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Faisal YusofCopyright © 2003 TWI Ltd
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Welding InspectionWeldability
Course Reference WIS 5
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Plain Carbon Steels
Steels are classified into groups as follows
1. Low Carbon Steel 0.01 – 0.3% Carbon
2. Medium Carbon Steel 0.3 – 0.6% Carbon
3. High Carbon Steel 0.6 – 1.4% CarbonPlain carbon steels contain only iron & carbon as main alloying
elements, traces of Mn, Si, Al, S & P may also be present
Classification of SteelClassification of Steel
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IRON CARBON DIAGRAM
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TTT DIAGRAM
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Diagram showing the Relationship between Carbon Content, Mechanical Properties, Microstructure and Uses of Plain Carbon Steels in the Normalised Condition
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Iron & Carbon as a main alloying elements
Alloy steels are divided into 2 groups
1. Low Alloy Steels < 7% extra alloying elements
2. High Alloy Steels > 7% extra alloying elements
Classification of SteelClassification of Steel
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(a) substitutional (b) interstitial
Solid solution
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G Y Carbon: Major element in steels, influences
strength,toughness and ductility Manganese: Secondary only to carbon for strength toughness and ductility, secondary deoxidiser and also acts as a desulphuriser.
Silicon: Primary deoxidiser
Molybdenum: Effects hardenability, and has high creep strength at high temperatures. Steels containing
molybdenum are less susceptible to temper brittleness than other alloy steels.
Chromium: Widely used in stainless steels for corrosion resistance, increases hardness and strength but reduces ductility.
Nickel: Used in stainless steels, high resistance to corrosion from acids, increases strength and toughness
Steel Weld MetallurgySteel Weld Metallurgy
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G Y Aluminium:Deoxidiser,grain refinement
Sulphur: Machineability
Tungsten: High temperature strength
Titanium: Elimination of carbide precipitation
Vanadium: Fine grain – Toughness
Copper: Corrosion resistance and strength
Steel Weld Metallurgy
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G Y Increased strength: C, Si, Cu, Mn, Mo (also Nb and V;
their exact effect depends on other factors also such as the rolling temperature and time, amount of carbon and nitrogen present, etc.)
Hardening capacity: C, Mn, Mo, Cr, Ni, Cu
Toughness: Ni, grain refinement (achieved via the presence of Nb, V, Al, Ti)
Elevated Temperature Properties: Cr, Mo, V
Atmospheric corrosion Resistance: Cu, Ni
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Steel Weld MetallurgySteel Weld Metallurgy
The type and number of elements present in the
material
The temperature reached during welding and or
PWHT.
The cooling rate after welding and or PWHT
The grain structure of steel will influence its weldability, mechanical properties and in-service performance. The grain structure present in a material is influenced by:
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Cooling RateCooling RateThe cooling rate of the weld zone depends on the following factors:
•Weld heat : Also call arc energy, is the amount of electrical
energy that is supplied to the welding arc
over a given weld length ( an inch or mm)
•Thickness of material
•Preheating
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Heat Affected ZoneHeat Affected Zone
The parent material undergoes microstructure changes due to the influence of the welding process. This area, which lies between the fusion boundary and the unaffected parent material, is called the heat affected zone (h.a.z.).
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Heat Affected ZoneHeat Affected Zone
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Heat Affected ZoneHeat Affected Zone
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Heat Affected ZoneHeat Affected Zone
Material composition
Cooling rate, fast cooling higher hardness
Arc energy, high arc energy wider HAZ
The HAZ can not be eliminated in a fusion weld
The extent of changes will be dependent upon the following :-
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Arc energyArc energy
Heat input = 1.6 kJ/mm
Amps = 200 Volts = 32Travel speed = 240 mm/min
Arc energy= Amps x volts Travel speed mm/sec X 1000
Heat input = 200 X 32 X 60 240 X 1000
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G Y High heat input - slow cooling
Low toughness
Reduction in strength
Heat InputHeat Input
Low heat input - fast cooling
Increased hardness
Hydrogen entrapment
Lack of fusion
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Carbon EquivalentCarbon Equivalent The CE of steel primarily relates to its hardenability.
Higher the CE, lower the weldability
Higher the CE, higher the susceptibility to brittleness
The CE of a given material depends on its alloying elements
The CE is calculated using the following formula
CE = C + Mn + Cr + Mo + V + Cu + Ni6 5 15
Hardenability:The relative ability of a ferrous alloy to form martensite when quenched from high temperatures.
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Weldability Weldability Weldability can be defined as the ability of a material to
be welded by most of the common welding processes,
and retain the properties for which it has been designed.
A steel which can be welded without any real dangerous
consequences is said to possess Good Weldability. A steel which can not be welded without any dangerous
consequences occurring is said to possess Poor
Weldability. Poor weldability normally generally results in
the occurrence of some sort of cracking problem
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Weldability Weldability
Weldability is a function of many inter-relatedfactors but these may be summarised as:
Composition of parent material
Joint design and size
Process and technique
Access
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Weldability Weldability It is very difficult to asses weldability in absolute terms therefore it is normally assessed in relative terms
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Weldability Weldability There are many factors which affect weldabilty e.g. material type, welding parameters amps, volts travel speed, heat input.
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Weldability Weldability Other factors affecting weldability are welding position and welding techniques.
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Weldability Weldability Basically speaking weldabilty is the ease with which a material or materials can be welded to give an acceptable joint
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CracksCracks
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Process Cracks Process Cracks
Hydrogen induced cold cracking (HICC)
Solidification cracking (Hot Tearing)
Lamellar tearing
Weld Decay
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Cracks Cracks
When considering any type of crack mechanism, three elements must be present for it’s occurrence:
Stress: stress is always present in weldments,through local expansion and contraction.
Restraint: may be a local restriction, or through the plates being welded.
Susceptible microstructure: the structure is often made susceptible to cracking through welding, e.g high hardness
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Hydrogen Hydrogen CracksCracks
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Hydrogen Cracking Hydrogen Cracking
Hydrogen causes general embrittlment and in welds may lead directly to cracking,
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A combination of four factors is necessary to cause HAZ hydrogen cracking
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Hydrogen Cracking Characteristics Also known as hydrogen induced cold cracking ,
delay cracking , underbead cracking and chevron. Hydrogen is the major influence to this type of
cracking. Source of hydrogen may be from moisture or
hydrocarbon such as grease , paint on the parent material, damp welding fluxes or from condensation of parent material
Hydrogen is absorbed by the weld pool from the arc atmosphere.
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from the solidified bead by the diffusion but some also diffuses into the HAZ of the parent metal.
• Type of cracking is intergranular along grain boundaries or transganular
• Requires susceptible grain structure, stress and hydrogen and low temperature is reached.
• Most likely in HAZ for Carbon Manganese steel and in weld metal for HSLA steel.
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Hydrogen induced weld metal cracking
Hydrogen induced HAZ cracking
Hydrogen Cracking Hydrogen Cracking Micro Alloyed Steel Carbon Manganese Steel
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Hydrogen Cracking Factors responsible:
Hydrogen cracking occurs when the conditions outlined in 1 – 4 occur simultaneously :
1.Susceptible grain structure – hardness value > 350 V.P.N
That part of HAZ which experiences a high enough temperature for the parent steel to transform rapidly from ferrite to austenite and back again,produces microstructures which are usually harder and more susceptible to hydrogen embrittlement.
2.Hydrogen level - > 15 ml/100g
This is inevitably present, derived from moisture in the fluxes used in welding and from other sources.
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Hydrogen Cracking 3.Temperature < 200oC for any steel and < 150oC for structural steel.
The greatest risk of cracking occurs when temperatures near ambient are reached and cracking may thus take place several hours after welding has been completed ( normally after 72 hours )
4.Stress > 50% yield strength of parent metal
These arise inevitably from thermal contractions during cooling and may be supplemented by other stresses developed as a result of rigidity in the parts to be joined.
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Pre heat, removes moisture from the joint preparations, and slows down the cooling rate
Ensure joint preparations are clean and free from contamination
The use of a low hydrogen welding process such as TIG or MIG/MAG
The use of Nickel and Austenitic filler metal
Ensure all welding is carried out under controlled environmental conditions
Ensure good fit-up as to reduced stress
The use of a PWHT with maintaining the pre- heat temperature
Avoid poor weld profiles
Use low hydrogen electrodes and baked as per manufacturer instructions
Hydrogen Cracking Hydrogen Cracking Precautions for controlling hydrogen cracking
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Solidification Solidification CracksCracks
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Solidification Cracking Characteristics Also known as hot cracking or center line cracking or crater cracking and liquation cracking
Solidification cracking is intergranular type of cracking that is along the grain boundaries of the weld metal.
It occurs during the terminal stages of solidification,when the stresses developed across the adjacent grains exceed the strength of the almost completely solidified weld metal.
Impurities such as sulphur and phosphorous and carbon pick - up from parent metal increase the risk of cracking
High joint restraint which produce high residual stress will
increase the susceptibility to this type of cracking.
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G Y • Occurs during weld solidification process from
liquidus to solidus and at the last area to solidified.• Steels with high sulphur content (low ductility at
elevated temperature ) whereby produce hot shortness to the weld metal
• FeS form films at the grain boundaries whereby reduce the strength of the weld metal.
• Addition of manganese will form MnS and forms globules instead of films( FeS)
• Occur longitudinally down center of weld• Welding process that most susceptible to this type
of cracking are SAW and MIG/MAG with spray transfer due to high dilution rate.
Solidification Cracking
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Intergranular liquid film along the grain
boundary
Solidification Cracking Solidification Cracking
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Weld Centerline
Solidification Cracking Solidification Cracking
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Solidification Cracking Factors responsible :Metallurgical factorsa) Freezing temperature range –higher freezing range more
susceptible to solidification cracking due to presence of FeS
b) Primary solidification Phase – Less than 5% delta ferrite
c) Surface tension – concave more susceptible than convex weld shape
d) Grain structure of fusion zone – Coarse columnar grain more susceptible especially with high energy welding process.
Mechanical factorsa) Contraction stresses – Thicker material more susceptible.
b) Degree of restraint – poor fit - up
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Use low dilution welding process
The use of high manganese and low carbon content fillers
Maintain a low carbon content
Minimise the amount of stress / restraint acting on the joint during welding
The use of high quality parent materials, low levelsof impurities
Use proper joint design, use Single J instead of single V
Clean joint preparations, free from oil, paints and any other sulphur containing product.
Joint design selection depth to width ratios
Solidification Cracking Solidification Cracking
Precautions for controlling solidification cracking
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Lamellar Lamellar TearingTearing
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Lamellar Tearing Characteristics Lamellar tearing has a step like appearance due to the solid inclusions linking up under the influences of welding stressesOccurs at beneath of HAZ or near HAZ It forms when the welding stresses act in the short transverse direction of the material (through
thickness direction) Low ductile materials containing high levels of impurities are very susceptible
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G Y • Occur only in rolled direction of the parent material
• Associated with restrained joints subjected to through thickness stresses on corners and tees
• Presence of elongated stringers such of nonmetallic inclusion such as silicates and sulfides parallel to steels rolling plane will produce poor through thickness ductility of the plate.
• Tearing will triggered by this such non metallic inclusion near the weld or it just outside HAZ during weld contraction.
Lamellar Tearing
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Step like appearance
Cross section
Lamellar Tearing Lamellar Tearing
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Tee fillet weld Tee butt weld (double-bevel)
Corner butt weld(single-bevel)
Lamellar Tearing Lamellar Tearing
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Critical area
Critical area
Lamellar Tearing Lamellar Tearing
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Lamellar TearingPrecautions for controlling lamellar tearing The use of high quality parent materials, low levels of impurities
( Z type material )
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Friction Welded Caps
Short Tensile Specimen
Through Thickness Ductility
Sample of Parent Material
A test for a materials susceptibility to lamellar tearing
Short Tensile TestsShort Tensile Tests
The results are given as a STRA valueShort Transverse Reduction in Area
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Lamellar TearingPrecautions for controlling lamellar tearing The use of high quality parent materials, low levels of impurities
( Z type material ) Change joint design
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Lamellar Tearing Lamellar Tearing
Modifying a Tee joint to avoid lamellar tearing
Susceptible
Susceptible Improved
Non-susceptible
Non-susceptible
Gouge base metal and fill with weld metal before welding the joint
Susceptible Less susceptible
Prior buttering of the joint with a ductile layer of weld metal may avoid lamellar tearing
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Lamellar Tearing Lamellar Tearing
Modifying a corner joint to avoid lamellar tearing
Susceptible Non-Susceptible
Prior welding both plates may be grooved to avoid lamellar tearing
An open corner joint may be selected to avoid lamellar tearing
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Lamellar TearingPrecautions for controlling lamellar tearing The use of high quality parent materials, low levels of impurities
( Z type material ) Change joint design Minimise the amount of stress / restraint acting on the joint during welding The use of buttering runs with low strength weld metal Hydrogen precautions e.g use low hydrogen electrodes Shift welding process such as Electro slag welding Use forging or casting joint. Place soft filler wire between the joint e.g T joint to reduce stresses during expansion and contraction of weld metal. Pre heating helps on removal of Hydrogen on the plate.
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Weld DecayWeld Decay
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Weld Decay Characteristics Weld decay may occurs in unstabilized austenitic stainless steels with carbon content above 0.1% Also known as knife line attack or crack Chromium carbide precipitation takes place at the critical range of 450oC-850oC (sensitising temperature ) At this temperature range carbon is absorbed by the chromium, which causes a local reduction in chromium content by promoting chromium carbides. Loss of chromium content results in lowering the materials resistance to corrosion attack allowing rusting to occur
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Precautions for Weld Decay Precautions for Weld Decay The use of a low carbon grade stainless steel e.g.
304L, 316L, 316ELC with carbon content < 0.03%
The use of a stabilized grade stainless steel e.g. 321, 347, 348 recommended for severe corrosive conditions and high temperature operating conditions
Standard grades may require PWHT, this involves heating the material to a temperature over 1100oC and quench the material, this restores the chromium content at the grain boundary, a major disadvantage of this heat treatment is the high amount of distortion
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Fatigue Fatigue CracksCracks
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Fatigue TestingFatigue Testing
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Fatigue Cracks Fatigue Cracks Fatigue cracks occur under cyclic stress
conditions
Fracture normally occurs at a change in section, notch and weld defects i.e stress concentration area
All welded materials are susceptible to fatigue cracking
Fatigue cracking starts at a specific point referred to as a initiation point
The fracture surface is smooth in appearance sometimes displaying beach markings
The final mode of failure may be brittle or ductile or a combination of both
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Initiation points / weld defects
Fatigue fracture surface smooth in appearance
Secondary mode of failure ductile fracture rough fibrous appearance
Fatigue Cracks Fatigue Cracks
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Fatigue Cracks Fatigue Cracks
A fatigue failure on a small bore pipe work
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Precautions against Fatigue Cracks Precautions against Fatigue Cracks
Toe grinding, profile grinding.
The elimination of poor profiles
The elimination of partial penetration welds and weld defects
Operating conditions under the materials endurance limits
The elimination of notch effects e.g. mechanical damage cap/root undercut
The selection of the correct material for the service conditions of the component
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Any Questions?Any Questions?
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cracking to occur
QuestionsQuestions
QU 2. State four precautions to reduce the chance of hydrogen cracking
QU 3. In which type of steel is weld decay is experienced and state how it can be prevented
QU 4. State the precautions to reduce the chances of solidification cracking
QU 5. State four the essential factors for lamellar tearing to occur