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: halvaee@ut.ac.ir (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 Tehranforexperimental and nancial supports.References[1] BrienRL. Editor. WeldingHandbook. Miami (FL, USA): AmericanWeldingSociety: 1991.[2] Kerstens NFH, Richardson IM. Heat distribution in resistance upset buttwelding. J Mat Proc Tech 2009;209:2715872.[3] SongQ, ZhangW, BayN. Anexperimental studydeterminestheelectricalcontact resistance in resistance welding. Weld J 2005:736.[4] KanneJr WR. Solidstateresistanceupset welding: aprocesswithuniqueadvantages for advanced materials. In: The 2nd advanced joining technologiesfor new materials conference, Cocoa Beach: FL; 1994[5] MiyazakiY, IchikawaM, SaitoT. 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