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lack of fusion in welded joints

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·Home ·Table of Contents ·General Lack of fusion in welded joints Gabriel RIHAR Welding Institute, Ptujska 19, SI1000 Ljubljana Contact Abstract. For an efficient examination of welded joints, thorough knowledge not only of nondestructive testing methods but also of the defects to be detected is required. In the welding literature, however, there are very few data on lack of fusion, which is one of the most serious defects. There are also no efficient testing methods available for its detection. Our studies were to provide as much data as possible on lack of fusion. It was investigated with which welding processes lack of fusion is most frequent, what its position in a welded joint is and what its physical properties are. It was also studied what kind of indications the lack of fusion gives with different nondestructive methods. Testing was carried out with typical welded joints. Metallographic and mechanical testing was also used in order to make a comparison with nondestructive testing. Various types of lack of fusion were classified into characteristic groups, i.e. the types including voids and nonmetallic inclusions and the types of pure lack of fusion which cannot be detected by nondestructive testing. Key words: welding, NDT, weld defects, lack of fusion. 1 Introduction It is known from the welding practice that the lackoffusion defects often occur in a welded joint. Many a failure of a welded structure thus occurs due to undetected lack of fusion in welded joints. Lack of fusion is one of the most serious weld defects. It produces the notch effect. There are, however, no efficient nondestructive methods available for its detection. If lackoffusion defects and cracks are compared, it is evident that much more attention is paid to cracks than to lack of fusion although lack of fusion is as serious a defect as a crack. In the welding literature numerous papers deal with cracks but very few with lack of fusion. Only some older documents of the International Institute of Welding were found to report on lack of fusion [14]. It was, therefore, decided to study lack of fusion more closely. It was to be found out where it occurs most frequently, under which conditions it forms and how it can be detected. 2 Definition of lack of fusion In welds, incompletely fused spots, called lack of fusion, persist. A weld can lack union with the parent metal or with a previous weld bead. An adhesion joint forms, which can be rather strong in certain cases. It is much like a brazed joint or joint formed in metallisation. The purer lack of

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  • HomeTableofContentsGeneral

    Lackoffusioninweldedjoints

    GabrielRIHARWeldingInstitute,Ptujska19,SI1000Ljubljana

    Contact

    Abstract.

    Foranefficientexaminationofweldedjoints,thoroughknowledgenotonlyofnondestructivetestingmethodsbutalsoofthedefectstobedetectedisrequired.Intheweldingliterature,however,thereareveryfewdataonlackoffusion,whichisoneofthemostseriousdefects.Therearealsonoefficienttestingmethodsavailableforitsdetection.

    Ourstudiesweretoprovideasmuchdataaspossibleonlackoffusion.Itwasinvestigatedwithwhichweldingprocesseslackoffusionismostfrequent,whatitspositioninaweldedjointisandwhatitsphysicalpropertiesare.Itwasalsostudiedwhatkindofindicationsthelackoffusiongiveswithdifferentnondestructivemethods.

    Testingwascarriedoutwithtypicalweldedjoints.Metallographicandmechanicaltestingwasalsousedinordertomakeacomparisonwithnondestructivetesting.Varioustypesoflackoffusionwereclassifiedintocharacteristicgroups,i.e.thetypesincludingvoidsandnonmetallicinclusionsandthetypesofpurelackoffusionwhichcannotbedetectedbynondestructivetesting.Keywords:welding,NDT,welddefects,lackoffusion.

    1Introduction

    Itisknownfromtheweldingpracticethatthelackoffusiondefectsoftenoccurinaweldedjoint.Manyafailureofaweldedstructurethusoccursduetoundetectedlackoffusioninweldedjoints.

    Lackoffusionisoneofthemostseriouswelddefects.Itproducesthenotcheffect.Thereare,however,noefficientnondestructivemethodsavailableforitsdetection.

    Iflackoffusiondefectsandcracksarecompared,itisevidentthatmuchmoreattentionispaidtocracksthantolackoffusionalthoughlackoffusionisasseriousadefectasacrack.Intheweldingliteraturenumerouspapersdealwithcracksbutveryfewwithlackoffusion.OnlysomeolderdocumentsoftheInternationalInstituteofWeldingwerefoundtoreportonlackoffusion[14].Itwas,therefore,decidedtostudylackoffusionmoreclosely.Itwastobefoundoutwhereitoccursmostfrequently,underwhichconditionsitformsandhowitcanbedetected.

    2Definitionoflackoffusion

    Inwelds,incompletelyfusedspots,calledlackoffusion,persist.Aweldcanlackunionwiththeparentmetalorwithapreviousweldbead.Anadhesionjointforms,whichcanberatherstrongincertaincases.Itismuchlikeabrazedjointorjointformedinmetallisation.Thepurerlackof

  • fusionis,themoredifficultitistodetectit.

    Withregardtothepositionofthelackoffusiondefectsinaweld,threetypesoflackoffusionaredistinguished[5]:

    1. lackofsidewallfusion,2. lackofinterrunfusion,3. lackoffusionattherootoftheweld.

    Astotheappearanceofthefractureface,onedistinguishesthelackoffusionduetounmeltedoxideinclusionsandthelackoffusionduetomeltedoxideinclusions.Thelackoffusiondefectsduetounmeltedoxideinclusionsconsistofoxidesandnonmetallicinclusions.Lackoffusion,ofwhichthreetypes,i.e.IIWreferences4011,4012,and4013,aredistinguishedinastandardshouldnotbemixedupwithlackofpenetration,i.e.IIWreference402[6].Thedefectslocatedatthesurfaceareefficientlydetectedbyavisualinspection.Lackofpenetrationinsidetheweld,however,canbedetectedbyXrayorultrasonicinspectionmethods.

    Astothepossibilityofdetecting,differenttypesoflackoffusioncanbeclassifiedintotwogroups,i.e.,theoneinwhichlackoffusionincludesvoidsornonmetallicinclusionswhichcanbedetectedbynondestructivemethods,andtheoneinwhichthelackoffusionshowsnodiscontinuityinthematerialsinceitisastructuraldefectandthuscannotbedetectedbynondestructivemethods.

    3Characteristicsoflackoffusion

    Itwasfoundinmetallographicexaminationsthatinaweldthreetypesoflackoffusioncanbefound:

    1. purelackoffusionorlackoffusionduetomeltedoxideinclusions,2. openlackoffusion,3. lackoffusionconsistingofnonmetallicinclusions.

    Thepurelackoffusionisastructuraldefect.Inthiscasethemoltenmetalstickstotheparentmetalwhichhasnotmeltedenoughduringwelding.Ajointbetweenthesolidphaseandtheliquidoneforms.Itislikeabrazedjoint.Thistypeoflackoffusioncannotbedetectedbynondestructivetestingmethodsbutwithamicroscopicinspection.Astraightfusionlineindicatesthattheremaybethelackoffusionbetweentheparentmetalandtheweld.Theinterrunlackoffusionisevenmorehidden.Itcanbedetectedonlybyanaccuratemicroscopicinspectionwitha50timesmagnification.AnexampleofthepurelackoffusionisshowninFig.1.

    Fig1:Purelackoffusionbetweenthefinalrunandtheparentmetal.a)macrograph:x3.5micrograph:

  • x100.

    Becauseofinternalstressesproducedduringweldsolidificationandcooling,thefacesstickingtoeachotherwillseparate.Avoidhavingawidthofonlysomehundredthsofamillimeterforms.Thisgapintheweldisverymucklikeacrack.Itcan,however,bedetectedbynondestructivetestingmethods.Suchatypeoflackoffusionisdifficulttodistinguishfromacrack.AnexampleoftheopenlackoffusionisshowninFig.2.

    Fig2:Openlackoffusionbetweenthecentralandfinalruns.a)macrograph,x3.5b)micrograph:x100.

    Wherethelackoffusionisthereareveryoftenalsooxidesandnonmetalicinclusions.SuchacaseisshowninFig.3.Iftheoxidelayerdoesnotmelt,theinclusionsareuniformlydistributedacrosstheentiresurfaceofthelackoffusiondefect.Iftheymelt,however,thenonmetallicinclusionsbecomespherical.

    Fig3:Inclusionsatthefacesstickingtogether.a)Themacrographshowslackoffusionbetweenthecentralandfinalrunsb)Themicrographshowsinclusionsatthefacesstickedtogether.

    4Locationoflackoffusiondefects

    Thelackoffusionisaplanardefect.Itmayappearattheedgeoftheparentmetalorbetweenruns.Thelackoffusionbetweentheparentmetalandtheweldmetalshowsaflatface.Thelackofinterrunfusion,however,showsanirregularshape.

    Thelackoffusionisusuallytobefoundattheweldinside.Itrarelyreachesthefinalrunsortherootrun.LocationoftypicaltypesoflackoffusionareshowninFig.4.

  • Fig4:Lackoffusioninaweld:lackofsidewallfusion(above),lackofinterrunfusion(below).

    5Detectionoflackoffusionbynondestructivetestingmethods

    Asalreadymentioned,thenondestructivetestingmethodscommonlyusedintestingofweldedjointsareineffectiveindetectionofthelackoffusiondefects.Inroutinetesting,thelackoffusionisusuallynotdetected.Whenthelackoffusionissuspectedinaweld,thetestingmethodsusedshouldbesuitablyadapted.Attentionshouldbepaidtoallindicationsincludingthosewhichdonotexceedtheirlevelsofacceptability.

    Accountshouldbetakenthatthelackoffusionmayappearingasshieldedarcweldingwithaconsumableelectrodeandgaswelding.Thereisagreatchanceoftheappearanceoflackoffusionindownhillwelding.

    Itisveryusefultoobtaindataontheweldstructureandweldingconditionspriortowelding.Informationshouldbesoughtwhetheranymechanicalandmetallographicexaminationsoftheweldedjointstobeexaminedhavebeenperformed.

    6Nondestructivetestingmethodsforthedetectionoflackoffusion

    Theradiographicexaminationmakespossiblethedetectionoflarge,distinctivetypesoflackoffusioninwhichthereareinclusionsandvoidsofconsiderablesize.Theexaminationdetectstheinclusionsbutnotthelackoffusionitself.Attentionshouldbepaidtoalldefectslocatedatthefusionlineandbetweenindividualruns.Theradiographictestingmethodsarenotsuitablefordetectionofthelackoffusion.

    Fordetectionofthelackoffusion,theultrasonicexaminationisthemostsuitableofalltheavailablenondestructivetestingmethods.Alltypesoflackoffusion,withtheexceptionofthepurelackoffusion,canbedetected.Ourinvestigationsshowedthatconsiderablylargelackoffusiondefectsgiveweakultrasonicindicationswhichareinpracticeconsideredminoracceptabledefects.Aconsiderablypoorreflectionofultrasonicwavesisobtaineddueto:

    thestickedsurfaceconsistsofanumberofsmalldefectswhichpassovertothepurelackoffusion

  • thelackoffusionlocatedattheedgesofaVweldcoincideswiththeangleofreflectionofultrasonicwaves.

    Fig5:Ultrasonicexaminationofthelackoffusionbetweentheparentmetalandtheweldmetal.a)examinationfromallfourscanningdirectionsb)examinationfromthecoverlayer.

    Becauseoftheabove,weak,repeatingindicationsshouldalsobeconsideredwheninspectingweldsinwhichthelackoffusionissuspected.Theultrasonicwavesshouldbedirected,ifpossible,perpendiculartothestickingfaces.Itcanbesupposedthatthelackoffusionmayappearattheedgeoftheparentmetal.Inthecasewhentheaccessofultrasonicwavesispossiblefromallfoursides,thedirectpathasshowninFig.5aisused.Theexaminationcanbeaccomplishedonlyfromthecoverlayer.Inthiscasethesinglebouncetechniqueisusedtoexaminetheupperpartoftheweld(Fig.5b).

    Thepenetranttestingmethodsareusedprimarilywhentightnessofweldedjointsistobeensured,e.g.intestingoftanks.Theypermitdetectionofthelackoffusionreachingthesurface.

    Allindicationsattheedgeofrunsindicateapossibilityofthelackoffusion.Specialattentionshouldbepaidtofilletwelds.Thelackoffusiondefectexistinginaweldmayopenwhenloadedandproduceleakingofatank.

    Thelackoffusionisatypicallywelder'sfault.Bymonitoringtheweldingprocessanexperiencedsupervisorcanfindtheformationoflackoffusion.Attentionshouldbepaidtothepositionoftheweldpoolwhichmustnotrunaheadofthearcandfilltheweldgroove.Attentionshouldbepaidwhethertheweldersufficientlymeltsthejointedges.Thelackoffusioncanbesuccessfullydetectedbythevisualinspectionduringwelding.Thevisualinspectionafterweldingisnotefficient.

    7Conclusions

    Inweldsboththelackoffusionwhichisastructuraldefectandthelackoffusioninwhichthemetaljointisinterruptedbyvoidsandnonmetallicinclusionsmaybefound.Thepurelackoffusioncanbedetectedonlybythevisualinspectionduringwelding.Theothertypesoflackof

  • fusionmaybedetectedbyultrasonictestingmethods.Thelackoffusionreachingthesurfacemaybedetectedbytheliquidpenetrantormagneticparticletestingmethods.Becauseofanunfavourablepositionofthelackoffusionandunfavourablepropertiesofthestickingfaces,weakindicationsareobtainedwithconsiderablylargedefectstherefore,acceptancecriteriafortheweldscontainingthelackoffusionshouldbemademorestrict.

    References

    1. N.Yamauchi,Y.Inaba,T.Taka:FormationMechanismofLackofFusioninMAGWelding.IIWDoc.21252982.InternationalInstituteofWelding,1982.

    2. CausesforWeldDefects.IIWDoc.XIIB04683.InternationalInstituteofWelding,1983.3. GasshieldedMetalArcWeldingofSteel.DirectionsforExecutionofProcess.AvoidanceofLack

    ofFusion.IIWDoc.XIIB04983.InternationalInstituteofWelding,1983.4. R.KillingandH.Hantsch:BeitragzurFragederBindefehlerempfindlichkeitbeimMetall

    AktivgasschweienmitFlldrahtelektroden.SchweienundSchneiden,45(1993)12,689693.5. MultilingualCollectionofTermsforWeldingandAlliedProcesses/InternationalInstituteof

    Welding.Part1.GeneralTerms.Institutzavarilstvo,Ljubljana,1988.6. EN26520:Classificationofimperfectionsinmetallicfusionwelds,withexplanations.

    AIPnD,createdbyNDT.net |Home||Top|


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