839
PACSnumbers:06.60.Vz,42.62.Cf,62.20.me,62.20.mm,81.20.Vj,81.40.Np,81.70.Bt
The
Fatigue
Strength
of
AISI
430—304
Stainless
Steels
Welded
by
CO
2Laser
Beam
Welding
U.Caligulu,M.Turkmen*,A.Ozer**,M.Taskin,andM.Ozer***
FiratUniversity,FacultyofTechnology, DepartmentofMet.andMaterialsEng., 23119Elazig,Turkey
*KocaeliUniversity, HerekeVocationalSchool, 41800Kocaeli,Turkey **
GaziUniversityTechnicalSciencesVocationalSchool,
OstimYenimahelle,Ankara,Turkey
***
GaziUniversity,TechnologyFaculty,
DepartmentofMet.andMaterialsEng., 06500Besevler,Ankara,Turkey
Inthisstudy,thefatiguestrengthofAISI430—304stainlesssteelsweldedby
CO2 laser beam welding (LBW) is investigated. Laser welding experiments
are carried under helium atmosphere at 2000, 2250 and 2500 W welding
powerswith100cm/minweldingspeed.Theweldingzonesareexaminedby
optical microscopy, scanning electron microscopy, and energy dispersive
spectroscopyanalysis.Fatiguetestsareperformedusinganaxialfatiguetest
machine,andthefatiguestrengthisanalyseddrawingS—Ncurvesand
criti-cally observingfatiguefracture surfacesof thetestedsamples. The
experi-mental resultsindicatethatmechanical propertiesandmicrostructural
fea-tures are affected significantly bywelding power. The fatigue strength of
CO2 laserweldedsamplesincreaseduetohigherdeeppenetrationinwelding
zonewithincreasingweldingpowerinchosenconditions.Thebestproperties
are observed with the specimens welded at 2500 W heat input and 100
cm/minweldingspeed.
ВційроботідослідженовтомнуміцністьнеіржавійнихсталейAISI430— 304,зваренихпроменемCO2-лазера.Експериментизлазерного зварюван-ня виконувалися в атмосфері гелію за потужности зварювання у 2000, 2250 і2500 Втіз швидкістюзварювання у 100 см/хв.Зонизварювання досліджувалися методами оптичної мікроскопії, сканівної електронної мікроскопії та енергодисперсійноїспектроскопії. Дослідженняна втому виконувалисязвикористанняммашинидлявипробуваньнавіснувтому, автомнаміцністьаналізувалася шляхом побудови S—N-кривихта крити-Фотокопирование разрешено только в соответствии с лицензией Напечатано в Украине.
чногоспостереженняповерхоньвтомногоруйнуваннядосліджених зраз-ків. Експериментальні результати показують,що механічні властивості та мікроструктурні особливості значно залежать від потужности зварю-вання. Втомнаміцність зразків, зварених CO2-лазером,зростає завдяки збільшеннюглибинипроникненнявзонізварюваннязізбільшенням по-тужностизварюваннязаобранихумов.Найкращівластивості спостеріга-лисяузразків,зваренихзапідведеноїтеплотипри2500Втташвидкости зварюванняу100см/хв. Вданнойработеисследованаусталостнаяпрочностьнержавеющихсталей AISI 430—304, сваренныхлучомCO2-лазера. Экспериментыпо лазерной сваркевыполнялисьватмосферегелияпримощностяхсварки2000,2250 и2500Втискоростисварки100см/мин.Зонысваркиисследовались ме-тодами оптической микроскопии, сканирующейэлектронной микроско-пиииэнергодисперсионнойспектроскопии.Испытаниянаусталость про-изводились с использованием машины для испытаний на осевую уста-лость,аусталостнаяпрочностьанализироваласьпутёмпостроения S—N-кривыхикритическогонаблюденияповерхностейусталостного разруше-ния исследованныхобразцов.Экспериментальныерезультаты показыва-ют, чтомеханические свойстваи микроструктурныеособенности значи-тельно зависят от мощности сварки. Усталостная прочность образцов, сваренных CO2-лазером, возрастаетблагодаряувеличению глубины про-никновенияв зоне сваркис увеличениеммощности сварки при выбран-ныхусловиях.Наилучшиесвойстванаблюдалисьуобразцов,сваренных приподводимойтеплотес2500Втискоростисварки100см/мин.
Key words:fatiguestrength,weldingzone,stainlesssteels,AISI430, AISI
304,CO2 LBW.
(ReceivedDecember2,2014; in final version, April 16, 2015)
1.INTRODUCTION
Stainlesssteelsareiron-basealloyscontaining8—25%nickelandmore
chromium thanthe 12%necessarytoproduce passivitybutless than
30%.Thesteelsresistbothcorrosionandhightemperature.Stainless
steelscanbedividedintofivetypesasferritic,austenitic,martensitic,
duplex,andprecipitation-hardening.Ferriticstainlesssteelsare
wide-lyusedduetothefactthattheircorrosionresistanceishigheratroom
temperatureandtheyaremuchcheaperthantheotherstainlesssteels.
Ferriticstainlesssteelscontain16—30%Crwithintheirstructuresin
respect of additionofthe alloy element. Thistypeof thesteel canbe
shapedeasilyandresistatmosphericcorrosionwellandthankstothese
characteristics,ithasawiderangeofapplicationinarchitecture,
inte-riorandexterior decoration,kitchenutensils,manufacturingofwash
boilersanddryingmachines,foodindustry,automotiveindustry,and
Austenitic-stainless steel is preferred more than other
stainless-steel types due to easiness in welding process. Then, some negative
metallurgic changes are taken into consideration in welding of the
steels.Thesearethe following ones:deltaferritephase,sigmaphase,
stress-corrosion cracking, and chrome—carbide precipitate between
grainboundariesat450—850CofCr—Niausteniticsteelssuchas18/8
joinedbyfusionweldinginlongwaitingtime.Stainlesssteelscan
gen-erally be welded with all methods of fusion welding and solid state
welding.Outofthefusionweldingmethods,electricarcwelding,
sub-merged arc welding, MIG, TIG,plasma welding, electron beam
weld-ing,resistancewelding,andlaserwelding,etc.arewidelyused.Inthe
fusionweldingmethodsforjoiningstainlesssteel,brittleintermetallic
compounds phases are producedinthe fusionzone,which reduce the
strength of theweldingjoint.However,inthe LBWjoining of
stain-less steel, because these phases are reduced, it improves the
perfor-manceofthestainlesssteeljoint[5—10].
Laser beam welding, using laser beam of high energy density as a
heat source, is a highly efficient and precise weldingmethod. It has
someexcellences,suchashighenergydensity,focalization,deep
pene-tration, high efficiency and strong applicability, and itis widely
ap-plied to welding zone requiring the high precision and high quality,
including aviation and space flights, automobile, microelectronics,
lightindustry,medicaltreatmentandnuclearindustry.Aslaserbeam
weldingis afast butunbalancedheat-circulation process,larger
tem-peraturedegreesappeararoundtheweld,thereforetheresidualstress
anddeformationofdifferentextentcanalsoappearinthepostwelding
structure.Allofthesephenomenabecomeimportantfactors,
influenc-ingthequalityofweldingstructureandtheusablecapability.
Under-standing thethermal processofwelding iscrucial toanalyse the
me-chanicalweldingstructureandmicrostructureaswellascontrollingof
weldingquality[11—14].
Fatigue is progressive localized permanent structural change that
occurs in a material subjected to repeated or fluctuating strains at
stresses havingamaximumvaluelessthanthetensilestrengthofthe
material.It hasbeenestimatedthat 90%ofthefailures,whichoccur
inengineeringcomponents,canbeattributedtofatigueandhigh
por-tionofthefatiguefailuresareassociatedwithaweldmentwithstress
concentrations and possible weld defects [15]. Fatigue affecting
fac-tors are the following: material type, the effects of composition and
structure,theeffectsofsurfaceproperties,theeffectofthenotch,the
effectsofstrains,theeffectofthecorrosion,theeffectoftheworking
environment,constructionandmontageerrors,theeffectof
tempera-ture, the effect of frequency (test speed), and dimensional factors.
Stresses,to beapplied topartsduring operation, may bedifferent in
typesoffatiguetestspecies.Theyareaxial,bending,torsionand
com-pound fatiguetests. Fatigue, crackinitiation, crack propagationand
breakage occur in three stages. Fatigue does not cause a significant
plasticdeformationofthematerial.Fatiguecracksoccurinsuch
plac-es, wherestress concentration,sharp edges,scratches,nicks and
cor-rosionandfatiguemoveatcertainspeedsandcausethefailureof
ma-terial. Therefore, the design of parts working under dynamic loads
shouldavoidsharpedges,whichcausestressconcentrationand
notch-es. Inaddition, thepolishedsurfaces ofcomponentsexposedtocyclic
loadingcanextendfatiguelife[16—18].
Inordertooptimizeweldingconditionsformaterialswithdifferent
propertiesoradvancedmaterials,experimentalinvestigationsarestill
required.Alongwithagreatdealofresearchesdevotedtothe
weldabil-ityof dissimilarmaterialsand propertiesestimationforwelding
pro-cess, the further investigation on practical applications of failure is
alsobecomingconsiderablyimportantandurgent.Itiswellknownthat
themajority offailuresthatoccur inthestructuresare caused by
fa-tigue failureinweldments duetofatiguetypeofloading experienced
bythestructures[19].Veryfewstudieshavebeencarriedoutto
evalu-atethefatiguepropertiesofAISI430ferriticstainlesssteelweld[20].
The fatigue processand its mechanism are largely influencedby the
presence of materialnon-homogeneities. Ithas beenfoundby several
researchers that generallyfatigue cracks originateeither in theheat
affectedzoneorintheweldmaterialsduetofatigueloadingandcould
be the potential source of catastrophic failures in some unfortunate
situations[21].
Curcio et al. [22] analysed the parameters of various materials at
laserweldingandreportedthatweldingpower,weldingspeed,
shield-ing gas,gasnozzle,andtheprocessoffocusingwereamongthese
pa-rameters. Zambonet al.[23] analysedthe microstructureand tensile
stages of AISI904L super austenitic stainless steel during CO2 laser
weldingandreportedthat therewasadecreaseinflowresistanceand
detachtensileanddependingonthistherewasanincreaseinhardness
valuebecauseofrapidcoolingofthefusionzoneduring welding.
Ber-trandetal.[24]analysedtheNd:YAGlaserweldingparametersofthe
stainlesssteelandreportedthatlaserpowerwas600—2700Wandthe
weldingspeedwas3—10m/minandflowspeedoftheshieldinggaswas
similartothesurfacecontaminationor itchangeddepending on
vari-oussurfacetensions.El-Batahgy[25]analysedtheeffectsofaustenitic
stainlesssteelsonsurfaceofhardnessandfusionzoneoflaserwelding
parametersand reportedthatpenetrationincreaseddependingonthe
increasedweldingpowerandweldingspeed,therewasnotasignificant
difference betweenheliumandargonshieldinggases,andmechanical
features(tension,hardness,bending)andfusionzonewerenot
microstructural characteristicsof lowcarbonsteelsonNd:YAGlaser
weldingandreportedthatdependingontheincreasedenergyinputand
processspeed,thehardnessvalueofthematerialatweldingzonealso
increased.Vuraletal.[27]analysedtheeffectofweldingnugget
diam-eteronthefatiguestrengthoftheresistancespotweldedjointsof
dif-ferent steel sheetsand reportedthat galvanized steelsheet
combina-tion has the highest fatigue limit and crack growth rate of the spot
welded galvanized AISI 304 joining type is slower than that of base
metals given in literature. C and m coefficients of Paris—Erdogan
equation for spot welded AISI 304 galvanized steel sheet joints were
obtained. Yuri et al.[28]analysed theeffect ofwelding structureon
high-cycle andlowcyclefatiguepropertiesforMIGweldedA5083
al-uminium alloys at cryogenic temperatures and reported that in the
high-cycle fatigue tests the ratios of 106 fatigue strength to tensile
strengthforA5183weldmetalswereslightlylowerthanA5083,inthe
low-cyclefatiguetestsfatiguelivesforA5183weldmetalswere
slight-lyshorterthanthoseforA5083.Jangetal.[29]analysedtheeffectsof
microstructureandresidualstressonfatiguecrackgrowthofstainless
steelnarrowgapweldsandreportedthatthefatiguecrackgrowthrate
depended onbothdendrite alignment andresidualstressdistribution
intheweld,sothefatiguecrackgrowthrateswerethehighestnearthe
boundary between theinner weld deposit andthe outer weld deposit,
where the dendrites were well aligned and the high tensile residual
stressexisted.Tabanetal.[30]analysedthelaserweldingofmodified
12%Crstainlesssteel,regardingstrength,fatigue,toughness,
micro-structure,and corrosion properties,andreported thatfatigue
behav-iour ofclean1.4003ferritic stainlesssteelweldis excellentprovided
welddefectsareomittedandallexcessofweldmetalisremoved
appro-priately or atleast the transition from weld tobase metalis suitably
smoothened. Under these conditions, the fatigue strength of butt
weldsiscomparablewiththatofthebasemetal.Themajorwithdrawal
ofthestainlesssteelisthetendencytograincoarseningattheHTHAZ.
Inthisstudy,thefatiguestrength ofAISI430—304stainlesssteels
weldedbyCO2 laserbeamweldingisinvestigated.
2.EXPERIMENTAL
AISI430steelscompriseapproximatelyone-halfoftheSAE-AISItype
400seriesstainlesssteels.Theyare knownwiththeirexcellentstress
corrosion crackingresistanceandgoodresistancetopittingand
crev-ice corrosion in chlorine environments. Welding is known to reduce
toughness, ductility and corrosion resistance because of the grain
coarsening and carbide precipitations. The grain size gradually
in-creases from the edge of Heat Effected Zone (HAZ) to the fusion
post-weld heattreatmenttominimizestressthatcanleadtocracking[31].
AISI304steelscompriseapproximatelyone-halfoftheSAE-AISItype
300seriesstainlesssteels.Theyare knownwiththeirexcellentstress
corrosion crackingresistanceandgoodresistancetopittingand
crev-ice corrosion in chlorine environments. Austenitic-stainless steel is
preferredmorethanotherstainless-steeltypesduetoeasinessin
weld-ing process. Then, somenegative metallurgic changes are taken into
consideration in welding of the steels. These are delta ferrite phase,
sigma phase, stress-corrosion cracking, chrome-carbide precipitate
betweengrainboundariesat450—850CofCr—Niausteniticsteelssuch
as18/8joinedbyfusionweldinginlongwaitingtime.Weldingof300
series usually requirespreheatandpost-weldheat treatmentto
mini-mize stress that canleadtocracking [32,33].Thechemical
composi-tions,mechanicalpropertiesandphysicalpropertiesofbothsteelsare
giveninTables1,2,and3,respectively.
Steel plates to be joined were cut at 45904 mm3 dimensions. 45
mmwidthisselectedtoachievethestandardlengthof90mmfatigue
TABLE1.ChemicalcompositionofAISI430ferriticstainlesssteelandAISI
304austeniticstainlesssteel.
Weight(%)Composition
Materials Fe C Si Mn P S Cr Ni
AISI430 Balance 0.055 0.045 0.420 0.031 0.008 17.0 —
AISI304 Balance 0.08 1.00 2.00 0.045 0.03 20.0 10.0
TABLE2.MechanicalpropertiesofAISI430ferriticstainlesssteelandAISI
304austeniticstainlesssteel.
Materials TensileStrength,
MPa YieldStrength 0.2%(MPa) Elongation, % Microhardness (RockwellB) AISI430 527 316 30 170 AISI304 590 295 55 130—180
TABLE3.PhysicalpropertiesofAISI430ferriticstainlesssteelandAISI304
austeniticstainlesssteel: –thermalexpansioncoefficient(20—800C), –
thermalconductivity(20C),–electricalresistance(20C),E–elastic
mod-ulus(20C).
Materials ,106 ,W/(m·C) ,nm E,kN/mm2
AISI430 13 24 600 225
test piece. The laser welding experiments were carried under helium
atmosphere with Trumpf Lazercell 1005 laser welding machine at
2000, 2250 and 2500 W heat inputs, 100 cm/min welding speed.
SchematicillustrationofLBWisgiveninFig.1[20].
TheLaserBeamWeldingwascarriedoutunderheliumatmosphere,
2000, 2250 and 2500 W different heat inputs and 100 cm/min
con-stantweldingspeed.Theweldedspecimenswerecutperpendicularlyto
theweldingzoneforpostevaluationsofweldintegrity.Afterthe
pro-cess,thesamplesweregroundtoan80—1200-gritandetchedby
polish-ingthemwith0.3mAl2O3 diamondpasteandaquaregion,
respective-ly. For microstructural examination, AISI 430 material was etched
electrolyticallyinasolutionof30%HCl10%HNO3 30%H2O
solu-tionandAISI304materialwasetchedelectrolyticallyinasolutionof
50%HNO3 50%H2O solution. Then, the microstructures of welds
were studied by means of scanning electron microscopy (SEM)
equipped with energy depressive spectrometer (EDS). XRD analysis
wasperformedtoidentifythestructuralphases.
Bendingfatiguetestswereconductedusingasinusoidalloadof
fre-quency50HzandloadratioR1,atroomtemperature,considering
asfatiguestrength(Fig.2).Sixgroupsoffatiguespecimenswere
pre-pared toobtainS—Ncurves. Theweldedinterfacewasthencut
longi-tudinallyforinvestigation ofthestructuralproperties (Fig.3).
Frac-tography was also employed in evaluation of the fractured fatigue
specimens.
3.RESULTSANDDISCUSSIONS
3.1.Microstructure
SEM imagesofwelded jointsunder heliumatmosphereat2000,2250
and 2500 W heat inputs and at 100cm/minconstant welding speeds
are presented inFigs. 4, a—c, respectively. Grain coarsening was
ob-served atHAZatbothsidesofweldinterfaceandfinecracks wereno
tracedatAISI430ferriticstainlessand AISI304austeniticstainless
steel side. Naturally, thearea ofweld (width) and heat-affected zone
haveincreasedatincreasedpowersbecauseofhighheatinputatweld
seam.SimilarresultswereachievedattheresearchofKaraaslanetal.
andKönigetal.[31,36].
ItwasdeterminedthatinthejointofAISI430ferriticstainlessand
AISI304austeniticstainlesssteelsampleswithlaserbeamweldingat
2500W weldingpower,at100cm/min weldingspeed,and under
heli-um shielding gas atmospheres, the penetration was completed and
blanksorporositiesintheweldwerenotobserved.Ahomogeneous
dis-persionwith smallgrainsbeginningfromthezonewithcoarse grains
on both sides appropriate for the original structure of the materials
wasalsoobserved.GraincoarseningwasobservedatHAZatbothsides
Fig. 2.Schematicillustration of fatigue test apparatus [34].
ofweldinterfaceatAISI430ferriticandAISI304austeniticstainless
steels sideat200—300mdistances. Ingeneral, coefficientof
ther-malexpansionofausteniticstainlesssteelishigherthanthatof
ferrit-ic stainlesssteel. Therefore, onbothsides oftheweldingseam, there
wasnoaspecificgrainhypertrophy,deformationandcrackformation
onthemainmaterials.TheSEMphotographsindicatethat
microstruc-tural features are affected significantly by welding power. The best
propertieswereobservedatthespecimensweldedat2500Wheatinput
andat100cm/minweldingspeed.
3.2.FatigueTest
Fatigue fractures, plastic deformation, tensile stress and fatigue
cy-cledstressesalsooccurbytheaction.Therefore,astoavoidthe
occur-renceofthesethreefactors,fatiguecrackswillnotoccur.Undera
giv-enappliedstressamplitudecycles,theinitiationofcracksinthe
mate-rial deformation and the tensile stress occurring in intermittent
Fig.4.SEMimagesofweldedsamplesjoinedatheatinput:2000W(a),2250
treatment is effective intheprogression ofcracks. Therefore,one of
the mainfactors affectingfatigue mechanismis plasticdeformation.
Figure 5shows fatigue test results forlaser beam welded joints.
Ac-cording to the fatigue test results, the fatigue endurance increased
withincreasingtheweldingpower.AISI430—304/2500Wjoint
exhib-itedbetterfatiguedatafromotherweldedjointtypes.Fatiguetestwas
conductedunderyieldstresses.
3.3.Fractography
Plastic deformation detected in the specimens after the welding was
notobserved.Thisbehaviourcanalsobeseeninthemicrographsofthe
fracturedsurfaces (Figs.6,a—c).Atfatigue strengthtests, fractures
were observedatAISI430sideclose tointerface.Afterwelding, itis
seenthatfatigueendurancecanbeincreasedwithincreasingthe
weld-ingpower,mainlyinbrittlefracturemannerbecauseAISI430steelis
a brittlesteel (Figs.6, a—c).Under givenconditions(2000, 2250and
2500 W heat inputs, 100 cm/min welding speed) fatigue strengths
Fig.5.Fatiguetestresultsforweldedjoints.
Fig.6.Micrographsofthefatiguefracturesurfacesofthespecimensobserved
wereclosetoparentmaterial,andslightlyincreasedatincreased
weld-ingpowers.
Thiscanbeexplainedwiththeincreasedheatinput.Thediffusionof
carbon toAISI430sidecauses totheformationofchromiumcarbide
precipitation atthegrainboundaries,whichare knowntobe
deleteri-oustothefatiguelifeofmaterial.Thecarbidesatthegrainboundaries
provide preferential sites for cavity nucleation in ferritic stainless
steels under fatigue conditions, thereby reducing the fatigue life [5,
20].
3.4.EDSAnalysis
IntheEDSanalysis,inthe200mdistance,fromAISI304austenitic
stainless steel to AISI 430 ferritic stainless steel 16% Chromium,
1.5% Carbon and 1% Nickel diffusion, the equal distance occurred
fromAISI430steeltoAISI304steelchromiumandcarbondiffusion.
As carbon is an interstitial element, stainless steel side will diffuse
more easily. It is seen that chromium—nickel diffusion can be
de-creasedbydecreasingtheweldingpower(Fig.7).
IntheXRDanalysis,likeFe, Ni—Cr—Fe,Fe—Cr—Co—Ni—Mo—Wand
Cr2Fe6.7Mo0.1Ni1.3Si0.3 phases were determined (Fig. 8). Intermetallic
phaselikeCr23C6 (chromium carbur),-ferriteand sigmawere not
ob-served.
4.CONCLUSIONS
In this study, the fatigue strength of AISI 430—304 stainless steels
weldedby CO2 laserbeamweldingwasinvestigated.Thefollowing
re-sultswereobtained.
— AISI430ferritic stainless steelcan bejoinedwithAISI 304
aus-tenitic stainless steel by laser welding process using helium
atmos-phere, at 2000, 2250 and 2500 W heat inputs, and 100 cm/min
con-stantweldingspeed.
— Atfatigue tests, fractureswere observed close toAISI 430side.
Afterwelding,itwasseenthatfatigueendurancedecreasedduetothe
decreasing oftheweldingpower. Thisis causedby thefactthat AISI
430 is brittlesteel.However,fatigue endurance increaseddueto the
increasingtheheatinput.
—Thefatiguestrengthincreasesduetohigherdeeppenetrationwith
increasing welding power. The best properties were observed at the
specimens welded at 2500 W heat input and at100 cm/min welding
speed.
— It was determined that the fusion zone was on average 1000—
mindepth.
— Coefficient of thermal expansion of austenitic stainless steel is
higherthanthatofferriticstainlesssteel.Therefore,onbothsidesof
the welding seam, there was no specific grain hypertrophy,
defor-mationandcrackformationonthemainmaterials.Thebestproperties
in terms of microstructure were observedat the specimen bonded at
2500 Wheatinput,at100cm/min.weldingspeedandhelium
inggas.
—IntheEDSanalysis,inthe200mdistancefromAISI304
austen-iticstainless steeltoAISI430ferriticstainless steel16%chromium,
1.5% carbon and 1% Nickel diffusion, the equal distance occurred
fromAISI430steeltoAISI304steelchromiumandcarbondiffusion.
As carbon is an interstitial element, stainless steel side will diffuse
more easily. It is seen that chromium—nickel diffusion can be
de-creasedbydecreasingtheweldingpower.
— In the XRD analysis, like Fe, Ni—Cr—Fe, Fe—Cr—Co—Ni—Mo—W,
andCr2Fe6.7Mo0.1Ni1.3Si0.3 phasesweredetermined(Fig.8).
Intermetal-lic phaselikeCr23C6 (chromium carbur),-ferriteand sigmawere not
observed.
ACKNOWLEDGEMENTS
The research was supported by the Scientific and Technological
Re-search Council of Turkey (Project No: TUBITAK-108M401 and
FUBAP:1566).
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