Chiang Mai J. Sci. 2013; 40(5) 831

Chiang Mai J. Sci. 2013; 40(5) : 831-838http://it.science.cmu.ac.th/ejournal/Contributed Paper

Characteristics of Microstructure, Microhardness,and Oxidation of FSW and MIG Welded SteelsSeul Ki Kim, Seung Boo Jung and Dong Bok Lee*School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746,South Korea.*Author for correspondence; e-mail: dlee@skku.ac.kr

Received: 03 July 2012Accepted: 14 February 2013

ABSTRACTThe SUS 409L stainless steel plates were joined by FSW and MIG welding

techniques. The welding electrode used for MIG welding was SUS 430 stainless steel. Inthe case of the MIG welding, the hardness was the highest in the weld metal, decreased inHAZ, and rose slightly in the base metal. The weld metal had the highest hardnessowing to the large residual stress, and its refined microstructure. The weld metal hadbetter oxidation resistance than the HAZ and base metal because of its high Cr contentcompared with the HAZ and base metal. In the case of the FSW, the microstructureconsisted of the stir zone, thermo-mechanically affected zone (TMAZ), and base metal.The stir zone had a relatively high hardness, owing to intense plastic deformation anddynamic recrystallization. It had better oxidation resistance than the base metal.

Keywords: steel, oxidation, microstructure, microhardenss, oxidation, FSW, MIGwelding

1. INTRODUCTIONFriction stir welding (FSW) is a

relatively new solid-state joining methodthat has received significant attention owingto its potential application in theautomobile and other industries (Figure 1).A non-consumable cylindrical rotatingtool, with a protruding probe centered ona circular face, plunges in the overlappingabutting plates to be joined, and traversesalong the line of joint [1-3]. The frictionalheat generated softens the workpiece, andthe rotating probe causes material flowin both the circumferential and axialdirections. The forging pressure exerted bythe tool shoulder creates frictional heat and

welding pressure, resulting in theformation of a solid-state bond around theprobe. Compared with electrical resistancewelding, which is the most commonly usedwelding method in the automobileindustry, FSW offers a significant energysaving, and it is an environmentallyfriendly method due to the absence of anyfume or spark. FSW has been successfullyapplied for aluminum joining [3], andsound, reliable joints were obtained.However, studies about steel joining arestill limited.

On the other hand, metal inert gas(MIG) welding is a semi-automatic or

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automatic arc welding process, in which theweld is shielded by a shielding gas (argon,helium, CO2, argon+oxygen, or other gasmixtures) (Figure 2). A consumable wireelectrode, having chemical compositionsimilar to that of the parent material, iscontinuously fed through a welding gun.The arc heats and melts both the workpiecesand the electrode wire. The fused electrodematerial is supplied to the surface of theworkpieces, fills the weld pool and formsthe joint. Today, MIG welding is the mostcommon industrial welding process,preferred for its versatility, speed and therelative ease of adapting the process torobotic automation [4-6].

Currently, both FSW and MIGwelding have been applied to weld theexhaust systems that consist of ferriticstainless steels in the automobile industry.An exhaust system consists of several partsthat have different functions, and thusrequire welding of different materials [7].During the MIG welding, the jointsundergo melting and quenching. During

the FSW process, the joints undergointense plastic deformation and dynamicrecrystallization at elevated temperatures.It is well known that the weldedjoints frequently suffer from cracking,which is accelerated by oxidation at hightemperatures [8]. Particularly, HAZs (heataffected zones) in welded joints are theweakest part with low toughness, and serveas frequent crack initiating points. Hence,welded joints require careful examination.Therefore, the microstructure, micro-hardness, and oxidation characteristics ofFSW and MIG welded steel joints areessential properties for commercialapplication. In this study, SUS 409L ferriticstainless steels were welded by the FSWand MIG welding process, and thecharacteristics of microstructure, micro-hardness, and oxidation of the welded jointswere investigated. The SUS 409L steels arecurrently used in the exhaust system of theautomobiles because of their resistance tooxidation and fatigue.

Figure 1. Schematic diagram of the frictionstir welding (FSW) process.

Figure 2. Schematic diagram of the metalinert gas (MIG) welding process.

2. EXPERIMENTAL PROCEDURECold rolled SUS 409L stainless steel

plates with a thickness of 3 mm were joinedby FSW and MIG welding techniques. TheMIG welding of 409L plates was performedusing the SUS 430 stainless steel welding

electrode under Ar+2% O2 shielding gases(Figure 2). Such a weld procedure is usedto weld the exhaust system in theautomobiles. FSW for the two 409Lstainless steel plates was done at a weldingspeed of 40 mm/min and a tool rotation

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speed of 700 rpm (Figure 1). The employedrotating tool was the WC-12%Co cementedcarbide. The chemical composition of SUS409L and SUS 430 employed in this studyis listed in Table 1. To understand the effectof oxidation on the welding, theweldments were cut to a size of 10x10x3mm3, ground to a 1000-grit emery paper,

cleaned in acetone, and heated at 800oC upto 200 hr in air inside an electrical furnace.The heat treatment was performed inorder to find the effect of heating onthe microstructure, microhardness, andoxidation behavior of FSW and MIGwelded steel plates.

The microstructure of the samples was

Table 1. Chemical composition of SUS 409L and SUS 430 stainless steels (wt.%).

Fe Cr Si Ti Mn C409L bal. 10.96 0.464 0.225 0.304 0.0078430 bal. 16.5 0.5 - 0.5 0.06

inspected by means of an opticalmicroscope (OM), and a scanning electronmicroscope (SEM). The etchant used formicrostructural inspection was (5mlHCl+5ml HNO3+1g picric acid+200mlethanol) solution. The Vickers micro-hardness of weldments was measured 3times for each sample by applying 1 kg loadfor 10 sec.

3. RESULTS AND DISCUSSIONFigure 3 shows the etched micro-

structure and microhardness of the MIGwelded steels. In Figure 3(a), the base metal(BM), weld metal (WM), and HAZ aredenoted. The base metal (SUS 409L)consisted of equiaxed ferritic grains. Theweld metal (SUS 430) displayed a finemicrostructure, due to the rapid coolingafter welding. The HAZ next to the basemetal consisted of fine grained HAZ(FGHAZ), whereas the HAZ next tothe weld consisted of coarse grainedHAZ (CGHAZ). Particularly, mature,polyhedral grains were seen in CGHAZ.The enlarged image of the weld metalshown in Figure 3(b) indicates that the weldmetal had a Widm nstatten structure thatconsisted of fine or coarse laths. Laths are

intersecting irregularly. On the otherhand, it is known that the formation ofintergranular Cr-rich carbides such asCr23C6 deteriorates the corrosion resistanceby depleting Cr around grain boundaries.Hence, Ti is added in 409L in order topreferentially react with undissolved,excess carbon and nitrogen to form TiCand Ti(C,N), which suppresses theprecipitation of harmful Cr-rich carbides.In Figure 3(b), neither Cr-rich carbidesnor titanium carbides were recognizable.Probably, Cr and Ti were supersaturatedin the observed ferritic grains or Cr-richcarbides and titanium carbides were toosmall to observe in Figure 3(b). Figure 3(c)shows the microhardness variation alongthe base metal, FGHAZ, CGHAZ, and theweld metal. The width of the narrow HAZwas about 1 mm. The hardness was thehighest in the weld metal, decreased inHAZ, and rose slightly in the base metal.The FGHAZ had the lowest hardness. Itis noted that microhardness depends on thegrain size, residual stress, the number, sizeand distribution of the secondary phasesor precipitates, and the fraction of retainedtempered martensite [2, 7-10]. All these areclosely related with heat generated during

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welding and subsequent cooling rates.Apparently, the softening of the HAZoccurred. The high hardness of the weldmetal is attributed to the residual stressthat was generated by the rapid solidifica-tion from liquid to solid during weldingand subsequent cooling. The refinedmicrostructure of the weld may haveadditionally increased the hardness.

Figure 4(a) and (b) show SEM top viewof the MIG welded steels after oxidationat 800oC for 25 and 50 h, respectively. Inthe right and left side of SEM images ofthe weldment, enlarged images of thesurface scales are displayed. The grindingmarks are clearly recognizable in the weldmetal (left side image of Figure 4(a)), owingto thinness of the formed scale. In the leftside image of Figure 4(b), some oxideparticles scattered over the thin scale that

Figure 3. MIG welded steels. (a) etched optical microstructures of the base metal, weldmetal, and HAZ, (b) etched SEM microstructure of the weld metal, (c) Vickersmicrohardness along the points 1~6.

Figure 4. SEM top view of the MIG weldedsteels after oxidation at 800oC for (a) 25 h,and (b) 50 h in air.

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still exhibited the grinding marks. Bycontrast, thicker oxide scales formed overthe HAZ and base metal, as shown in theright side images of Figure 4. The oxidegrains grew coarser, as oxidation progressed.The weld metal clearly had better oxidationresistance than the HAZ and base metal.The main reason is the higher Cr amountin the weld metal (430) compared with theHAZ and base metal (409L).

Figure 5(a), (b), and (c) show top view,the cross-sectional image, and micro-hardness of the MIG welded steels,respectively, after oxidation at 800oC for

Figure 5. MIG welded steels afteroxidation at 800oC for 200 h in air. (a) SEMtop view, (b) etched optical microstructure,and (c) Vickers microhardness along thepoints 1~8.

200 h in air. The surface was completelycovered with a thick oxide scale, owingto the increased extent of oxidation(Figure 5(a)). Hence, it was impossible todifferentiate the original base metal, HAZ,and the weld metal from examinationof the SEM top view. The etched cross-sectional image of these three regions isshown in Figure 5(b), which still retainsthe original microstructure that was shownin Figure 3(a). However, owing to heatingat 800oC for 200 h, grain coarseningoccurred in Figure 5(b), and themicrohardness of the base metal, HAZ,and the weld metal decreased in Figure 5(c)when compared with the microhardnessvalues shown in Figure 3(c). Themicrohardness decreased on moving fromthe weld metal, the HAZ, and base metal.In Figures 3(c) and 5(c), the weld metal washarder than the HAZ, due to its finemicrostructure and large residual stresscaused by rapid quenching after welding.

Figure 6(a) shows the etched opticalmicrostructure of the upper 409L (plate 1)and lower 409L (plate 2), which werewelded by the FSW method. It is a cross-section perpendicular to the weldingdirection of the weld. In Fig. 6(a), points1, 2-6, and 7 roughly correspond to thestir zone (SZ), thermo-mechanicallyaffected zone (TMAZ), and base metal(BM), respectively [10]. The TMAZ, whichoccurs on both sides of the SZ, is differentfrom HAZ in that HAZ is subjected to athermal cycle but is not deformed duringwelding. In TMAZ, grains were plasticallydeformed, and finer than those of the basemetal. Particularly, grains were quite fineat the point 2. Such a fine microstructureoriginated from the low heat input in theFSW process. By contrast, grains at theHAZ were coarse in MIG welding as shownin Figure 3(a). This reduces the ductility

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of the welds. Another advantage of FSWis that FSW is a solid state bondingmethod. Hence, the sensitization in theHAZ that develops in MIG weldingbecause of formation of the Cr depletedzone can be avoided. The frictional heatgenerated by the rotating tool softens theworkpiece so that the softened material ispushed and stirred to form a metallurgicalbond around the rotating tool. Theboundary between the SZ and TMAZ onthe advancing side of the rotating tool wassharp [10]. In the SZ (point 1), severalconcentric rings, which have been referredto as an “onion-ring” structure, were seen(Figure 6(b)). The SZ underwent intenseplastic deformation and metal flow atelevated temperatures, resulting in thegeneration of quite fine grains. Such a finemicrostructure in friction stir weldsproduces good mechanical properties. Atpoint 2 in TMAZ, substantial plasticdeformation and metal flow occurred.No defects such as pores or cracks wereseen in Figure 6(b), which indicates thatFSW is a proper method to weld 409Lsteels. Microhardness at the points 1~7 isdenoted in Figure 6(a). SZ (point 1) hada relatively high hardness, owing tointense plastic deformation and dynamicrecrystallization. Point 2, which was theadvancing side of the weld metal [2], hadthe highest hardness, owing to the severeplastic deformation and the accompanyinglarge residual stress. Around the point 1and 2, martensitic transformation may haveoccurred. The frictional heat generatedduring FSW softened the workpiece so thatthe hardness kept decreasing on movingfrom point 3 to point 6. Point 7 belongsto the base metal. Unlike MIG welding,no liquids formed during FSW. Unlikethis study, the microhardness of the SZ,TMAZ, and HAZ was higher than the base

Figure 6. Etched images of FSW steels. (a)optical microstructure and Vickersmicrohardness along the points 1~7, (b)enlarged SEM microstructure of the point‘1’.

metal in the case of FSW of 409M stainlesssteel [10]. This strongly indicates that themicrostructure and hardness depend on thealloy chemistry and welding parameters.

Figure 7(a) and (b) show the SEM topview of the stir zone (SZ) and base metal(BM) of FSW’ed steel, respectively, afteroxidation at 800oC for 50 h in air. Grindingmarks were still retained in Figure 7(a),whereas mature oxide grains covered theentire surface in Figure 7(b). Hence, it isseen that SZ has better oxidation resistance

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than BM, probably due to its refinedmicrostructure. It is noted that grainboundaries are short-circuit diffusion pathsso that protective oxide scales that consistedprimarily of Fe and Cr can develop quicklyon SZ when compared with BM.

Figure 8(a) shows the SEM top viewof FSW’ed steel after oxidation at 800oCfor 200 h in air. The surface was completelycovered with the oxide scale. Preferentialoxidation along the grain boundariesoccurred. In Figure 8(b), the etched cross-sectional images of the upper 409L plateand lower 409L plate are shown, along withmicrohardness values at points 1~7. Thegrains of the upper plate were coarser thanthose of the lower plate. It seemed thatTMAZ and, moreover, SZ underwentmuch faster grain growth than the basemetal, owing to large residual stress. Allthe microhardness values denoted inFigure 8(a) were smaller than those denotedin Figure 6(a), owing to grain growth and

Figure 7. SEM top view of FSW steels afteroxidation at 800oC for 50 h in air. (a) stirzone (SZ), (b) base metal.

Figure 8. FSW steels after oxidation at800oC for 200 h in air. (a) SEM top view,(b) etched optical microstructure andVickers microhardness along the points1~7.

the accompanying stress relieving. Thehardness along the points 1-7 varieddepending on location, due to combinedeffects of frictional heating, mechanicaldeformation by the rotating tool, andmicrostructural change. Basically, therewas not much variation in microhardnessamong the points 1-7, as shown inFigure 8(b).

4. CONCLUSIONSIn the case of the MIG welded steels,

the hardness was the highest in the weldmetal, decreased in HAZ, and rose slightlyin the base metal. The fine grained HAZhad the lowest hardness. The weld metalhad a Widm�nstatten structure thatconsisted of fine or coarse laths. When theMIG welded steels were heated at 800oC,the microhardness decreased in the orderof the weld metal, HAZ, and base metal.The weld metal was always harder than theHAZ, due to its fine microstructure and

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large residual stress. In the case of theFSW’ed steels, the microstructure consistedof the stir zone (SZ), thermo-mechanicallyaffected zone (TMAZ), and base metal.Grains in TMAZ were fine and plasticallydeformed. In SZ, intense plastic deformationand metal flow occurred, resulting in thegeneration of quite fine grains. Thefrictional heat generated during FSWsoftened the workpiece. The SZ had betteroxidation resistance than the base metal.When heated at 800oC, TMAZ and,moreover, the SZ underwent faster graingrowth than the base metal.

ACKNOWLEDGEMENTSThis work was supported by the

Human Resource Development Project ofthe Korea Institute of Energy TechnologyEvaluation and Planning (KETEP) grantfunded by the Korea governmentMinistry of Knowledge Economy (No.20114010203020).

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