Testing & Evaluation of Indications

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TESTING & EVALUATION OF INDICATIONS IN METALLIC MATERIALS

(Training Module) Page 1

Whenever there is a transformation in the homogeneity and uniformity of attributes inside of an object, it can

invariably be attributed to the presence of discontinuities or imperfections (absence of substance) in the

materials. Starting up from the dislocations and atomic structure irregularities, the discontinuities can takeseveral designs and sorts such as fuel inclusions (micro-porosity ,porosity, blowholes, pipes, voids), cracks,

metallic inclusions, absence of penetration, lack of fusion, shrinkage, laps and seams, etc.

Discontinuities can be divided into a few standard classes inherent, processing, and support (in-service).

(i) Inherent discontinuities are generally formed when the metal is molten. There are two additionally sub

classifications. Inherent wrought discontinuities relate to the melting and solidification of the original ingot

previous to it is formed into slabs, blooms, and billets. Inherent cast discontinuities relate to the melting,casting and solidification of a cast report.

(ii) Processing discontinuities (primary and secondary) are usually linked to the many production processes

such as machining, forming, extruding, rolling, welding, heat healing, and plating. In the course of the

manufacturing procedure, numerous discontinuities that had been subsurface will be made open to thesurface by machining, grinding, etc.

(iii) Support (in-service) discontinuities are relevant to the many service ailments, such as tension, corrosion,

tiredness and erosion. The discontinuities may well alter the local anxiety distribution and, in addition, may

well have an impact on the mechanical or chemical (corrosion resistance) attributes.

Discontinuities should be characterized not only by their nature, but also by their form. Planar formdiscontinuities, such as cracks, laminations, incomplete fusion, and inadequate joint penetration, produce

significant notch effects. Three-dimensional discontinuities make nearly no notch influence, but amplifystresses by minimizing the weldment area. For that reason, the attributes of discontinuities which will need

to often be thought to be, consist of the size, acuity or sharpness, orientation with respect to the principalfunctioning worry and residual strain, spot with respect to the exterior surfaces and the crucial sections of theframework.

TERMS

The terms used during the testing of parts are sometimes misused and misleading. More than one inspector

has found himself or herself in a difficult position when explaining that the defect found was only a

nonrelevant indication.

This misuse of terms clouds the reliability of the test and can cause the customer to question the test results.

It is therefore important to understand that when a test is conducted there is a progression of terms to identifythe results.

When a test such as a visual, magnetic particle test or dye-penetrant test is conducted, indications may be

seen. These indications must be interpreted as either relevant or nonrelevant. The basic difference between

a relevant or nonrelevant indication is a discontinuity (relevant) or an indication formed by the way the test

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was performed or the normal configuration of the part, such as a safety wire hole in the head of a bolt

(nonrelevant).

If the interpretation of the indication results in it now being classified as a relevant discontinuity, then

further evaluation must be conducted. This further evaluation determines if the discontinuity is a defect. The

primary difference between a discontinuity and a defect is the acceptability of the item. A discontinuity

whose size, shape, orientation or location make it detrimental to the useful service of the test object or which

exceeds the accept / reject criteria of an applicable specification is considered a defect. However, there are

other factors to consider. There can be a situation where a crack in a part may eventually be detrimental, but

is within acceptable limits. In such cases, the test interval may be reduced to closely monitor the crack for

growth.

In addition to understanding the terms used during the testing process, Level II personnel should understand

the basic categories of discontinuities and when and how they are formed in metals.

DISCONTINUITY CATEGORIES

There are four broad categories for discontinuities:

1. Inherent Discontinuities

2. Primary Processing Discontinuities

3. Secondary Processing Discontinuities

4. In Service Discontinuities

INHERENT DISCONTINUITIES

Inherent Discontinuities occur as the base metal is taken from a molten state. Discontinuities formed during

this process include blow holes, cold shuts, hot tears, inclusions, pipe, porosity and segregation.

This group refers to the discontinuities that originate during the initial casting process (when the metal is

casted into ingots for further processing) and it also includes the discontinuities that are produced when the

metal is casted as parts of any given shape. The initial casting discontinuities are usually removed by

cropping the ingots but some of them remain and further change their shape and nature during the

subsequent manufacturing operations.

Cold Shut

Cold shut occurs usually during the casting of parts because of imperfect fusion between two streams of

molten metal that converged together. It could be on the surface or subsurface. It could be attributed to

sluggish molten metal, surging or interruption in pouring, or any factor that prevents the fusion of two

meeting streams.





Pipe

During solidification of molten material it shrinks causing an inverted-cone shaped cavity in the top of theingot. It could be on the surface or subsurface. If this defected region is not cut out completely before further

processing (rolling or forging) it will show up in the final product as an elongated subsurface discontinuity.

Also, pipe could occur during extrusion when the oxidized surface of the billet flows inwards toward the

center of the extruded bar.

Shrinkage Cavities

Shrinkage cavities are subsurface defects that are found in casted parts. They are caused by the lack of

enough molten metal to fill the space created by shrinkage (similar to pipe in an ingot).





Micro-shrinkage Cavities

Micro-shrinkage cavities are aggregates of subsurface defects that are found in casted parts. They are usually

found close to the gate and they occur if metal at the gate solidifies while some of the metal beneath is stillmolten. Also, micro-shrinkage could be found deeper in the part when molten metal enters from the light

section into heavy section where metal could solidify in the light section before the heavy section.

Hot Tears

Hot tears occurs when low melting point materials segregate during solidification and thus when they try toshrink during solidification cracks and tears will develop because the surrounding material has already

solidified.

Also, hot tears occur at the joining of thin sections with larger sections because of the difference of the

cooling rate and thus solidification.





Blowholes and Porosity

Blowholes and porosity are small rounded cavities found at the surface or near surface of castings and they

are caused by the entrapped gasses that could not escape during solidification. Blowholes are caused by

gases released from the mold itself (external gases) while porosity is caused by gases entrapped in the

molten material (internal gases). During subsequent manufacturing operations these gas pockets get flattened

or elongated or fused shut.

Nonmetallic Inclusions

Nonmetallic (or slag) inclusions are usually oxides, sulfides or silicates that remained with the molten metal

during original casting. The properties of those inclusions are different from the metal and usually they have

irregular shapes and discontinuous nature therefore they serve as stress raisers that limit the ability of the

material to withstand stresses.

Segregation





Segregation is localized differences in material composition (and thus mechanical properties) caused by the

concentration of some alloying elements in limited areas. These compositional differences may be equalized

during subsequent hot working processes but some still remain.

PRIMARY PROCESSING DISCONTINUITIES

Primary Processing Discontinuities occur during the forming or fabrication stage of making a part. Forming

and fabrication operations include: rolling, forging, drawing, extruding and welding in the case of

fabrication. Discontinuities that can be formed during these operations include seams, laminations, laps,

cooling cracks, bursts, stringers, cupping, slugs, gouging and hydrogen flakes. This group refers to the

discontinuities that originate during hot or cold forming processes (extrusion, forging, rolling, drawing,

welding, etc.). Also, some of the inherent discontinuities in the material could propagate and become

significant.

Seams

Seams are elongated surface discontinuities that occur in bars during rolling or drawing operations. They

result due to under-filled areas that are closed shut during rolling passes. Those under-filled areas may result

because of blowholes or cracks in the material. Also seams may result from the use of faulty, poorly

lubricated or oversized dies. Seams are formed either during the rolling or drawing process. Figure 1 shows

how a seam is formed during a rolling operation. Seam indications can appear as very fine cracks, but

normally as edges that can be intermittent in length.





Figure 1: Formation of a seam: (a) underfill results when there is not enough metal to fill the rolls; and (b) a

seam in the finished bar occurs when underfill is squeezed tight on a subsequent rolling pass.

Laminations

Laminations are thin flat subsurface separations that are parallel to the surface of plates caused by the rolling

of a base metal that had inherent discontinuities such as pipe, porosity, inclusions or blowholes. Lamination

indications are often found on the edge of the material after it has been cut and are parallel to the direction of

rolling.

Forging and Rolling Laps





Laps are elongated surface discontinuities that occur during rolling or forging operations due to the presence

of some excessive material (fin) that is folded over. They may result because of oversized blanks or

improper handling of the material in the die or a misaligned die thereby squeezing out material that is rolled

or forged over. Figure 2 shows how a lap is formed. Lap indications are normally very fine and not jagged.

Figure 2: Formation of a lap: (a) an overfill produces excess metal squeezed out of the rolls causing a fin;

(b) a lap results when the projection is folded over and forced back into the bar’s surface during a subsequent

pass.

Internal or External Bursts

Internal bursts are found in bars and forgings formed at excessive temperatures due to presence of inherentdiscontinuities that are pulled apart by the tensile forces developed during the forming operation.

External bursts occur when the forming section is too severe or the sections are thin. Bursts are formed

when a metal is worked at the improper temperature or at too high a rate of speed or material feed. Burst

indications can occur in different directions and can be very deep.

Cooling Cracks, as a primary processing discontinuity, may form during a rolling operation when cooling

of the material is not properly controlled. The uneven cooling causes internal pressures that are released by





the cracking of the material. Cooling crack indications are normally forked and do not necessarily follow any

pattern or direction in the part. Cooling cracks may occur on the surface of bars after rolling operations due

to stresses developed by uneven cooling. They run in the axial direction (similar to seams) but unlike seams,

they do not have surface oxidation (see figure 3).

Stringers

Stringers are elongated subsurface discontinuities that are found in bars (they run in the axial direction).

They result from the flattening and lengthening of nonmetallic inclusions during the rolling process.

Cupping





Cupping is a subsurface discontinuity that may occur in bars during extrusion or sever cold drawing. It is a

series of cone-shaped internal ruptures that happen because the interior of the material cannot flow as fast as

the surface where that causes stress build-up and thus rupture.

Slugs

Slugs are surface discontinuities found on the inner surface of seamless (extruded) tubes. They occur when

some metallic pieces that are stuck on the mandrel, are torn and fused back on the inner surface of the tube.

GougingGouging is surface tearing found on the inner surface of seamless (extruded) tubes and it is caused by

excessive friction between the mandrel and the inner surface of the tube.





Hydrogen Flakes

Hydrogen is available during manufacturing operations (from decomposition of water vapor or hydrocarbons

“oil”, atmosphere, etc.) and it dissolves in material at temperatures above 200° C. Hydrogen flakes are thin

subsurface discontinuities that develop during cooling of large size parts produced by forging or rolling

because of the entrapment of hydrogen resulting from rapid cooling.

FORGING DISCONTINUITIES

Forging operations can develop discontinuities unique to the forging operation. Such discontinuities includeforging bursts, forging laps and flash line tears.

Forging Bursts are the rupture of the material caused by working the material at an improper temperature or

at too high a rate of speed or material feed. These ruptures can be internal or external. The external bursts

can be easily detected by magnetic particle inspection (MPI). The indications can be very jagged and vary in

length and width. Internal bursts are not normally detected by MPI or PT.





Forging Laps are caused by the misalignment of the dies as the material is worked, blanks that are too large

for the dies or improper placement of the blanks in the dies. (See Figure 2) Forging lap indications are

usually smooth and straight or slightly curved.

Flash Line Tears are formed when a small amount of material from the die is squeezed between the dies

and forms what is known as flash. After the forging operation the flash is trimmed from the forging. If the

trimming operation is not done properly a tear or crack will form.

CASTING DISCONTINUITIES

Casting operations involve the pouring of molten metal into molds to form objects. These operations can

result in cold shuts, hot tears or shrinkage cracks, among others, being formed in the final product.

Cold Shuts are formed when molten metal splashes inside of the mold and solidifies on the mold wall

before the remaining molten metal is poured. Cold Shuts vary in size based on the size or shape of the mold,

the amount of molten material being poured, or the temperature conditions of the molten material or the

mold. Cold Shuts often look very similar to a forging lap.

Hot Tears are caused by uneven cooling between different sections of the casting. They usually occur at the

junction of thick and thin sections of a casting. Hot tear indications look very much like cracks.

Shrinkage Cracks occur as the molten material cools. As the material cools, it gets smaller or contracts,

causing stress factors within the casting. These stresses are sometimes relieved by the formation of shrinkage

cracks.

WELDING INDICATIONS





Welding Indications occur during the primary process stage and usually consist of lack of penetration or lack

of fusion in the weld. Both of these indications are usually subsurface. Lack of penetration will normally

appear to progress down the center of the weld bead. The lack of fusion indication will normally be to either

side of the center of the weld bead. In addition to these two welding indications, welding operations can alsocause cracks, crater cracks, porosity, inclusions, overlap and undercut, among others.

A welder's primary concern in any kind of work is ensuring his weld is sound. For this reason, it's important

for an inspector examining the weld to be able to spot a variety of weld discontinuities, including:

1. Porosity.

2. Incomplete fusion.

3. Incomplete joint penetration.

4. Unacceptable weld profiles.

5. Cracking.

The first step toward understanding weld discontinuities is to examine some welding terminology.

Discontinuity is defined as an interruption of the typical structure of a material, such as a lack of homogeneity in its mechanical, metallurgical, or physical characteristics. A discontinuity could be the resultof a defect, but isn't necessarily a defect.

A defect, on the other hand, is a discontinuity that by nature or accumulated effect (for example, total crack

length) renders a part or product unable to meet minimum applicable acceptance standards or specifications.

A defect results in rejection of the part or product.

Because we're examining these phenomena outside the requirements of specific welding codes or standardsand don't discuss their limitations in terms of these documents, we'll use the word discontinuities to describe

them. The first type of weld discontinuity we'll address is porosity.

PorosityPorosity is defined as a cavity-type discontinuity formed by gas entrapment during solidification (small

cavities or bores, which mostly have spherical shape, that are found on the surface of the weld or slightly

below surface when some constituents of the molten metal vaporize causing small gas pockets that getentrapped in the metal as it solidifies). These trapped gases in the molten weld may form bubbles or pockets

as the weld solidifies. These small bores could have a variety of shapes but mostly they have a spherical

shape. The distribution of bores in weld metal could be linear (linear porosity) or they could be clusteredtogether (cluster porosity).

The four main reasons for the presence of gases that cause porosity are:

1. Dirty base material contaminated with hydrocarbons such as oil, grease, or paint.

2. Moisture on the joint surface or electrode in the form of water or hydrated oxides or water leaks

from poorly maintained cooling systems that can introduce hydrogen into the welding process.

3. Insufficient or improper shielding caused by an inadequate shielding gas flow rate; gas that's

contaminated from its source or from its delivery system; or wind or draft that prevents the gas

from adequately protecting the molten weld metal.

4. Incorrect welding conditions or techniques.





5. Porosity often is classified by its shape and distribution within the weld, such as uniformly or

randomly scattered, cluster, or linear. Each of these porosity distributions may have different levels

of acceptance within a welding code or standard.

The most practical methods for controlling or eliminating porosity are to use clean base materials, properlystore uncontaminated welding consumables, adequately maintain welding equipment, use proven welding

procedures, and weld in acceptable environmental conditions.

Incomplete Fusion and Incomplete Joint Penetration

Because these terms are sometimes misused, it's important to understand the difference between these twoweld discontinuities.

Incomplete fusion (Lack of Fusion) is a weld discontinuity in which fusion doesn't occur between the weld

metal and fusion faces or adjoining weld beads. This absence of fusion can occur at any location within theweld joint and be present in fillet welds or groove welds.





Incomplete fusion may result when the temperature of the base material or previously deposited weld metalis not elevated to its melting point during the welding process. Incomplete fusion often is found on one leg

of a fillet weld and is caused by an incorrect welding angle, which distributes heat non-uniformly between both sides of the joint. It also may be caused by oxides or other foreign material on the surface of the base

material.

This typically occurs when welding large components that could dissipate heat rapidly especially when it is

at a relatively low temperature before welding. Incomplete Fusion (Lack of fusion) is often seen at the beginning of the first pass and in such case it is commonly called a cold start. Also, lack of fusion could

occur when the surface a previous pass is not properly cleaned from slag where slag reduces the heating of

the under-laying surface.

Incomplete joint penetration (Lack of Penetration) is a discontinuity in a groove weld in which the weld

metal doesn't extend through the joint thickness [insufficient (less than specified) penetration of the weld

metal into the root of the joint]. It's the failure of the filler metal or base metal to fill the root of the weldcompletely.

Some common causes of incomplete joint penetration are improper welding parameters such as; lowamperage, oversized electrode or improper angle, high travel speed, or inadequate surface pre-cleaning. A

bad groove weld design or a fit-up that is unsuitable for the welding conditions. Incomplete joint penetration

can occur if the root face dimensions are too large, the root opening is too small, or the included angle of aV-groove weld is too narrow. All of these joint design problems restrict the weld's ability to penetrate the

joint's thickness.

Incomplete joint penetration can be prevented with the correct joint design and fit-up in accordance with

welding procedure requirements.

Understanding these weld discontinuities will help welding inspectors identify them and, more important, prevent them from occurring in production. Using welding inspection as a preventive tool within the qualitysystem is more efficient than using it only as an appraisal technique to sort bad welds from good welds.





Unacceptable Weld Profiles

The profile of a completed weld may have a considerable effect on the performance of that weld in service.Welding inspectors must identify discontinuities through visual inspection and evaluate their acceptance or rejection according to the applicable welding code or standard acceptance criteria.

Unacceptable weld profiles can cause a reduction in base material thickness, reduction in weld size, or stress concentrations on the weld or plate surface. These types of weld discontinuities often seriously detract

from the overall performance of a welded component in service.

Some weld profile discontinuities are undercut, overlap, insufficient throat, and excessive convexity.

Undercut. Undercut is defined as a groove melted into the base metal adjacent the weld toe, or weld root,

and left unfilled by weld metal.





Undercut The term undercut describes two specific conditions. The first is the melting away of the base material atthe side wall of a groove weld at the edge of a bead, which produces a sharp recess in the side wall in the

area where the next bead is to be deposited. This type of undercut can entrap inclusions within the recess,

which then may be covered by a subsequent weld bead.

This condition usually can be corrected by grinding down the recess before depositing the next bead. If theundercut is slight, however, an experienced welder who knows how deep the arc will penetrate may not need





to remove it. Undercut of the side wall of a groove weld won't affect the completed weld if the condition is

corrected before the next bead is deposited.

The second condition is reduction of the base metal's thickness at the line where the weld bead on the final

layer of weld metal ties into the surface of the base metal. This position is known as the toe of the weld. Thiscondition can occur on a fillet weld or a butt joint.

The amount of undercut permitted at the surface of the completed weld usually is specified within thewelding code or standard being used. The maximum permissible undercut requirements for completed welds

should be followed stringently, because excessive undercut can seriously affect the performance of a weld,

particularly if it's subjected to fatigue loading in service.

Both types of undercut usually are caused by an incorrect welding technique, incorrect electrode positioning,or incorrect travel speed. High currents and a long arc increase the probability of undercut.

Overlap

Overlap is defined as a protrusion of weld metal beyond the weld toe, or weld root. This condition occurs in

fillet welds and butt joints and produces notches at the toe of the weld that are undesirable because of their





resultant stress concentration under load. This discontinuity can be caused by incorrect welding techniques

or insufficient current.

Insufficient Throat

Insufficient throat usually occurs in fillet weld and butt joint profiles that are concave. Excess concavityreduces throat thickness, which considerably reduces weld strength. This condition usually is caused byexcessive welding current or arc lengths.

Excessive Convexity

Excessive convexity can produce a notch effect in the welded area and, consequently, concentration of stress under load. For this reason, some codes and standards specify the maximum permissible convexity of

a weld profile. Insufficient current or incorrect welding techniques typically cause this condition.

Cracking

Cracks in a weldment are probably the most dreaded of all the weld discontinuities. Because so manymaterials and applications are used in welding, cracking is a complex subject.





The base material's crack sensitivity may be associated with its chemistry and its susceptibility to theformation of elements that reduce its ductility. Excessive stresses in the weld joint, particularly if the

material is in a crack-sensitive condition, can cause cracking to occur.

The welding operation itself can produce stresses in and around the weld, introducing extreme localizedheating, expansion, and contraction.

Cracking often is caused by stress concentration near discontinuities in welds and base metal and near mechanical notches in the weldment design. Hydrogen embrittlement, a condition that causes a loss of

ductility and exists in weld metal because of hydrogen absorption, can contribute to crack formation in some

materials.

Hot and Cold Cracks

Cracks are classified as one of two types: hot or cold.

Hot cracks develop at elevated temperatures, propagate between the grains of a material, and commonlyform during solidification of weld metal.

Cold cracks develop after solidification of the weld, as a result of stresses, and propagate both betweengrains and through grains. Cold cracks in steel sometimes are called delayed cracks and often are associated

with hydrogen embrittlement.

Hot cracks and cold cracks can be further categorized as base material cracks and weld metal cracks.

Base Material Cracks. Heat-affected zone (HAZ) cracking most often occurs with base material that can behardened. High hardness and low ductility in a HAZ often are the result of a metallurgical response to

welding thermal cycles. In ferritic steels, hardness increases and ductility decreases with an increase in

carbon content and a faster cooling rate.

The HAZ hardness depends on the base material's ability to be hardened, which in turn depends on the basematerial's chemical composition. Carbon has a predominant effect on steel's hardenability. For instance, cast

iron contains between 2 and 4.5 percent carbon, which gives the alloy high hardness and low ductility.

Welding this material without seriously considering cooling rates and residual stress invariably will result in base material cracking.

Cold Cracks

Cold cracks, also known as delayed cracks, are hydrogen induced surface or subsurface cracks that appear inthe heat affected zone or the weld metal during cooling or after a period of time (hours or even days). The

sources of hydrogen which leads to this type of cracks may include moisture in the electrode shielding, theshielding gas or base metal surface, or contamination of the base metal with hydrocarbon (oil or grease).





Hot Cracks

Hot cracks include several types of cracks that occur at elevated temperatures in the weld metal or heataffected zone. In general, hot cracks are usually associated with steels having high sulfur content. The

common types of hot cracks include:

Solidification Cracks: This type occurs near the solidification temperature of the weld metal. They arecaused by the presence of low melting point constituents (such as iron sulfides) that segregate during

solicitation then the shrinkage of the solidified material causes cracks to open up.

Centerline Crack is a longitudinal crack along the centerline of the weld bead. It occurs because thelow melting point impurities move to the center of the weld pool as the solidification progresses from

the weld toe to the center, then shrinkage stresses of

the solidified material causes cracking along the centerline. The likelihood of centerline crackingincreases when the travel speed is high or the depth-to-width ratio is high.

Crater Crack which occurs in the crater formed at the crater formed at the termination of the weld pass. Crater cracks are mostly star shaped and they are caused by three dimensional shrinkage

stresses. The likelihood of crater cracks increases when welding is terminated suddenly.





Liquidation Cracks: This type, also known as hot tearing, occurs in the heat affected zone when thetemperature in that region reaches to the melting temperature of low melting point constituentscausing them to liquidate and segregate at grain boundaries. As the weld cools down, shrinkage

stresses causes the formation of small micro-scale cracks which later might link up due to appliedstresses to form a continuous surface or subsurface crack.

Lamellar TearingLamellar tearing is a subsurface discontinuity that occurs in rolled plates having high content of nonmetallic

inclusions. Those inclusions have low strength and they are fattened during roiling, thus they can be tornunderneath the welds because of shrinkage stresses in the through thickness direction.

Inclusions

Inclusions refer to the presence of some material that is not supposed to be present, in the weld metal.

Slag Inclusions: This type of inclusions mostly happens in shielded metal arc welding (SMAW) and it

occurs when the slag cannot float to the surface of the molten metal and get entrapped in the weld metalduring solidification. This could happen when; the solidification rate is high, the weld pool viscosity is high,

an oversized electrode is used, or slag on the previous pass was not properly removed.





Tungsten Inclusions: This type of inclusions can be found in weld metal deposited by gas tungsten arcwelding (GTAW) as a result of allowing the tungsten electrode to come in contact with the molten metal.

Oxide Inclusions: This type of inclusions results from the presence of high melting point oxides on the basemetal which mixes with the molten material during welding.

Weld Metal Cracks

Weld metal cracks can be divided into three types:





1. Transverse, which are perpendicular to the direction of the weld.

2. Longitudinal, which travel in the same direction as the weld and often are confined to the center of

the weld. This type of crack may be an extension of a crack that originally initiated at the end of a

weld.

3. Crater, which can be formed by an abrupt weld termination if a crater is left unfilled with weldmetal. These cracks usually are star-shaped and initially extend only to the edge of the crater.

However, they can propagate into longitudinal weld cracks.

Dealing With Cracks. Cracks are unacceptable discontinuities and are detrimental to weld performance. A

crack, by its nature, is sharp at its extremities, so it acts as a stress concentration. The stress concentrationeffect of a crack is greater than that of most other discontinuities.

Cracks have a tendency to propagate, contributing to weld failure under stress. Regardless of their size,cracks aren't permitted in weldments governed by most fabrication codes. They must be removed by

grinding or gouging, and the excavation filled with sound weld metal.

Successful welding procedures incorporate the controls that are necessary to overcome the tendency for crack formation. Such controls are preheating temperature, interpass temperature, consumable type and

preparation, and post weld heat treatment.

Welding inspectors are responsible for evaluating these procedural controls during inspections to ensurewelding is performed to minimize the possibility of weld cracking.

Detecting and Evaluating Discontinuities from Welding Operations

Weld discontinuities often are detected through visual inspection. Some are detected with inspectionmethods, such as radiography, ultrasonics, liquid penetrant, and magnetic particle inspection.





The maximum acceptable limitations for these discontinuities depend on the performance requirements of

the welded component and are specified in the appropriate welding code, standard, or specification. The

welding inspector often is required to determine the extent of discontinuities and to establish their

acceptance or rejection based on the relevant acceptance criteria.

SECONDARY PROCESSING DISCONTINUITIES

This group refers to the discontinuities that originate during grinding, machining, heat treating, plating and

related finishing operations. Secondary processing discontinuities are formed during the manufacture or

fabrication of products. These secondary processing operations involve taking products that have been

forged, casted, extruded, or made by another primary processing method and performing finishing

operations such as machining, heat treatment and grinding.

Machining Tears

During machining operations metal surfaces can be torn or burned. The tearing of the metal surface results

from a tool bit dragging the metal surface instead of cutting it. A dull tool bit usually causes this tearing of

the surface, although an improperly shaped tool bit can cause metal tears also. The burning of the metal

surface can be caused by performing the machining operation at the improper speed or by a dull bit. Such

discontinuities serve as stress raisers and can lead to premature failure of a component especially when it is

subjected to fatigue loading. Machining tears are normally very smooth and parallel to the direction of

machining.

Heat Treatment (Quenching) Cracks

While heat treat cracks can be formed during the heating of the product, they often form during the coolingdown operation [especially when harsh media is used for quenching (such as cold water, oil quenching is

less harsh)]. One type of crack is the quench crack that is formed when the heated part is cooled too rapidly

by quenching it in a cooling medium. During quenching the material at the surface cools immediately upon

contacting the liquid while the material inside take relatively longer time. This difference in cooling rate

causes residual stresses in the component and could also result in cracks at the surface if the residual tensile

stress is higher than the strength of the material. In steels, austenite is transferred into ferrite and martensite

upon cooling. This transformation results in volume increase and thus causes tensile stresses at the surface

layer since the material at the surface transformed and solidified before material at the core. Other cracks can

be formed when a heat treated part of varying thicknesses is allowed to cool in an uncontrolled manner. Heat

treat cracks are normally jagged and do not follow any defined pattern.

Grinding cracks

These are caused by the overheating of a localized area by excess grinding at a right angle to the grinding

direction. Grinding cracks usually appear as very fine localized cracks with no discernible pattern. They

have also been described as having the appearance of lightning in the sky, or as a series of parallel





indications, or when severe, as a lattice network. Such cracks might be caused by the use of glazed wheels,

inadequate coolant, excessive feed or grinding depth. Grinding cracks are not normally deep, however, they

are very serious because they do form stress risers in the product.

Pickling Cracks

Pickling is chemical surface cleaning operation (using acids) used to remove unwanted scale. Picking cracks

are hydrogen induced cracks caused by the diffusion of the hydrogen generated at the surface into the base

metal. Such cracks mostly occur in materials having high residual stresses such as hardened or cold worked

metals.

Plating Cracks

Plating cracks are surface discontinuities that can develop due to the penetration of hydrogen or hot plating

material into the base metal. Also, some plating materials (such as chromium, copper and nickel) produce

residual tensile stress which can reduce the fatigue strength of a component.

IN SERVICE DISCONTINUITIES

This group refers to the discontinuities that originate or develop while the component is in service. The

service conditions (loading, mechanical and chemical environment, maintenance) of a component affect its

expected life. Although most of service discontinuities might look somehow similar but they are caused by

different failure mechanisms.

In service discontinuities are formed while the product or part is in use. There are basically two types of inservice discontinuities: fatigue cracks and corrosion cracking.

Cracking can be caused by overloading or cyclic loading of a part. The crack indication (fatigue cracks) has

a very distinctive jagged appearance. This can be detected by MPI / PT.

Corrosion is the basic process by which a material returns to its natural form. In some cases where a metal is

exposed to a corrosive environment and stress loads, stress corrosion cracking will form. This can be

detected by MPI / PT.

Fatigue CracksWhen a component is subjected to fatigue stress (cyclically applied stress), fatigue cracks can develop and

grow and that will eventually lead to failure (even if the magnitude of the stress is smaller than the ultimate

strength of the material). Fatigue cracks normally originate at the surface but in some cases can also initiate below surface. Fatigue cracks initiate at location with high stresses such as discontinuities (hole, notch,

scratch, sharp corner, porosity, crack, inclusions, etc.) and can also initiate at surfaces having rough surface

finish or due to the presence of tensile residual stresses.





According to Linear-Elastic Fracture Mechanics (LEFM), fatigue failure develops in three stages:

Stage 1: development of one or more micro cracks due to the cyclic local plastic deformation at a

location having high stress concentration.

Stage 2: the cracks progress from micro cracks to larger cracks (macro cracks) and keep growing

making a smooth plateau-like fracture surfaces which usually have beach marks that result from

variation in cyclic loading. The geometry and orientation of the beach marks can help in determiningthe location where the crack originated and the progress of crack growth. The direction of the crack

during this stage is perpendicular to the direction of the maximum principal stress.

Stage 3: occurs during the final stress cycle where the remaining material cannot support the load,thus resulting in a sudden fracture.

The presence of the crack can (and should) be detected during the crack growth stage (stage 2) before the

component suddenly fails.

Creep Cracks

When a metal is at a temperature greater than 0.4 to 0.5 of the melting point and is subjected to a high

enough value of stress (lower than the yield strength at room temperature but it is actually higher than theyield strength at the elevated temperature), it will keep deforming continuously until it finally fractures. Such

type of deformation is called creep and it is caused by the continuous initiation and healing of slipping

dislocation inside the grains of the material.

According to the rate of progress of the deformation, three stages of creep deformation can be distinguished:

Initial stage (or primary creep): the strain rate is relatively high but slows with increasing time due towork hardening.

Second stage (or steady-state creep): the strain rate reaches a minimum and becomes steady due tothe balance between work hardening and annealing (thermal softening). The characterized "creepstrain rate" typically refers to the rate in this secondary stage.





Third stage (or tertiary creep): the strain rate exponentially increases with stress because of necking

phenomena and finally the component ruptures.

Creep cracks usually develop at the end of the second stage (the beginning of third stage) and theyeventually lead to failure. However, when a component reaches to the third stage, its useful life is over and

thus creep should be detected (by monitoring the deformation) during the second stage which takes the

longest time period of the three stages. For steels, adding some alloying elements such as molybdenum and

tungsten can enhance creep resistance. Also, heat treatments that produce coarse grains (such as annealing)

can also life under creep conditions.

Stress Corrosion Cracks

Stress corrosion cracks are small sharp and usually branched cracks that result from the combined effect of a

“static” tensile stress and a corrosive environment. The stress can either be resulting from an applied load or

a residual stress. Stress corrosion cracks can lead to a sudden failure of ductile materials without any previous plastic deformation. The cracks usually initiate at the surface due to the presence of pre-existing

discontinuity or due to corrosive attack on the surface. Once the cracks initiate the surface, corrosive

material enters the cracks and attacks the material inside forming corrosion products. The formation of the

corrosion products (which have a larger volume than original metal ) inside the tight cracks causes a

wedging action which increase the stress at the crack tip and causes the crack to grow. The corrosive

environment varies from material to material; for examples saltwater is corrosive to aluminium and stainless





steel, ammonia is corrosive to copper alloys, and sodium hydroxide is corrosive to mild steel. The resistance

to corrosion can be improved by plating the surface of a component by appropriate material which does not

react with the environment.

Hydrogen Cracks

Hydrogen cracking, also known as hydrogen embrittlement, results from the presence of hydrogen medial

and usually occurs in conjunction with the presence of applied tensile stress or residual stress. Hydrogen can be already present in the metal due to previous processes such as electroplating, pickling, and welding in

moist atmosphere or the melting process itself. Also, hydrogen can come from the presence of hydrogen

sulfides, water, methane or ammonia in the work environment of a component. Hydrogen can diffuse in the

metal and initiate very small cracks at subsurface cites (usually at the grain boundaries) subjected to high

values of stress. The presence of such cracks at several locations causes ductile materials to show brittle

fracture behaviour.





NONRELEVANT VERSUS RELEVANT

A nonrelevant indication is an indication caused by something that does not interfere with the use of a part,

whereas a relevant indication is caused by something that may interfere with the use of the part. These two

very simplistic definitions point out the major factor that separates nonrelevant and relevant indications, the

potential impact of the indication on the usefulness of the part.

Nonrelevant indications can be caused by any of the following situations:

1. Sharp Fillets

2. Part construction or geometry

3. Heat Treatment boundaries

4. Grain boundaries

5. Dissimilar metals

6. Pressed fits

7. Lack of process control

In each case the indication must be evaluated to determine its true cause and, if required, must be corrected.

The correction may be necessary because the nonrelevant indication may mask a relevant indication.

In contrast, a relevant indication is one caused by an unintentional discontinuity in the part. While the

indication may be relevant, it may not be considered a defect. A relevant indication caused by a

discontinuity is only a defect when it interferes with the usefulness of the part or does not meet theacceptance criteria.

ACCEPTANCE AND REJECTION CRITERIA





The average technician does not have a problem determining the difference between a nonrelevant and

relevant indication. However, the determination of whether the relevant indication is a defect can be

difficult. It would be simplistic to say that all parts with relevant indications should be rejected. The resulting

loss of useful product, labour hours, materials and profits would soon confirm that technicians must be provided with acceptance and rejection criteria. For these reasons, and others, most specifications require

that procedures contain acceptance and rejection criteria for a part.

AIDS FOR INDICATION EVALUATION

Aids used to assist inspectors in the evaluation of an indication can be very valuable. For example, by using

a magnifying lens a technician can better see details that may assist in identification.

Another aid in indication interpretation is the viewing of the indication as it forms. Removing the indication

and re-inspecting to determine if the indication is relevant or nonrelevant.

DISCONTINUITY EVALUATION

The process of discontinuity evaluation as presented here should not be confused with establishing the

acceptance / rejection criteria. The process of discontinuity evaluation presented in this section is based on

the principle that the technician needs to understand the basic facts about the part being tested.

Understanding these basic facts helps the inspector determine the difference between nonrelevant and

relevant indications and properly identify relevant indications that can cause a part to be rejected.

HISTORY OF PART

Knowing the history of a part may be unnecessary when testing production line parts. However, for thetesting of in service parts, this knowledge can be vital. Parts that are to be tested may have been subjected to

excess stress, which may cause discontinuities not normally associated with the routine testing of the part.

Historical knowledge allows the inspector to know that there is a special handling requirement for the part.

MANUFACTURE PROCESS

Knowledge of the manufacturing process helps the technician to better understand the different types of

discontinuities he or she may need to identify. For example, knowing that a forged part was tested after the

forging process lessens the need to look for forging related discontinuities when, later in the manufacturing

process, that same part is tested for heat treatment discontinuities.

POSSIBLE CAUSES




Understanding the possible causes for a part’s failure helps the technician concentrate the testing effort. For

example, in the casting process the part may have several areas where there are several changes between

thick and thin sections. By knowing where hot tears can develop during the cooling process, the technician

can concentrate added attention to these areas.

USE OF PART

The use of a part and the history of the part can be closely related. It is important to know how the part is

used while in service. With that information, the technician can better understand how and where a crack

may form. For example, by knowing where a part is used in a corrosive environment and under cyclic stress

loading, the inspector can place added emphasis on looking for the early signs of stress corrosion cracking.

TOLERANCES

It has already been pointed out that relevant indications may be valid discontinuities in a part but not defects.

However, in some situations those discontinuities may have tolerances placed on them. For example, a part

may have repair tolerances for cracks found in certain areas. If the crack exceeds the repair tolerance, the

part is rejected. However, if the crack is within the repair tolerance it should not be rejected but properly

identified and sent to the appropriate area for rework and retesting.

Documents

Testing & Evaluation of Indications