ndt intro. flaws

8/7/2019 NDT Intro. Flaws

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Introduction

NDT personnel should be familiar

with the test method, know thepotential types of flaws in a particular

material and be familiar with their

effect on the structural integrity of theitem.


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Introduction

Failure of critical components

Catastrophic results

Loss of life

Loss of production

Loss of revenue Environmental damage


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Non Destructive TestingThe presence of flaws in critical

components increases the

compromise of system integrity.

NDT often provides the only method of

identifying damage and irregularities inmaterials which would lead to failure.


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FlawsDetection

InterpretationEvaluation


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FlawsMisinterpretation and/or improper

evaluation of flaws or improperperformance of NDT can have

catastrophic results


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FlawsTypes

Corrosion

Inherent flaws

Primary processing flaws

Secondary processing flaws In-service flaws


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CorrosionElectrochemical deterioration of a

metal resulting from chemical

reaction with the surrounding

environment.


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Inherent Flaws

Present in metal as the result of its

initial solidification from the molten

state.

Cont.


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Inherent Flaws

Cont.

pipe shrinkage cavity that forms at the top

of an ingot during metal solidification Shows up as irregular voids in finished

product

Cont.


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Inherent Flaws

Cont.

porosity occur when gas bubbles aretrapped as the molten metal in an ingotmold solidifies

shows up as laminations or seams in thefinished product

Cont.


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Inherent flaws Cont.

inclusions impurities

slag sulfides

cont


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Inherent flaws

Cont.

Shrinkage cracks

can occur in castings due to stressescaused by the metal contracting as it coolsand solidifies


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Primary Processing FlawsFlaws which occur while working the

metal down by hot or cold

deformation into useful shapes suchas bars, rods, wires, and forgedshapes

cont


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Primary Processing Flaws Cont

seams surface flaws, generally long,

straight, and parallel to the longitudinalaxis of the material

Can originate from ingot blowholes and

cracks and are formed by the drawing orrolling processesCont.


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Primary Processing Flaws Cont.

laminations are formed in rolled plate,

or sheet occur from internal fissures, i.e. porosity

Cont.


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Primary Processing Flaws Cont

cooling cracks stress cracks caused by

uneven cooling

generally longitudinal

long but not necessarily straight

Cont


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forging laps results from metal being

folded over and forced into the surfacenot forming a solid piece

caused by faulty dies, oversized blanks,etc.

can occur on any area of the forgingCont.


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cold shut failure of metal to fuse

occurs in castings when part of the metal

being poured into the mold cools and

does not fuse with rest of the mental into a

solid piece


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incomplete weld penetration

failure of the weld metal to penetrate

completely through a joint before

solidifying

Cont.


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lack of fusion

occurs in welds where the temperature hasnot been high enough to melt the parentmetal adjacent to the weld

sidewall lack of fusion

inter-run lack of fusionCont.


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Primary Processing Flaws cont.

weld undercutting surface flaw running

in the toe of the weld caused by welding at too high a

temperature

decreases the thickness of the parent materialat the toe of the weldCont.


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weld crater cracks (star or spider cracks)

occur at the end of a weld run

Cont.


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weld heat-affected zone cracks

occur because of stress induced in the

material adjacent to the weld by its

expansion and contraction from thermal

changesCont.


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slag inclusion of nonmetallic solid

material within the weld forms on the surface of the weld during

the welding process

becomes imbedded when not removedprior to the next welding run


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Secondary Processing or

Finishing Flaws This category includes those flaws

associated with various finishing

operations heat treating

grinding

Cont.


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Finishing Flaws Heat treating cracks are caused by

stresses setup by unequal heating or

cooling material thickness changes

stress risers

Cont.


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Finishing Flaws Grinding cracks

thermal type cracks similar to heat treatingcracks and can occur when hardenedsurfaces are ground

usually shallow and very sharp, acts as astress riser as a potential source of fatiguefailure

Cont.


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In-Service Flaws

In-service flaws are formed after allfabrication has been completed and thepart has gone into service stress corrosion cracking

stress cracking

fatigue cracking

Cont.


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In-Service Flaws

Stress corrosion cracks can developwhen parts are placed under tensionstresses and are exposed to a corrosiveenvironment

Cont.


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In-Service Flaws

Stress cracking can occur when a partis stressed beyond the level for which itwas designed

Cont.


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In-Service Flaws

Fatigue cracks can occur in parts thathave been subjected to repeated orchanging loads while in service and aresurface connected will start at highly-stressed areas such as

stress risers

weld toe and HAZ undercut

lack of fusion arc-strike

chipping hammer damage

grinding cracks


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Factors and Considerations

Factors and considerations affecting theinspection critical nature of component

type of defect sought possible locations and orientations of flaws

maximum acceptable limits in size andallowable number of defects


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Codes and Specifications

Acceptance / Rejection criteria

Codes

example:

ASMESection V, Article IV

ASME B31.1

ASME B31.3

AWS D1.1


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Codes and Specifications

Know the code!

ndt intro. flaws

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