Microstructure and mechanical properties of resistance upset
butt welded 304austenitic stainless steel jointsM. Sharitabara, A.
Halvaeeb,, S. KhorshahianaaMetallurgy and Materials Engineering
Division, Sistan & Baluchestan University, Zahedan, IranbSchool
of Metallurgy and Materials Engineering, University College of
Engineering, University of Tehran, P.O. Box: 11365-4563, Tehran,
Iranarti cle i nfoArticle history:Received 26 October 2010Accepted
3 March 2011Available online 10 March 2011Keywords:A. Ferrous
metals and alloysD. WeldingE. MechanicalF.
MicrostructureabstractResistance upset welding (UW) is a widely
used process for joining metal parts. In this process, current,time
and upset pressure are three parameters that affect the quality of
welded products. In the presentresearch, resistance upset butt
welding of 304 austenitic stainless steel and effect of welding
power andupset pressure on microstructure, tensile strength and
fatigue life of the joint were investigated. Micro-structure of
welds were studied using scanning electron microscopy (SEM). X-ray
diffraction (XRD)
anal-ysiswasusedtodistinguishthephase(s)thatformedatthejointinterfaceandinheataffectedzone(HAZ).
Energy dispersive spectroscopy (EDS) linked to the SEM was used to
determine chemical compo-sition of phases formed at the joint
interface. Fatigue tests were performed using a pullpush fatigue
testmachineandthefatiguepropertieswereanalyzeddrawingstress-numberof
cyclestofailure(SN)curves. Also tensile strength tests were
performed. Finally tensile and fatigue fracture surfaces were
stud-ied by SEM. Results showed that there were three different
microstructural zones at different distancesfrom the joint
interface and delta ferrite phase has formed in these regions.
There was no precipitation ofchromiumcarbideat thejoint
interfaceandintheHAZ. Tensileandfatiguestrengths of
thejointdecreased with welding power. Increasing of upset pressure
has also considerable inuence on tensilestrength of the joint.
Fractography of fractured samples showed that formation of hot
spots at high weld-ing powers is the most important factor in
decreasing tensile and fatigue strengths. 2011 Elsevier Ltd. All
rights reserved.1. IntroductionResistance upset welding is a
solid-state welding process whichinvolves the interaction of
electrical, thermal, mechanical and met-allurgical phenomena. In
this process, the joining surfaces are keptat aforcedcontact;
followedbyahighelectriccurrentpassingthroughtheworkpieces.
DuetothecontactresistanceandJouleheating, a vast amount of heat is
generated at the faying surfaces.Before, duringandafter applyingthe
electric current, force isapplied to maintain the electric current
continuity and to providethepressurenecessarytoformtheweldzone.
Themetal atthejoint is heated to a temperature where
recrystallization can rapidlyoccur across the heated surfaces. In
this process, similar to otherresistance welding processes, there
is no requirement to any extra-neous material such as ller material
or shielding gas [1]. In thiswelding process there are two types of
resistances namely contactresistance and bulk resistance. At the
earlier stages of the
welding,contactresistanceplaysthemainrolebutgraduallyitdecreasesandtheroleof
bulkresistancebecomes moreimportant [2,3].Kanneexpressedthat
incomparisonwithfusionweldingpro-cesses, the chemical composition
and metallurgical properties arenot signicantly changed leading to
better mechanical properties.Simplicity,welding speed,capability of
remote control and inde-pendence of welding quality from the
operator skill are the otheradvantagesofthisprocess[4]. Miyazaki
etal., Kang, KanneandSharitabar and Halvaee stated that resistance
upset welding is asuitable welding process for applications such as
sealing of atomicwaste containers, welding of automotive parts and
joining of stain-lesssteels, lowcarbonsteels, superalloys,
aluminumalloysandparts made of dissimilar materials [47]. The
general congurationof parts and equipments in upset welding is
shown in Fig.
1.Stainlesssteelsplayanimportantroleinthemodernworld.Austeniticstainlesssteelsrepresent
morethan2/3of thetotalstainless steel production. These stainless
steels are preferred morethan other stainless steel types due to
their good weldability [8].But there are some negative
metallurgical changes during weldingof these steels which should be
considered. They are [9,10]:(a) formation of delta ferrite phase,
(b) formation of sigma phase,(c) stress corrosion cracking, (d)
precipitation of chromium carbideat grain boundaries and (e)
formation of hot cracks.Nikitin et al. and Nikitin and Bses stated
that fatigue behavior ofaustenitic stainless steel welds is
strongly affected by stress ampli-0261-3069/$ - see front matter
2011 Elsevier Ltd. All rights
reserved.doi:10.1016/j.matdes.2011.03.007Corresponding author.
Tel.: +98 2161114104; fax: +98 2188006076.E-mail address:
[email protected] (A. Halvaee).Materials and Design 32 (2011)
38543864ContentslistsavailableatScienceDirectMaterials and Designj
our nal homepage: www. el sevi er . com/ l ocat e/ mat destude,
temperature, frequency and welding conditions [11,12]. Mostof the
service failures are expected to occur either in the HAZ or inthe
weld metal. These failures are most frequently associated
withdefects or microstructural in-homogeneities. But with
variations inwelding conditions, changes in the type and the amount
of defectsand in-homogeneities lead to variations in fatigue
behavior of thejoint
[13].Plasticdeformationofmeta-stableausteniticsteelsleadstoaphasetransformationfromparamagneticaustenitetoferromag-netic
martensite [14,15]. Smage showed that consequences of
thistransformation for the application of these materials can be
posi-tive or negative. Increment of the strength, e.g. the
transformationinduced plasticity (TRIP) effect and increase in the
lifetime in thehigh cycle fatigue (HCF) range are advantages in
contrast to localincrease of the hardness and related reduction in
ductility [14].Because of high cooling rate, short welding time and
formationof the joint in solid state in resistance upset but
welding, there ispossibility for elimination of some of these
metallurgical changesin welding of austenitic stainless steels by
UW.The literature in the upset welding eld is not very
extensive.The rst reported work on development of UW was the
researchdone at NASA Lewis Research Center. In This project Holko
focusedon magnetic resistance upset welding of stainless steel 304
plateswith different thicknesses [16]. Resistance welding of
nuclearwaste containers was another application of this technology
whichrequired design of new equipment able to deliver currents of
up to400,000 A at 64,000 kgf. The same application was further
reportedby Kanne [17]. He examined the properties of upset welded
cylin-drical and spherical components. He pointed out that
advantagesofUW, comparedtofusionweldingprocesses,
includefewerde-fects and stronger welds with a faster and more
reliable process.Cannell used UW for welding canisters made of 304L
stainless steel[18]. Bezprozvannyi at PatonWeldingInstitute
reportedupsetwelding of high-speed steel to carbon steel with a
current regula-tion system for controlling special cyclic welding
[19]. The effect ofvariationinupsetbuttweldingparameters,
suchascurrentandweldinglengthonthehardnessof
differentregionsoftheHAZ,microstructure and toughness of the weld
in high strength low-al-loy steel weldment were studied by Ghosh
and Gupta [20]. Miya-zaki et al. examinedtheupset weldabilityof
Nb-bearinghighstrengthsteel of the600 MPalevel.
Theyfoundthatthehigherwelding current density requires shorter
upset length (the
lengththattwosamplespenetrateintoeachotherduringwelding)forproducing
a high quality weld. Also they reported that the requiredupset
length can be reduced using lower welding forces [5]. Kannealso
reported applicability of the UW process to weld a variety
ofstainless steels (including A-286), super alloys (including TD
nick-el), refractorymetals(includingtungsten)
andaluminumalloys(including 2024) [4]. Shieh and Chang presented a
study of upsetwelding process in wire drawing; obtaining the
optimum
parame-tersoftheoperationforabetterdistributionofhardnessinthewire
[21]. Further, Cannel et al. wrote on the optimization and
reli-ability of UW process [22]. In a study by Kang et al. the
upset wel-dability and formability of a particular kind of material
(SPCC) wasinvestigated. The results showedthat the formabilityof
upsetwelded SPCC steel sheets were slightly lower than that of the
par-ent material [11]. Applications of upset welding processes were
re-cently extended to cast iron parts by Shakhmatov and
Shakhmatovand dissimilar austenitic to martensitic stainless steels
by Sharit-abar and Halvaee [7,23]. They found that a good
metallurgical bondcanbe produced betweenaustenitic and martensitic
stainlesssteels by resistance upset welding. Also in thepastdecade
somework has beencarried outonnumerical simulation
ofresistanceupset welding. Recently, Kerstens
andRichardsonreportedanexperimental study of weld development
during resistance upsetbutt weldingprocess. Theyalsomade asimplied
thermal niteelement model to explore the inuence of welding
conditions onheating [2]. In a very recent study Hamedi et al.
considered numer-ical simulationand experimental investigationof
UWprocessparametersincluding heatingand post-weldheatingcurrent
andtheir corresponding duration as well as interference of the part
fea-tures that form the joint and effect of these parameters on
tensilestrength of a low carbon low alloy oil pressure sensor. They
foundthat both numerical and experimental results suggest an
optimumset of welding parameters, i.e. time and electrical current
that yielda maximum value for the tensile strength of the joint.
Also the ef-fects of post-weld heating time and current on the
tensile strengthshowed that these parameters had a remarkable
effect on improv-ing tensile strength of the weldment [24].In this
research, resistance upset butt welding of 304 austeniticstainless
steel and effect of welding power and upset pressure
onmicrostructure, tensileandfatiguelifeofthejointwereinvesti-gated
in order to correlate the weld quality to the variation of thesetwo
parameters and introducing optimum welding conditions.2.
Experimental procedureChemical composition of AISI 304 stainless
steel used in this re-searchwas0.04%C, 0.48%Si, 1.75%Mn, 18.15%Cr,
8.2%Ni, 0.045%P,0.016%S, 0.7%Cuand0.11Mo.
Alsoyieldandtensilestrengthsofthe steel used were 242 and 658 MPa
respectively.Start material was heated 10 min at 1060 C and cooled
in air todiminish cold work effects due to mechanical processing
prior towelding. The welding machine used in this research had been
man-ufacturedbyElectro-TechnoTakCompany(Tehran, Iran)
anditsmaximumpower was 25 KVA. Thensurfaces of samples wereground
by 1000 mesh grinding paper to remove oxide layer formedduring heat
treatment. Two rods of 50 mm length and diameter of8
mmwereclampedinupsetweldingmachineforeachstateofwelding. Table 1
shows welding conditions and measured param-eters during welding.
Primary and upset pressures were the sameand were applied by a
mechanical system. Firstly, heating
pressurewasappliedonthefayingsurfaces. Thenelectrical
currentwaspassed through the bars in contact. During welding, the
electricalpotential was measuredbyanAVOmeter andweldingpowerwas
calculated using following equationP VI 1Fig. 1. Schematic
illustration of resistance upset welding process.M. Sharitabar et
al. / Materials and Design 32 (2011) 38543864 3855where P is the
welding power (VoltAmpere), I is the current inten-sity (Ampere)
and V is the electrical voltage (Volt) [2]. Also, weldingtime which
is the passing time of electrical current was measuredby the AVO
meter.Tensiletest wascarried outbyMST30/MH machine at2
mm/mindisplacement rateonweldedsamples. This test was per-formed
according to ASTM-E8 standard [25] while the joint inter-face was
held in the middle of the tension samples and the ashwasremoved.
Theexaminationmethodinfatigueinvestigationwasthesinglefactormethod,
i.e. foreachseriesofexperimentsone factor was varying while
theother parameter was kept
con-stantatpreviouslyoptimizedlevelsoftensilestrength. Theaimof the
authors was to investigate the effect of UW process param-eters on
fatigue properties of 304 austenitic stainless steel joints.
Inother words, the authors wanted to see how variations in
weldingparameters can affect the fatigue life of the joint. For
this purpose,three different stress amplitudes were selected higher
than
yieldstrengthofthealloytorepresenttheresultsofthetestsasSNcurves.
ThePullpushfatiguetest was performedaccordingtoASTM-E60692 standard
[26] while the joint interfaces were heldinthemiddleofthesamples.
ThesetestwascarriedoutbyIN-STRON 8502 fatigue testing machine at R
= 1, frequency of 2 Hzand 320, 370 and 430 MPa stress amplitudes.
The
metallographicsampleswerecutlongitudinallyandtheinterfaceswerestudiedby
SEM Cam Scan MV2300 and Energy dispersive X-ray lined upto the SEM
after preparation and etching with Kalling No. 2 agent(5 g CuCl2,
100 ml HCl and 100 ml Ethanol). XRD test was fullledby wave length
of Coka 1:7889 nm: Finally fracture surfaces werestudied by SEM.3.
Results and discussionLeberetal.
showedthatcoldworkcausesformationofnon-homogeneities such as shear
bands, mechanical twins and defor-mationinducedmartensite inthe
microstructure of austeniticstainless steels. Presence of
non-homogeneities in the microstruc-ture leads to decreasing
corrosion resistance of these alloys [15].Therefore in many
applications, austenitic stainless steels are
usedinnormalizedheattreatingconditionaftercoldworking. There-fore,
inthisresearchnormalizingheattreatmentwasperformedon samples before
welding to improve corrosion resistance.Fig. 2a and b shows
microstructure of base metal before and afternormalizing
respectively. It is observable that the microstructureconsists of
high density of mechanical micro and macro twins.
Alsosometransformationinducedmartensiteisformedintwinsandtwin-matrixboundaries(Fig.
2a). Afterannealing, equiaxegrainsand annealing twins were formed
in the microstructure (Fig. 2b).3.1. Study of microstructureFig.
3ashowsmacrostructureof halfof thejointinterfaceinsample A1B3. Fig.
3bdshows the microstructures of differentzones formed at the joint
interface. As can be seen, three differentmicrostructural
zoneshavebeenformedintheinterfaceduetothermalgradientbetweenthejoiningfacesandelectrodes.
Theyare:1. Widmanstttenausteniteformationzone(WAZ),
anellipticalzoneatthecenterofthejointinterfaceconsistedofdifferentmorphologies
of austenite (Fig. 3b). In addition to allotriomor-phic austenite
(Ac) localized in grain boundaries and intergran-ular austenite
(Ic), unusual austenite morphology is found.According to
literatures, this microstructure is Widmanstttenaustenite (Wc)
[27,28]. Small amounts of lathy d-ferrite couldbe found within
Widmansttten austenite laths. The
Wid-manstttenaustenitestructureismorecommoninausteniticstainless
steels solidifying asd-ferrite [28]. Woollin, expressedthat Like
Widmansttten ferrite found in carbon steels, appear-ance of
Widmansttten austenite is one of narrow wedges ema-nating either
directly from a grain boundary or fromallotriomorphic ferrite
(allotriomorphic austenite incase ofWidmansttten austenite) [27].2.
Dynamicrecrystallizationzone(DRZ) aroundtheWAZcom-posed of ne
austenite grains and delta ferrite phase is formedat the grain
boundaries (Fig. 3c).3. Partially recrystallization zone (PRZ)
which contains recrystalli-zation lines along the drawing direction
during
manufacturingofthebaranddeltaferriteformedalongsomeoftheselines(Fig.
3d).According to statements of Kerstens and Richardson and
Song[2,3], because electrical resistance of faying surfaces is
higher thanbulkresistanceofthematerials,
duringweldingcontactsurfacesarehotterthanbulkof
thesamplesandtheirtemperatureroseup to liquid + delta ferrite zone
in FeCrNi phase diagram. Whentheupsetpressurewasapplied,
theliquidmetalattheedgesofcontact surfaces of samples was rejected
as ash and was replacedwith mushy metal led to formation of upset
in contact area. But inthe center of the contact surfaces, the
liquid metal was trapped
andsolidied.DuetohighCrequivalenttoNiequivalentratio(Creq/Nieq =
1.91), the solidication microstructure is fully ferrite[28,29]. In
austenitic stainless steels, Delta ferrite is not stable atroom
temperature. So it transforms to austenite phase during cool-ing.
Southwick and Honeycombe concluded that decomposition ofd-ferrite
to austenite occurs by two different mechanisms depend-ing upon the
transformation temperature. At high temperature,
thereactionoccursbyadiffusional nucleationandgrowthprocesswhereas
at low temperature the austenite phase forms by a
displa-civemechanism. ItisbelievedthattheWidmanstttenaustenitegrows
by a displacive mechanism whereas allotriomorphic
austen-iteisconsideredtobeareconstructivetransformationproduct[30].
Menezes etal. have reported that in bead on plateweldingof
two-phase ferriticaustenitic stainless steels, residual
compres-sive stresses were formed near the ferrite to austenite
transforma-tion temperature. These stresses increased the
probability offormationof Widmanstttenaustenite [31]. During
solidstatetransformation of ferrite to austenite in resistance
upset weldingof this steel, allotriomorphic austenite
formedinferrite grainboundaries. But
thetransformationacrosstheentiregrainwassuppressedbyhighcoolingrateof
upsetbuttweldingresultinglow diffusion rate and low driving force
due to low transformationTable 1Selected conditions and measured
parameters during welding.SamplenameWeldingcurrent
(A)Weldingpressure (MPa)Weldingtime (S)Heat input(V.A.S)A1B11500
1.01 1.5 4500A2B12000 1.01 1.42 5680A3B12500 1.01 1.33 6650A4B13000
1.01 1.21 7260A1B21500 1.15 1.51 4530A2B22000 1.15 1.41
5640A3B22500 1.15 1.33 6650A4B23000 1.15 1.19 7140A1B31500 1.27 1.5
4500A2B32000 1.27 1.40 5600A3B32500 1.27 1.31 6550A4B33000 1.27
1.20 7200A1B41500 1.41 1.49 4470A2B42000 1.41 1.42 5680A3B42500
1.41 1.32 6600A4B43000 1.41 1.21 72603856 M. Sharitabar et al. /
Materials and Design 32 (2011) 38543864temperature. This
causedresidual ferritetotransformtoWid-mansttten austenite by
displacive mechanism. Compressive stres-ses during transformation
in this welding process also encouragedferrite to Widmansttten
austenite transformation (Fig. 3b).Joining surfaces havethe highest
temperature during weldingand with increasing distance from the
weld interface, temperaturedecreases [2,28]. Around the interface,
temperature rose up to aus-tenite + deltaferriteinFeCrNi
phasediagramresultingtotheformation of ferrite at austenite grain
boundaries (Fig. 3c). Becauseof high cooling rate of the joint, the
possibility of ferrite to austen-ite transformation was low and
some ferrite was remained in grainboundaries. So microstructure of
this region consisted of austeniteand delta ferrite [27]. Fuller et
al. concluded that presence of d-fer-rite in grain boundaries
prohibits grain growth and so the grainsarene.
Alsothisphaseactsasacrackgrowthinhibitorandre-ducesthepossibilityof
intergranular fracture[32]. But LippoldandKotecki expressedthat
presence of delta ferrite decreasesformability of austenitic
stainless steels and increases probabilityFig. 2. Microstructure of
base metal (a): before and (b): after annealing heat treatment.Fig.
3. (a): Macrostructure of resistance upset butt welding joint of
304 stainless steel. (b)(d): different microstructural zones formed
in the joint.M. Sharitabar et al. / Materials and Design 32 (2011)
38543864 3857of precipitationof carbides [28]. Onthe other hand,
dynamicrecrystallizationdue tohot deformation may be oneof
themostimportant
factorsindecreasingthegrainsizeinthisregionasmentioned by Humphreys
[33].Fig. 3d shows how recrystallization is limited to a series of
linesalong the drawing direction during manufacturing of the bars
anddelta ferrite is formed along some of these lines. It is
observed thatdirectionof partial
recrystallizationliesonthedirectionof theshear bands formed due to
mechanical working during manufac-turing of the bars. Cizek stated
that these bands are composed ofne dislocation containing cells
which grow parallel to each otherand pass through the grains. These
cells have high angle
non-crys-tallographicgrainboundarieswiththematrix[34].
Thepossiblereason for formation of this region is that heat
treatment in this re-search could not remove these cells due to
high density of
disloca-tionsandlowstackingfaultenergyinausteniticstainlesssteels.Duringwelding,
becauseof stressconcentrationinthesebands,density of dislocation
increased. So, heat and pressure caused dy-namic recrystallization
along these bands. But around thebands,possibility of dynamic
recrystallization was low due to low densityof dislocations.
Alsobecauseof thehighdislocationdensityintheseshearbands;
diffusionrateof
ferritepromotingelementssuchaschromiumandsegregationof
theseelementswashighleadingtoformationofhightemperaturedeltaferritealongtheshear
bands and in austenite grain boundaries [28].Microstructural
analysis of samples welded according to differ-ent welding
conditions represented in Table 1 showed that increas-ing of
welding power at a constant welding pressure (e.g. samplesA1B2A4B2)
caused widening all different zones formed in the jointinterface.
As stated before, higher welding power produced higherheat at the
joint interface resulting to the formation of consider-ableamount
of liquidmetal at theinterfaceandwideningtheWAZ. Also, The heat
generated by bulk resistance during weldingincreased at higher
welding powers leading the wider zone of basemetal temperature to
rise to austenite + ferrite region in FeCrNiphasediagram.
Thiscausedwidening thezonethat candynami-cally recrystallized
during welding according to following equationZ e0expQ=RT 2where Z
is ZenerHolloman parameter, e0is strain rate, Q is activa-tion
energy, R is gas constant and T is temperature. According to
thisequation, higher temperature during hot deformation reduces Z
andincreases the possibility of dynamic recrystallization and
thereforewidening DRZ and PRZ [33]. On the other hand, higher heat
gener-ated by bulk resistance of samples decreased temperature
gradientbetween the joint interface and bulk of the samples. This
reducedthe cooling rate leading to decreasing the amount of grain
boundaryferrite near the interface and grain growth in this
region.By using higher welding pressure at constant welding
powers(e.g. samples A2B1A2B4), widthof the WAZdecreased. But
itcausedwideningDRZandPRZ. Higherweldingpressureslowerthe interface
resistance according to Eq. (3) and increase the rollof bulk
resistance on heat generation during welding and forma-tion of
lower amount of liquid at the interface.Rc q=2pHB=Fups1=23where Rc
is contact resistance, HB is Brinell hardness, q is
specicresistivity and Fups is upsetting force [2]. Also, high
welding pressurecausedrejectionof liquidmetal formedat
theinterfaceduringupsettinganddecreasedwidthoftheWAZathighweldingpres-sures.
On the other hand, increasing the roll of bulk resistance onheat
generationat highpressuresledtowideningtheareathatcan dynamically
recrystallize according to following equationZ C1 sinhC2rn4where
C1, C2 and n are constants and r is stress exerted during
hotdeformation [33]. According to this equation, higher welding
pres-sures increasedthe possibilityof dynamic
recrystallizationandwidening DRZ and PRZ.3.2. Phase analysisFig. 4
shows the result of XRD analysis for weld metal and HAZ.Only there
are austenite and delta ferrite phases in these
regionsandnoprecipitationofchromiumcarbides(Cr23C6)phaseisob-served.
This is because of high cooling rate of weld interfaces
fromthetemperaturerangeof chromiumcarbideprecipitation(450850
C).Fig. 5a and b shows EDX analysis of the black phase in Fig.
3c.Alsochemical compositionof thisphaseisshowninTable2. Itcan be
seen that the amount of Cr increased and Ni decreased inthis phase
and its chemical composition is in the ferrite region ofthe FeCrNi
phase diagram at room temperature [28].3.3. Mechanical
properties3.3.1. Tensile propertiesFig.
6showsthattensilestrengthofthejointdecreaseswithwelding power.
Welding power of 3 KVA maintained enough heatFig. 4. XRD analysis
of the joint interface.3858 M. Sharitabar et al. / Materials and
Design 32 (2011) 38543864to produce a mushy zone and a complete
metallurgical joint. Butwith increase in weldingpower, thegrains
within the HAZgrawand the joint strength decreased according to
HallPetch equation.Ontheotherhand,
higherheatinputproducedathighweldingpowersincreasedresidual
liquidatthejointfaceandproducedconsiderable amount of Widmansttten
austenite phase. Plateformof
thisphaseincreasedstressconcentrationinthisregionand therefore
decreased joint strength.Formation of hot spots at the joint
interface also reduced jointstrength at high welding powers. Fig. 7
shows a hot spot in the cen-ter of the joint interface of sample
A4B1 formed due to heteroge-neous distribution of electrical
current and heat.Using nite element method (FEM) and experimental
investiga-tions, Kerstens andRichardson[2] showedthat
heterogeneousheating and formation of hot spots occur because of
non-uniformcurrentdensitypassingthroughthematerial.
Thisheterogeneitycan be formed by in-homogeneities in the material
and local
vari-ationsintheinterfaceresistanceresultingfromcontaminations,non-uniform
deformation or surface imperfections. A non-uniformupsetting
pressure distribution over the joint may also play a role.If the
upsetting pressure is not uniform, then according to Eq. (3)there
will be a difference in contact resistance over the joint
area.Non-uniformity in current distribution also may arise as a
resultofcontactresistancevariationsattheelectrode/sampleinterfacedue
to the electrode surface condition and contaminations or
vari-ations of the clamping force. With increasing of welding
power, theprobability of hot spot formation rises due to increase
in the heatinput to the joint interface.Fracture surface of sample
A4B2 (welded at high welding power)is shown in Fig. 8. The fracture
mode is completely ductile in thissteel. It can be seen that
formation of hot spot at the interface ofthis sample caused crack
initiation and reduced the joint strength.Fig. 9 shows that with
increasing of the upset pressure, strengthof the joint increases
rstly and then decreases. Contact resistancerises with decrease in
upset pressure according Eq. (3). Experimen-tal investigations of
Song et al. [3] also showed that contact resis-tance increases with
decreasing of welding pressure. So, lowwelding pressures leads to
widening the area for formation of
Wid-manstttenausteniteanddecreasingjointstrength[35]. AlsoatFig. 5.
(a): Black phase formed in the microstructure and (b): EDX line
scan analysis of the black phase in the microstructure.Table 2EDX
Chemical composition of black phase in the microstructure.Element
Cr Ni Si Mn FeWt% 23.93 4.71 0.46 1.61 BalFig. 6. Effect of welding
power on tensile strength of the joint.M. Sharitabar et al. /
Materials and Design 32 (2011) 38543864 3859low welding pressures,
good metallurgical bond between samplesdid not happen and therefore
the strength of the joint is low. Withincreasingof upset
pressureupto1.27 MPa, amount of liquidmetal formed at the joint
interface decreased. Also, large amountof liquid was rejected in
upsetting stage due to high welding pres-sure. This caused thinning
the area for formation of Widmanstt-tenausteniteandincreasingjoint
strength(sampleA1B3). Alsoincrease in upset pressure led to
formation of good metallurgicaljoint. Byusing weldingpressurehigher
than1.27 MPa; allliquidand mushy metals were rejected from the
joint interface as ashandanincomplete joint was
formedwhichreducedthe jointstrength (sample A1B4). On the other
hand, effect of welding pres-sure ontensile
strengthdecreasedwithincreasing of weldingpower (Fig. 9).
Thismaybeduetotheformationofhotspotsathighweldingpowers.
Becauseofthepresenceofweldingdefectat the joint area, effect of
microstructure on mechanical
propertieswasreducedledtodecreasingtheeffectofweldingpressureontensile
strength at high welding powers.Fig.
10showsfracturesurfaceofsampleA3B1(weldedatlowweldingpressure).
InFig.
10atherearedifferentcrackinitiationsitesatthefracturesurface.
Thehighermagnicationsof zones13 are shown in Fig. 10bd
respectively. It is observed that pres-enceof
plateformWidmanstttenaustenitephaseat thejointinterface caused
formation of large voids due to stress concentra-tion and reduced
the joint strength at low welding pressures.3.3.2. Fatigue
propertiesFig. 11 show the effect of welding power on fatigue life
of thejoint at different stress amplitudes in samples welded
with1.27 MPa welding pressure. It is observed that fatigue life
ofweldedsamplesislowerthanbasemetalanddecreasesslightlyFig. 7. SEM
macrostructure of a hot spot in sample A4B1 and formation of cracks
inthis region.Fig. 8. Fracture surface of sample A4B2, a: lower
magnication b: higher magnication showing hot spot on this
surface.Fig. 9. Effect of upset pressure on tensile strength of the
joint.3860 M. Sharitabar et al. / Materials and Design 32 (2011)
38543864withincreasingofweldingpowerfrom3000to5000 V.A.
Butinsamples welded with welding power of 6000 V.A,fatigue life
de-creases remarkably in all stress amplitudes. Slight decrease in
fati-gue strengthwithincreasing of welding power from3000 to5000
V.A may be due to grain growth in the HAZ of welded sam-ples.
AsstatedbyHertzberg, accordingtoHallPetchequation,coarse grain size
reduces tensile, fatigue strength and consequentlyfatigue life
[34].As stated before, the probability of hot spot formation
increasesat higher welding power due to higher heat input to the
joint area.Because of the melted and solidied microstructure and
presenceof cracks into the hot spots, stress concentration
decreases the fa-tigue strength. Fig. 12a shows the main crack
initiation site on thefatiguefracturesurfaceof thesampleweldedat
highweldingpower (6000 V.A) and tested at 320 MPa stressamplitude.
In theFig. 12b and c the higher magnication of this site and
dendritesarisenfrommeltingandsolidifyinginthehotspotsareshownrespectively.
Therefore it can beconcluded that formation ofhotspots is the main
reason in decreasing of the fatigue life at weldingpower of 6000
V.A.Variation in welding pressure at welding power of 3 KW had
noconsiderable inuence on fatigue life of the joints and therefore
itis not represented here.Fig. 13a and b showthe fatigue fracture
surfaces at the stage II offatigue crack propagation in samples
A2B3tested at 320 and430 MPa stress amplitudes respectively. There
are secondary crackson the fracture surface of the sample tested at
320 MPa (Fig. 13a).But no secondary cracks are observed at the
fracture surface of thesample tested at 430 MPa(Fig. 13b).
Observation ofcracks couldbe associated with the partially
transformed martensite phase inaustenitematrix. Becauseof
non-uniformmicrostructure, localstresses may be concentrated at
these locations causing secondarycracks to initiate [36]. But at
the stress amplitude of 430 MPa, thephenomenon of the self-heating
of the specimens was much morepronounced and affected deformation
behavior of the sample. Sur-face temperature of the samples tested
at 320, 370 and 430 MPastress amplitudes was measured by
thermocouples. It is observedthat it rose up to 40, 69 and 85 C
respectively (the Md(30/50)(C)temperature for investigated steel is
47 C where Md(30/50)(C) isthetemperatureatwhich50 vol%
a-martensiteisformedinthisFig. 10. (a): Fracture surface of sample
A3B1 (b)(d) higher magnication of crack initiation sites on the
fracture surface.Fig. 11. Effect of welding power on fatigue
strength of the joint.M. Sharitabar et al. / Materials and Design
32 (2011) 38543864 3861steel after a true tensile strain of 30%)
[8]. This relatively high tem-peratureinsampletestedat430
MPastressamplitudeinhibitedmartensite transformation. In addition,
plastic deformation bymigrationof
dislocationswasfacilitatedattheserelativelyhightemperatures
(thermal activation).Fig. 14 shows the sub-surface fatigue crack
formed in the Wid-mansttten austenite formation zone. Fatigue
process and itsmechanisms are largely inuenced by the presence of
the materialin-homogeneities. Since d-ferrite is basically
different from austen-ite matrix in crystallography and chemical
composition, it is likelytoprovidecracknucleationsites. Goyal et
al. [13]
conductedaseriesofexperimentstoinvestigatetheeffectofd-ferriteonthecontinuous
cycling fatigue properties. Using nite element model(FEM)
calculationandtransmissionelectronmicroscopy(TEM)studies, they
showed that stress concentration at the
delta/gammainterfaceoccursduetoincompatibilityandconsequentlyactsascrackinitiationsite.
Therefore, presenceofd-ferritebetweentheWidmansttten plates is one
of the reasons for formation ofsub-surfacecracks. Alsoplateformof
Widmanstttenphaseledto stress concentration in this region and
increased probability ofsub-surface crack formation. Fatigue crack
formed in the dynamicrecrystallized zone is shown in Fig. 15. It is
observed that there isFig. 12. Fatigue fracture surface of the
sample welded at high welding power.Fig. 13. Fatigue fracture
surface of the sample A2B3 tested at two different stress
amplitudes (a): 320 MPa and (b): 430 MPa.3862 M. Sharitabar et al.
/ Materials and Design 32 (2011) 38543864no fatigue crack deection
by d-ferrite phase in this region. There-foreit canbeconcludedthat
presenceof
d-ferriteinausteniticstainlesssteelweldscausesfatiguecrackinitiationanddoesnothave
any considerable effect on crack path at high stressamplitudes.4.
ConclusionsInthepresentinvestigation,
resistanceupsetbuttweldingof304 austenitic stainless steel and
effect of welding power and up-set pressure on microstructure,
tensile strength and fatigue prop-erties ofthejoint
wereinvestigated. Theobtained results canbesummarized as follows:1.
In resistance upset butt welding of 304 stainless steel, three
dif-ferent microstructural zones were formed at the joint
interfaceduetothermal gradientbetweenthejoiningfacesandelec-trodes.
Thesezonesare: Widmanstttenausteniteformationzone, dynamic
recrystallization zone and partially recrystallizedzone. Also delta
ferrite phase formed in these regions.2.
Increaseofweldingpowerraisesheatinputtothejointareaand widens all
different microstructural zones at the joint inter-face. Also
higher heat input increases probability of hot spot for-mation.
Tension tests results showed that tensile strength of thejoint
decreases withincreaseofwelding powerand hotspotsformed at high
welding powers are the most important factorsin decreasing of the
joint strength.3. X-ray diffraction analysis showed that there is
no precipitationof chromium carbide in the HAZ due to high cooling
rate of thejoint area from the chromium carbide formation
temperature.4. With increasing in welding pressure, area for
formation of Wid-mansttten austenite decreases leading to higher
tensilestrength of the joint. But effect of welding pressure on
tensilestrength decrease at high welding powers due to formation
ofhot spots.5. Fatigue test results indicatedthat fatigue life of
the jointdecreases withwelding power due to graingrowthintheHAZ.
Formation of hot spots is the other reason for decreasingfatigue
strength at high welding powers. Microstructural analy-sisof
thefatiguesamplesshowedthat presenceof
d-ferritebetweenWidmanstttenausteniteplatescaninitiatefatiguecrack
and dose not have any considerable effect on crack pathat high
stress
amplitudes.AcknowledgmentsTheauthorsgratefullyacknowledgetheextensivesupportoftheElectro-TechnoTakCompanyandUniversityof
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