Defect Related Failures

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramcosemployees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,or disclosed to third parties, or otherwise used in whole, or in part,without the written permission of the Vice President, EngineeringServices, Saudi Aramco.

Chapter : Materials & Corrosion Control For additional information on this subject, contactFile Reference: COE10604 S.B. Jones on 874-1969 or S.P. Cox on 874-2488

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Defect-Related Metal Failures

Engineering Encyclopedia Materials & Corrosion Control

Defect Related Metal Failures

Saudi Aramco DeskTop Standards

CONTENTS PAGES

TYPES OF MANUFACTURER DEFECTS .............................................................. 1

Casting Defects................................................................................................ 1

Centrifugal Castings............................................................................. 2

Processing Defects .......................................................................................... 2

Additional Defects........................................................................................... 5

TYPES OF FABRICATION DEFECTS..................................................................... 6

Common Welding Defects .............................................................................. 6

Dimensional Defects ............................................................................ 6

Structural Discontinuities..................................................................... 7

Weld Cracking ..................................................................................... 8

High Hardness Welds .......................................................................... 9

Additional Metal Defects............................................................................... 10

Quench Cracking ............................................................................... 10

Cladding............................................................................................. 10

Workmanship..................................................................................... 10

Shipping, Storage, and Handling ....................................................... 10

GLOSSARY ............................................................................................................. 11

REFERENCES.......................................................................................................... 14

ADDENDUM ........................................................................................................... 15



Saudi Aramco DeskTop Standards 1

TYPES OF MANUFACTURER DEFECTS

Defects in metals account for a large number of field failures. The defects often can be tracedback to the manufacturer of the material or the fabricator of the component/equipment.

Metals and alloys that are used in the petroleum industry usually originate as castings thatwere poured from molten metal. Complex component shapes that require little or no finishingare produced by means of casting techniques such as sand, shell, and investment molding. Inother cases, molten metal is cast in ingots, which are further processed into wrought productssuch as bars or tubes with improved homogeneity and mechanical properties. A steelmakingflowline is included in the Addendum.

Casting Defects

Several types of defects are inherent to castings. Failures most commonly occur when thedefects are of such size that the remaining sound metal is insufficient to maintain the loadrequirements of the part. In some cases, defects provide stress risers which lead to crackpropagation. In other cases, surface defects cause flow-induced erosion or they provideregions for concentration of corrodants, which cause accelerated corrosion.

Common castings defects include the following:

Sand or nonmetallic inclusions, which occur when slag particles, or sand thathas broken away from the mold become entrapped in the casting duringsolidification. These defects can occur at any location within the casting, andthey must be identified by metallography or scanning electron microscopy(SEM) that employs energy dispersive X-ray spectroscopy.

Porosity, which is caused by gas bubbles that become trapped within themolten metal and cannot escape during solidification. Examples of gases thatare generated during pouring are vaporized moisture or binders in mold sands.Large pores on the casting surface are called blowholes. Porosity may occur inindividual regions or it may be randomly distributed through the casting.Additional types of porosity are described as pinholes, interdendritic porosity,and microporosity.

Shrinkage, which occurs from thermal contraction during solidification.Thinner sections of a casting solidify rapidly and shrink, and thereby causemolten metal to feed from the thicker, molten sections. As a result, shrinkagecavities form in the thicker sections. These cavities usually are more irregularthan gas porosity.

Hot tears, which occur primarily at corners or abrupt changes in casting sectionthickness. They are caused by shrinkage and stress concentration due todifferential thermal contraction. Hot tears appear as cracks which are oxidizedsignificantly.




Cold shuts, which are surface or internal discontinuities that are caused by a hotstream of molten metal as it flows into or over an already solidified surface.Other discontinuities can result when chaplets or chills inside the mold fail tomelt and become part of the casting.

Misruns, which occur when a casting is not fully formed as a result ofpremature solidification of the molten metal before the mold is filled. Misrunstypically exhibit rounded edges or corners that may be rejectable for certainapplications.

The above defects represent some of the most common problems with castings. Since thereare numerous specific defects and there are sometimes different names that are used for thesame defect, the International Committee of Foundry Technical Associations (CFTA) hasstandardized nomenclature for casting defects. The standardized tables are included in theAddendum.

In the analysis of failures in castings, it often is prudent to radiograph the casting to locatedefects. If a casting had one defect that caused a failure, it often has additional, similardefects. After defects are located, metallographic examinations and elemental analyses areused to categorize the defects. Experience indicates that when defects are found in a castingsuch as a valve body or pump casing, similar valves or pumps from the same foundry willalso contain defects and, therefore, these valves or pumps will require inspection.

Centrifugal Castings

High temperature tubing that is used in the petroleum industry is often centrifugally cast fromhigh alloy steel. The process involves the pouring of the molten alloy into molds that arespinning at high velocities. The resulting product has superior properties compared to staticcastings. While centrifugal castings usually contain far fewer internal defects, poor melt shoppractices can result in surface defects that cause failures. When orders are rushed, numerousdefects can result. In one case, a batch of centrifugal tubes was found to contain surface pitsand imperfections, hot cracks (formed while pulling tubes from molds), dimensionalproblems, and weld defects (tubes are welded together to make up coils).

Processing Defects

Wrought alloy products typically are produced from castings that are called ingots. The ingotsare then hot rolled into semifinished forms that are called blooms, billets, or slabs, or forgedinto more complex shapes. Blooms and billets are further hot rolled into bars and tubes, andslabs are rolled into sheets. Final finishing may also involve cold rolling, cold drawing, andextruding.

The above forming processes can create various defects, which are mostly surface-related, inthe finished products. The most common defects are seams and laps, where the metal isfolded over, which result in unwelded discontinuities that are similar to cold shuts. Rolled-in




scale, scratches, roll marks, and internal inclusions are additional defects that are found infinished products.




Failures that result from processing defects are analyzed through the use of ordinarymetallographic and fractographic methods. Defects in rolled, drawn, or extruded productsusually run longitudinally for great distances, and regions away from the failure typicallyreveal indications of the defects. On the surfaces, the defects may appear as thin lines orscratches. Transverse cross sections provide information on the nature and depth of the defect.Seams or laps appear in cross sections as straight or curved cracks, as shown in Figure 1. Inthis case, the defect was on the ID surface of the seamless tube, and it caused a catastrophicfailure. Defects do not necessarily have to run deeply below the surface to cause problems.The sharp tips of the defects are stress risers, which promote crack propagation, and the defectregions often provide crevices for contaminant concentration, corrosion, and environmentalcracking.

Figure 1. ID Surface Defect in Seamless Steel Tube




Additional Defects

Refined metals and alloys have specified composition ranges or limits for various elements.While occurrences are infrequent, cast or wrought components may not meet compositionalspecifications, and failures can result from degraded mechanical and physical properties. Themanufacturer of the alloy is responsible for producing heats of material with compositionswithin specifications.

Improperly applied and/or mixed (improperly identified) materials are more common causesof failure than are off-chemistry alloys. Mixed materials can occur at the manufacturers shop,fabricators shop, or at various warehouses and supply companies. In recent years, there havebeen specific equipment problems that have involved substandard bolts and valves. For theabove reasons, compositional checks should almost always be included in failure analyses.

Welded pipe and tubing have failed in service due to poor welds. Quality problems withelectrical resistance welded (ERW) pipe in particular have resulted in failures.




TYPES OF FABRICATION DEFECTS

Common Welding Defects

Failures in welds account for a high percentage of the total number of failures in equipment.Defects occasionally occur in shop welds, but more defects are found in field welds.Defective welds may be produced from improper weld technique, incorrect filler, or incorrectheat treatment. All welding must follow a welding procedure and each procedure needs to bequalified prior to use. The WPS (Welding Procedure Specification) and PQR (WeldingProcedure Qualification Record) are the two pieces of documentation that are required.Common welding defects and their descriptions are listed below:

Dimensional Defects

When weldment dimensions do not meet specifications, structures and components are proneto failure by overload. Common dimensional defects are as follows:

Weld Discontinuities due to Incorrect Joint Preparation: Incorrect preparationof a joint with the proper dimensions can result in weld discontinuities andpossible failure. Typical single V joint design dimensions are shown inFigure 2. The joint geometry is keyed to weld composition, thickness, andwelding process.

Figure 2. Typical Single V Joint Design Dimensions




Weld Discontinuities due to Joint Misalignment, which refers to offset ormismatch across butt joints. This misalignment causes stress risers at the toeand root of the weld.

Undersized Welds, which result in inadequate strength for the intendedapplication.

Structural Discontinuities

Types of structural discontinuities are as follows:

Porosity and Slag Inclusions, which affect the soundness of welds just as in thecase of castings.

Incomplete Fusion, which results when the weld metal does not fuse to the basemetal or when successive weld passes do not fuse to each other.

Incorrect Weld Profiles (Figure 3), which can result in incomplete fusion orslag inclusions when successive layers are deposited. Improper profile can alsolead to stress risers. Undercutting, underfill, and overlap are common causes offailures.

R

RNote: R denotes reinforcement.

Acceptable Butt Weld Profile

Excessive Convexity

Insufficient Throat

Excessive Undercut

Overlap

Unacceptable Butt Weld Profiles

Figure 3. Acceptable and Unacceptable Weld Profiles




Weld Cracking

Cracks occur in the weld metal, base metal, and heat-affected zone (HAZ) of a weldmentwhen localized stresses exceed the ultimate tensile strength of the metal. Cracking is oftenassociated with stress risers at discontinuities or mechanical notches in the weldment.Hydrogen embrittlement often contributes to crack formation in steel. Cracking also can resultfrom insufficient preheat, excessive interpass temperatures, using the wrong filler metal, rapidcooling, or poor fit-up.

Cracks can be classified as either hot or cold types. Hot cracks develop at elevatedtemperatures, commonly during solidification of the weld metal. Hot cracks may beassociated with impurities and high restraint. Cold cracks result from thermal stressesdeveloped during cooling and are often associated with hardenable alloys, high restraint,and/or hydrogen embrittlement. Hot cracks propagate intergranularly, while cold cracks canbe either transgranular or intergranular.

In multiple layer welds, weld cracking is most likely to occur in the first weld layer (root passor root bead). Unless the cracking is repaired, crack propagation occurs through subsequentpasses as the weld is completed.

Three types of cracks that can occur in weld metal are as follows:

Transverse Weld Cracks, which appear perpendicular to the axis of the weldand, in some cases, extend beyond the weld into the HAZ and base metal. Thistype of crack is more common in joints that have a high degree of restraint.

Longitudinal Weld Cracks, which are found mostly within the weld metal andare confined to the center of the weld. Such cracks may be the extension ofcracks that were formed at the end of the weld or in the root of the weld.

Crater Cracks, which occur whenever the welding operation is interrupted anda crater is formed. The cracks are usually star-shaped and progress only to theedge of the crater. However, crater cracks can be initiation sites for largerlongitudinal and transverse cracks. Crater cracks are found most frequently inalloys with high coefficients of thermal expansion, such as austenitic stainlesssteels.




Weld cracking that is caused by hydrogen embrittlement can also occur in the HAZ and basemetal. This cracking generally occurs at temperatures below 120 C (248 F) andimmediately upon cooling or after a period of several hours. Such cracking is known byseveral names, including underbead, cold, and delayed cracking.

Underbead Cracking (Figure 4) typically occurs in the HAZ, may be transverseor longitudinal, and is almost always associated with hardenable steel alloys.

Figure 4. Underbead Cracking

Lamellar tearing is a form of cold cracking that results from high stress in thethrough-thickness direction. Lamellar tears are generally terrace-likeseparations in base metal that are caused by welding thermal stresses.Inclusions are often associated with the cracking. Fractographic examination ofthe cracks reveals a fibrous appearance.

High Hardness Welds

Carbon steel generally is easy to weld. However, if the welding process is not followedcorrectly, or the carbon content of the steel is too high, high hardness in the weld metal orheat affected zone may occur. Hardnesses above approximately 200 HB or 225 HB, which isdetermined by using the Brinell hardness scale discussed in COE 106.07, generally areconsidered "high." High hardnesses can result in cracking in service, especially if the servicecontains hydrogen sulfide or caustic. Proper weld procedures must be used to avoid highhardness welds. If the hardness continues to be too high, a post weld heat treatment (PWHT)should be performed. Alloy steels harden very easily and almost always require a post weldheat treatment.




Additional Metal Defects

Incorrectly following heat-treating requirements/specifications can result in corrosion ormechanical failures in service. Quench cracking is one of the most serious problems withhardenable steels.

Quench Cracking

This type of cracking can occur when ferritic steels are heated above approximately 816 C(1500 F) and rapidly cooled (quenched). As the steel is heated, the ferrite and pearlitemicrostructure transforms to austenite. Upon quenching, brittle martensite is formed andaccompanied by internal stresses that are due to the thermal gradients. The internal stressesplace the surface of the piece in tension, which results in immediate or delayed cracking.

Quench cracks usually originate at the surface and propagate toward the center of the piece.Cracks are very fine and clean after quenching. If the piece was tempered after cracking,oxide may be observed within the cracks. Fractographic examination of the crack surfacesreveals a shiny, crystalline appearance since quench cracks are always intergranular.

Cladding

Cladding of vessels and equipment is performed for corrosion or erosion resistance. Whetherthe clad is applied by co-rolling, explosive bonding welding, or casting, it is important that asound interface is formed between the layers to avoid delamination. Weld overlays, as well asweld joints where clad sections are fabricated, are especially prone to failure due to defects.

Failures in weld overlayed equipment result from many of the same defects, especiallyporosity, slag inclusions, and brittle cracking, that are found in other welded joints.

Workmanship

While most failures can be explained in terms of a scientific mechanism, the root cause ofmany failures is poor workmanship. Lack of reasonable cleanliness can result incontamination and corrosion. Careless work habits can result in failures. For example, failureshave been analyzed where catastrophic cracks grew from defects such as arc strikes on theworkpiece.

Shipping, Storage, and Handling

When it is not well protected, equipment is often damaged during shipment or installation.Excessive vibration and exposure to the atmosphere during transit have caused fatiguecracking and stress corrosion cracking. Bumping and dropping components during installationhave caused overload and impact failures.




GLOSSARY

billet A semifinished product with a smaller rectangular cross sectionthan a bloom, that is produced by hot rolling, forging, orextrusion.

bloom A semifinished product with a rectangular cross section that isproduced by hot rolling an ingot.

castings (cast alloys/Static castings)

Parts or components that are solidified (molded) directly fromliquid metal into the desired shape.

chaplets Solid metal that is used to support internal parts of the mold.(Similar to chills.)

centrifugal castings Cylindrical shapes that are formed by solidification in a rotatingmold, and result in improved physical properties.

chills Solid pieces of metal that are added to a mold to increase the rateof solidification in regions of the casting. The chill becomes partof the casting.

cladding The attachment of a special alloy to a component surface toimprove corrosion or erosion (wear) resistance.

cold cracks(cold cracking)

Cracking in weld regions after the weld has cooled.

cold drawing Final forming by pulling bars or tubes through a die of a desiredcross section.

cold shuts Discontinuities that are formed in castings when molten metalsolidifies over previously solidified metal.

crater cracks Cracks that occur in the final molten puddle that remains at theend of a weld pass.

defect Flow in a part or piece of equipment that exceeds the limits ofstandards.

extrusion Hot or cold forming of metal or plastic by pushing billetsthrough a die of the desired cross section.

flaw Detectable imperfection in a part or piece of equipment.




forged (forging) Hot forming of metal into desired shapes by means ofcompressive stresses.

Heat-Affected Zone(HAZ)

Region that is adjacent to welds and in which the base metalmechanical properties and microstructural features were alteredby heating.

hot cracks (hot cracking) Cracking in weld regions that occurs while the region is at hightemperatures due to welding.

hot tears Casting defect that is caused by shrinkage and stressconcentration due to differential thermal contraction.

indication A flaw or defect detectable by NDE.

incomplete fusion Lack of metallurgical bonding (mixing of molten metals) of weldmetal to base metal or between weld passes.

lamellar tearing Base metal cracking that is parallel to the rolling direction ofplates, and is caused by thermal stress.

misrun Casting defect that results from incomplete filling of the mold.

nonmetallic inclusions Foreign particles that have been entrapped in castings duringsolidification.

porosity Casting defect that is caused by gas bubbles.

quench cracking Cracking, that is caused by internal stresses, which were createdby rapid cooling from a high temperature.

rolling Hot or cold forming of metal by compressing metal betweenmechanical rolls.

shrinkage Casting defect that is caused by thermal contraction duringsolidification.

underbead cracking Cracks that occur in the HAZ of hardenable steel welds.

Welding ProcedureQualification Record(PQR)

A record of welding data that is used for a test coupon. Thesupporting data used to establish a WPS.

Welding ProcedureSpecification (WPS)

A document that provides instruction to the welder on how tomake a weld that complies with the code.




workmanship Quality that is imparted to a product by the craftsman.

wrought products(wrought alloys)

Metal forms that are produced by hot working aftersolidification. Wrought materials with improved propertiesgenerally are more homogeneous than are castings.




REFERENCES

1. D. B. Roach and F. H. Beck, Performance and Reliability of Corrosion-Resistant Alloy Castings, MTI Manual No. 5: Phase 1 Causes ofUnsatisfactory Performance, and MTI Manual No. 6: Phase 2 CastingDiscontinuities, Materials Technology Institute of the Chemical ProcessIndustries, Columbus, Ohio, USA, 1981.

2. American Society for Metals (ASM), Failure Analysis and Prevention,Metals Handbook, Ninth Ed., Vol. 11, Metals Park, Ohio, USA, 1986.

3. The Making, Shaping, and Treating of Steel, Tenth Ed., Association of Ironand Steel Engineers, Herbick and Held, Publishers, Pittsburgh, Pennsylvania,USA, 1985.

4. R. D. Port and H. M. Herro, The Nalco Guide to Boiler Failure Analysis,McGraw-Hill, Inc., New York, New York, USA (1991).

5. American Society for Metals (ASM), Fractography, Metals Handbook, NinthEd., Vol. 12, Metals Park, Ohio, USA (1987).




ADDENDUM

Documents

Defect Related Failures