Case
study
A
failure
study
of
the
railway
rail
serviced
for
heavy
cargo
trains
Y.D.
Li
a,b,*
,
C.B.
Liu
c,
N.
Xu
a,
X.F.
Wu
a,
W.M.
Guo
a,
J.B.
Shi
aa
ShandongProvincialEngineeringandTechnologyCenterofMaterialsFailureAnalysisandSafeEvaluation,ShandongAnalysisandTest Center,ShandongAcademyofSciences,Jinan250014,PRChina
b
TechnicalR&DCenter,DalipalPipeGroupCo.,Ltd.,Cangzhou061000,PRChina
c
TechnicalR&DCenter,LaiwuIron&SteelGroupCo.,Ltd.,Laiwu271100,PRChina
1. Introduction
With thedevelopment of high-speedand heavy-loadrailway, muchhigher requirements are putforward for the comprehensivepropertiesofsteelrails[1].Sincefractureaccidentsofsteelrailthreatenthesafetyofrailwaytransportations directly,moreandmoreattentionispaidtothequalityofrailsteel.Thefatiguefractureisthemainfailureformofsteelrail. Despitesubstantialadvantagesindesign,materialsandnon-destructiveinspection,fatiguepropagationinandfailureof railwaycomponentsremainsanimportantissueforsafetyengineeringwhichisalsoemphasizedbyanumberofaccidents overthelastdecades[2,3].Atthebackgroundofanincreasedvolumeoftraffic,highertrafficspeedsandhigheraxleloads, reliabledamagetolerancedesignandeffectivemaintenancemethodshavetobeestablished.Therefore,failureanalysisof thesteelrailisverycritical.
In addition tothe fatigue load, rails are also subjected to other highmechanical loads and harsh environmental conditions.Themainloadingcomponentsarerollingcontactpressure,shearandbendingforcesfromthevehicleweight, thermalstressesduetorestrainedelongationofcontinuouslyweldedrailsandresidualstressesfrommanufacturing(roller straightening)andweldinginthefield(Fig.1)[2].Weldingisindispensableintherailwayrail.Today,themostcommonrail
CaseStudiesinEngineeringFailureAnalysis1(2013)243–248
ARTICLE INFO
Articlehistory: Received28June2013
Receivedinrevisedform4September2013 Accepted26September2013
Availableonline4October2013 Keywords: Railwayrail Overload Failureanalysis Cleavagefracture Fatigue ABSTRACT
Inthis casestudy, a failedrailway rail whichwas used for heavycargo trainswas
investigated in order to find out its root cause.The macroscopic beach marks and
microscopicfatiguestriationswerenotobservedbymacroandmicroscopicobservations.
Thechevronpatternswereobservedbymacroobservations.Thecrackoriginwasatthetip
ofchevronpatterns.Thefan-shapedpatterns,cleavagestepandtheriverpatternswere
observedatthecrackorigin,whichdemonstratedthefeatureofcleavagefracture.The
metallurgicalstructuresatthecrackoriginwerepearliteandferritenetworks.Thecrackis
supposedtobeinitiatedfromtheweakerferritenetworks.Givenallofthat,thefailed
railwayrailisconsideredtobecausedbyoverload.Itisofgreatimportancetoimprovethe
weldingtechnology,andcontroltheloadoftraininordertopreventsimilarfailurein
future.
ß2013MartinHewisonTheAuthors.PublishedbyElsevierLtd.
* Correspondingauthorat:ShandongProvincialEngineeringandTechnologyCenterofMaterialsFailureAnalysisandSafeEvaluation,ShandongAnalysis andTestCenter,ShandongAcademyofSciences,Jinan250014,PRChina.Tel.:+8653182605313;fax:+8653182964889.
E-mailaddresses:[email protected],[email protected](Y.D.Li).
ContentslistsavailableatScienceDirect
Case
Studies
in
Engineering
Failure
Analysis
j ou rna l h ome pa ge : w ww . e l se v i e r. co m/ l oc a te / cse f a
2213-2902 ß2013MartinHewisonTheAuthors.PublishedbyElsevierLtd.
http://dx.doi.org/10.1016/j.csefa.2013.09.003
Open access under CC BY-NC-ND license.
weldingprocessesareflash-buttwelding,aluminothermicwelding,gaspressureweldingandenclosedarcwelding.Among themaluminothermicweldingofrailsisusedwidelywithintherailwayindustryforin-trackweldingduringre-railand defectreplacement.Theprocessprovidesflexibilityandlowcapitalcost,butsuffersfromvariablequalityinfinishedwelds, duetotheinherentlimitationsoftheprocessesused,andtheiroperatordependency[4].Therefore,itishardertocontrolthe quality of welded points. Statistics show that during 18-month period aluminothermic weld failures comprise approximately 75%of allbroken railreportsfor theNewmanmainline[4].In responsetofailuresin aluminothermic welds,andinrecognitionthatsuchweldsrepresentoneofthemainrisksforacatastrophicderailment,itisnecessaryto developanimprovedrailweldingprocesswhichmeetstheperformancedemandsofhigheraxleloads.
Inthepresentpaper,afracturedrailwayrailwhichwasusedforcargotrainswasanalyzedtofindoutitsfailurecause.In theend,a conclusionwasreachedafterperformingmacroscopicinspections,chemical analysis,SEMobservations,and metallographicexaminations.
2. Background
Firstly,itisnecessarytocollectasmuchaspossibletheinformationontheprevioushistoryofthefracturedrailwayrail. Thefracturedrailwayrailwasprovidedbytherailwayadministrationstaff.Accordingtotheintroductions,therailwayrail cameintoserviceinMay2005.Therailwayrailswerebutt-weldedtogetherbythealuminothermicweldingprocess,andthe postweldheattreatmentwasnotconducted.TherailwayrailwasfoundtobefracturedinDecember2011whenitwasin winter.During2005–2011,thisroutewasservicedforheavycargotrains.ThetypeoftherailsteelwasGBP60U75Vas introducedbytherailwayadministrationstaff.
3. Results
3.1. Macroscopicinspections
ThemacroscopicfracturemorphologyofthefracturedrailwayrailwasshowninFig.2.Thedimensionsoftherailwayrail whichwereprovidedbytheclientwereshowninFig.2(a).Itwasfoundthatthefracturesurfacewasbasicallycleanandfresh, whichdemonstratedthatthecorrosionofthefracturesurfacewasnotheavy.ThetopinFig.2(a)wastherailheadcontacted withwheelrim,thebottomwastherailbottomcontactedwiththeballast,themiddlepartbetweenrailheadandrailbottom wastherailweb.Wecouldalsofoundthatthefracturesurfaceattherailbottomwasclosetotheweldbeadandrelativelyflat (Fig.2(b)),andthatthefracturesurfacewasapproximatelyperpendiculartothelongitudinaldirectionoftherailwayrail.The fracturesurfaceattherailwebwasapproximatelyarchedwhenobservedfromtheprofile(Fig.2(b)).Thefracturesurfaceatthe railheadwasinclinedatabout608tothelongitudinaldirectionoftherailwayrail.Thefracturesurfaceattherailheadwasvery rough;therefore,itwasdeducedthatthispartmightbecausedbythefinalinstantfractureinsteadofthefractureorigin.There wasadarklyfan-shapedareaattheleftbottomofFig.2(c).Asmallbrightspotwasobservednexttothedarklyfan-shapedarea (Fig.2(c)).Asintroducedbytherailwayadministrationstaff,thebrightspotwasnotseparatedyetwhentherailwayrailfailed.
Fig.1.Loadingcomponentsactingatacontinuouslyweldedrailduringvehiclepassing[2]. Y.D.Lietal./CaseStudiesinEngineeringFailureAnalysis1(2013)243–248 244
Sincetherailwayrailwassubjectedtocyclicloadingandservedabout6years,itisrationaltoconsiderthatthisrailway railmightbefailedduetofatigue,eveningigacyclefatigueregime[5–7].Thearcboundaryoffan-shapedarealookslikea beachmarkwhenobservedmacroscopically,asseeninFig.2(c).Thenitcanbededucedthatthecrackoriginmightbeatthe cornerofdarklyfan-shapedarea(viz.,thesmallbrightspotasshowninFig.2(c)).However,takingintoaccountthatthesmall brightspotwasnexttodarklyfan-shapedarea,itisobviouslytodeducethatthesmallbrightspotwouldnotbethecrack origin. Thebeachmarks whichwere theclassical features ofmetal fatiguewerenot observed fromthemacroscopic observations(thearcboundaryoffan-shapedareaisactuallynotabeachmark,wewilldiscussthathereinafter).However, thechevronpatternscanbeclearlyobservedatthefracturesurfaceofrailbottom(Fig.2(d)).Therefore,wecandeducethat thecrackoriginshouldbeatthetipofchevronpatterns(area1),andthecrackgrowthdirectionisalongthediverging directionofriverpatterns,asshowninFig.2(d).Itcanbededucedfromtheflatfractographyandchevronpatternsthatthe macrofracturefeatureisbrittlefracture.
3.2. Chemicalanalysis
The sample used for chemical analysis which was sampled from railhead was analyzed by ZSX Primus II X-ray fluorescencespectrometer.TheresultswereshowninTable1.Itwasdemonstratedthatthechemicalcompositionsofthe
Fig.2.Themacroscopicmorphologiesofthefailedrailwayrail.(a)Thefrontview,(b)thelateralview,(c)themacroscopicmorphologyoftherailbottom, and(d)thechevronpatternsandcrackoriginatthefracturesurfaceofrailbottom.
Table1
Thechemicalcompositionsoffailedrailwayrail(mass%).
Steels C Mn Si S P V
Therailsteel 0.74 0.96 0.68 0.0035 0.016 0.062
GBP60U75V 0.71–0.80 0.70–1.05 0.50–0.80 0.030 0.030 0.040–0.12 Y.D.Lietal./CaseStudiesinEngineeringFailureAnalysis1(2013)243–248 245
rail steel were in accordance with the standard of P60U75V [8]. Therefore, the compositions of the rail steel were normal.
3.3. SEMobservations
Thefracturesurfaceattherailbottom,viz.surfaceS1(asshowninFig.2(c)),wascutfromthefailedrailwayrail,andthen cleanedbyalcoholanddichloroethane.After that,surfaceS1wasobservedbyZEISS-SUPRA55fieldemissionscanning electronmicroscope(FESEM)indetail.Themorphologyinsidethedarklyfan-shapedareaisshowninFig.3(a)whichshowsa relativelyflatsurface.ThequalitativechemicalcompositionsofthisareaareanalyzedbyEDX,alsoshowninFig.3(a).Higher oxygencontentsweredetected,whichdemonstratesthatthisareawasoxidizedheavily.Thetypicalmorphologyinarea1is showninFig.3(b)whichshowsthetypicalfeatureofcleavagefracture.Thefan-shapedpatterns,cleavagestepandriver patternswhicharethetypicalfeatureofcleavagefractureareobservedinthisfigure.Fatiguestriationswhichwerethe typicalmicroscopicfeatures ofmetalfatiguewerenotobservedin thefracture surface.Themicrofractographyofthe chevronpatternsareaisshowninFig.3(c)whichissimilartoFig.3(b).Thefracturesurfaceoutsidethedarklyfan-shaped areaiscleanandfresh,almostnooxygenisdetected.Combinedwiththeexperimentalresultsoutsideandinsidethedarkly fan-shapedarea,itcanbededucedthatthedarklyfan-shapedareamightbeanincompletefusionareaduringwelding. 3.4. Metallurgicalobservations
Firstly,surfaceS2(asshowninFig.2(c))waspolishedtoobservethedistributionofinclusions.ItwasshowninFig.4that somebiggerslaginclusionswithsharpangularshapewereobservedatsurfaceS2closetothefracturesurfaceattherail bottom.Thesizeoftheseslaginclusionswasaboutatleast126
m
mdefinedbyMurakami’seffectiveprojectiveareamodel [9].EDXdemonstratedthatthecompositionoftheseslaginclusionswasalumina.Afteretchedby3%nitalthemetallurgical structureofsurfaceS2closetothefracturesurfacewasobservedbyopticallymetallurgicalmicroscope(OMM).ContinuousFig.3.Themicroscopicmorphologiesofthefracturesurface.(a)Insidefan-shapedarea,(b)area1,and(c)theareaofchevronpatterns. Y.D.Lietal./CaseStudiesinEngineeringFailureAnalysis1(2013)243–248
ferritenetworksandpearlitecolonieswereobserved,asshowninFig.5(a).ItwasalsodemonstratedfromFig.5(a)thatthe sizeofpearlitecolony,viz.theareasurroundedbycontinuousferritenetworks,wasratherheterogeneous.Thebiggestsizeof thepearlitecolonywasabout726
m
mindiameter,thesmallestsize68m
m.Aftermakinganobviousmarkatthecrackorigin site(area1)onsurfaceS1,surfaceS1waspolishedandetchedby3%nitalinordertoobservethemetallurgicalstructures. Themetallurgicalstructuresinarea1werepearlite,continuousferritenetworksandamassofferritefragmentsdistributed insidethepearlitecolonies,asshowninFig.5(b).Consideringtheweakerstrengthofferritedistributedlikenetscompared withpearlite;itcanbededucedthatthecrackmightbeinitiatedfromtheferritenetworks.4. Discussionandanalysis
AsintroducedinSection1,therailwayrailwasmainlysubjectedtocyclicloading.Inthiscasestudy,themacroscopic beachmarksandmicroscopicfatiguestriationswerenotobservedatthefracturesurface.Inaddition,thetypicalchevron patternswereobserved.Andthefeatureofcleavagefracturewasobservedatthetipofchevronpatterns.Givenallofthat,we candrawa conclusionthattherailwayrailismainlycaused byoverloadeventhoughitissubjectedtocyclicloading. Consideringtheabnormalmetallurgicalstructures(ferritenetworksdistributedalongthegrainboundaries)atthecrack origin,thecrackissupposedtobeinitiatedfromtheweakerferritenetworkswhicharecausedbywelding.Therefore,itis muchneededtoeliminatetheferritenetworksbyimprovingtheweldingtechnology.
4.1. Stressanalysis
Inthiscasestudy,thisrailwayrailwasmainlysubjecttoalternatebendingstressduetovehicleweight,asshowninFig.1. ItisshownfromFig.1thattherailheadwassubjecttocompressivestressandtherailbottomwassubjecttotensilestress.
Fig.5.(a)ThemetallurgicalstructuresofsurfaceS2closetothefracturesurfaceand(b)themetallurgicalstructuresinarea1. Fig.4.ThemorphologiesoftheslaginclusionsandEDXresults.
Therefore,therailbottomwassubtletofailfromthepointofviewofmechanics.Therailbottomcanbeapproximately consideredtosubjectfatigueloadinthelongitudinaldirectionofrailwaywithR=0(Rwasstressratio),andtheapplied stressatthelowerbottomofrailwasapproximatelythemaximum.Inviewoftheheavilyincompletefusionarea(darkly fan-shapedareainFig.2(a))atthebottomcornerofrailbottom,thecrackwassupposedtobeinitiatedfromthisincomplete fusionarea.Nevertheless,infactitisnotthecase.Theresidualstressmustbeconsidered.Inadditiontotheresidualtensile thermalstressduetotrackinstallationandtemperature(asshowninFig.1),theweldingresidualstresswasalsoofgreat importance.Theweldingresidualstresswasusuallydetrimental;therefore,manyrailfailureswereinitiatedfromtheweld [4].Theresidualstressandappliedstressduetothetrain’sgravitycanbesuperimposedtogether.Thesuperimposedstress willinducethefailureoftherailundercertainconditions.
4.2. Theroleoffatigue
Therailway railis failedby overload; however,wecannot completelydeny therole offatigue.The damagetrace generatedbycyclicloadingwasnotobservedatthecrackgrowtharea;however,thefatiguedamagewasalmostinevitable bytakingthelongerservicelifeintoaccount(about6years).Ontheotherhand,usuallythefatiguedamagecannotbeeasily detectedduetothecomplicatedworkingsituationsofacomponent.
4.3. Suggestions
Thefailedrailwayrailwascaused byoverload.Thecrackoriginwastheferritenetinducedbyinadequatewelding technology.Therefore,inordertopreventsimilarfailuresinfuture,theweldingprocessmustbeimproved.Forexample,pro andpostweldheattreatmentsshouldbeconductedtoeliminatethebiggerslaginclusionsandferritenetworksalongthe grainboundaries.Ontheotherhand,itisofgreatimportancetocontroltheloadoftrain.
5. Conclusions
Thisfailedrailwayrailiscausedbyoverload.Thecrackisinitiatedfromtheweakerferritenetworkswhichareinducedby inadequateweldingtechnology.Itisofgreatimportancetoimprovetheweldingtechnology,andcontroltheloadoftrainin future.
Acknowledgments
ThisworkwasfinanciallysupportedbyNationalNaturalScienceFoundationofChina(Grantno.51101094)andFund projectofTechnologyDevelopmentofShandongAcademyofSciences(Grantno.KJHZ2011-04).
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