NE 110 – Introduction to NDT & QA/QC

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NE 110 – Introduction to NDT & QA/QC. Classification of Discontinuities Prepared by: Chattanooga State Community College. Review of Definitions. - PowerPoint PPT Presentation

Text of NE 110 – Introduction to NDT & QA/QC

  • NE 110 Introduction to NDT & QA/QCClassification of Discontinuities

    Prepared by:Chattanooga State Community College

  • Review of Definitionsdiscontinuity An interruption (crack, forging lap, seam, inclusion, porosity, etc.) in the normal physical structure or configuration of an articleIt may or may not be a defectdefect a discontinuity that interferes with the usefulness of a part or exceeds acceptability limits established by applicable specificationsNot all discontinuities are defects

  • More Definitionssurface discontinuity a discontinuity open to the surfaceMay be observed through VT, PT, or MTMore likely harmful to an article than a subsurface discontinuitysubsurface discontinuity a discontinuity that is not open onto the surface of an articlenonrelevant indication - indications that are not due to discontinuitiesMay be due to abrupt changes in article shape, surface roughness, etc.

  • Classification of Defects by OriginTypically grouped into 3 stages:

    InherentProcessingService-Induced (also call Inservice)

  • Inherent Discontinuities1st stage of discontinuityRelated to the melting and original solidification of metal in the ingot (i.e. original steel-making process)InclusionsPorosityPipe

  • Steps in the Steelmaking ProcessIron ore, coke, and limestone are fed into the top of a blast furnace Coke is a solid carbon fuel obtained from coalLimestone is calcium carbonateAs the coke burns, the oxygen is removed from the iron oreThe limestone reacts with the molten iron to remove impuritiesThe impurities form a slag which floats on the surface of the metal pool

  • Steps in the Steelmaking ProcessThis slag is periodically removed from the furnaceThe molten iron is drawn from the furnace and poured into ingot molds called pig ironPig iron contains 3-5% carbon; further refining is required to remove the excess carbon and improve the properties of the steel

  • How Steel is Made

    Steel-Making Video

    Types of Furnaces Blast Furnace (shown) Basic Oxygen Furnace Electric Furnace

  • InclusionsNon-metallic impurities, such as slag, oxides, and sulfides that are present in the original ingotThrough rolling of raw material into billets or bar stock, these impurities form stringers, or lines Often irregularly shaped and in groupsUsually not objectionable, except when they occur in critical areas, on highly-stressed surfaces, or in unusual numbers

  • Inclusion Example

  • PorosityDue to gas bubbles being trapped in the material prior to solidificationSimilar to bubbles in a carbonated drinkBubbles generally round or spherical in shape

  • PipeA discontinuity in the center of the ingot, caused by cavities formed during solidificationBecomes elongated in rolling operationsNot usually visible on the surface of the article

  • Processing Discontinuities2nd stage of discontinuityFormed by forming or fabrication operationsTypically subdivided into:primary processing discontinuities casting, rolling, forgingsecondary processing discontinuities machining, grinding, heat treating, welding, plating

  • Primary Processing Discontinuities - CastingInclusionsPorosity Cold shutsHot tearsShrinkage cavities

  • Casting Discontinuity ExamplesSand Inclusion


  • Primary Processing Discontinuities - CastingCold shut due to lack of fusionbetween two streams ofmetal as it flows into the cast

    Hot tear due to differencein cooling rates between thinand thick sections (a crack formed while the casting is cooling)

  • Primary Processing Discontinuities - CastingShrinkage cavity due to inadequate metal to fill the cast mold following shrinkage of the cast upon cooling

  • Primary Processing Discontinuities - FormingRolling DiscontinuitiesLaminationsImproperly fused layers of materialCan be seen from the cross-sectional view of rolled materialsStringers, pipe also caused by rollingForging DiscontinuitiesBursts - surface or internal ruptures due to forging at improper temperaturesLaps folding of metal in a thinplate onto surface of forging

  • Secondary Processing DiscontinuitiesMachiningTears due to use of dull tools or cutting too deepGrindingCracks Due to localized overheatingHeat TreatingStress cracks due to unequal heating or cooling

  • Secondary Processing Discontinuities - WeldingWeldingUndercut Lack of fusionLack of penetrationInclusionsPorosityStress cracks Crater cracks

  • Secondary Processing Discontinuities - WeldingIncomplete fusion

    Lack of penetration

  • Secondary Processing Discontinuities - WeldingSlag Inclusion

  • Secondary Processing Discontinuities - WeldingPorosity

  • Service-Induced DiscontinuitiesDiscontinuities caused by fatigue, corrosive environments, or overheating

    Fatigue CracksDevelop at areas of high stress concentrations such as holes, fillets, keyways, etc.May be due to mechanical or thermal fatigueOnce crack initiates, it can quickly propagate resulting in failure

  • Mechanical FatigueFailure of a part under repeated fluctuating stresses Applied stresses are below the tensile strength of the materialEx. Repeated loading and unloading of flow through a pipe1842 - Versailles train wreckDerailment due to broken locomotive axleCrack growth occurred due to repeated loading in this area of high stress concentration

  • Mechanical Fatigue1954 de Havilland Comet plane crashesTwo separate crashes due to metal fatigue caused by repeated pressurization and depressurization of the cabinFailure occurred at cracks that started at riveted joints

  • Thermal FatigueDue to repeated heating and cooling of a partCracks resulting from large thermal gradients are usually multiple and often have an alligator skin appearance

    To reduce chance of failure, materials must be loaded below their fatigue strength

  • Corrosive EnvironmentsCorrosive environments can lead to:EmbrittlementWhen a material loses its ability to elastically deform it breaks rather than stretchesStress corrosion cracking (SCC)Combination of stress and a corrosive environmentDifferent materials react to different corrosives

  • Corrosive EnvironmentsDecember, 1967 collapse of Silver Bridge at Point Pleasant, WVRust in an eyebar joint caused a stress corrosion crack Initial crack 0.1 deepHigh bridge loading and low temperatures caused crack to propagate to point of failure46 people killed

  • Material LossesService-Induced Discontinuities also due to loss of materialWearErosionCorrosion

  • Assignment

    Discontinuities worksheet