Case Study 1-Final Report-FA-PT. Sulzer Turbo Services Indonesia

8/16/2019 Case Study 1-Final Report-FA-PT. Sulzer Turbo Services Indonesia

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FAILUREANA LYSIS

REPORT

Case StudyGroup 1

StudBolt Failure

duetoHydro e!E"#rittle"e!t

Te$%!i$al Report

Metallurgy and MaterialDepartmentmakes no representations or warranties,eithe

r expressed or implied,


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and specificallyprovides the results ofthis report “asis” basedon

the informationprovided by clien t


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Executive Summary

PT.SulzerTurboServicesIndonesiareportedthatitsstudbolthad failedinservice. AsreportedbyPT.SulzerTurboServicesIndonesia,thefailure occurredonthe

threadpartofthestud bolt.Thethreadwaslocatedonbothendsofthestudboltbut thefailed

partonly occurredonone endofthestudbolt.Ourstudy showedthatthe studbolt failed

due to hydrogenembrittlementthatresultedfromitslonge posureto hydrogengas

atmosphere duringoperation. Thepresenceofthe areawitha number

oflocalizedprecipitatesinthe failedstudboltacceleratedthe failurebecausethis

localizedprecipitates wasthemostfavorablesitefor hydrogenmoleculesthatcame from

the inner container to re!combined and then produced the pressurized

hydrogengas in themetal. Once this pressurized hydrogengas e isted,it wea"ened

the bondingbetween metalatoms.As aresult,thestudbolt materialfracturedin a

brittlemode.Inmostcases,oncethehydrogengasis

formedinthemetalandandwea"ensthemetal atombonding,thecrac" propagationwould

occurautomatically withoutthe e ternal stress becausethis produced hydrogen has

enough pressureto create andpropagate thecrac"s.


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Background Information

PT. SulzerTurboServicesIndonesiareportedthatitsstud bolthad failedin

service.AsreportedbyPT.SulzerTurboServicesIndonesia,the failureoccurredon

thethread part ofthestud bolt. Thethreadwaslocatedon both ends ofthestudbolt

butthe failedpartonly occurredononeendofthestudbolt.Principally,theboltis

usedto#ointwo parts li"eflange or others. Thebolt hastwo$%&similar nuts on each

endwhichistightenedduring application.'igure(illustratesthe principalwor"of

studboltin#oiningmethod.Thefailedsamplewasthensubmittedto)*P'Afor

failure analysis$'ig. %&.

Objectives

The assessmentwas carried out in *etallurgyand *aterial +epartment,

niversity ofIndonesia,inordertofindouttherootcausefailureofstudboltandto ma"e

recommendations.

Thescope ofwor" wasasfollows-

(. .%.

/.

4. ………………………………………………………………………….

InvestigationResults

Visualexamination

The failedstud bolt was visually e amined and photographed in as!received

condition$'ig.(&.'orabetter e amination,thefracturesurfaceofthefailedstudbolt

wasobservedbyopticalmicroscopeatmagnificationof01$'ig.%&.Asobserved

fromthemacrophotography,thecrac"seemstobeinitiatedfromaspotatthe

threadandtopropagateinacircumferentialmode.Thecrac"surfacehadawide

smooth area$crac"propagationroute& androugharea$final rupture&.


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Chemical Com osition!est

Inorder toobtaininformation aboutthechemical composition,thesamplewasthen

sub#ectedtothechemicalcompositiontest usingOptical 2missionSpectroscopy

$O2S&.Table(showsthechemicalcompositionoffailedsamplein comparisonwith

AST*A3456rade 4/7.Theresults revealthatthere isaslight differencebetween

thenic"el$8i&contentsinthetestedsample andtheoneinthestandard. Thetested

samplecontains 3.(/98i,whichis 7.(/9 higher thanthe ma imum nic"elcontent

allowedforAST*A3456rade/7 material$/.7!3.798i&.Theother elementscomply

withthestandard materialre:uirement, as canbeseeninthetable.Thisslight

differenceinnic"elcontentmay notsignificantly impairthemechanicalproperties of thestud bolt.8ic"el isintentionally addedtointroduce the toughness intothis type of

material.

!ensile !est

The tensiletest was conducted to obtain the information about mechanical

properties ofthe bolt, i.e.tensilestrength, yieldstrengthand elongation. Table%

showsthetensile propertiesofsampleincomparisonwithAST* A3456rade4/7.

Thetensilestrengthofthe failedtestedsampleis(%53*Pa,whichisabout43*Pa below

thestandard material.Itsyieldstrengthis(%%3*Pa,whichis33*Pahigher than

overthestandard materialanditselongationis (7.;49$7.;49higherthanthe

standardmaterial.

"ardness test

Toobtainanother mechanical propertiesinformation,hardnesstestwasperformed

onthefailedsample.Table/showsthehardnessvalueofsampleincomparison

withAST*A3456rade4/7.Theresultshowsthatthehardness valueisslightly below

theminimumvaluere:uiredby thestandard.Thefailedsamplehasa hardness

valueof/;


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Informationobtainedfromthe microstructure photosisthat the basematerialofthe

studboltcontains martensite phases.Thecrac"propagating along thegrain

boundarywas observed onthefailedsample.

$ractogra hyanalysis by SE# and E%S

The fracturesurface wasobservedusingScanning2lectron*icroscopy $S2*&to

gathermoreinformationabouttheinitialcrac"sandotherrelatedfactorsaffecting the mode

offailure. The2+Sanalysiswasalsoperformedonthe fracturesurface to

analyzeanypossiblecontributionofenvironmentormicrophaseofthematerialto the

failure.Thecrac"propagatingfromthethreadwasobservedonboththefracture

surfacesofthefailedsample.'igure ; and> showthe propagatingcrac".

The fracturesurface observed fromS2*imagesindicatesthatitisbrittle fracture

sincenoevidence ofdimpleslocatedinthe fracturearea. Thepresencesofdimples

indicatethat the fracturemodeisductile. ?rittlefracturecanbe intergranularor

transgranular.Intergranularbrittle fracturecanbetheresultofembrittlement along

thegrainboundaries.


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Precipitateswerefoundwithinthematri ofbase metalsinceitisaprecipitate!

hardenedstainlesssteel@itmainly composed of'e!)r.?utan anomalywasfound

thattheprecipitateswithinthefailedsamplewereconcentratedat onepoint.Some

precipitateswereaccumulated at some spot onthe thread edge.Table 5showsthe

2+Sresultsofthetestedspot.Theseinhomogeneouslydispersedprecipitateswere

observedusing?ac"scatteredSecondary2lectron$?S+&.Somecrac"swerefound

tobeinitiatedfrom these precipitates.

?ased onthe2+Sresultsshowed onTable 5and3,thecompositionof'e and)r

thedar" spotandthe basemetal is :uitethesame.Itcanbeconcludedthatthedar"spotsare precipitatethatcomposedof'e and)r.Precipitates, incaseforthis stainless

steel, areintroducedtogivestrengthtothematerial. Some disadvantages

arethenfoundiftheyarenotwelldispersed$inhomogeneous&.

The accumulated precipitates could generate wea" areasincestress concentration

willbelocatedonthatspot.Thiswillbetheinitialspotforfatiguecrac"propagation.

Afatiguecrac"willtypicallyinitiateatadiscontinuityinthematerialwherethecyclic

stressisma imum.Sincethestudbolde periencedthetensilestressandalsovibrationduringapplication,thecombinationoftensilestressandvibrationcould

facilitate thefatiguefailure tooccur..


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%iscussion

Thestudboltiscommonly usedasa fastener to#ointwoparts altogether@thepart suchas

flangeis#oinedusingthis"indofbolt$see $ig&' &.?ased onthechemical

compositiontest$see !able' & andmetallographice amination$see $ig& ()*+ , &,

thestud boltwasmadeofprecipitated!hardenedstainlesssteel

withmartensitephasematri .The2+Simage analysis results showed

thattheprecipitates$see $ig&

'' & mainly composed of'e!)rthatcame fromthealloying elementsofthestainless

steel.Inpractice,whenthenutisfastened,astress$tensile&isthenintroducedto

thestudbold. Sincetheboltwasapplied onaturbine part,thevibration during itsoperationcouldbealsotransferredtotheboltgeneratingacyclicload. The

combinationbetweenatensilestressandavibrationcouldpossiblytriggerthe

fatiguefailure.

Theboltswereappliedontheturbinesystemconsumingfuelmainlycomposed of

nitrogen$8 %&andhydrogen$< %&withsmall fractionofargon$Ar&,methane$)< 5&,

carbondio ide$)O %&andcarbonmono ide$)O&.Thecorrosivegasinthissystem

is)O %.


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thethread$see $ig& '1 &@someprecipitateswereaccumulated generating stress

concentrationspotonthethread$see $ig&'-+'. &.Thehard precipitated particles could

beafactor forcrac"initiationsinceama imumstresswillbemainly accumulatedat or

focusedonthoseparticleswhenthestressis wor"inginthe material@ moreover,this

effectwillbecatastrophic ifthe precipitatesare accumulated atcertainspotonthe

bolt.?asedontheS2* observation,anumberofprecipitates

werefoundatthethread$see $ig& '. &@that ma"ethis locationasthewea"estspot andcould

fail duetocyclic load$fatigue&.8evertheless,it mightbe notindicatingthat

fatiguewastherootcauseofthefailure.Itwaspossiblytriggeredby otherinitiation

sinceverylittle evidencefounduponthefacture surface.

Theindicationofembrittlement duetohydrogenwas also observed.?ased ontheS2*images,thecrac"spropagated alongthegrainboundary. 8oevidences of

dimplewere observedonthsductilematerialthusindicating thatthecrac"s

propagatedduetoembrittlement$see $ig&'( &.Theatmospherecontainshydrogengas$< %&

thatpossiblybethesourceofhydrogenatomsdiffusingintothemetal

matri creatingembrittlement.

Ascan beseeninTable 4, hydrogengasma"esup almost 059 ofthetotal gascompositioninthesystem.Inthiscase,the othergases may beconsideredasthe

impurities. Severalliteratureshavereportedtheeffectsofhydrogenembrittlement of

gaseousimpurities.6asimpuritiescaninhibit,accelerate, orhavenoeffectupon

hydrogenembrittlement.Asdocumentedelsewhere,)< 5 and8 % gaseshaveno

effectonhydrogenembrittlement,< %Sand)O %gases act as accelerators of hydrogen

embrittlement 4B.

Along e posureofthemetalli"ethestud bolttothehydrogengasinthecontainer could

promotethehydrogenatomsto

migrateintothemetalmatri .Someofthehydrogenatomsre!combinedwithcarbon atoms

toformmethane gas$)< 5&inside minusculevoids.The

methanegascreatedpressurewithinthe minusculevoids$see $ig&', &

thatcouldreducetheductility ofa metalcreatingembrittlement.8ot only

withintheminuscule voidswherehydrogenwaspresentbut also withinthegrain

boundariessincetherewerealsocavitiesdue tothedisorder arrangementofatomicdirection.


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Thefailedstudboltwase posedtotherelatively hightemperature $around(47C)&.

Accordingtotheliterature,underambient temperature,hydrogenatomscanbe

absorbedtothe metallatticeanddiffusethroughthegrainboundaries.Thecrac" inducedby

stress underthisconditionwillpropagatetransgranularly.Otherwise, under

hightemperature,theabsorbed hydrogenatomstendtogatherinthegrain

boundariesandthecrac"swill propagateintergranularly 5B.Someresearchers

describethetheappearanceas beingsimilartobro"enroc"candy >B.Thistypeof bro"en

roc" candy!li"e fracture can be seen in 'igs. %7 and %(.In addition,the

hydrogeninthecontainer also diffusesinto the metal,gravitating towardsthe area

withthegreateststress ;B.Inthecaseofthis failedstudbolt,the areawiththe

greateststress isthe areacontaininga localized precipitation$see $ig& '' &.

AS*2?oilerandPressureDessel)ommittee (7Bestablishedthe hydrogenstorage

container pro#ectteamin%775statesthatthe ma imumwor"ing pressure forthis

systemis(7/*pa,whichismuchhigherthantheactualwor"ingpressurein SulzerEs

system,i.e. around4*pa$around47?ar&. Thus,thereis noissueof overpressurecontributingto thefailure.

Conclusion

Thestudboltfailed duetohydrogenembrittlementthatresulted fromitslong e posureto

hydrogengas atmosphereduring operation.Thepresence ofthearea

withanumberoflocalizedprecipitatesinthefailedstudbolt acceleratedthefailure

becausethislocalizedprecipitates wasthemostfavorablesitefor hydrogenmoleculesthat camefromtheinnercontainertore!combined andthenproducedthe

pressurizedhydrogengasinthe metal.Once this pressurizedhydrogengas e isted,

itwea"enedthebonding betweenmetal atoms.Asaresult,thestudboltmaterial

fracturedina brittlemode.Inmostcases,oncethehydrogengasisformedinthe metal

andandwea"ensthe metal atombonding,thecrac"propagationwouldoccur

automaticallywithoutthe e ternalstress becausethisproducedhydrogenhas enough

pressuretocreateand propagate thecrac"s.


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Recommendations

(. sethematerial withtheunwantedprecipitates as lowas possible.

%. Athouroughmaterial selectionstudyalongwith thelaboratorytestare

commonlyre:uiredforthematerials thatwould becontinuouslye posedto

hydrogengas.


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Reference

(. 'atigue'ailure =esultingfromDariableFoading$ h ttp -GGh ig he re d .m c g r a w!

h ill.c om Gsi te sG d lGf r e e G77 0/ ( %( > /% G/4 3 0 4 4 Gc ha p te r7 4 .p d f &

%. 'ractography,AS*

Case Study 1-Final Report-FA-PT. Sulzer Turbo Services Indonesia

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