Section 4 - Overview of Damage Mechanisms

8/13/2019 Section 4 - Overview of Damage Mechanisms

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Copyright 2005, TWI Ltd World Centre for Materials Joining TechnologyWorld Centre for Materials Joining Technology

Damage Mechanismsand FFS AssessmentDamage Mechanisms

and FFS Assessment


IntroductionIntroduction

Essential to know actual and likelyEssential to know actual and likelydamage mechanisms for any FFS, todamage mechanisms for any FFS, toclarify the aims of any assessment:clarify the aims of any assessment:

How to find the damage?How to find the damage? What caused the damage?What caused the damage?

Failure mode (leakage or rupture)?Failure mode (leakage or rupture)?

Consequence of failure?Consequence of failure?

InIn--service monitoring?service monitoring?

Life prediction?Life prediction?


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Pre-service damagePre-service damage

Many flaws are created during fabrication,Many flaws are created during fabrication,

erection, transportation, etc:erection, transportation, etc:

Casting solidification flawsCasting solidification flaws

Laminar tearing after weldingLaminar tearing after welding

Hydrogen cracks (cold cracking)Hydrogen cracks (cold cracking)

OutOut--ofof--roundness and forming flawsroundness and forming flaws

Heat treatment flaws and embrittlementHeat treatment flaws and embrittlement

Mechanical damage (dents and gouges)Mechanical damage (dents and gouges)


Welding flawsWelding flaws

Slag entrapmentSlag entrapmentLack ofLack of

side wallside wall

fusionfusion

Inter run issuesInter run issues

Lack of rootLack of rootfusionfusion

PorosityPorosity

UndercutUndercut

HAZHAZ

HydrogenHydrogen

crackcrack

SolidificationSolidification

crackcrack


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FFS considerations (PS flaws)FFS considerations (PS flaws)

NonNon--destructive techniques havedestructive techniques have

become more sensitive over the yearsbecome more sensitive over the years

Are these flaws from original production, orAre these flaws from original production, orhave they grown in service?have they grown in service?

Welding flaws can often, simply beWelding flaws can often, simply be

assumed to be crackassumed to be crack--like flaws inlike flaws in

assessments, although some areassessments, although some are

volumetricvolumetric


In-service damageIn-service damage

SurfaceSurface--connected crackingconnected cracking

Subsurface crackingSubsurface cracking

MicrofissuringMicrofissuring,, microvoidmicrovoid formationformation

Metal loss due to corrosion, erosionMetal loss due to corrosion, erosion

Metallurgical changesMetallurgical changes


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API 571, Process flow diagramAPI 571, Process flow diagram


DimensionalDimensional

Bulging coke drum shellBulging coke drum shell


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Metal loss (1/2)Metal loss (1/2)

Pitting corrosionPitting corrosion

(isolated or(isolated or

scattered)scattered)


Metal loss (2/2)Metal loss (2/2)

Corrosion underCorrosion under

insulationinsulation

Tube bendTube bend

erosionerosion


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Stress corrosion crackingStress corrosion cracking

Requires stress, susceptible materialRequires stress, susceptible material

and aggressive environmentand aggressive environment

Typical examples:Typical examples:Amine SCCAmine SCC

Caustic SCCCaustic SCC

Sulphide SCCSulphide SCC

PolythionicPolythionicAcid SCCAcid SCC

Chloride SCCChloride SCC


Sour service damageSour service damage

Low temperature hydrogen relatedLow temperature hydrogen related

Hydrogen diffusion can cause:Hydrogen diffusion can cause:

BlisteringBlistering

Hydrogen induced cracking (HIC)Hydrogen induced cracking (HIC)

Stress orientated hydrogen induced crackingStress orientated hydrogen induced cracking(SOHIC)(SOHIC)


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FatigueFatigue

BeachmarksBeachmarks

StriationsStriations


Metallurgical changesMetallurgical changes

Strength, ductility, toughness andStrength, ductility, toughness andcorrosion resistance may change incorrosion resistance may change inservice, due to:service, due to:

Hydrogen embrittlementHydrogen embrittlement

Strain age embrittlementStrain age embrittlement

Temper embrittlementTemper embrittlement

Sigma phase embrittlementSigma phase embrittlement


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Section 4 - Overview of Damage Mechanisms

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