its atom contains 26 protons and 26 electronsthere are also 30 neutrons in the core giving an atomic weight of 56.

Element Proton (+) Neutron Electron (-) Atomic WeightAluminium 13 14 13 27

Carbon 6 6 6 12Copper 29 35 29 64

Iron 26 30 26 56Silver 47 61 47 108


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CASTING

Protons have a positive charge and electrons a negative charge equal but opposite value. Thenumber of protons and electrons is the same giving the atom a neutral charge. The nucleus of theatom also contains a number of neutrally charged particles called neutrons and the number ofprotons and neutrons gives the element its atomic weight.

The amount of metals present in the earth’s crust is variable, for instance, iron is 4.3%,aluminium 7.4%, copper 0.01%, silver 0.0001%. They are not, however, spread evenly so someareas are rich and some contain none.

The ores are mined at source and by stages separated from the unwanted material for refining.Different materials have different methods of refining, for the purposed of these notes we willfollow the process which produces engineering steel, steel being an alloy of iron, carbon andother materials to tailor its required properties.

When iron ore has been concentrated to a usable value, it is 'smelted' in a blast furnace to give apure metal which can be mixed with other elements to give the required properties.

Blast Furnace

The iron ore is fed into the blast furnace along with coke and limestone which are heated toabove the iron's melting point this then falls to the bottom of the furnace along with the slag fortapping off when required. The iron at this stage is known as pig iron containing up to 4% carbonand other impurities including unwanted material making it brittle and requiring furtherprocessing.

In the steel making process materials are added which form 'compounds' with the impurities.These impurities are then removed from the steel as gas or slag, the output from this refining isnow called steel and may be cast into an 'ingot' for later processing or into a continuous casting(concast) machine.

Ingot castings tend to be used for short runs or special materials, whereas continuous castingsallows continuous production of a standard material specification and a range of simple crosssectional shapes.


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The steel making process

The Basic Oxygen Steelmaking (BOS) process is the steel refining process commonly used todaywhere the molten iron from the blast furnace is mixed with scrap steel. This is to aid the coolingprocess as very high temperatures are produced in the BOS process.

Basic Oxygen Steelmaking (BOS)

Most of the impurities which are contained in the pig iron are removed by the oxidation process,where oxygen is blown through the molten metal by a water cooled lance and reacts with theimpurities as it passes through. The carbon contents is lowered and changes the pig iron to a lowcarbon steel. However the oxygen also reacts with the iron to produce dissolved iron oxide whichwould release gases on cooling and cause unsound castings or ingots. The addition of manganesetransfers the oxygen to a manganese oxide which is removed with the slag. The steel is tappedfrom the furnace when it is at the correct temperature and composition. The furnace is tilted andthe molten metal is run out via the tap hole into a ladle. Once the steel has been removed, thefurnace is turned upside down for the slag to run into another ladle which is further used in theproduction of cement or in building roads. Quantities of 250 tons of steel may be processed in alittle as 40 minutes. The process generates large quantities of heat so up to one fifth of thecharge is made up of steel scrap partly to control temperature. Electric arc furnace are used forsmaller quantities of high grade steels. The electric arc furnace usually melts scrap metal that hasbeen shredded which has a known content and has already been refined as above and so cancontrol final analysis more closely. The furnace is charged with the scrap metal, the roof of thefurnace is then swung back over the furnace to allow meltdown to start. The electrodes arelowered onto the scrap metal and an arc is struck this then starts the melting process. Once thetemperature and chemistry of the steel is correct it is tapped off into ladles through tilting thefurnace. Further reduction in gases can be made by subjecting the liquid to a vacuum to draw offthe gases. This is called vacuum degassing.


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Electric Arc furnace

Charging

Usually 2 baskets - first part melted, then second charged.

Melting

Use of oxygen and fuel + arc to melt then C boil 0.5/0.6% carbon required. Reduces Si to SiO2Mnto MnO, P to P2O5 these in the slag. Boil purges the melt of N2 and H2.

After the oxidising stage the slag is removed and the bath 'blocked' (deoxidised) with FeSi or FeMn or Ae or a combination of these desulphurisation can then be achieved in the ladle with CaSi.

Note: electric arc furnaces are very versatile with the range of steels from low C to stainlesssteels and super alloys.


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Composition of steel

Steel is an alloy of iron. Carbon is added to produce desirable strength, toughness, ductility andhardness. The amount added determines which of these properties becomes dominant.

For example:

Low carbon 0.1 - 0.3% - car bodies

Medium carbon 0.4 - 0.6% - general engineering

High carbon 0.7 - 1.1% - drills and cutting tools

Other elements are blended with the steel as can be seen below to give additional properties.

Silicon - deoxidiser

Manganese - deoxidiser, desulphuriser

Aluminium - refines grain, deoxidiser

Chromium - improves hardness, wear resistance, corrosion resistance

Nickel - improves strength and ductility, corrosion resistance

Molybdenum - improves creep resistance

Vanadium - improves strength, toughness and ductility

When steel solidifies the atoms form into regular structures called crystals. These crystals forminto groups called grains. Steel therefore has a grain structure and where the crystal structurechanges we have a grain boundary.

Steel usually exists in one of two different crystal structures and it is these structures that give itits strength. The first is Body Centred Cubic (BCC) consisting of a cube with an atom at eachcorner and one in its centre. Materials with these structures are magnetic.

Body Centred Cubic


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The second in the Face Centred Cubic (FCC). This also consist of a cube with atoms at each cornerplus one in the centre of each face. The materials with this structure are non-magnetic.

Face Centred Cubic

Steel at room temperature may consist in a variety of forms depending on composition and heattreatment. Normally it will consist of a mix of ferrite and cementite, the mix giving hardness andductility. Grains containing alternate layers of ferrite and cementite are called Pearlite.

The amount of the above constituents vary with carbon content.

Heat treatment can vary the hardness and other properties by changing the formation of thestructure. An extreme case is that by rapid cooling, Martensite is formed which is very hard butbrittle.

Ingot production

The molten metal from the steel making process is teemed into moulds to solidify. Ingot mouldscan be separated into 2 basic types, wide end up (WEU) and narrow end up (NEU) and in crosssection are usually shaped to avoid sharp corners which could lead to cracking.


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Before teeming additions are made to the metal to add alloying elements or to remove gases insolution (molten metal will dissolve gases forming iron oxide) the amount they will dissolvebecoming less as the temperature falls so the gases come out of solution and form bubbles.Gases evolved are normally CO (Carbon Monoxide) due to carbon/ oxygen reactions which re-form as CO2 (Carbon Dioxide).

Formation of these gases may be suppressed by adding de-oxidation materials such asferromanganese, aluminium, silicon.

Rimming steel ingots

These ingots are produces by semi-killing the steel in the ladle with silicon (Si), just sufficient toallow oxygen to react with the carbon in the rim of the ingot producing blowholes of CO2 and apure iron rim free from carbon. The subsurface CO2 gas welds up on rolling.

This is mainly used for sheet, although used in plate form for non-critical applications.

Killed steel ingots

These ingots are produced by fully killing the steel prior to transfer of the liquid steel from ladleto ingot mould. All carbon/ oxygen reactions are killed by either silicon or aluminium, or acombination of both. Aluminium also refines the grain structure producing fine grained steels.

Semi killed ingots have a reduced level of additions to allow for some reactions.

All engineering grades of steel, high alloy and tool steels are killed steels.

Grain Structure of a Casting

The solidification from a liquid to room temperature occurs on 3 stages - contraction of the liquidsteel, liquid to solid contraction and contraction of the solid room temperature.

During the liquid to solid contraction crystals begin to grow from the mould face. The firstcrystals are formed close to the wall of the mould and comprise of the chilled layer. These havethe smallest grain structure.

The liquid to solid contraction is the next stage and it is the growth if dendritic crystalsperpendicular to the mould wall. They grow in an elongated manner and are called columnarcrystals. The factors that affect these crystals are thermal properties of the mould, the liquidus tosolidus range of the metal, the thermal conductivity of the solidifying metal and the teemingtemperature.


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When the temperature of the remaining liquid starts to fall and the cooling rate slows, directionalgrowth stops and the final solidification takes place with the formation of equiaxal grains.

Solidification of the metal in an ingot mould is accompanied by a reduction in volume as thetemperature falls. As the metal cools from the outside first, the last liquid is at the centretowards the top and it is in this area that the final shrinkage takes place. Any impurities in themetal also float to the top due to their lower specific density and melting point and gather at thetop centre of the ingot.

In a narrow end up ingot, the solidifying metal contracts during cooling to form sinks in the top ofthe ingot this is known as 'primary pipe' and the shrinkage within the ingot is known as'secondary pipe'. One way of reducing this is to place a refractory top on to the iron mould toreduce heat loss, this is often referred to as a 'hot top'.

Some impurities or evolved gas may be trapped in the ingot which may form defects during laterprocessing.

Cross section of a casting

1100⁰C 1200⁰C 1300⁰C

Plan section of a casting


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Continuous casting

Also known as 'concast' produces continuous lengths of simple shapes which are cut into therequired lengths for further processing.

Molten metal from a ladle is teemed into a 'tundish' which feeds metal into a vertical, openbottom mould. This mould is water cooled copper flux lined and oscillates (to prevent sticking),the molten metal solidifies at the surface of the mould and withdrawn from the bottom. Thecentre solidifies as it passes through water cooling jets and through rolls to support and curve itinto the horizontal plane but has no significant effect on the as cast grain structure, it is thenflame cut to length.

Concast machines may produce several strands simultaneously giving a high production rate.

Because of the shapes produced it is possible to omit the first stage of rolling that is necessarywith ingots.

The diagram shows the ladle which is kept at the required temperature whilst being transferredfrom the steel furnace to its position at the top of the tower structure.

Continuous casting of steel (vertical)

Sand casting

Casting is the teeming of molten metal into a mould, where solidification occurs. Almost everyfinished metal product has been cast at some time during its manufacture, and it may containevidence of this in its structure, or segregation, voids or surface defects.

The cavity in the sand is formed by using a pattern (an approximate duplicate of the real part)which are typically made out of wood and sometimes metal. The cavity is contained in anaggregate housed in a two part box. The core is made of sand and inserted into the mould toproduce the internal feature of the part such as holes or internal passages. Core print is theregion added to the pattern, core or mould that is used to locate and support the bodies of largecores within the mould and subsequently melt to become part of the casting.


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A riser is an extra void created in the mould to contain excessive molten material. The purpose ofthis is to feed the molten metal to the mould cavity as the molten metal solidifies and shrinksthereby preventing voids in the main casting.

The molten material is poured in the pouring basin (feeder) which is part of the gating systemthat supplies the molten material to the mould cavity. The vertical part of the gating system iscalled the sprue and the horizontal portion is called the runners and finally the points where it isintroduced into the mould are called gates. Additionally extensions to the gating system arecalled vents that provides a path for the build of gases and displaced air to be vented to theatmosphere.

The casting cavity is usually made oversize to allow for the metal contraction as it cools down toroom temperature. This is achieved by making the pattern oversize to account for the shrinking.The shrinkage allowances are only approximate, because exact allowances are determined by theshape and size of the casting, different parts of the casting might require a different shrinkageallowance and some materials expand and contract more than others.

Typically the sand casting is in two halves and the upper half is known as the cope and the lowerhalf is known as the drag. The parting line on the surface of the casting where excess materialexists is known as a fin, it is where the cope and drag separate.

Sand mould

On removal of the casting from the mould, the excess material on the casting i.e. feeders andrisers, are removed by grinding. This removal process is generally referred to as fettling.


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Shell moulding

A ferrous or aluminium pattern is made resembling the cast item to be manufactured. Thepattern is heated to between 150-370˚C and has a coating of silicone applied which acts as arelease agent. A fine sand mixed with a thermo-setting binder is then blown over the heatedpattern to provide optimum coverage prior to placing into a sand box containing the samematerial. Further heating is applied to complete resin curing.

The shell is then taken from the pattern in two halves and suitably mounted to receive moltenmaterial to produce castings within very fine tolerance.

The shell moulding process

Investment casting

Investment casting is also known as the Lost Wax process. Metals that are hard to machine orfabricate are good candidates for this process and intricate shapes can be made with a highdegree accuracy. This can also be used to make parts that cannot be produced by normalmanufacturing techniques such as turbine blades that have complex shapes, or airplane partsthat have to withstand high temperatures.

The types of materials that can be cast are aluminium alloys, bronzes, tool steels, stainless steelsand precious metals. Parts made with investment castings often do not require any furthermachining, because of the close tolerances and surface finish that can be achieved.

The mould is made by making a pattern using wax or some other material that can be meltedaway. This wax pattern is dipped in refractory slurry, which coats the wax pattern and forms askin. This is dried and the process of dipping in the slurry and drying is repeated until a robustthickness is achieved. After this, the entire pattern is placed in an oven and the wax is meltedaway. This leads to a mould that can be filled with the molten metal. Because the mould isformed around a one piece pattern (which does not have to be pulled out from the mould as intraditional sand casting process) very intricate parts and undercuts can be made.

The materials used for the slurry are a mixture of plaster of Paris, a binder and powdered silica, arefractory, for low temperature melts. For higher temperature melts, sillimanite and aluminia-silicate is used as a refractory, and silica is used as a binder.


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Depending on the fineness of the finish desired additional coatings of sillimanite and ethyl silicatemay be applied. The mould thus produced can be used directly for light castings, or be reinforcedby placing it in a larger container and reinforcing it with more slurry.

Just before the teeming the mould is pre-heated to about 1000˚C (1832˚F) to remove anyresidues of wax and harden the binder. Teeming can be done using gravity, pressure or vacuumconditions. Attention must be paid to mould permeability when using pressure, to allow the airto escape as the teeming is done.

Traditional Investment Casting

Die casting

Die casting is primarily used to make castings with aluminium, magnesium alloys and low meltingpoint materials.

The molten metal is forced into the die cavity of special steel dies at pressures between 0.7-700MN/mm2.

There are two types of process:

1. Hot chamber process - a piston forces the hot molten metal into the die cavity andmaintains pressure until the metal solidifies. Ideal for zinc, tin and lead materials.

2. Cold chamber process - molten material is teemed into a cold piston aperture and thenis injected into initially cold die-plates. Ideal for aluminium, magnesium alloys andcopper base alloys.


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The cold chamber process

Expandable-pattern casting (lost foam process)

The pattern used in this process is made from polystyrene (this is the light, white packingmaterial which is used to pack electronics inside boxes). Polystyrene foam is 95% air bubbles, andthe material itself evaporates when the liquid metal is teemed on it.

The pattern itself is made by moulding the polystyrene beads and pentane which are put insidean aluminium mould and heated; it expands to fill the mould, and takes the shape of the cavity.The pattern is removed and used in the casting process as follows:

The pattern is dipped in a slurry of water and clay (or other refractory grains); it is driedto get a hard shell around the pattern.

The shell-covered pattern is placed in a container with sand for support, and liquid metalis teemed from a hole on top.

The foam evaporates as the metal fills the shell; upon cooling and solidification, the partis removed by breaking the shell.

The process is useful since it is very cheap, and yields good surface finish and complex geometry.There are no runners, risers, gating or parting lines - thus the design process is simplified. Theprocess is used to manufacture crank-shafts for engines, aluminium engine blocks, manifolds, etc.


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Expandable mould casting

Centrifugal Casting

In centrifugal casting, a permanent mould is rotated about its axis at high speeds (300 to 3000rpm) as the molten metal is poured. The molten metal is centrifugally thrown towards the outermould wall, where it solidifies after cooling. The casting is usually a fine grain casting with a veryfine-grained outer diameter, which is resistant to atmospheric corrosion, a typical situation withpipes. The inside diameter has more impurities and inclusions, which can be machined away.

Typical materials that can be cast with this process are iron, steel, stainless steels, and alloys ofaluminium, copper and nickel. Two materials can be cast by introducing a second material duringthe process. Typical parts made by this process are pipes, boilers, pressure vessels, flywheels,cylinder liners and other parts that are axi-symmetric.

Centrifugal Casting


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CASTING DEFECTS

The following defects may be present in the material before any further processing operationssuch as forging or rolling have begun. All casting defects are therefore inherent.

Porosity

This is formed by gas which is insoluble in themolten metal. The gas is trapped within themetal when it solidifies and remains in the formof spherical or tubular cavities.

Airlocks

A cavity formed by air which has been trappedin the mould by the metal during pouring.

Blowholes

These are small holes near to or on the surfaceof the casting. They are caused by gas evolutionfrom the decomposition of grease, moistureetc. but not from the mould itself.

For example, during the sand casting operation,moisture from the mould produces steam, thisis normally forced through the mould due tothe absorbent nature of the sand butsometimes the steam cannot get through to theoutside and is forced back into the casting,blowing holes in the casting surface. There is anincreased possibility of this occurring in handproduced sand moulds on the cross sectionalchanges, where the operator has compressedthe material too much whilst trying to pick upthe change of section.


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Non-metallic inclusions

Non-metallic inclusions are impurities suchas slag, oxides and sulphides, which exist inthe molten metal and finally the solidifiedmetal.

Pipe/shrinkage defects

This is a cavity in the centre of theingot/casting caused by shrinkage duringsolidification. A primary pipe defect issurface breaking and a secondary pipedefect is one that exists sub-surface. Thetop of an ingot casting is removed to getrid of the primary and secondary pipedefects (if existing) prior to rolling...

Other shrinkage defects may occur insteel castings where there is a localizedvariation in section thickness. Shrinkagedefects are not normally associated withgas, but a high gas content will magnifytheir extent.

Interdendritic shrinkage: very smallshrinkage cavities associated withdendrite solidification.


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Segregation

Segregation is chemical heterogeneity, or thenon-uniform distribution of the alloys orimpurities. Pure metals do not exhibitsegregation.

In carbon steels, the elements which segregateare those that are either insoluble or formlower freezing point complexers, e.g. sulphur,phosphorus, carbon, manganese and silicon.

Cold shuts

A cold shut is an area where two or morestreams of metal meet within the mouldhowever they do not fuse together this maybe surface breaking or sub-surface in acasting. Cold shuts may result from splashing,surging, interrupted teeming or the meetingof two streams of molten metal coming fromdifferent directions, usually where castingtemperature is too low.

Hot tears (cracks)

Cracks caused by non-uniform coolingresulting in stresses which may or may notrupture the surface of the metal while itstemperature is still in the brittle range. Theyappear as ragged lines of variable width andnumerous branches.

The tears may originate where stresses areset up by the more rapid cooling of thinsections that adjoin heavier masses of metal,which are slower to cool. Curved surfacesand corners tend to promote hot tearing.


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WROUGHT PRODUCTS

A wrought product is a worked product, primarily produced by hot working, e.g. forging orrolling, although cold working is possible in some cases and is classed as a high energy formingprocess. Forging is usually used for higher strength applications in comparison to the castingprocess.

FORGING

Forging is one of the oldest forms of metal working processes known to man and is simplyillustrated by the blacksmith using a hand held hammer and anvil working the hot metalsupported by tongs.

At the other extreme, very precise pairs of dies may be used with hydraulic presses to produceprecision finished components.

Forging is often used when strength and toughness are needed from the components. Manybasic components, e.g. nuts, bolts and rivets are mass produced by forging.

Because the base material is more malleable at high temperatures, hot forging* is easier toperform. Cold forging requires much greater forces to distort the material to shape, but will becapable of finer finished tolerances and a higher surface profile than a hot worked material.

Forging refines the grain structure and improves physical properties of the metal. With properdesign, the grain flow can be orientated to take account of the direction of principal stressesencountered in actual use. Grain flow is the direction of the pattern that the crystals take duringplastic deformation. Physical properties (such as strength, ductility and toughness) are muchbetter in a forging than in the base metal, which has crystals randomly orientated.


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Direction of Grain Flow

In order to make hollow items it may be required to pierce a solid forged block. This is achievedby forcing a punch through the blank. The end product may be a flat item such as a greater blank,a large diameter ring or a length of tubing. To control the inner diameter of items either aninternal roll (rings) or mandrel is used.

Although more complicated and expensive than cutting from plate (gear blanks and rings) orrolling and welding plate (tubing), forging offers control of fibre flow and thus directionalstrength.

A gear blank cut from the plate would have weak planes in line with the plate rolling direction,whereas a forged gear blank would have the strong planes radiating from the centre givingmaximum strength and toughness in the essential plane.

Forging can be carried out by many methods, some of which are listed below:

Hammer forging

For components to be used in industry, hand forging is rarely used; however, automatic hammerprocesses may be encountered.

Hammer forging uses the energy derived from the mass and velocity of the hammer contactingthe stationary work piece.

There are two main types of hammer used in industry:

1. Gravity drop hammers (drop forging) - this is where a forging ram is raised againstgravity by chain, belt, air and stream etc., and is then allowed to fall freely to contact theworkpiece.

2. Power drop hammers - these are similar to the above but the power (down stroke) is apressurised ram that intensifies the impact. The ram is air, steam or hydraulicallyoperated, this allows for more control and hence more versatility.


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Hammer Forging

Press forging

Press forging is when force is imparted by a pressing action, generally applied to non-ferrousmaterials and allows a deeper flow of the metal; the specific methods used are:

1. Mechanical presses - these have a crank or eccentric type of actuation. They are limitedby the inherent length of stroke but high forces can be generated. Special componentdies are fitted to meet component manufacturing requirements.

2. Screw press - this uses the stored energy of a flywheel or centrifugal mass; the mass offlywheel or centrifugal mass is a limitation in itself. This system uses die inserts and isonly suitable for light work.


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Press Forging

Open die forging

Two flat dies are placed in the two forging faces and a suitable heated section of material isplaced on the lower die. The faces are brought together and the material is shaped bycompression, sometimes termed upsetting.

Open die forging

Closed die forging

Used when special shapes are required. A sequence of forging sets have to be used: the materialis shaped, part formed, formed again, possibly pierced and finally trimmed to producecomponents almost to size. Closed die forging produces excess metal called flash which has to betrimmed off.

To ensure the die is fully filled by the base material in all areas, the blank (base material beingformed) contains more material than the finished forging, this excess material is squeezed into anarea around the die called the gutter, this excess material is now referred to as flash and isremoved afterwards via fettling.

Closed die forging

Cold forging

This produces a good surface finish and dimensional accuracy. Material must be capable of beingcold worked.


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Rolling

Rolling practice

When an ingot or continuously cast section is to be further processed, the first operation isusually rolling. Rolling modifies the shape to one that can be either the finished product or issuitable for further processing.

Rolling also modifies and homogenises the structure from the as cast state.

Primary rolling (roughing/cogging) is usually only applied to ingots to bring them to a suitable sizeand shape. This size and shape may also be a direct product of continuous casting so concastproducts are not subject to primary rolling.

Further stages of rolling (secondary) produce progressively more useful shapes. The product ofthis rolling may be blooms, billets, slabs, plate, I or H beams.

Blooms, billets and slabs

The following terms are commonly used in the iron and steel industry:

Square bloom - Semi-finished product with sides generally greater than 120 mm. Rectangular bloom - Semi-finished product with cross sectional area greater than

14,400 mm2 and with a ratio width to thickness greater than 1:1 and less than 2:1. Square billet - Semi-finished product with sides generally equal to or greater than 50

mm and less than or equal to 120 mm. Rectangular billet - Semi-finished product with cross-sectional area equal to or greater

than 2500 mm2 and less than or equal to 14,400 mm2 and with a ratio of width tothickness greater than 1:1 and less than 2:1.

Round billet - Semi-finished product with diameter equal to or greater than 75 mm. (Upto 75 mm is termed round bar)

Slab - Semi-finished product of thickness equal to or greater than 50 mm and with awidth to thickness ratio equal to or greater than 2:1.

Flat slab - Slab with a width to thickness ratio greater than 4:1.

Rolling may be carried out 'hot' or 'cold'. Hot means that the process is carried out at atemperature above the 'recrystallisation' temperature. Above this temperature, grains that havebeen distorted and elongated are re-formed as small equiaxed grains. Rolling temperature iscritical as grain growth continues at higher temperatures which could lead to excessive grain sizeand consequently weak material.

From the above simple shapes a multitude of end products may be produced.

Primary rolling usually takes place between a pair of large diameter work rolls, the arrangementbeing called '2 high'. Each pair of rolls is held in a 'stand'. Rolling may take place back and forththrough these rolls (reversing mill) or through a series of roll stands (roll train).

As thickness is decreased the length will increase. Initial (primary) rolling from ingot (or concastbillet or bloom) is carried out hot and reductions in thickness of over 500mm per pass can beachieved.

Hot rolling is carried out initially to make the material easier to deform and to allow modificationof the structure to enhance properties. Ingots are preheated in a soaking pit to around 1300


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degrees C. When hot working steel the material reacts with the oxygen in the atmospherecreating scale on the material surface and has to be periodically removed to avoid being rolledback in to the surface, this is achieved by

Scale busting rolls i.e. the plate is bent then straightened and the hard inflexible scalebreaks off.

"Scarfing" a generic term for using heat to remove surface imperfections in this casescale has a different coefficient of expansion and contraction rate to the base steel andfalls away, sometimes with a tremendous release of energy.

Blasting, normally with water not at high pressure but simply to induce a cooling effect. Other techniques may include pickling (acid) or mechanical techniques such wire

brushing, grinding, needle guns or machining.

The process is carried out between two large rolls (up to 50 inches in diameter and each weighingup to 20 tons) called a "two high mill".

Two High Mill

These may be reversing rolls with reductions taking place on consecutive passes through the rollsin opposite directions. Width is controlled by side rolls.

From this stage the steel goes to "three high mills" which are not reversing, the direction beingchanged by raising or lowering a table.

These use less power and have less wear and tear as they run continuously in one direction.

As the process continues, more accuracy is needed and rolls are given backing rolls to controldistortion. Mill stands then become '4 high', '6 high' or may be complicated 'cluster mills' with20+ rolls in them for producing thin foil.

For greater control over dimension, surface finish and properties, a cold rolling process is carriedout which will use mills having four or more rolls per set.

By using smaller work rolls there is less surface area in contact with the metal and less energyrequired but to prevent bending (and maintain dimensional tolerances) backing rolls are used.


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Cold rolling gives good dimensional control and surface finish.

Four High Mill

Effects of Hot Rolling


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HOT ROLLING PRACTICES

COLD ROLLING PRACTICES


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EXTRUSION AND DRAWING PROCESS

Extrusion

Extrusion is the process which pushes material through a shaped die. A lubricant is oftenemployed to reduce friction through the dies and varies with the material extruded i.e. glass maybe used as a lubricant when dealing with some steels. The lubricant is generically referred to as"Soap". There are two main types of extrusion process:

1. Direct extrusion - a hot billet is placed in a chamber, then forced out under pressurethrough a die opening.

2. Indirect (or reverse) extrusion - the billet is held within the chamber, the die holder isthen forced into the billet extruding the shaped section.

Extrusion is normally carried out at elevated temperatures termed hot extrusion to increase theductility of the material and therefore the ease in which the extrusion can be achieved. Afterextrusion a thin residual shell known as "Skull" is sometimes left in the chamber, the existence ofskull confirms that the extruded material is free of oxides.

This process is normally associated with non-ferrous materials and classified as a high energyprocess to form the product, it is rarely used for high melting point materials unless difficult toforge or roll.

Direct extrusion

Indirect extrusion


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WROUGHT PRODUCT DFECTS

Laps

Forging laps are caused by metal being foldedover and flattened but not fused onto thesurface of the forging. Laps can be produced byusing faulty/ oversized dies/ misaligned diesparticular reference to the closed die forgingprocess or too greater reduction attemptedwhen rolling plate/strip causing a back up ofmaterial which eventually is pulled through therolls folding back onto the plate/strip surface.

Bursts

Forging bursts are surface or internal rupturescaused by processing at too low a temperature,excessive working or metal movement duringforging.

Laminations (in flat plates)

Laminations are planar voids usually alignedparallel to the surface of the material. Theymay be the result of any original castingdefect enlarged and flattened.

Slugs

A slug is a piece of foreign material which hasbeen processed or rolled into the surface of awrought material.


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Stringer

As a billet is rolled into a bar section, any non-metallic inclusions are squeezed out into longerand thinner defects. These are called stringers.

Seams and laps (in bar stock)

During the rolling operation, faulty, oversized dies,surface cracks or any surface irregularities maycause laps or seams.

Banding

As the ingot is forged and rolled, thesegregations are elongated and reduced incross-section. If further processing is carriedout, they may appear as very thin parallellines or bands and is generally known asbanding. Banding is not usually consideredsignificant. However, this could cause non-relevant MT indications.

Flakes

Internal ruptures usually associated with hydrogen and nitrogen and often found in heavy alloysteel forgings. Flakes are caused during cooling and, being internal are seldom found by magneticor penetrant testing.

Clinks

A form of cracking occurring due to a rapid thermal gradient, so called due to the sound createdas the material spontaneously cracks, usually associated with large forgings or high strengthmaterials when rapidly cooled for high (working) temperatures.

Slivers

Where the metal has been scarfed, usually to remove visible surface defects and burning torcheshave not evenly melted/ removed the surface it can leave ridges on the surface and when thematerial is then re rolled the ridges are pushed into the material surface being fused at one endonly to the base material, this is also sometimes referred to as fash.


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Chevron Cracks (Cone Cracking)

Associated with the extrusion processwhen the material deformation rate at thecontact of the dies is excessively differentto that of the central area of the productresulting in stresses beyond the plasticflow capability of the material and arupture occurs, often at regular intervalsalong the length of the product.


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WELD TERMINOLOGYGENERAL

Welding is the process of joining two or more pieces of material together by bringing the atomsof each piece into such close contact that an atomic bond takes place, i.e. the separate piecesfuse together to form one.

This process is not restricted to metals, many materials such as plastic and glass can also bewelded.

The first welding carried out was called forge welding. As the name implies, it was used in theforge or smithy by blacksmiths. The method involves heating the pieces of iron to be joined tored heat and hammering them together. Because no melting of the materials is involved, theprocess is termed hot solid phase welding or welding with pressure.

Fusion welding is the alternative process to welding with pressure.

Welding with pressure is used to obtain a welded joint between two materials without meltingthem. The process involves the use of high pressures to bring the materials into close enoughcontact for an atomic bond to be obtained.

To achieve an atomic bond, the pressure applied must cause plastic deformation of the surfacesbeing welded in order to break up and remove the oxides on the surfaces. The weld is obtainedby atomic diffusion followed by crystal growth across the surfaces being joined.

The application of heat, or the generation of heat due to frictional effects, has the effect ofreducing the amount of plastic deformation required to produce a bond.

Welding with pressure has a low heat input when compared to fusion welding, this isadvantageous for many welding applications. Welding with pressure can also join togetherdissimilar metals which are difficult to weld with any fusion welding process. However, fusionwelding processes are more widely used than the welding processes involved with pressure.

The fusion process relies on the properties of molten materials to easily form atomic bonds.When a material melts, the lattice structures which form the material are destroyed, allowing theatoms to easily mix together. Upon cooling and solidification, the atoms re-form into new latticestructures. These structures may well be different to the original lattice for various reasons,including the rate of heating, the temperatures reached, the rate of cooling, and any additionsmade to the molten material. Therefore the finished weld may have properties quite differentfrom the parent materials.

Fusion welding processes require a local application of heat in order to bring the material to atemperature at which it will fuse, for steels this is approximately 1400˚C to 1500˚C. Thetemperature in the molten weld pool may be in the 2500˚C to 3000˚C range. The averagetemperature in the arc is 6000˚C. This heat energy is dissipated into the surrounding atmosphereand parent material on either side of the weld.

Additions to the weld may be made unintentionally by exposing the molten material to theatmosphere. The gases which form the air (primarily nitrogen and oxygen) are readily combinedwith the molten metal and undesirable nitrides and oxides may be formed. It is thereforedesirable to shield the molten weld metal from the air; most fusion welding processesincorporate a system to protect the weld pool from atmospheric contamination.


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Weld Cladding Definition

Weld cladding is a recognised method of protecting against erosion and corrosion. Industry hasdeveloped, and is continuously improving, techniques for the application of cladding to a widevariety of components.

Weld cladding is a means of depositing a metallic layer onto a substrate to enhance theproperties. The most common purpose is to enhance corrosion resistance and is commonlyapplied to boiler walls and roofs in utility boilers and energy-from-waste boilers. Cladding hasbeen successfully deposited over large surface areas and in intricate, hard-to reach places in highvalue plant items. Additional applications include deposition of weld metal buttering to reinstateminimum design thickness requirements on vessels and tanks. The benefit of weld cladding isthat it is a fused bond onto the substrate creating an integral layer with the component,eliminating the risk of the layer spalling or detaching during service and aiding volumetricinspection.

MAIN WELDING METHODS (PROCESSES)

SHIELDED METAL ARC WELDING (SMAW) also called MANUAL METAL ARC WELDING (MMA)

GAS METAL ARC WEDING (GMAW)

FLUX CORED ARC WELDING (FCAW)

SUBMERGED ARC WELDING (SAW)

GAS TUNGSTEN ARC WELDING (GTAW)


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TYPES OF JOINTS

JOINT PREPARATION


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WELD ZONE TERMS

WELD ZONE

FUSION ZONE

ACTUALTHROAT

DESIGNTHROAT

ROOT GAPROOT FACE(LANDING)

ROOT

ORIGINALFACE

WELDJUNCTION

INCLUDED ANGLE

PARENTMETAL

HEATAFFECTED

ZONE

DESIGNTHROAT

ACTUALTHROAT

FUSIONZONE

WELDJUNCTION

ORIGINALFACE

HEATAFFECTED

ZONEROOT

PARENTMETAL

WELDMETAL


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WELDING PROCESSESOXY-GAS WELDING

The term oxy-gas welding is a generic term for a fusion welding process which uses a fuel gas andoxygen to provide a flame hot enough to weld the materials to be joined.

Acetylene is the only fuel gas, when mixed with oxygen, which gives sufficient thermal energy forthe commercial welding of steels; a flame temperature of 3100˚C is produced. Oxy-acetylenewelding is suitable for the welding of most metals including carbon steels, stainless steels, castiron, bronze, copper, aluminium etc. For all materials except the carbon steels the use of a flux isrequired.

The main area of application for oxy-gas welding is on metals less than 5 mm thickness, althoughthicker sections may be welded.

The main disadvantage of oxy-gas welding is the slow speed of travel (and therefore heat input),this causes a wide HAZ, possibly undesirable metallurgical changes and distortion.

In recent years the process has declined in popularity, mainly due to the development of othermore efficient processes such as TIG, MIG/MAG and plasma arc.

Process technique

The high temperature flame is used to bring a small area of the parent metal up to the meltingpoint, a separate filler wire is then dipped into the molten pool and a portion is then melted off,this mixes with the base metal to provide the weld.

Two main welding techniques are used for oxy-gas welding:


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1. Forehand technique:The filler wire precedes the blowpipe along the seam to be welded. The forehandtechnique is for general purpose work.

2. Backhand technique:The backhand technique is vice versa to the forehand technique, i.e. the blowpipeprecedes the filler wire along the welded joint. This technique can be used on thickersections and, with modifications, on positional work.

The oxy-acetylene flame

There are three distinct flame types which can be set with oxy-acetylene and these are asfollows:

1. The neutral flame:The neutral flame is combined from equal quantities of oxygen and acetylene and has adistinct inner white cone with a rounded tip. This flame is the most frequently used. It issuitable for all carbon steels, cast irons, low alloy steels and aluminium.

2. The carburizing (carbonizing) flame:The carburizing flame has a slight excess of acetylene and is identified by the featheraround the inner white cone. The flame is suitable for the welding of high carbon steelsand for hard surfacing applications. Some welders prefer a very slightly carburising flamewhen welding aluminium as it ensures that there is no chance of excess oxygen beingpresent to contaminate the weld pool.

3. The oxidizing flame:The oxidising flame has an excess of oxygen and is identified by an inner white conewhich is shorter and sharper than the neutral cone. This flame is suitable for all brass,bronze, zinc applications, i.e. bronze welding and brazing.


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MANUAL METAL-ARC (MMA) WELDING OR SHIELDED METAL ARC WELDING (SMAW)

Manual metal arc welding is the most versatile of the welding processes, suitable for almost allthickness and types of ferrous and most non-ferrous metals. Welding can be carried out in allpositions relatively economically with reasonable ease of use, although the eventual weld qualityis dependant mainly upon the skill of the welder.

Manual metal arc welding is an arc welding process, the heat being provided by an electric arcwhich is itself formed between a flux coated consumable electrode and the metal being welded.The arc has an average of around 6,000˚C which is more than sufficient to melt the parent metalconsumable electrode and flux.

Power requirements

MMA welding is carried out using either AC or DC. In the case of DC, Positive (+VE) or Negative(-VE) polarity may be used. The actual current form selected is dependent upon the compositionof the electrode flux coating and the specified requirements of the weld. AC transformers are themost cost effective form of power source.

Power for MMA can be obtained from either transformers, transformer-rectifier, generators orinverters.

Regardless of type, the welding plant must provide the following:

a) A high open circuit voltage (OCV) to initiate the arc, e.g. 65-90 volts, and a lower arc orwelding voltage to maintain the arc, e.g. 20-40 volts; therefore the plant must have adrooping characteristic.

b) A reasonable range of current must be available; 30-350 amps is typical. Approximately500 amps would be the maximum capable of being handled manually.


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c) Arc stability. A rapid arc reignition (arc recovery) must be available after short circuitingwithout excessive current surges which can cause spatter.

d) A current which remains almost constant even if, as is usual, the operator varies the arclength during welding, so that consistent electrode burn off rate and weld penetrationcharacteristics are maintained.

Current (amperage)

The welding current, measured in amperes, controls electrode burn off rate and depth ofpenetration. The possible effects of having an incorrect amperage when using MMA are shownbelow:

Amperage too low Poor penetration or fusion, unstable arc, irregular beadshape, slag inclusions, porosity, electrode freezes to the weld,possible stray arc strikes.

Amperage too high Excessive penetration, burn throughs, porosity, spatter, deepcraters, undercut, electrode overheats, high deposition(positional welding difficult).

Voltage

The welding potential (voltage) controls the weld pool fluidity. The possible effects of having anincorrect voltage when using MMA are shown below:

Voltage too low Poor penetration, electrode freezes to work, possible strayarcs, fusion defects, slag inclusions, unstable arc and irregularbead shape.

Voltage too high Porosity, spatter, arc wander, irregular bead, slag inclusions,very fluid weld pool, positional welding difficult.

Speed of travel

The speed of travel affects heat input and therefore also effects metallurgical-and mechanical-conditions. The possible effects of having an incorrect welding speed when using MMA areshown below:

Travel speed too fast Narrow thin bead, slag inclusions, fast cooling, undercut, poorfusion/penetration.

Travel speed too slow Excessive deposition, cold laps, slag inclusions, irregular beadshape.


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Current type

The current type, and more specifically its polarity, determines the heat distribution at the arc.

DC electrode positive

An electrode connected to the DC +VE pole will have two thirds of the available energy-which ismainly heat-developing in the electrode tip with the remaining one third of the energy in theparent material.

This connection produces a wide, shallow weld pool with a broad HAZ which together slow downthe rate of cooling and reduce the possibility of hydrogen entrapment and/or the development ofa brittle metallurgical structure.

DC electrode negative

An electrode connected to the DC -VE pole has reversed energy distribution compared to DC +VEand therefore has one third of the energy develops at the electrode and two thirds of the energyin the parent material.

This creates a rapid development of the weld pool which is narrow, deep and fast freezing withlimited HAZ. Using this polarity with certain electrodes, may lead to hydrogen entrapment and abrittle metallurgical structure which is more susceptible to cracking during contraction or whenexternal stresses are applied.

AC

In an AC arc the polarity is reversing 100 times per second (50 CPS). This has the effect ofequalising the heat distribution; half the heat at the electrode and half in the parent material.

The weld zone and mechanical characteristics are therefore midway between those producedwith electrode DC +VE and electrode DC -VE.

Consumable electrodes

Three electrode types/coverings are commonly used:

Rutile, Cellulose, Basic.


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GAS METAL ARG WELDING (GMAW)METAL INERT-GAS (MIG) AND METAL ACTIVE-GAS (MAG) WELDING

MIG and MAG welding may be considered together because the welding equipment, includingpower source, is essentially the same. It is the shielding gas and consumables (filler wires) whichdiffer.

The MIG/MAG welding process uses a bare wire consumable electrode to provide the arc andweld metal. The wire, typically 0.8-1.6 mm diameter, is continuously fed from a coil through aspecially designed welding gun.

Because the process is fluxless, it is necessary to eliminate the possibility of atmosphericcontamination by introducing a shielding gas. For some materials, argon is an efficient shieldinggas, being inert, it does not chemically react with the weld metal. When an inert gas is used forshielding the welding process is known as metal inert-gas (MIG) welding.

Different shielding gases change the electrical properties of the arc, this influences metal transferproperties, heat input, penetration and weld profile characteristics.

The shielding gas selected will depend on the material to be welded, the corresponding fillerwire, and the required characteristics of the weld. For example, carbon steel-as an electrode-cannot be transferred successfully through a pure argon shielded arc; a very irregular weld profilewith poor fusion would result.

Carbon steel can be transferred successfully through an arc using carbon dioxide (CO2) as theshielding gas. CO2 is an active gas, i.e. it chemically reacts with the weld pool to produce an oxide,and therefore extra deoxidizers must exist in the wire for an acceptable weld to be produced.This process is widely referred to as CO2 welding but is also called metal active-gas (MAG)welding. This latter terminology also applies to the process when other active gases. Gasmixtures are used, e.g. 75% argon, 25% CO2.


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Shielding gasesThe gas shield fulfils two main functions:

It provides a suitable ionisable atmosphere for the electric arc, It protects the weld pool from atmospheric contamination.

Wire consumable

The solid wire consumable used for MIG/MAG welding should conform to BS EN ISO 14341Welding Consumables. Wire electrodes and deposits for gas shielded metal arc welding of non-alloy and fine grain steels or other agreed specification.

Because of the porosity problems which can occur when welding carbon steels with the MAGprocess, fully deoxidized (killed) wire, such as silicon manganese, should be used.

Gas Application Examples

Pure argon Aluminium copper, 9% nickel steel

Argon + 1% to 5% oxygen Stainless steel

CO2 (carbon dioxide) C steel up to 0.4% C, Low alloy steel

Argon + 5% to 25% CO2 Carbon and low alloy steels

Argon + 5% hydrogen Nickel and its alloys

Argon + 15% Copper and its alloys

75% helium + 25% argon Aluminium and copper

75% helium + 25% argon + CO2 trace Austenitic stainless steel

Example gases and applications for MIG/MAG welding

Note: A H2 trace may be added to most gases to increase arc voltage and therefore overalldeposition rates.


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Metal transfer modes

Metal transfer for MIG/MAG welding may be achieved in one of four ways:

Dip transfer (semi-short circuiting arc), Globular transfer, Spray transfer, Pulsed transfer.


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In Short Circuit Transfer Mode both low amperage and low arc volts are required so that theconsumable wire electrode touches the weld pool and short circuits. This is followed by a short,rapid rise in current which causes the tip of the wire to melt off creating an arc which graduallyreduces in length until it short circuits again and the process is repeated. Because this transfermode produces a relatively cool arc, it can be used on thinner sections and for all positionalwelding, including vertical down welding.

Globular Transfer Mode occurs in the intermediate range between spray and dip transfer. Thistransfer mode has no manual application area in MIG/MAG welding and only limited success onmechanized and automatic set-ups.

Spray or Free Flight Transfer is accomplished when a high welding current is used, the weldmetal transfers across the arc in the form of a fine spray. This type of transfer gives highdeposition rates and deep penetration welds. The spray transfer mode is suited to thickmaterials, and except for the light alloys may only be used in the flat or horizontal weldingpositions.

Pulsed Transfer is a modified form of spray transfer which effectively uses both the dip and spraytransfer modes in one operation. Pulses of high powered spray transfer current aresuperimposed over a constant low semi-short circuiting background mode. This results in a lowerheat output compared to true spray transfer but is greater than with dip transfer; this permitshotter welding which allows for high deposition rates and all positional welding. The mainadvantage of the pulsed transfer mode is that poor fusion of root runs is virtually eliminated.There is also regular penetration, no spatter, good profile and the welds are high quality.

Power requirements

Power for MIG/MAG welding is usually electrode DC +VE of a flat (constant voltage)characteristic, this can be obtained from a generator or transformed-rectifier.

Advantages and disadvantages

The advantages and disadvantages of the MIG/MAG welding process particularly when comparedto MMA welding can be summarized as follows:

Advantages

Minimal wastages of consumable electrode, No frequent changing of consumable electrode, Little or no interpass cleaning required (no slag produced), Heavier weld beads are produced, Faster welding process, Low hydrogen process - preheat may not be required.

Disadvantages

Increased risk of porosity - due to displacement of the gas shield, More maintenance of plant involved, High risk of lack of fusion. MIG is ideal for materials with high oxidation rates i.e. Magnesium, Titanium and

Aluminium.


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GAS TUNGSTEN ARC WELDING (GTAW)

TUNGSTEN INERT GAS (TIG) OR TUNGSTEN ACTIVE GAS (TAG) WELDING

General

The TIG welding process uses a non-consumable tungsten electrode to provide an arc. Fillermetal, when required, is fed from a separate filler rod in a manner similar to oxy-acetylenewelding. A shielding gas, e.g. argon, is fed through the welding gun to the weld area and providesa gas shield to prevent contamination by the atmospheric gases. No fluxes are used with theprocess.

Although initially developed for the light alloys, i.e. aluminium and magnesium, TIG welding maybe used on a large variety of metals particularly those with high oxidation rates.

The manual TIG process is expensive when compared to most other manual arc weldingtechniques and is generally only used on carbon steels when high metallurgical and mechanicalproperties are required for the weld. An example application is for the deposition of high qualityroot runs on pipework; the fillers and cap are usually deposited by a more cost effective processsuch as MMA or MAG.

When high quality root runs are to be deposited, a back purge is used to prevent oxidizing(coking) of the weld metal.

When access to the weld area is difficult, e.g. with deep vee preparations or corner welds, thetungsten electrode stick-out length can be increased providing a gas lens is fitted to stiffen thegas shield to prevent turbulence, which would otherwise lead to oxidation of the weld metal.

It is possible to automate the TIG process and many systems are in current use, particularly onpipe where the welding head travels in fixed rings around the joint, the electrode may bestationary or may oscillate from side to side. On root beads it is usual to pulse the current tocontrol the penetration.


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Tungsten electrodes

There are two classifications for tungsten electrodes:

1. Plain (unactivated) tungstenPlain tungsten electrodes tend to laminate in use and can cause tungsten inclusions inthe weld. This type of electrode is rarely used and is suitable for lower quality generalpurpose welds on all metals.

2. Activated tungstenThe addition of either thoria or zirconia to the tungsten gives considerable advantagesincluding increased electron emission for better arc striking, re-ignition and stability,particularly with low current values. There is also a reduction in the possibility oftungsten inclusions in the weld.

1% Thoriated tungsten electrodes: used with electrode DC -VE for the welding of allmetals except the light alloys (aluminium and magnesium).

2% Thoriated tungsten electrodes: as above, but for applications where loweramperages are used and improved arc stability is required.

Zirconiated tungsten electrodes are specifically used with AC for the welding of the lightalloys.

Selection of current type

In selecting the type of current to be used for TIG welding, consideration has to be given to thematerial being welded and the requirements of the arc. Sometimes arc stability is one of theprime importance, but occasionally the removal of surface oxide, i.e. a cleaning action, takespriority. Tungsten has a good ionization potential, i.e. electrons and therefore current flow, areeasily produced; this produces an inherently stable arc. Electrons flow from negative to positive,therefore natural stability will also be achieved with electrode DC -VE however, because mostmetals have some natural ionization potential, then stability will also result with electrode DC+VE but the arc voltage will be higher.

When electrode is negative it is at the cool end of the arc, when it is positive it is at the hot end ofthe arc. Tungsten electrodes usually require a clean sharp tip to be maintained during welding.Welding with electrode DC +VE can overheat and melt the tip, which becomes globular in shaperesulting in an uncontrolled arc and possible tungsten inclusions in the weld metal. For mostmetals electrode DC -VE is used, the exceptions are aluminium, magnesium, and their alloys. Thewelding of the light alloys requires an electric arc which is capable of removing the oxide filmwhich has a higher melting point than the material from which it was formed.

There is a scavenging action achieved with electrode DC +VE which does not exist with DC -VE,therefore from a cleaning point of view, this connection is the one most suitable for the weldingof the light alloys. However, electrode DC +VE polarity will melt the electrode tip as stated earlier,unless a low current with a very large electrode is used, but this is unsuitable as it creates anunstable arc. A compromise is met by using alternating current, so that for 50% of each currentcycle the electrode is positive, therefore cleaning and welding takes place, and for the other 50%of the time the electrode is negative and is cooled down, therefore melting of the electrode isprevented. In AC arcs, because of the reversal of polarity, the heat distribution is even.


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Protection of the molten pool

The gas shield fulfils two main functions:

It provides a suitable ionisable atmosphere for the electric arc. It protects the weld pool from atmospheric contamination.

Gas type and gas flow rates are important considerations. Excessive gas pressure can causerippling of the weld pool and give a coarse finish to the weld bead.

Three gases may be considered for TIG welding: argon, helium, and nitrogen.

1. ArgonThe inert gas argon provides a very cost effective gas shield for all metal types, itproduces a smooth, quiet arc with low arc volts which makes it ideal for light gaugematerial or positional welding. It improves the cleaning action when used with AC onlight alloys.The addition of between 1% and 5% of the active gas hydrogen will raise the arc voltageand give deeper penetration or increases welding speed on stainless steel, or on carbonsteels that can accept the extra hydrogen content on the weld/HAZ.

2. HeliumThe inert gas helium is lighter than argon, therefore requires higher flow rates (2 to 2.5times) to give the same effective shielding. Helium creates a higher arc voltage which isuseful for welding thick sections and metals with a high thermal conductivity.When used with AC on the light alloys there is less cleaning action when compared toargon. Helium is also more expensive than argon.

3. NitrogenInert at room temperature, nitrogen combines with oxygen at arc temperatures andbecomes active, therefore it is unsuitable for the majority of metals but gives goodresults on copper as it increases arc voltage which creates more heat and is far morecost effective than argon or helium.

Filler material

The filler material used for TIG welding should conform to BS EN ISO 14341 WeldingConsumables. Wire Electrodes and Deposits for Gas Shielded Metal Arc Welding of Non Alloy andfine Grain Steels (or other national/international specifications).

Because of the porosity problems which can occur when welding carbon steels with the TIGprocess, killed or fluffy deoxidized wire-such as silicon manganese-should be used. For very highquality welds, triple deoxidized silicon/manganese/aluminium wire is recommended.


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Power source requirements

A high OCV of around 90 volts is required for TIG welding to ensure arc stability at all times. Thepower source, which may be a generator, transformer or transformer-rectifier must be of adrooping characteristic to maintain a relatively constant current value, the operator beingresponsible for arc length control.

To assist arc initiation, to prevent tungsten inclusions in the weld and to prevent damage to theelectrode tip, a high voltage, high frequency current is superimposed at the start of all DCwelding operations.

These characteristics are permanent when AC is used, to assist arc reignition at the beginning ofeach positive half cycle.


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PLASMA ARC WELDING (PAW)

Plasma arc welding is basically a modification of the TIG process, the majority of the equipmentbeing similar, but with modifications to the power source and torch design. PAW can becomplementary to, or used as a substitute for, TIG welding, offering greater welding speed, lesssensitivity to process variations and consequently better weld quality.

The welding capability range is much greater than TIG, particularly for low material thicknesswhere micro-plasma units can operate as low as O.1 amps, for the welding of very thin materialsand high conductivity materials.

The PAW process has the ability to produce welds by the keyhole technique, this is used closedsquare butts on material 1.5-10.0 mm thick. Full penetration in a single pass is achieved withconsiderably reduced distortion compared to more conventional welding processes.

PAW may require the use of a separately fed filler wire or may be used autogenously.

Method of operation

The welding torch consist of a non-consumable tungsten electrode set back into a constrictednozzle through which the plasma gas flows, this nozzle lies within another nozzle through whichthe shielding gas flows.


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Gas is fed into the inner nozzle under low pressure and passes through the electric arc where itbecomes ionized before being forced through the nozzle constriction. This increases the gaspressure and thus the temperature which is in the range of 10,000-17,000˚C. This superheatedionized gas is referred to as plasma.

Power source and equipment

A conventional TIG power source, i.e. transformer/rectifier capable of operating in the rangefrom 5 to 200 amps, may be used with an additional plug in plasma arc module, althoughpurpose built units are available.

Shielding and plasma gases used are pure argon, helium or argon/helium/hydrogen mixturesdependent upon the material type being welded.

The electrode should be connected to the negative pole when DC is being used. When AC is used,a square wave form is recommended to give instant reversal of current.

Methods of arc transfer

Two means of arc transfer are used in plasma arc welding, these being the transferred arc andnon-transferred arc processes.

With the transferred arc process, the work piece forms part of the circuit. The arc transfers fromthe electrode to the work piece via the plasma gas; this results in additional heat output. Thecombined temperature of both arc and plasma is in the region of 17,000˚C.

With the non-transferred arc process, the arc is initiated between the electrode and theconstricting nozzle within the torch and only plasma gas (no arc) exits the nozzle; the work doesnot form part of the circuit. The plasma temperature is in the range of 10,000˚C.


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SUBMERGED ARC WELDING

Submerged arc welding uses a continuously fed bare wire consumable electrode, 1.6 to 6.4 mmdiameter, to produce a weld pool which is protected from atmospheric contamination by aseparately supplied shielding flux in fused or agglomerated form.

It is possible to feed more than one consumable wire electrode into the weld pool at the sametime to increase production rates by up to a factor of eight times compared to using a single wire.

Submerged arc welding is normally fully mechanised, but may be used manually or in a fullyautomatic mode. The arc and molten weld metal are completely submerged beneath the layer ofshielding flux and are not visible to the eye, however, protection against the arc light is advisable.The flux also provides additives to the weld, removes impurities from the weld and provides athermal blanket (slag) protecting the weld as it cools down. The remaining unfused flux isrecovered for re-use after the removal of impurities and sieving.

It may be specified that the flux used can only contain a limited amount of recycled flux e.g. amaximum of 25%. If this is the case the recycled flux must always be thoroughly mixed in with thenew flux before use. An advantage of the submerged arc welding process is that very highwelding currents can be used to produce the rapid deposition of heavy weld beads without thespatter. Although it is possible to use 5,000 amps or more to produce for example a 37 mm thickweld in one pass, it is more usual to restrict the current to around 1000-2000 amps and deposit amulti-run weld because of the improvement in metallurgical properties.

Power source and equipment

Both AC and DC power sources are used with SAW with a typical current output of 400-1500amps. Both drooping characteristic and flat characteristic power sources are used. Because of thehigh current draw off, a 100% duty cycle capability is recommended.

Flat characteristic DC power sources are the type most commonly used for applications wherethe current does not exceed 1000 amps, they are also the best for the high speed welding ofthinner steel sections.


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Above 1000 amps and for the thicker sections, drooping characteristic AC is preferred and worksbest if the sine wave are square because polarity reversal is instantaneous. AC is also preferablefor multi-wire techniques and applications where arc-blow is a problem.

Single, twin or triple wire feed systems are commonly used, all feeding into the same weld pool.All the wires may be live, or dead fillers may be applied. In a multi-wire application, the leadingwire is usually DC +VE polarity, this will limit the risk of burn through, although deep penetrationwill be achieved because of the high current used. AC would normally be used for the remainingwire(s), or dead fillers could be used, or a combination of both.

Application areas

Submerged arc welding is widely used in ship building, structural steel work, general engineeringapplications, and for the fabrication of pipes and pipelines, e.g. double jointing stations. Carbonsteel, alloy steel and stainless steels are the main materials welded using this process. Because ofthe heavy deposition rates and fluid slag, it is only possible to weld in the flat or horizontalvertical position. However, circumferential welds may be made on pipes and vessels. For thisapplication the welding head remains stationary while the work piece rotates beneath it.

Wires to BS EN ISO 14171, Fluxes to BS EN 760

Weld quality and properties are influenced by the choice of wire and flux. The determination ofthe best wire and flux combination to use to give optimum qualities is often a case of trial anderror. The BS EN ISO 14171 - Wire electrodes and flux combinations for submerged arc welding ofnon-alloy and fine grain steels, gives requirements for the wire and flux, designates a codingsystem for SAW wires and fluxes, and also offers guidance on choice.

Fluxes

Fluxes for SAW are divided into two types:

Fused - granulated, Agglomerated - powdered.

Fluxes can be further classified depending on their basicity or acidity.

Fused fluxes

Fused fluxes are manufactured as follows: the ingredients are mixed and melted at a hightemperature, the mixture is then poured onto large chill blocks or directed into a stream of waterto produce granules which have a hard glassy appearance. The material is then crushed, sievedfor size, and packaged.

Advantages of fused fluxes include:

Good chemical mix achieved, They do not attract moisture (not hydroscopic) this improves handling, storage, use, and

weldability. Any moisture present is easily removed by low temperature drying, The easy removal of impurities and fine particles etc. when recycling.

The main disadvantage is the difficulty in adding deoxidants and ferro-alloys. These would be lostduring the high temperature manufacture. The maintenance of a controlled flux depth isconsidered critical.


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Agglomerated fluxes

All the flux materials are dry mixed and then bonded with either potassium or sodium silicate,they are then baked at a temperature below the fusion or melting point and therefore remain asa powder which is sieved for size and packaged.

Advantages of agglomerated fluxes include:

Easy addition of deoxidants and ferro-alloys,

Disadvantages include:

Tendency for flux to absorb moisture and a difficult redrying procedure, Possibility of molten slag causing porosity, Difficult re-cycling, i.e. the removal of impurities and sieving.

Flux basicity or classifications

A certain amount of oxygen will exist during welding, some will remain in the weld metal either ingaseous form or as oxide inclusions. The oxygen can be controlled by chemical reactions with themolten flux.

Basic oxides tend to be more stable than acidic oxides. Generally the higher the basicity of a flux,the less the production/formation of oxygen (porosity) and oxide inclusions, leading to animprovement of weld metal strength.

Fluxes for SAW may be classified as follows:

Acid-general purpose use and for dirty (rusty) steel IMPROVING QUALITY Neutral Semi-basic Basic High basicity - maximum weld toughness and performance


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ELECTROSLAG WELDING (ESW)The main application area of the electroslag process is the joining of plates approximately 10 mmthick and above, although plates in excess of 50 mm thick are more likely to be welded using thisprocess. Carbon steel, low alloy steels and austenitic stainless steels are the only materialsweldable with the electroslag process.

Welding is carried out only when the plates are in the vertical, near vertical position. A square cutjoint is always used. Once welding has started it must be carried out to completion becauserestarts produce defective areas. The process is used on ships, pressure vessels, steel castings,structural steel etc..

For welds up to 75 mm thick, the ESW process uses less weld metal and 90% less flux than SAW;plates 75-300 mm thick are welded at 600-1200 mm/hr. Angular distortion is eliminated.

Electroslag welds are relatively defect free, slag entrapment, porosity and lack of fusion defectsare almost non-existent. Electroslag welds normally require post-weld heat treatment especiallyon the thicker materials, due to the resultant coarse grain structure.

A flat characteristic power source is required. A typical 3 mm diameter wire will require 40 voltsand 600 amps.

Method of operationESW is a fusion welding process which uses the combined effect of current and electricalresistance to produce a conducting bath of molten slag which melts both the filler wire(s) and thesurfaces of the work pieces to be welded. The weld pool is also shielded by this slag which coversthe full surface of the weld and rises as the weld progresses up the joint.

The process is initiated by an arc, usually struck on wire wool type material, which is itself laidonto a starting block which supports the initial liquid material.Powdered flux is placed at the bottom of the joint, this is liquified by the arc which is thenextinguished by the now conductive, though highly resistive, molten slag. All the current nowpasses through this molten slag, the resistance creating heat.

In order to retain the molten mass of flux and weld metal, water cooled copper shoes are fittedeither side of the joint and walk or slide progressively upwards as the welding proceeds.


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Process optionsTwo variations of ESW are in general use. These are non-consumable guide and consumableguide processes.

Non-consumable guide processIn this technique, one or more wires, depending upon metal thickness, are fed into the moltenslag through a guide or guides which are constantly maintained approximately 75 mm above themolten slag. One electrode is required for each 60 mm of metal thickness. If an oscillating orpendulum technique is used this can be increased to 120 mm. This method of ESW is suitable formaterial thicknesses ranging from 10 to 500 mm thick.

Consumable guide processWith this method, filler metal is supplied by both the electrode and its compatible metal guide.The metal guide directs the wire to the bottom of the joint and extends for the full weld heightwhich may be as much as 10 meters. The guide is consumed as welding progresses upwards andcan provide from 5 to 15% of the filler metal. One electrode/ guide is required for each 60 mm ofweld metal but this increases to 150 mm if an oscillating technique is used. The consumableguide technique is suitable for material of unlimited thickness.


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Specialist welding systems

Ultrasonic WeldingThe components usually of thin sectionare vibrated to achieve a clean contactwith each other at the atomic level. Thissystem is mainly employed for materialsless than 2.5 mm in thickness and also inthe joining of plastics.

Explosive WeldingWhere large areas if dissimilar materialsare required to be joined this method isoften employed. A controlled explosionbrings the two materials into close contactto each other achieving an atomic bondwhilst pushing out the contaminates.

Diffusion WeldingThis method presses the two cleansurfaces together whilst raising thetemperature, normally carried out in aninert atmosphere to reduce possiblecontamination.


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STEEL WELD METALLURGY

GRAIN STRUCTURESThe grain structure of a material will influence its weldability, its mechanical properties and in-service performance. The type and number of grain structures present in a material will beprimarily influenced by three factors: (1) the elements in the material, (2) the temperaturesreached during welding and/or post-weld heat treatment and (3) the cooling rates produced.Single or multiple grain structures may be present in a material in its final state.

AusteniteAustenite is the high temperature form of Fe (pure iron) found in C, C-Mn and alloy steels whichexists above 723˚C. The temperature at which the steels are fully austenitic depends on carboncontent, e.g. low carbon <0.1%C - over 910˚C, 0.8%C about 730˚C.

The cooling rate from the austenite region determines the hardness of the steel at roomtemperature. Very slow cooling produces very soft steels; medium cooling rates produce soft tomedium steels; fast cooling can produce very hard and brittle steels depending on the carboncontent and thickness of the steel.

FerriteFerrite is essentially pure iron at room temperature, it contains either very little or no carbon.This grain structure is formed from the austenite region by holding at a temperature whichdepends on the content of the steel, e.g. 910˚C for low carbon steel. Ferrite is very soft andductile and has low tensile strength but has good machining properties.

PearlitePearlite forms from the austenite region under slow cooling and consist of plates of ferrite andcementite, it is harder than ferrite because of the layers of hard cementite it contains. Pearlite isthe most frequently encountered grain structure in a constructional steel.

BainiteBainite forms from the austenite region when the cooling rate is too fast for the pearlite to form,it is harder and usually tougher than pearlite. Bainite often forms in the HAZ area of C-Mn steelwelds.

MartensiteMartensite is a very hard and brittle grain structure but it can be tempered in order to improvetoughness. It is formed from the austenite region by quenching or very fast cooling. This grainstructure can only be formed in plain steels when sufficient carbon exists, usually over 0.3%. Foralloy steels this figure may be much lower because other alloys in the steel - especially chromium- also have an influence. Unless specifically designed into the steel, the presence of martensiteshould be avoided.


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THE HEAT AFFECTED ZONE (HAZ)

During welding using a fusion welding process, there is a huge temperature difference betweenthe weld and parent material. Because of this temperature difference, the material immediatelyadjacent to the weld undergoes microstructural changes. This area, which lies between the fusionboundary and the unaffected parent material, is called the heat affected zone (HAZ)

The extent of the changes in microstructure will depend on the following:a) Material composition; especially carbon content.b) Heat input. The higher the heat input or arc energy, the wider the heat affected zone.

Metallurgical properties will also be affected.

Arc energy (kJ/mm) =

c) The rate of cooling. The faster the rate of cooling the harder the heat affected zone,especially if the carbon equivalent of the steel is high.

The HAZ of a fusion weld on steel consists of up to four separate regions of microstructure, theactual condition will be dependent upon the alloying elements present and the thermalconditions applied during welding. The following grain structures - starting from the areaimmediately adjacent to the weld - are typically present on a 0.15%C steel:

1. A coarse grained region (heated between 1100˚C and melting point).2. A grained refined region (900 to 1100˚C).3. A region of partial transformation (750 to 900˚C).4. A region of spheroidization (just below 750˚C).

On C-Mn and low alloy steels, the HAZ of the weld tends to be more brittle, i.e. it has a lowernotch toughness, than the actual weld metal. The HAZ area is therefore more prone to cracking,especially when hydrogen is included, although it must be noted that the tensile strength of theHAZ is normally high in comparison with the weld and parent material. Unfortunately, if a fusionwelding process is being used, the heat affected zone cannot be eliminated, although it can becontrolled using a properly applied welding procedure.

THE EFFECT OF HYDROGEN IN STEEL

The presence of hydrogen causes general embrittlement in steel and during welding may leaddirectly to cracking of the weld zone. The following terms are forms of hydrogen relatedproblems:

Hydrogen induced cold cracking (HICC), Fissures/ micro-fissures, Chevron cracks, Fish-eyes.


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Mechanism

The following text describes the mechanism believed to be involved with the information ofHydrogen induced cold cracking (HICC) in steel:

Hydrogen enters a weld via the welding arc. The source of hydrogen may be from moisture in theatmosphere, contamination on the weld preparation, or moisture in the electrode flux. With themain MMA and SAW processes, the selection of flux type will also affect the H2 content.

The intense heat of the arc is enough to breakdown the molecular hydrogen (H2) into its atomicform (H). Hydrogen atoms are the smallest known to man and therefore can easily infiltrate theiron atoms while the weld is still hot. When the weld area is hot, the iron atoms are more mobilethereby producing larger gaps between themselves, i.e. the steel is in an expanded condition.

As the weld cools down, most of the hydrogen diffuses outwards into the parent material andatmosphere, but some of the hydrogen atoms become trapped within the weld zone. This is dueto the iron atoms settling as the weld cools, therefore the gaps between them become smaller,i.e. the steel is contracting.

Below 200˚C, the element of hydrogen prefers to be in its molecular form (H2), the individualatoms of hydrogen are attracted towards each other as the weld cools and they congregate inany convenient space as microscopic gas bubbles.

When the hydrogen molecules exist in large numbers, a lot of pressure is exerted - 60,000 to200,000 PSI. Because of this internal pressure, the adjacent grain structure may react in one oftwo ways:

1. It may deform slightly to reduce the pressure. This will occur if the surrounding metal isductile, e.g. pearlite;

2. It may separate completely to reduce the pressure, i.e. crack. This will occur if thesurrounding metal is brittle, e.g. martensite.

Weld fractures associated with hydrogen are more likely to occur in the HAZ as this area tends tohave increased brittleness. It must also be observed that it usually takes an external stress toinitiate and propagate a crack. Lower temperatures will decrease the fracture toughness of thesteel and at the same time increase H2 pressure.

Conclusion: before hydrogen cracking occurs, the following criteria must exist:

Hydrogen; A grain structure susceptible to cracking, this normally means brittle but not necessarily;

martensite grain structures, which are brittle, are very susceptible to cracking; Stress; A temperature <200˚C

To reduce the chance of hydrogen cracking:

Ensure joint preparations are clean, Preheat the joint preparations; Use a low hydrogen welding process, or if using MMA, use hydrogen controlled

electrodes; Use a multi-pass welding technique; Use H2 release post-heat treatment.


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Hydrogen scales

The following chart shows terminology used by the International Institute of Welding (IIW) andBS EN 1011 Part 2 with regard to hydrogen levels per 100 grams of weld metal deposited:

Diffusible hydrogen contentHydrogen scale

Ml/100 g of deposited metal> 15 A

10 ≤ 15 B5 ≤ 10 C3 ≤ 5 D

≤ 3 E

Hydrogen content of weld processes

The hydrogen content in a specific weld depends on a variety of factors such as the degree ofcontamination on the weld preparation, the arc length used, the amount of water vapour in theimmediate environment and cooling rate of the weld. However, it is still possible to approximatehydrogen contents of welds made under typical well controlled conditions. The amount ofhydrogen remaining in a weld - assuming no hydrogen release post-heat treatment process hasbeen used - will depend largely on the welding process used. Shown below are welding processeswith hydrogen levels achieved per 100 grams of weld metal deposited:

1. TIG <1 ml is possible.2. MIG/MAG < 2 ml is possible.3. ESW > 3 ml is likely.4. MMA < 5 ml possible for high temperature baked basic electrodes,

but could be as much as 70 ml for certain cellulose electrodes.5. Submerged arc > 5 ml but could be as much as 50 ml. Dependant on flux type

and heat treatment of flux.6. Flux cored MAG > 10 ml is likely.

THE CARBON EQUIVALENT OF STEEL

Preheat temperatures on steel pipe and many steel structures are arrived at by taking intoconsideration the carbon equivalent (Ceq%) of the material, the material thickness and the arcenergy or heat input (kJ/mm). Reference may be made to standard specifications, e.g. BS EN1011 - Process of arc welding of carbon and carbon manganese steels, which define preheattemperatures based on Ceq%, thicknesses and arc energy.

The welding inspector would usually find the preheat temperatures to be used from the relevantwelding procedure.

The Ceq% of a steel primarily relates to its hardenability. If a steel has a relatively high Ceq% itwill be more susceptible to hardening in the heat affected zones of any welds made, incomparison with welds made on steels of low Ceq%. Hardenability affects weldability, thereforematerials of high Ceq% are considered more difficult to weld.


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The Ceq% of a material depends on its alloying elements; the typical elements in a high-gradecarbon manganese steel are as follows:

Iron (Fe) Silicon (Si)

Carbon (C) Titanium (Ti)

Manganese (Mn) Niobium (Nb)

Chromium (Cr) Aluminium (Al)

Vanadium (V) Tin (Sn)

Molybdenum (Mo) Sulphur (S)

Nickel (Ni) Phosphorus (P)

The Ceq% of a steel is usually calculated from the I.I.W. carbon equivalent formula:

Ceq% = C + + +

Only carbon and manganese have any significant effect on the final Ceq% figure on carbon/carbon manganese steels, therefore the formula may sometimes be shortened to:

Ceq% = C +

The manganese content is divided by 6 because it has one sixth of the effect of carbon in relationto hardenability.

A carbon equivalent value less than approximately 0.4% would be considered low for a low alloysteel (this includes C-Mn steel).

A typical specification example of preheat temperatures for C-Mn steel 8-20 mm thick is asfollows:

For Ceq < 0.4% - minimum preheat 50˚CFor Ceq > 0.4 < 0.48 - minimum preheat 100˚CFor Ceq > 0.48% - minimum preheat 200˚C

EXAMPLE:

What is the Ceq% of a steel which contains 0.12 carbon and 1.3 manganese?

a) Ceq% = C + Ceq% = C +

b) Ceq% = 0.12 + .c) Ceq% = 0.12 + 0.216d) Ceq = 0.336%

Thicker materials normally require higher preheat temperatures, however, for a given Ceq% andarc energy, the preheat temperature is likely to be the same for wall thicknesses up toapproximately 20 mm.


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HEAT TREATMENT

To reduce the risk of cracking in the HAZ, preheat, controlled interpass temperatures andpossibly post-heating may be applied to the weld areas.

Heat treatment is an expensive operation and is therefore only carried out when necessary i.e. ifthere is a significant chance that adverse metallurgical structures and/ or cracks could occur.

Preheat

Preheat is the application of heat to a joint prior to welding. Preheat is usually applied by a gastorch or induction system.

Preheating has many advantages:

Preheat slows down the cooling rate of the weld and HAZ, which reduces the risk ofhardening and also allows absorbed hydrogen a better opportunity of diffusing out,thereby reducing the chance of cracking. Basically speaking, the application of a preheathelps to counteract the adverse metallurgical effects produced by welding on thematerial.

Preheat removes any moisture in the region of the preparation. Preheat improves the overall fusion characteristics during welding. Preheat ensures more uniform expansion and contraction and lowers the stress

between the weld and parent material.

Preheat temperatures may be measured by the use of a touch pyrometer (thermocouple) ortemperature indicating crayons (Tempil sticks). Temperature indicating crayons exist in twoforms; the type that melt and the type that change colour. The method of temperaturemeasurement to be used is sometimes stated in the specification for the work being carried out.

Preheat temperatures are measured at intervals along or around a joint to be welded. Thenumber of measurements taken must allow the inspector to be confident that the requiredtemperature has been reached over the full area to be welded. Specifications sometimes specifythat the preheat temperature must be maintained over a specified distance from the joint faces,e.g. 50 - 100 mm.

The preheat temperature should be taken immediately prior to welding. If a gas heat source hasbeen used, sufficient time must be allowed for the temperature to equalize throughout thethickness of the components to be welded, otherwise only the surface temperature will bemeasured. Time lapses vary depending on specification requirements, e.g. BS EN 1011 states 2minutes for a 25 mm wall thickness.

The temperature of the joint during welding and between passes is known as the interpasstemperature. It is often specified that the interpass temperature must not drop below theminimum preheat temperature.


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Post-heat treatment

Post-heat treatment in this context is a process in which metal in the solid state is subjected toone or more controlled heating cycles after welding. The post heat treatment of welds (PWHT) isnormally carried out for the purpose of stress relief, i.e. the reduction of localised residualstresses. Post-heat treatment may also be used to produce certain properties, such as:

Softening after cold working. Hardening to produce improved strength structures giving ranges of strength with

toughness. Tempering to improve hardened structures giving ranges of strength with toughness.

Another PWHT process which may be used is for hydrogen release only.

The relevant variables for a PWHT process which must be carefully controlled are as follows:

Heating rate, Temperature attained, Time at the attained temperature, Cooling rate - in certain circumstances.

Stress relievingUsed to relax welding stresses without any significant effects on the component's metallurgicalstructure because austenite is not produced.

Stress relief is achieved by heating to 550-650˚C, holding for the required time, e.g. 1 hour per 25mm thickness, and then cooling down in air. Local heating is carried out with gas flame or electricelements; whole components may be stress relieved in a furnace.

AnnealingFull anneal - is used to produce a very soft, low hardness material suitable for machining orextensive cold working. A full anneal is achieved by very slow cooling after the steel has beenheated to above 910˚C and made fully austenite. By the time the steel has been very slowlycooled down to 700˚C, all the austenite changes to ferrite and pearlite with extensive graingrowth. The component is cooled down in air from 680˚C.

Sub-critical anneal - this process is also known as spheroidizing and is used to produce a soft, lowhardness steel - cheaper than full anneal. Temperatures must not rise above 700˚C. A sub-criticalanneal is achieved by heating to 680-700˚C, holding for sufficient time for full recrystallisation tooccur, i.e. new ferrite grains to form; the component is then air cooled in most circumstances.

Note: Any temperatures quoted in the following sub-sections apply to C-Mn steels.Temperatures may differ for other steels.


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NormalisingNormalising is used to maintain and improve mechanical properties and to modify grainstructures by making them more uniform giving a refined structure avoiding grain growth.

Normalising is achieved by heating the steel until it is fully austenitic - the same temperature asthat used for full anneal - soaking for the minimum time necessary to achieve a uniform throughthickness temperature and then air cooling.

Hardening/quenchingHardening is achieved by very fast cooling from the austenite region.

The steel is first heated to produce austenite; it is then allowed to soak at this temperature toproduce grain uniformity, and then fast cooled by quenching into oil or water (brine) to achievethe desired hardness.

After quenching, the steel is highly stressed, very hard and brittle with a high tensile strength.Quenched steel is very prone to cracking and therefore required tempering.

TemperingTempering is used to produce a range of desired mechanical properties to meet specificrequirements.

Tempering is achieved by slowly heating the hardened steel to a temperature between 200-650˚C to produce the required tensile strength and toughness properties; the component maythen be air cooled.At 200˚C, the quenching stresses are reduced and the steel will give maximum tensile andhardness with a reduced risk of cracking.

Increasing the tempering temperature reduces the hardness and tensile strength whilstincreasing the toughness and ductility. At 650˚C, a full temper is produced, giving a very finegrained soft steel with a spheroidized structure.

Hydrogen releaseBoth normalising and annealing heat treatment processes will help to release hydrogen from aweld area. However, there may be a situation where only hydrogen release is required. This maybe performed by heating the weld area to 150-200˚C and soaking for approximately 10-24 hours.


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WELD DEFECTSTERMINOLOGY

Weld defect (BS EN SIO 6520-1: 2007):

"An unacceptable imperfection"

Imperfection (BS EN ISO 6520-1: 2007):

"A discontinuity in the weld or a deviation from the intended geometry"

Weld defect

"A discontinuity the size, shape, orientation or location of which makes it detrimental to theuseful service of the part in which it occurs, i.e. a discontinuity out of specification."

Discontinuity (American):

"Any interruption in the normal physical structure or configuration of a part, such as cracks, lackof fusion, inclusions or porosity. A discontinuity may or may not affect the usefulness of a part."

WELD DEFECTS

Cracks

Definition: A linear discontinuity produced by fracture. Cracks may be longitudinal, transverse,edge, crater, centreline, fusion zone, underbead, weld metal, or parent metal.

There are many types of cracks, some of which only occur with certain types of material. Listedbelow are some crack types encountered.

1. Weld metal cracks:

a) solidification cracking (hot tearing),

b) hydrogen induced cold cracking (HICC):

i. Macro cracks,ii. Fissures,

iii. Micro fissures,iv. Chevron cracks.

2. Heat affected zone cracks:

a) Liquation cracks,b) Reheat cracks,c) HICC

3. Parent material / HAZ:

a) Lamellar tearing


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Because of the various crack types and causes, a welding inspector, under most circumstances,need only talk in terms of the position of a crack, e.g. longitudinal centre line crack; longitudinalcrack in the HAZ of the root (root underbead crack); transverse crack; crater crack etc..

A crack is the most serious type of defect. If a crack exists in the weld zone, the applicationspecification may require the entire weld to be removed (cut out) and rewelded, rather thancarry out a localised weld repair. Some application specifications will permit a localised weldrepair on the cracked area, but very few specifications will allow the acceptance of a detectedcrack, no matter how small.

Incomplete Root Penetration (Lack of Penetration)

Definition: The failure to extend into the root of a joint.

Causes:

a) Root faces too large,b) Root gap too small,c) Arc too long,d) Wrong polarity,e) Electrode too large for joint preparation,f) Incorrect electrode angle,g) Travel speed too high for current.


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Root concavity

Definition: A shallow groove that may occur in the root of a butt weld.

The terms suckback and underwashing may also be encountered.

Causes:

a) Root face too large,b) Insufficient arc energy,c) Excessive back purge pressure with TIG welding.

Lack of fusion

Definition: Lack of union in a weld:

Between weld metal and parent metal, Between parent metal and parent metal, Between weld metal and weld metal.

Lack of fusion can be sub-divided as shown in the diagram:

Causes:

a) Contaminated weld preparation - prevents the melting of material beneath,b) Amperage too low,c) Amperage too high - may cause welder to increase his travel speed resulting in a lack of

melting on the underlying metal,d) Excessive inductance in MIG or MAG dip transfer welding.

See also causes for incomplete root penetration.

LACK OF INTER-RUN FUSION

LACK OF SIDEWALL FUSION LACK OF ROOT FUSION


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Undercut

Definition: An irregular groove at the toe of a run in the parent material, or in previouslydeposited weld, due to welding.

Internal Undercut

External/Crown Undercut

Causes:

a) Excessive welding current,b) Welding speed to high,c) Incorrect electrode angle,d) Excessive weaving,e) Electrode too large.

Incompletely filled groove

Definition: A continuous or intermittent channel in the surface of a weld, running along itslength, due to insufficient weld metal.

Causes:

a) Insufficient weld metal deposited.b) Improper welding technique.


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Shrinkage groove

Definition: A shallow groove caused by concentrationin the metal along each side of a penetration bead.

Gas pores/ porosity

Definition: A gas pore is a cavity, generally under 1.5 mm in diameter, formed by entrapped gasduring the solidification of molten metal.

Definition: Porosity is a group of gas pores.

Other terms which relate to entrapped gas in welds are:

Blowhole - a cavity generally over 1.5 mm. Wormhole (piping) - an elongated or tubular cavity. Hollow bead - elongated porosity in the root bead (pipeline terminology). Herringbone porosity - wormholes side by side taking on a herringbone pattern.

Causes (all types):

a) Excessive moisture in the flux,b) Excessive moisture on the preparation,c) Contaminated weld preparation: scale, oxides etc.,d) Use of flow welding current,e) Arc length too long (especially with basic hydrogen controlled electrodes),f) Damaged electrode flux,g) Incorrect weaving technique,h) Removal of gas shield, e.g. wind on site.

SHRINKAGE GROOVE UNDERCUT


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Slag inclusion

Slag is defined as: a fused, non-metallic residue produced from some welding processes.

A slag inclusion is an entrapped non-metallic deposit in the weld originating from the weldingflux. Linear slag inclusions, or slag lines, almost exclusively exist at the toes of a weld pass.Equiaxed slag inclusions may exist anywhere in the weld.

Causes:

a) Insufficient cleaning between passes,b) Contaminated weld preparation,c) Welding over an irregular profile,d) Incorrect welding speed,e) Arc too long.

Tungsten inclusion

Definition: An inclusion of tungsten from the electrode in tungsten inert-gas welding.

Copper inclusion

Definition: An inclusion of copper due to the accidental melting of the contact tube or nozzle inself-adjusting and controlled-arc welding, or to pick-up by contact between the copper nozzleand the molten pool in MIG/MAG.


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Burn through

Definition: A localised collapse of the molten pool due to excessive penetration, resulting in ahole in the weld run.

Burn throughs are usually associated with the roots of butt welds.

Causes:

a) Excessive amperage during the welding of the root or hot pass on butt welds,b) Excessive root grinding, which may cause the second pass to burn through,c) Improper welding technique.

Crater pipe

Definition: A depression due to shrinkage at the end of a weld run, where the source of heat wasremoved.

Crater pipes must not be confused with burn throughs or gas pores.

Causes:

a) Deoxidation reactions and liquid to solid volume change.


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Arc strike (stray flash)

Definition: Damage on the parent material resulting from the accidental striking of an arc awayfrom the weld.

Arc strikes may have a very brittle structure, especially on steels with a high carbon equivalent.

Causes:

a) Electrode straying parent material,b) Electrode holder with poor insulation touching the workpiece,c) Poor contact of the earth clamp.

Spatter

Definition: Small droplets of electrode material which have strayed away from the arc, whichmay not have fused to the parent plate.

Causes:

a) Excessive arc energy,b) Excessive arc length,c) Damp electrodes,d) Arc-blow.


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CLASSIFICATION AND SIGNIFICANCE OF DEFECTS

Defects may be classified depending on their shape:

Planar defects: linear from at least one dimension, crack like, e.g. cracks, lack of fusion.Relative to other defects usually have a high significance.

Linear volumetric defects: e.g. slag lines, elongated porosity.

Equiaxed defects: rounded and non-linear, e.g. gas pores, slag inclusions. Relative toother defects, these defects usually have a low significance.

The position of a defect in the cross-section of a weld is also an important consideration. Stress ismore concentrated at a surface and corrosion may also be taking place in the region, therefore anon-planar defect breaking the surface may be classed as highly significant defect.

The actual acceptance or rejection will depend on the defect acceptance levels listed in therelevant specification.

DEFECT ACCEPTANCE LEVELS

General

Defect acceptance levels are included in certain specifications - especially in applicationspecifications. The tolerances are usually determined by the use of fracture mechanics, a subjectwhich uses mathematical calculations and mechanical tests, in order to arrive at maximum defectdimensions permissible prior to remedial action.

Some specifications contain defect acceptance tolerances which are stricter than others,depending on the critically rating of the structure or application to which the specificationsapplies.

Overall critically rating depends on a variety of factors including: the stresses involved, e.g. pipepressures; the environment, e.g. contact with corrosive chemicals; erosion, e.g. from fluid flow.

The following lists shows a variety of applications in a descending order of criticality:

1. Vessels and pipework for radioactive substances.2. Pressure vessels welded to PD 5500 or the ASME boiler code.3. Pipework welded to BS 2633.4. Pipelines welded to BS 4515, API 1104, BGC PS P2.5. Bridges, ships and general construction.

REPAIR WELDING

If not properly controlled, it is possible for a weld repair to be more detrimental than the originaldefect, due to adverse microstructural effects in the HAZ, especially on high Ceq% materials,therefore separate repair welding procedures are normally required.

Some specifications may not permit the same weld area to be repaired more than once, againdue to adverse metallurgical effects.

Some specifications may limit the length of repairs, taking into consideration resultingmetallurgical changes and stresses acting on the rest of the unopened weld.


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Removal of defective areas

The specification or procedure will govern how the defective areas are to be removed.

At least one of the following removal methods will be used:

Grinding, Gouging using an arc process, Gouging using a fuel gas process, Chipping, Machining, Filing.

Prior to repair welding, the defective area would normally be removed or blended withoutimpinging onto the parent material, i.e. the original weld joint profile would normally have to bemaintained. If unacceptable defects exist in the parent material, e.g. surface laminations, arcstrikes, gouges etc., the action to be taken will again depend on the specification requirements.Some specifications will allow grinding and re-welding, whilst others will require the defectivearea to be cut off if the material thickness is reduced below certain limits after grinding/blending.


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CRACKINGCracks found within the zones may be divided into two broad categories:

1. Weld process cracks: attributable to the weld process itself.2. Service induced cracks: attributable to some external influence during service such as

vibration or cyclic thermal stresses.

Weld process cracks are categorized/ termed in many ways, but there are essentially only fourcrack types caused through welding:

1. Solidification cracks.2. Hydrogen induced cracks (HIC).3. Lamellar tearing.4. Re-heat cracks.

There are also many ways to categorize/ term service induced cracks. The following list identifiesthe main types:

1. Brittle fracture.2. Ductile fracture.3. Fatigue fracture.4. Creep fracture.5. Stress corrosion cracking.6. Hydrogen cracking induced by corrosion.

Cracks may also be termed in relation to their direction or shape:

Longitudinal with the weld axis. Transverse with the weld axis. Branched. Multi-directional. Chevron.

Terminology may be smooth or jagged in profile. Some cracks have branches, some are moremulti-directional and some occur intermittently.

WELD PROCESS CRACKS

Solidification cracking

Cracking that takes place during the weld solidification process is termed either hot cracking orsolidification cracking and occurs in all steels which have a high sulphur content - sulphur causeslow ductility at elevated temperatures.

In order for a crack to develop the solidifying metal must be subjected to a high tensile stress,this may be present as a result of weld metal contraction combined with high restraint.Solidification cracks usually occur longitudinally down the centre of a weld because of thesegregation of impurities and have a blunt profile compared to other crack types. A crater crack isa type of solidification crack and is often star shaped, hence the alternative definition - star crack.

If a high longitudinal stress was present this may cause transverse cracks to develop, e.g. on largesubmerged arc welds.


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Liquation cracking

Solidification cracks in welds may be due in part to the presence of materials within the metalwhich have a lower defined melting point than that of the metal itself. These low melting pointmaterials usually accumulate at the grain boundaries and can cause problems in the HAZ near thefusion boundary where melting of the parent metal does not occur, but where the temperature ishigh enough to cause melting of the grain boundary. If this melting occurs in the presence of ahigh tensile (contraction) stress, then the boundaries will be pulled apart and a liquation crackoccurs. Within the steel itself, sulphur is the major liquation material. If the welding involves avery high heat input, the sulphur in the HAZ is taken into solution by the surrounding steel andprecipitates out during cooling as sulphides, causing embrittled grain boundaries whichsignificantly weaken the steel. If such an occurrence has happened, the steel is said to be burned.Copper pick-up may also cause this particular form of liquation cracking. Liquation cracks are verysmall and can initiate hydrogen cracking.

Hydrogen induced cracking

See other course notes.

Lamellar Tearing

Lamellar tearing has a characteristic step like appearance. It may occur in the parent plate or HAZof steels with poor through thickness ductility where the fusion boundary of the weld is parallelwith the plate/pipe surface, i.e. lamellar tearing only occurs in the rolled direction of the parentmaterial. It is usually associated with restrained joints that are subjected to through thicknessstresses on corners, 'T, K, Y' configurations, tees or fillet welds joining thick plate which have ahigh sulphur content, although other non-metallic inclusions may also play a part. The presenceof hydrogen increases a steel's susceptibility to lamellar tearing quite significantly.

The through thickness ductility of the parent material may be assessed by using the short tensiletest - see BS EN 1011-2.

A buttering run is welded onto parent material susceptible to lamellar tearing. This weld metalhas higher ductility than the filler metal. Weld concentration which can cause the parent metal totear now only causes the buttering pass to deform thus preventing lamellar tearing fromoccurring.


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Prevention of lamellar tearing

Buttering the surface of the susceptible plate with a low strength weld metal has been widelyemployed. As shown for the example of a T butt weld (Fig. 5) the surface of the plate may begrooved so that the buttered layer will extend 15 to 25mm beyond each weld toe and be about 5to 10mm thick.

Re-heat cracking

Re-heat cracking - also known as stress relaxation cracking - mainly occurs in the HAZ of welds,particularly in low alloy steels during post weld heat treatment or service at elevatedtemperatures.

Most alloy steels are subject to an increase of embrittlement of the coarse grained region of theHAZ when heated above 600˚C, the problem is worse with thicker steels containing Cr, Cu, Mo, V,Nb, and Ti; S and P also have an influence. Typical steels susceptible would be the 2 Cr-Mo-Vtypes, e.g. creep resisting steels.

During post weld stress relief and at high operating temperatures, the residual stresses will berelieved by creep deformation which involves grain boundary sliding and grain deformation. If,due to high creep strength, these actions cannot occur, the grain boundaries may open up intocracks.

Re-heat cracks most frequently occur in areas of high stress concentration and existing defects.They are not unknown in the weld area where the cracks may originate from sharp profiles, e.g.incomplete root penetration or at the toes of badly shaped fillet welds.

Precautions against re-heat cracking include toe-grinding, elimination of partial penetrationwelds, the rejection of poor weld profiles, the selection of steels resistant to liquation cracks, theuse of the lowest strength weld metal acceptable and controlled post weld heat treatment.

Note: A crack which has only been found after post-weld heat treatment is not necessarily a re-heat crack.

Buttering with low strength weld metala) general deposit on the surface of thesusceptible plate

b) In-situ buttering


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SERVICE INDUCED FAILURES

Brittle and ductile failure

See other course notes.

Fatigue cracks

Fatigue cracking is a service failure which occurs under cyclic stress conditions. It normally occursat a change in section, e.g. groove, radius, step, weld toe etc., therefore design and workmanshipare important to minimise failure by fatigue.

All materials are susceptible to fatigue failure. Since design and workmanship play a major part,ferrous based materials have an endurance limit applied to one grade of steel in a specific heattreated condition operating within specific parameters, below this limit fatigue is unlikely tooccur. Other metals will all have the potential to fail by fatigue given the required conditions.

Fatigue failures start at a specific point and propagate with each stress cycle at a rate thatdepends on the applied stress, Fatigue failure is easily identified by beach markings on thefractured face. Final failure can be any other mode of fracture, e.g. brittle or ductile failure.

Thermal fatigue cracks may occur if the cyclic stresses are provided by frequent temperaturechanges producing fluctuating thermal stresses.

Corrosion fatigue failure results from a combination of cyclic stress and a corrosive environmentat the fatigue site.

Creep/ creep failures

Deformation by creep is the slow plastic deformation of a metal, under a constant stress at anytemperature. The plastic deformation is very small compared with normal tensile loading whilstthe temperature range for creep in a given material is between 0.5 to 0.7 of its melting pointexpressed in Kelvin (K). Creep may lead to fracture.

The stages of creep are:

1. Primary creep2. Secondary creep - steady state constant rate creep - the most important stage3. Tertiary creep - the stage when the rate of extension accelerates and leads to failure.

Creep may occur in any situation where a steady state of stress exists, e.g. ranging from leadpipes at room temperature to steam and power generating plant at 450˚C to 500˚C.

Materials such as Cr-Mo-V have been developed for high temperature service which resist creepby blocking the plastic deformation slip systems.

Stress corrosion cracking (Environmentally Assisted Cracking)

This type of cracking occur in materials in a state of tensile stress and in contact with a corrosivemedium. The level of stress which can cause the cracking may be well below the yield point ofthe material. Stress corrosion cracks are surface breaking and are found at any sharp change insection, notch or crevice, especially in structures which have not been stress relieved.

Both ferrous and non-ferrous materials are susceptible to stress corrosion cracking.


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WELD DECAY IN AUSTENTIC STAINLESS STEEL

Weld decay, also known as knife-line attack, occurs in unstabilised austenitic stainless steels, e.g.18-8 type, within the 600-850˚C range in the HAZ. At this temperature range carbon is absorbedby the chromium and chromium carbide is precipitated at the grain boundaries as the metal coolsdown. This causes a local reduction in chromium content which has the effect of lowering theresistance to corrosive attack allowing rusting to occur.

Weld decay is prevented in stabilised stainless steels by the addition of niobium or titanium, butthe most common method now used to prevent weld decay is to decarburize the molten steel tobelow 0.03% C.

GRINDING CRACKS

Shallow cracks formed on the surface of relatively hard materials due to excessive grinding heatto allotropic transformation of the surface material, or to the high sensitivity of the materialsurface containing high tensile residual stresses.

Grinding cracks typically are at 90˚C to the direction of grinding. They are perpendicular to thecomponent surface, have sharp edges and can propagate under cyclic loading.

Overheating can be caused by using the wrong grinding wheel, a dull or glazed wheel, insufficientor poor coolant, feeding too rapidly or cutting too heavily.


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OTHER PROCESSES AND TECHNOLOGIESCOMPOSITE MATERIALS

Composite materials are engineered materials made from two or more constituent materialswith significantly different physical or chemical properties which remain separate and distinct ona macroscopic level within the finished structure.

Composites are made up of individual materials referred to as constituent materials. There aretwo categories of constituent materials: matrix and reinforcement. At least one portion of eachtype is required. The matrix material surrounds and supports the reinforcement materials bymaintaining their relative positions. The reinforcements impart their special mechanical andphysical properties to enhance the matrix properties. The combination of the two differentconstituents produces material properties unavailable from the individual constituent materials,while the wide variety of matrix and strengthening materials allows the designer of the productor structure to choose an optimum combination. Engineered composite materials must beformed to shape. The matrix material can be introduced to the reinforcement before or after thereinforcement material is placed into the mould cavity or onto the mould surface. The matrixmaterial experiences a melding event, after which the part shape is essentially set. Dependingupon the nature of the matrix material, this melding event can occur in various ways such aschemical polymerisation or solidification from the melted state.

Shock, impact, or repeated cyclic stresses can cause the laminate to separate at the interfacebetween two layers, a condition known as delamination. Individual fibres can separate from thematrix e.g. fibre pull-out.

Composites can fail on the microscopic or macroscopic scale. Compression failures can occur atboth the macro scale or at each individual reinforcing fibre in compression buckling. Tensionfailures can be net section failures of the part or degradation of the composite at a microscopicscale where one or more of the layers in the composite fail in tension of the matrix or failure thebond between the matrix and fibres.

Some composites are brittle and have little reserve strength beyond the initial onset of failurewhile others may have large deformations and have reserve energy absorbing capacity past theonset of damage. The variations in fibres and matrices that are available and the mixtures thatcan be made with blends leave a very broad range of properties that can be designed into acomposite structure. The best known failure of a brittle ceramic matrix composite occurred whenthe carbon-carbon composite tile on the leading edge of the wing of the Space Shuttle Columbiafractured when impacted during take-off. It led to catastrophic break-up of the vehicle when itre-entered the earth's atmosphere.

POWDER METALLURGY

Powder metallurgy uses the sintering process for making various parts out of metal powder. Themetal powder is compacted by placing in a closed metal cavity (the die) under pressure. Thiscompacted material is placed in an oven and sintered in a controlled atmosphere at hightemperatures and the metal powders coalesce and form a solid. A second pressing operation,repressing, can be done prior to sintering to improve the compaction and the materialproperties.

The properties of this solid are similar to cast or wrought materials of similar composition.Porosity can be adjusted by the amount of compaction. Usually single pressed products have hightensile strength but low elongation.


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Powder metallurgy is useful in making parts that have irregular curves, or recesses that are hardto machine. It is suitable for high volume production with very little wastage of material.Secondary machining is virtually eliminated.

Typical parts that can be made with this process include cams, ratchets, sprockets, pawls,sintered bronze and iron bearings (impregnated with oil) and carbide tool tips.

POLYMERS

A polymer is a large molecule composed of repeating structural units typically connected bycovalent chemical bonds. While polymer in popular usage suggests plastic, the term actuallyrefers to a large class of natural and synthetic materials with a variety of properties.

Due to the extraordinary range of properties accessible in polymeric materials, they have cometo play an essential role in everyday life from plastics and elastomers to DNA and proteins thatare essential for life. A simple example is polyethylene, whose repeating unit is based onethylene monomer. Most commonly, the continuously linked backbone of a polymer consistsmainly of carbon atoms. However, other structures do exist; for example, elements such assilicon form familiar materials such as silicones, examples being silly putty and waterproofplumbing sealant.

Natural polymeric materials such as shellac, amber, and natural rubber have been in use forcenturies. A variety of other natural polymers exist, such as cellulose, which is the mainconstituent of wood and paper. The list of synthetic polymers includes synthetic rubber, Bakelite,neoprene, nylon, PVC, polystyrene, polyacrylonitrile, PVB, silicone, and many more.

Failure of safety-critical polymer components can cause serious accidents, such as fire in the caseof cracked and degraded polymer fuel lines, cracks in fuel lines can penetrate the bore of thetube and case fuel leakage. If cracking occurs in the engine compartment, electric sparks canignite the petrol and can cause a serious fire.

Chlorine-induced cracking of plumbing joints and pipes has caused many serious floods indomestic properties. Traces of chlorine in the water supply attacked vulnerable polymers in theplastic plumbing, a problem which occurs faster if any of the parts have been poorly extruded orinjection moulded.

TRIBOLOGY

Tribology is the science and technology of interacting surfaces in relative motion. It includes thestudy and application of the principles of friction, lubrication and wear.

The study of tribology is commonly applied in bearing design but extends into almost all otheraspects of modern technology.

Any product where one material slides or rubs over another is affected by complex tribologicalinteractions, whether lubricated like hip implants and other artificial prosthesis or unlubricatedas in high temperature sliding wear in which conventional lubricants cannot be used but in whichthe formation of compacted oxide layer glazes have been observed to protect against wear.

Tribology plays an important role in manufacturing. In metal-forming operations. Frictionincreases tool wear and the power required to work a piece. This results in increased costs due tomore frequent tool replacement, loss of tolerance as tool dimensions shift, and greater forces are


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required to shape a piece. A layer of lubricant which eliminates surface contact virtuallyeliminates tool wear and decreases needed powder by one third.

The tribological interactions of a solid surface's exposed face with interfacing materials andenvironment may result in loss of material from the surface. The process leading to loss ofmaterial is known as "wear". Major types of wear include abrasion, adhesion (friction), erosion,and corrosion.

Wear can be minimised by modifying the surface properties of solids by one or more of "surfaceengineering" processes (also called surface finishing) or by use of lubricants (for frictional oradhesive wear).

Engineered surfaces extend the working life of both original and recycled and resurfacedequipment, thus saving large sums of money and leading to conservation of material, energy andthe environment.

Methodologies to minimise wear include systematic approaches to diagnose the wear and toprescribe appropriate solutions.


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TERMINOLOGYAGE HARDENING

A process causing structural change which may occur gradually in certain metals and alloys atroom temperature or more rapidly at higher temperatures. The effects of Age Hardening arecaused by the precipitation from a super saturated solid solution after rapid cooling from hightemperature, to give an increase in the hardness of the material. Where the ageing occurs at anelevated temperature the process is often referred to as Precipitation Hardening.

ALLOTROPIC

To exist in several different crystalline structures, hence allows heat treatment to controlproperties, or to change the lattice structure in the 'Solid' state only (may give rise to anexothermic reaction) i.e. Iron.

ALPHA IRON (Ferrite)

Body centered cubic form of iron below 910˚C which is Magnetic and as such can be inspectedwith Magnetic Particle Inspection.

ANISOTROPIC

Properties vary with orientation or direction of testing.

ANNEALING

Performed by heating the material to a temperature of around 850˚C for Steels and allowing anycarbide to be taken in to solution. On cooling slowly in a furnace any stresses which were presentdue to cold working, welding etc. are removed to give a softer more machinable re-crystallizedstructure.

SUB CRITICAL ANNEAL (Stress relieving)

Is carried out at a temperature below that at which any carbide are taken into solution. As suchrecrystallization does not take place which results in a material not quite having the properties ofa full anneal.

AUSTENITE (Gamma Iron)

Face centered cubic form of iron above 910˚C which is nonmagnetic. By adding Manganese,Nickel and Chromium to steel a stable form of Austenite is formed at room temperature - i.e.Austenitic Stainless Steels.

BECKING (Mandrel Forging)

A process which produces rings by forging discs whose centre has been punched away on amandrel bar and working the wall thickness between the mandrel bar and the hammer.

BLOOM

An intermediate product which has been rolled or forged down from an ingot usually square insection which is to be further worked. The machinery which is used to create this product ishoused in an area usually designated a blooming mill or cogging mill. Another intermediateproduct is the rectangular slab which is worked in a slabbing mill.


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BLOW HOLE

Rounded or Elongated (Wormhole) smooth walled gas filled cavities in solid metals formed eitherby the trapping of gas evolved during solidification of the metal or by gas or steam from a mouldssurface.

BRAZING

The process of joining two pieces of metal by fusing a layer of brass or other non-ferrous metalbetween the adjoining surfaces.

CARBURIZING

The introduction by absorption of carbon into the surface layer of steel having low carboncontent. Also known as Case Hardening. The carbon may be obtained from either a solid liquid orgaseous carbon containing Iridium. After carburizing the surface of the product is capable of heattreatments such as hardening.

CHAPLETS

Metal supports used to hold the cores in position in the sand mould. They subsequently melt andbecome part of the casting.

CHILLS

These are metal inserts which are placed on the side of a sand mould to control the process anddirection of solidification and as such improve the integrity and soundness of a casting.

CHILL CRYSTALS

The first crystals which form very quickly on the inside surface of a metal ingot mould and havethe smallest structure of all the crystals in the ingot.

CLINK

A form of cracking occurring due to a rapid thermal gradient, so called due to the sound createdas the material spontaneously cracks, usually associated with large forgings or high strengthmaterials when rapidly cooled for high (working) temperatures.

COGGING

The production of slabs or blooms by either forging or rolling from an ingot. This is the first roughstage of working.

COLD CRACKING

Cracks in cold or nearly cold material due to excessive internal stress caused by contraction. Thismay be due to the unsuitable design of a mould and subsequent casting.

COLD DRAWING

A process of reducing the cross sectional diameter of tubes and wire with a subsequent increasein harden ability by drawing the material through dies without previously heating the material.


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COLD ROLLING

Rolling metal or thin sheet at a temperature below the recrystallization to provide a smoothsurface and/ or enhance tensile strength.

COLD SHUT

Any area within a casting where two areas show incomplete fusion. Examples include, unfusedChaplets and Chills.

COLD WORKING

Working below recrystallization temperature.

CONTINUOUS CASTING

The production of cast slabs or billets of long lengths by withdrawing from water cooled mouldssolidifying metal which is continually being added to.

CORNER CRACK

Formed by bad mould design allowing grains to join together at 90˚ to each other. Segregation isoften associated with corner cracking and can be reduced with better mould design.

CRATER CRACK

The small crater which occurs at the finish of a weld run due to contraction/ shrinkage.

CREEP CRACKING

Continuous deformation of a material under constant load.

CURIE POINT

The temperature at which alloys become non-magnetic on heating, this is typically in the range of2/ 3 that of the materials melting point.

CUPPING

Production of seamless pipe via pressing.

DEOXIDISATION

The operation which changes any dissolved oxygen into non-metallic inclusions by the addition ofAluminium (Al2O3,) or silicon (Si2O). This also prevents the carbon which is present from formingcarbon monoxide which gives rise to blow holes.

DIE CASTING (Chill Casting)

The process of teeming metals probably under pressure into two half metal moulds to generatethe final product. Generally used for lower melting point alloys such as Zinc, Lead and Aluminiumto give a highly precise component.


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DROP FORGING (Impact Forging)

The process of using repeated blows on metal between two dies, each of which contains half theimpression of the desired shape.

EXCESS PENETRATION BEAD

Weld filler material deposited which extends beyond the parent metal surface/ thickness in theroot, but within specified limits in the acceptance criteria.

EXTRUSION

The action of forcing a material through a restricted orifice (die) under pressure, causing a greatlyelongated section with the cross sectional shape of the die used.

FATIGUE

The effect on a metal of repeated cycles of stress. Fracture may result from the development of acrack which propagates under the repeated stress.

FEEDER HEAD

An extra portion into which liquid metal can be poured. This extra metal is available to fill thecavities below when shrinkage occurs in the casting.

FETTLING

The first cleaning process by which castings (closed die forgings etc have excess materialremoved and overall improve the manufactured surface profile).

FLAKES (Hair Line Cracks)

Occur due to entrapment of hydrogen on cooling. Casting in a vacuum or below a slag coating canreduce the possibility of flakes occurring, as can suitable heat treatment after production.

FLAME HARDENING

This is a method of local hardening by which the steel is hardened by an oxyacetylene torchwhich transverses the material at a pre-determined rate, related to the depth of hardening.Quenching is often carried out by a jet of water immediately following the heating torch.

FLASH

This is the metal which is squeezed out of a pair of forging dies which is excess to filling the twodies. In drop forgings this excess usually fills a flash line with the dies themselves at a junction ofthe parting line.

FIN

Similar to flash above but usually occurs on a casting again on the parting line between twohalves of the mould.


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FORGING

The art of working parts at a temperature above the recrystallization point between the hammerand anvil, either by sharp successive blows of short duration (hammer forging) or by a hydraulicpress where pressure is applied for longer periods of time (press forging).

FRETTING

When two surfaces press against each other whilst slightly moving, heat which is built upbetween them allows small particles from each surface to become stuck (welded) together andeventually break free. This type of debris removal can be a starting place for fatigue to occur.

GATE

This is the end of the runner section which connects to the mould and allows liquid metal toenter the mould itself. It is an area which gives a change in velocity and as such turbulent flowwithin the casting cavity. This site is favourable for the formation of micro shrinkage and porosity.

GATING SYSTEM

Refers to the feeding system which feeds a number of individual ingot moulds from once centredown gate.

GRINDING CRACKS

Fine shallow cracks caused by local overheating and cooling. Generally a loss of coolant is thereason for an irregular network of cracks.

HARD FACING

A method of improving wear resistance by the introduction of a hard protective coating ofsurface metal such as stellite, metal carbides or inconel, etc.

HARDENABILITY

This is the property which determines depth and distribution of hardness due to quenching froma high temperature. It is generally a function of carbon content and composition and is related tothe information of Martensite.

HETEROGENIUS

A non-uniform structure.

HOMOGENIUS

A uniform structure.

HOT SHORTNESS

Brittleness in metals with a chance of cracking at certain temperatures when under stress due tothe loss of ductility near the melting point.

HOT SPOTS

Highest temperature area after teeming, ideally kept close to the feeders it is the most likely areafor defects to occur.


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HOT TEAR

This crack manifests itself in castings near to or at a change in section due to the stresses whereby differing sections cool and contract at different rates. Hot tears can occur at any time oncesolidification has taken place.

HOT TOP

Extension of the ingot mould, used to retain heat in the upper part of the ingot as it solidifies anexothermic powder is usually applied, this will reduce the formation of secondary piping.

INCLUSIONS

Usually due to deoxidisation using Aluminium and Silicon to produce the non metallic oxidesSilicon oxide and Aluminium oxide. Also caused by manganese which combines with sulphur inpreference to iron forming manganese sulphide. All inclusions can cause a reduction in ductilityfatigue strength and tensile strength.

INDUCTION HARDENING

This process can surface harden or fully harden by heating the material to a temperature abovethe transformation range with an alternating magnetic field and then quenching immediately.

INGOT

A casting made in a cast iron ingot mould which is to be reworked by rolling or forging.

INTERCRYSTALLINE CORROSION (Intergranular Corrosion)

Also known as Weld Decay due to this phenomena occurring within the heat affected zone ofaustenitic welds. After heating within the range 500-800˚C. If the material is subjected to acorrosive atmosphere chemical attack can take place. This is due to the above temperatureallowing carbon to be deposited at grain boundaries as chromium carbide and so depleting theaustenite adjacent to the boundaries of chromium and leaving it susceptible to attack. Theaddition of small quantities of niobium or titanium will form carbides in preference to chromiumand will therefore not deplete the austenite of its chromium making it less susceptible tointergranular corrosion cracking.

ISOTROPIC

Composed of equiaxed grains.

LACK OF FUSION

A lack of bonding between 2 or more materials in a weld, this may be generated by incorrectwelding conditions such as, too low a current, too fast travel, incorrect edge preparation.

LACK OF PENETRATION

Lack of bonding of the original material and the weld metal in the root area; causes could bethrough using too large a diameter electrode; having too large a root face, having too narrow aroot gap or too low a current.


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LAMELLAR TEARING

Found in the rolled plate of configuration weld joints such as T, K or Y joints caused due to lack ofthrough thickness ductility which causes a tear to occur along the line of weakness plains presentin the plate.

LAP

This defect occurs when metal is folded on the surface when forging without being welded up onfurther working.

MALLEABILITY

The property which enables a metal to be mechanically deformed under compression such as byhammering or rolling without cracks occurring.

MARTENSITE

The hardest decomposition product of austenite, which is formed due to quenching from hightemperatures at a rate which is greater than its critical cooling rate and as such produces a brittlehard substance called martensite. Martensite is too brittle to be of use and materials aregenerally required to have further heat treatment processes to remove the structure.

MOULD

The cavity usually in two parts places together into which molten metal is poured to produce acast product.

NITRIDING

A surface hardening process by which the metal is heated within an ammonia atmosphere for anumber of hours. The steels which are affected by this process must contain amounts ofAluminium, Chromium, Molybdenum, Vanadium and tungsten which will form a stable hardnitrides.

NORMALISING

This process involves heating the material to above the transformation (recrystallization)temperature and holding for a length of time prior to air cooling. Unlike annealing a stress freestructure is not achieved due to the faster rate of cooling but internal stresses are relieved andgrain size is refined.

PIN HOLES

Minute gas cavities generally in light alloy castings due to the liberation of gases which had beeninitially absorbed by the materials making up the alloy.

PIPE

The shrinkage cavity which occurs on top of an ingot. Any cavity which is oxidized and open tothe atmosphere is called Primary Pipe and if it is formed subsurface called Secondary Pipe. Bothtypes can be reduced by using a feeder head.


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PRECISION CASTING (Lost Wax or Investment)

Refractory slurry forms the mould by flowing around an exact wax replica of the part to be made.After heating and subsequent removal of the wax the mould can be filled with molten metal togive a precise cope of the wax pattern.

QUENCH CRACK

A fracture resulting from thermal and transformation stresses induced during rapid cooling byimmersing a hot part into a quenching medium such as oil, water, brine, etc.

RESIDUAL STRESS

The stress which exists in parts by external loading such as cold corking or phase change. Stressesare also induced by processes such as castings and welding which transform liquid to solid.

RIMMEL STEEL

Steel which has not been deoxidized and the resultant oxygen reacts with carbon to form CO andCO2 gases during solidification this results in the formation of below holes.

RISER

The Riser on a casting acts as a reservoir (along with the feeder) head but also allows gases whichare formed to escape minimizing porosity and blow holes. It is also an indication that the cavity isfull.

ROLLING

A process similar to forging except that the material is elongated and reduced in section betweentwo rolls revolving in opposite directions. Rolling can also be carried out below therecrystallization temperature and as such is referred to as cold rolling.

SCALE

The oxidized surface of steel produced during hot working at elevated temperatures. The scaleconsists of stable iron oxides such as Fe2O3 and Fe3O4.

SEAMS

Elongated indications along rolled bar material where the surface has been pinched togetherwithout being welded together. They can also occur from oxidized surface blow holes which havebeen stretched and elongated through working the material.

SEASON CRACKING

This occurs in severely cold worked materials particularly copper and brass resulting from acombination of corrosion and internal stresses.

SEGREGATION

This is the heterogeneous (non-uniform) distribution of impurities or alloying elements. Not onlyis it dependant on the chemical composition but also on the cooling rate. For example close tothe surface of an item the impurities become trapped within the rapidly growing crystals. Further


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below the surface where cooling is slower the segregates tend to form together during the V andA type ghosting segregates.

SHELL MOULDING

A mixture of very fine sand and a resin bonding are mixed together over a heated metal patternat about 250˚C. In this way a thin walled half pattern is formed, which when used in a pair can befilled with molten metal.

SHOT BLASTING

A method of cleaning steel surface by abrasion, where steel shot is blasted onto the surface. Thesurface can become slightly work hardened with an increase in fatigue strength, due to thepeening effect of the shot removing sharp edges etc. on the surface of the material, however thiscan/ will also close small crevices and is not advisable prior to penetrant inspection without anacid etch taking place.

SKELP

Plate prepared to be made into pipe.

SOLIDIFICATION

'Directional solidification' solidifies from one end of a casting to the other. 'Progressivesolidification' solidifies from the outside of the casting to the centre.

SPATTER (Welding)

Globules of molten metal thrown out of the weld pool onto the parent metal remote from theweld. Causes could be using too high a current using contaminated consumables which give riseto explosions within the weld pool, or magnetic arc blow if using DC techniques on ferriticmaterial.

STAINLESS STEELS

Ferritic stainless steel - magnetisable/ non-hardenable, used for general cutlery consists of lowcarbon, 13% + Cr e.g. AISI 403≥10 - 15˚C 11.5 - 13% Cr.

Martensitic stainless steel - magnetisable/ may be hardened, used for cutting knives consists ofhigh carbon, 13% + Cr e.g. AISI 440 0.6 - 1.2% C 16-18% Cr

Austenitic stainless steel - non-magnetic/ non-hardenable, a general purpose stainless consists oflow carbon, 17-19 Cr, 8-10% Ni e.g. ≤0.15C, 17-19% Cr, 8-10% Ni 18% Cr, 8% Ni, 2.5% Mo this isoften used for corrosion resistance in sea water environments.

Duplex stainless steels - are a combination of both ferric and austenitic structures which are usedfor high temperature applications up to 600˚C (typically).

STRESS CORROSION

Also referred to as environment sensitive cracking. Deterioration in mechanical propertiesthrough the simultaneous action of static stress and an exposure to a corrosive atmosphere. It isoften accompanied by cracking which can be either transgranular or intergranular.


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STRESS RELIEF

A process whereby residual stress is reduced by heating within the range 600-650˚C for carbonsteel and holding for sufficient time to allow internal stress to be released by creep. This will befollowed by a controlled cooling to prevent further stresses being induced. Stress Relief istypically carried out after welding, cold working, casting etc.

SUBLIMATION

Changes from a solid directly to a gas.

SUPERHEAT

Additional heat above that necessary for melting.

TEMPERING

This is the process of heating hardened or mechanically worked steel at some temperature belowthe transformation temperature to remove brittleness and improve toughness so that thematerial can be usefully used. On heating bright steel between the temperature of 200˚C and400˚C the correct temper temperature may be indicated by the colour of the oxide layer whichforms on the surface.

TUBE

Generally a seamless hollow cylinder. If there is a seam joint it is generally referred to as a Pipe.

VACUUM DEGASSING

The process of casting steel in a vacuum vessel or refining the steel to remove gaseous products.Any molten stream of metal which is introduced into the degassing vessel will have any gasespulled out the steam. The molten steel can also be stirred using an inert gas such as argon orhelium which purges any gas out of the melt. One of the benefits of degassing is that non-metallic inclusion content is much reduced.

WORK HARDENING

The increase in hardness and strength produced by cold plastic deformation or mechanicalworking.

WELDING

A process by which two pieces of metal are joined by heat or pressure, or both with or withoutadditional filler metal, so that recrystallization takes place across the joint. Usually there is localfusion and heat for the process is obtained in a number of different ways, such as an electric arcwhich may be struck between an electrode and the metal to be joined or electrical resistance.Below are some of the more common techniques:

Oxy-gas welding (oxy acetylene)Manual metal arc weldingMetal inert gas weldingMetal active gas weldingTungsten inert gas weldingPlasma arc weldingSubmerged arc welding


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NORMATIVE DOCUMENTS

1. Normative document: Document that provides rules, guidelines or characteristics foractivities or their results.

The term normative document is a generic term that covers such documents as standards,technical specifications, codes of practice and regulations. [ISO GUIDE 2]

2. Standard: Document, established by consensus and approved by a recognized body, thatprovides, for common and repeated use, rules, guidelines or characteristics for activitiesor their results, aimed at the achievement of the optimum degree of order in a givencontext. [ISO GUIDE 2].

3. Code of practice: Document that recommends practices or procedures for the design,manufacture, installation, maintenance or utilization of equipment, structures orproducts.

A code of practice may be a standard part of a standard or independent of a standard. [ISOGUIDE 2].

4. Specification: The document that prescribes the requirements with which the productor service has to conform.

A specification should refer to or include drawings, patterns or other relevant documents andshould also indicate the means and criteria whereby conformity can be checked.[BS 4478: PART1].

5. NDT Procedure: A written description of all essential parameters and precautions to beobserved when applying an NDT technique to a specific test, following an establishedstandard, code or specification [PCN/ GEN].

6. NDT Instruction: A written description of the precise steps to be followed in testing toan established standard, code, specification or NDT procedure [PCN/ GEN].


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NON-DESTRUCTIVE TESTING

PENETRANT TESING (PT)

This type of testing uses the forces of capillary action to detect surface breaking defects. It isimpossible to detect defects which do not break the surface with this method, but it can be usedon both magnetic and non-magnetic materials providing they are non-porous.

There are several types of penetrant systems, this includes the following which are shown in adescending order of flaw detection sensitivity:

Post-emulsifiable - fluorescent Solvent based - fluorescent Water based - fluorescent Post-emulsifiable - colour contrast Solvent based - colour contrast Water based - colour contrast

Fluorescent penetrants require the use of an ultraviolet (UV-A) light to view indications, whilstcolour contrast penetrants are viewed with the naked eye.

One of the most common site used penetrant systems uses solvent based colour contrastpenetrants in aerosols. A typical sequence of operations on a steel test item is as follows:

1. Clean area using wire brush, cloth and solvent. On aluminium, other soft alloys andplastics, wire brushing should not be used, as there is a danger that surface breakingdefects may be closed.

2. Apply penetrant - leave for typically 15 minutes. Colour contrast penetrants arenormally red in colour and should remain on the part long enough to be draw into anysurface discontinuities. This time can vary from about ten minutes to several hoursdepending on the type of material and size/ type of defect sought.

3. Remove surface penetrant using cloth and solvent. Apply solvent to the cloth and notdirectly on to the work piece. Clean thoroughly.

4. Apply developer - leave for typically 15 minutes. The developer draws any penetrantremaining in any surface breaking discontinuities with a blotting action.

5. Interpret area. Any discontinuities are indicated by a red mark, e.g. line or dot against awhite background. Fluorescent penetrants would show green-yellow when viewed withan ultraviolet (UV-A) light.


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MAGNETIC PARTICLE INSPECTION (MT)

This method of NDT may detect surface, and in certain cases, slight sub-surface discontinuities upto 2-3 mm below the surface. MT can be used on ferromagnetic materials only.

A magnetic field is introduced into a specimen to be tested, fine particles of ferromagneticpowder, or ferromagnetic particles in a liquid suspension, are then applied to the test area. Adiscontinuity which interrupts the magnetic lines of force will create a leakage field, which has anorth and south pole on either side of it. This attracts the ferromagnetic particles in greatnumbers. The discontinuity may show as a black indication against the contrasting background -usually white contrast paint - or as a fluorescent indication which is usually green/ yellow againsta dark violet background.

When MT is carried out using fluorescent inks, the use of an ultraviolet (UV-A) light is necessaryto cause fluorescence of the particles, although there is no need to apply a contrast paint.

Fluorescent ink methods are more sensitive than black ink methods.

There are many ways to apply a magnetic field, e.g. a permanent magnet, coils, prods, cables andthreading bar.

Listed below is a sequence of operations to inspect a weld using a permanent magnet with blackink:

1. Clean area using wire brush and a cloth plus solvent if necessary.2. Apply a thin layer of white contrast paint.3. When the paint is dry, straddle the magnet over the weld.4. Apply ink (1.25 to 3.5% particles to a paraffin base).5. Interpret area.6. Too look for transverse defects, turn magnet approximately 90˚ and re-apply the ink.7. Interpret data.


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RADIOGRAPHIC TESTING (RT)

Principles

Radiography is carried out using x-ray machines or artificial gamma sources (radio-isotopes).

X-rays or gamma rays pass through the object to be radiographed and record an image on aradiographic film on the opposite side. The radiation reaching the film will be determined by theobject's thickness and density, e.g. lack of root penetration in a weld will increase the amount ofradiation falling on the film in that area due to a reduction in thickness.

It is the wavelength of the radiation which governs its penetrating power; this is governed by theKilovoltage (kV) when using x-rays, and isotope type with gamma rays. The intensity if theradiation is governed by the milli-amperage (mA) when using x-rays, and by the activity of thespecific isotope with gamma. Activity is measured in Curies (Ci) or Gigabecquerels (GBq).

A negative is produced when the film is processed. The thin areas of an object will be darker thanthe thicker areas, therefore most weld defects will show up dark in relation to the surroundingareas; exceptions are excess weld metal, spatter, tungsten and copper inclusions.

Radiographic quality

An overall assessment of radiographic quality is made by the use of image quality indicators(IGI's); these usually consist of seven thin wires decreasing in thickness. IQI(s) are pre-placed onthe weld being examined and therefore show on the radiographic image. The more wires visiblethe better the flaw detection sensitivity is likely to be.

The density - degree of blackness - of a radiograph is also measured by using a densitometer toensure it lies within a specified range of optimum quality.

Advantages and disadvantages

X-radiography requires bulky and expensive machinery in comparison with gamma radiography,but x-radiography generally produces better quality radiographs and is safer. X-ray machines canbe switched on and off, unlike gamma sources which permanently produce radiation andtherefore require shielding when not in use.

A major disadvantage with radiography is that it will only detect defects which have significantdepth in relation to the axis of the x-ray beam - roughly over 2% of the wall thickness in the sameaxis as the x-ray beam, i.e. radiography will not usually detect plate laminations, lack of inter-runfusion or cracks perpendicular to the x-ray beam.

A major advantage of radiographic testing is that a permanent record is produced, i.e. theradiograph.


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ULTRASONIC TESTING (UT)

This method uses the ability of high frequency sound waves, typically above 2 MHz (2,000,000CPS), to pass through materials.

A probe is used which contains a piezo electric crystal to transmit and receive ultrasonic pulses.Ultrasound hitting any air interface, or an interface with a different material density, which isperpendicular to the ultrasonic beam, is reflected back and displayed on a cathode ray tube(CRT). The actual display relates to the time taken for the ultrasonic pulses to travel the distanceto an interface and back, i.e. the longer the time, the further away the interface.

An interface could be the opposite side of the plate, therefore, wall thickness measurements caneasily be made.

Lamination checks are easily carried out using ultrasonic methods (opposite to radiography).Welds can be tested using angle type probes, although this requires more operator skill to applyand interpret results. Defects in welds usually can be located but the type of defect is sometimesdifficult to identify.

To detect a linear defect with radiography, the defects must have depth in line with the radiationbeam; the opposite is true for ultrasonic flaw detection, i.e. when using ultrasonic testing thedefects should ideally have their major face at 90˚ to the axis of the ultrasonic beam.

For the ultrasound to enter a material a couplant must be introduced between the probe and thespecimen, e.g. grease, oil, glycerine or water, because ultrasound does not travel very wellthrough air.

Ultrasonic equipment is quite portable, but one major disadvantage with most of the equipmentused is that no permanent record of results is produced. Equipment that is able to record resultsis currently expensive.


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EDDY CURRENT TESTING (ECT)

Eddy current testing uses the electromagnetic induction of electrical currents - eddy currents - ina material. The currents are affected by any section change in the material, e.g. the presence ofdefects. These current changes are detected by the test instrument, often by the use of a probewhich induced the currents initially, they are then displayed on a meter or a cathode ray tube(CRT).

Eddy current testing is quite versatile. It is used for coating thickness measurements, claddingthickness measurements and alloy sorting as well as flaw detection.

Eddy current testing is able to detect sub-surface discontinuities, but the depth of eddy currentpenetration is limited. It is excellent for surface flaw detection, but for all types of testing, canonly be used on conductive materials; both magnetic and non-magnetic.

Many different types of probe attachments are available, these include: internal bobbin-typecoils, external coils, knife edge probes and many unique designs for specific applications.


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SUMMARY OF DISCONTINUITIES

Discontinuity Location CauseCold shut surface or subsurface meeting of two streams of liquid metal that do not fuse togetherPipe surface absence of molten metal during the final solidification processHot tears surface restraint from the core of mold during the cooling processPorosity surface or subsurface entrapped gases during solidification of metalInclusions surface or subsurface contaminants introduced during the casting processSegregation surface or subsurface localized differences in material composition

Discontinuity Location CauseSeams surface elongation of unfused surface discontinuities in rolled productsLaminations subsurface elongation and compression of inherent discontinuitiesStringers subsurface elongation and compression of inherent discontinuitiesCupping subsurface internal stresses during cold drawingCooling cracks surface uneven cooling of cold drawn productsLaps surface material folded over and compressedBursts surface or subsurface forming processes at excessive temperaturesHydrogen flakes subsurface an abundance of hydrogen during the forming process

Discontinuity Location CauseCold cracking surface or subsurface atomic hydrogen, hardenable material and high residual stressHot cracking Solidification surface or subsurface low melting point constituents opening up during solidification Liquidation surface or subsurface segregation of material in the liquid state during solidificationLamellar tearing surface delamination of base material during solidification and coolingLack of fusion subsurface failure of the filler metal to coalesce with the base metalPorosity surface or subsurface entrapped constituents in molten weld metal during solidificationInclusions Slag subsurface improper cleaning of a previous weld pass Tungsten subsurface molten weld pool contact with filler metal and tungsten electrode Oxide surface mixing oxides on the base metal surface into the weld poolUndercut surface oversized weld poolOverlap surface insufficient amperage or travel speedLack of penetration surface failure of the weld material to penetrate weld preperation to root

Discontinuity Location CauseGrinding cracks surface localized overheating of the material caused by improper grindingHeat treating cracks subsurface uneven heating or coolingQuench cracks surface sudden cooling from elevated temperaturesPickling cracks surface residual stresses being relievedMachine tears surface improper machining practicesPlating cracks surface residual stresses being relieved

Discontinuity Location CauseFatigue surface cyclically applied stress below the ultimate tensile strengthCreep surface or subsurface material subjected to high temperatures and stressStress cracking surface combined effects of a static tensile load and corrosive environmentHydrogen cracking surface or subsurface combined effects of stress and hydrogen enriched environment

IN-SERVICE DISCONTINUITIES

I SEE DISCONTINUITIESINHERENT DISCONTINUITIES

PRIMARY PROCESSING DISCONTINUITIES

PRIMARY PROCESSING DISCONTINUITIES IN WELDS

SECONDARY PROCESSING DISCONTINUITIES


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INTERPRETATION VS. EVALUATION

INTERPRETATION

To decide what condition caused the indication.

FALSE INDICATIONS (Not caused by Discontinuities)

Can be caused by too high amperage (MT), thick background coating (MT), fingerprints, hair, lint,dirt, scale, rust (MT / PT) and does not necessarily break the surface continuity (MT / PT / UT /ECT /RT), electrical interference (UT / ECT), film marks (RT).

False indications SHALL be eliminated and the part re-tested.

NON-RELEVANT INDICATIONS (Caused by Discontinuities or may be a design feature)

Caused by design features such as rivets, grinding grooves, weld curves or indications smallerthan 1.5mm (ASME VIII) or an indication that is supposed to be there (part of manufacturingprocess).

RELEVANT INDICATIONS (Caused by Discontinuities – bad for part)

Caused by discontinuities and can affect the service life of a part.

All relevant indications MUST be evaluated according to Acceptance standards.

EVALUATION

To decide whether the indication is acceptable, rejectable or needs rework.

CLASSIFICATIONFirst determine whether the indication is round or linear (three times as long as wide).LINEAR INDICATION: L > 3WROUND INDICATION: L ≤ 3W

SPECIFICATIONSDesign engineers predetermine the acceptance criteria. Standards are written in clearspecifications and must be adhered to at all times.

REPORTSMeasure each relevant indication and fill out a detailed report. Mark out indications on testobject so they can be repaired or reworked.

Documents

1002 Rev.0 - Product Technology - Note Book