Materials and Design 31 (2010) 4592–4600

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Materials and Design

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An assessment of microstructure, hardness, tensile and impact strengthof friction stir welded ferritic stainless steel joints

A.K. Lakshminarayanan *, V. BalasubramanianCentre for Materials Joining & Research (CEMAJOR), Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar 608 002, Tamil Nadu, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 March 2010Accepted 22 May 2010Available online 27 May 2010

Keywords:D. WeldingE. MechanicalG. Metallography

0261-3069/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.matdes.2010.05.049

* Corresponding author. Tel.: +91 4144 239734 (O+91 4144 238080/238275.

E-mail addresses: akln2k2@yahoo.com (A.K. Lakyahoo.com (V. Balasubramanian).

Microstructure and mechanical characterization of friction stir welded 409M ferritic stainless steel jointwere carried out. Single pass welds free of volumetric defects were produced at a welding speed of50 mm/min and rotation speed of 1000 rpm. Optical microscopy, microhardness testing, transverse ten-sile, impact and bend tests were performed. The coarse ferrite grains in the base material are changed tovery fine grains consisting duplex structure of ferrite and martensite due to the rapid cooling rate andhigh strain induced by severe plastic deformation caused by frictional stirring. Tensile testing indicatesovermatching of the weld metal relative to the base metal. The joints are also exhibited acceptable duc-tility and impact toughness.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

409M grade ferritic stainless steels (FSS) widely used for themanufacture of coal wagons for transporting iron ore. The differentparts made from 409M grade are box body (including the inner andouter walls, floor plate and wagon under-frame), vertical side stan-chions and flab doors [1]. These steels generally conform in compo-sition to grades S41003 (ASTM A240), 1.4003 (EN 10088-2 and EN10028-7) and 3Cr12. The steel was developed from the ferriticstainless steel AISI 409 by careful balancing of ferrite (Cr, Si, Ti)and austenite (Ni, Mn, C, N) stabilising elements using Kaltenhaus-er’s relationship presented in Eq. (1) [2]

Kaltenhauser Ferrite Factor ðKFFÞ¼ Crþ 6Siþ 8Ti� 2Mn� 4Ni� 40ðCþ NÞ ð1Þ

The 409M grade ferritic stainless steels are designed to trans-form partially to austenite on cooling, passing through the dual-phase austenitic–ferritic phase field on the Fe–Cr equilibriumphase diagram, (Fig. 1). This partial solid-state phase transforma-tion of ferrite to austenite on cooling improves the weldabilityand as-welded toughness by restricting heat-affected zone (HAZ)grain growth [3]. In high purity Fe–Cr systems, the gamma loop ex-tends as far as about 13.5% Cr, after which the structure is fully fer-ritic at all temperatures. Due to low alloying content, the steel usedin the present study lies in the dual phase region, and the structure

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would therefore consist of a mixture of untransformed delta fer-rite, alpha ferrite which transformed from austenite on coolingand martensite, depending on the cooling rate [3] and that iswhy it is variously described as ‘‘ferritic” or ‘‘ferritic–martensitic”12% Cr stainless steel [4].

The 409M grade stainless steels are usually supplied in the fullyannealed and desensitized condition. Final annealing is performedat temperatures below the A1 (normally between 700 �C and750 �C) after air cooling or cold rolling [5]. Though the modified12% Cr ferritic stainless steels are having better weldability thanconventionally used ferritic stainless steels, the steel still suffersfrom grain growth in the heat-affected zone and weld metal regionresults in significant alterations in the mechanical properties [6].

Friction stir welding (FSW) is a novel solid-state joining processthat was invented in 1991, it can avoid many problems associatedwith fusion welding processes, thereby defect-free welds havingexcellent properties can be produced even in some materials withpoor fusion weldability [7]. Due to its many advantages, FSW at-tracts a great deal of attention in industrial fields, and is success-fully applied to the joining of the various aluminum alloys,magnesium and copper alloys. In recent years, FSW of high meltingtemperature materials such as steels, nickel and titanium alloyshas become a research hotspot [8]. The main obstacle to use FSWwith these higher melting point materials is the development oftool materials capable of surviving the high temperatures andforces generated by the process [9]. Considerable advances havebeen made, mainly through improved materials selection and tooldesign [10–13]. Tool material development work has looked pri-marily at refractory metals (e.g., W–Re) [10,11], polycrystallineboron nitride (PCBN) [12] and WC [13].

Table 2Welding conditions and process parameters.

Parameters FSW

Welding machine RV Machine Tools, IndiaWelding speed (mm/min) 50Tool rotational speed (rpm) 1000Axial force (kN) 33.5Tool material Tungsten alloyTool profile, shoulder and pin diameter (mm) Taper cylindrical, 20 and 8–6Pin length (mm) 3.7

Fig. 1. Fe–Cr equilibrium phase diagram showing the position of 409M ferriticsteel.

A.K. Lakshminarayanan, V. Balasubramanian / Materials and Design 31 (2010) 4592–4600 4593

Only three papers were reported on friction stir welding of fer-ritic stainless steels in the literature. Thomas et al. [14] reported afeasibility study on FSW of a ferritic–martensitic, low carbon,12 wt.% Cr stainless alloys, and of low carbon mild steel. The weld-ing speeds used were in the range of 102–240 mm/min to fabricatethe joints. The author reported that the detailed weld parameters,dimensions of the tool, and the tool material are still being furtheroptimized. They observed that the maximum weld temperatures ofthe order of 1100 �C and the hardness values of heat-affected zoneare higher than the weld metal region. The reason for this differ-ence is not clear from the limited microstructure presented byauthors. Park et al. [15] examined the microstructural evolutionand mechanical properties of friction stir welded 430 stainlesssteel. The joint was made using a welding speed and rotationalspeed of 80 mm/min and 550 rpm respectively and they found thatthe coarser elongated base material was changed to very fine du-plex microstructure of ferrite and martensite resulted in significantimprovement in mechanical properties. Thomas et al. [16] usedbobbin tool design to join 8 mm thick 12 Cr ferritic stainless steelplates and reported that sound welds were produced using a rota-tional speed of 584 rpm and 75 mm/min. During tensile test thejoints were found to fail in the plate away from the weld. FSW offerritic stainless steels leads to improvement in mechanical prop-erties by microstructural refinement compared to the fusion weld-ing processes. However, details of microstructures and mechanicalproperties of friction stir welded 409M grade ferritic stainlesssteels are not yet reported in literature. In this paper, microstruc-ture, microhardness, tensile, impact toughness and bend tests werecarried out in order to evaluate the joint performance and the weldzone characteristics of friction stir welded 409M grade ferriticstainless steel joints.

2. Experimental

The as-received base material (BM) used in this study was4 mm thick cold rolled, annealed and pickled AISI 409M grade fer-ritic stainless steel plates. The chemical composition of the base

Table 1Chemical composition of base metal used in this study (spectrometry results).

C Cr Ni Nb Cu Si Mn

0.026 11.40 0.4 0.009 0.365 0.45 1.15

metal presented Table 1 was obtained using a vacuum spectrome-ter (ARL, model 3460). Sparks were ignited at various locations,and their spectrum was analysed for the estimation of alloying ele-ments. Few welding trials were carried out and specimens wereextracted from various locations of the joint and subjected to mac-rostructural analysis. The specimen free of volumetric defect andlack of penetration was considered as the optimized welding con-dition. Also care was taken to avoid the tool pin failure and controlthe heat input by controlling the combination of tool rotationalspeed and welding speed. The welding conditions and optimizedprocess parameters presented in Table 2 were used to fabricatethe joints for further investigation. An indigenously designed anddeveloped CNC controlled friction stir welding machine was usedin position control mode to fabricate the FSW joint. Argon gasshielding was employed to prevent the oxidation of the plate sur-face. Fig. 2 shows the photograph during friction stir welding of409M joint and an example of weld bead appearance. The weldedjoints were sliced (as shown in Fig. 3) using abrasive cutting andthen machined to the required dimensions for preparing tensile,impact test, bend and metallographic specimens.

Two different tensile specimens were prepared as shown inFig. 4a and b, to evaluate the transverse tensile properties. Un-notched smooth tensile specimens were prepared to evaluate thetransverse tensile properties of the joints such as yield strength,tensile strength and elongation. Notched specimens were preparedto evaluate notch tensile strength and notch strength ratio(notched tensile strength/un-notched tensile strength) of thejoints. Small tensile specimens of which the gauge length was cov-ered by the weld nugget shown in Fig. 4c, were used to character-ize the tensile properties of the stir zone. Three replicates fortensile testing were prepared to minimize errors. Tensile testingwas carried out using 100 kN, electromechanical controlled univer-sal testing machine (FIE-Blue Star, India; model UNITEK-94100).ASTM E8M-04 guidelines [17] were followed for preparing andtesting the tensile specimens.

Charpy impact specimens were prepared to the dimensionsshown in Fig. 4d and e to evaluate the impact toughness of theweld metal and HAZ, and hence the notch was placed (machined)at the weld metal (weld centre) as well as in the HAZ. As the platethickness is small, subsized specimens were prepared. Impact test-ing was conducted at room temperature using a pendulum-typeimpact testing machine (ENKAY, India) with a maximum capacityof 300 J. The amount of energy absorbed in fracture was recorded,and the absorbed energy is defined as the impact toughness of thematerial. ASTM E23-04 specifications [18] were followed for pre-paring and testing the impact specimens. Face and root three-pointbend tests were performed as per ASTM E190-03 specifications[19]. A Vickers microhardness testing machine (SHIMADZU, Japan;

P S N Al Co Ti Va

0.4 0.16 0.04 0.01 0.2 0.008 0.017

Fig. 2. Photographs of the welding process and an example of weld bead appearance.

Fig. 3. Scheme of extraction of specimens for mechanical testing (A-Smooth tensile specimen, B- Notched tensile specimen, C & D– Impact specimen notch placed at theweldcentre and HAZ respectively, E- Transverse tensile gauge length covering nugget, F- Metallographic specimen, G - bend test specimen).

4594 A.K. Lakshminarayanan, V. Balasubramanian / Materials and Design 31 (2010) 4592–4600

model HMV-2T) was used for measuring the hardness of the weldwith 0.5 N load and 15 s. Microstructural examination was carriedout using an optical microscope (MEJI, Japan; model MIL-7100)incorporating image analysing software (Metal vision). The speci-mens for metallographic examination were sectioned to the re-quired size from the joint comprising weld metal, HAZ and basemetal regions, and polished using different grades of emery papers.Final polishing was done using a diamond compound (1 lm parti-cle size) in the disc polishing machine. The specimens were etchedwith standard vilelia’s reagent for 30 s [20]. The fractured surfaceof the tensile and impact tested specimens was analysed using ascanning electron microscope (JEOL, Japan; model 6410LV) at high-er magnification to study the fracture morphology and establishthe nature of the fracture.

XRD analysis was carried out in Theta–Theta (Vertical type), D/Max (Make: RIGAKU, Japan; Model: ULTIMA-III) with copper targetunder a working voltage of 40 kV and 40 mA working current. Scin-tillation counter detector was used with a scan range 3–154� (min.setup size 0.0002�). A qualitative analysis of the precipitate wascarried out by X-ray diffraction based on the identification of thepeaks after refinement, using PROFIT and PCPDFWIN software toadjust the exact peak angular position of the phases presented inthe FSW Joint. Based on the values of the peaks in the diffracto-grams, a comparison with JCPDS card values was made, in orderto identify the phases which could precipitate in the friction stirwelded 409M joints.

3. Results

3.1. Macrostructure

Macrostructure of the friction stir welded 409M grade FSS jointis shown in Fig. 5. The weld cross-section appears like ‘basin-shape’and no volumetric defect is observed. The macrostructure of theweld can be split into several distinct regions such as stirzone(SZ), a region either side of the stirzone which can be termed thethermomechanically affected zone (TMAZ), the inner or high tem-perature heat-affected zone (HTHAZ), Low temperature heat-af-fected zone (LHAZ), and the base metal (BM) itself.

3.2. Microstructure

Optical micrographs of base metal and at different regions ofFSW joint are displayed in Fig. 6. The base metal (Fig. 6a) exhibitsa microstructure of coarser ferrite grains approximately 30 lm indiameter with randomly distributed carbides. Very fine equiaxedferrite grains with grain boundary martensite of 4 lm is observedat the top of the stirzone (Fig. 6b), whereas semi elliptical bandedstructure of ferrite and martensite is observed in the bottom regionof the stirzone (Fig. 6c). In the TMAZ, a distorted structure withgrains re-oriented perpendicular to the transverse direction is ob-served (Fig. 6d and e). It should be noted that the TMAZ/SZ bound-

(a) Subsize smooth tensile specimen

(b) Subsize notched tensile specimen

(c) Tensile specimen gauge length covering weld nugget

(d) Impact specimen (notch at weld centre)

(e) Impact specimen (notch at HAZ) All dimensions are in ‘mm’

Fig. 4. Dimensions of specimens for mechanical testing.

Fig. 5. Macrostructure of friction stir welded 409M ferritic stainless steel (a – flow arm: shoulder influenced region, b – stir zone bottom (pin influenced region), C –thermomechanically affected zone, D – high temperature heat affected zone, G – low temperature heat affected zone, AS – Advancing side, RS – Retreating side).

A.K. Lakshminarayanan, V. Balasubramanian / Materials and Design 31 (2010) 4592–4600 4595

ary on the advancing side (Fig. 6e) of the tool is sharper than thaton the retreating side (Fig. 6d). The high temperature heat-affectedzone (HTHAZ) was observed to be similar to the base metal,whereas the microstructure of the low temperature heat-affectedzone (LTHAZ) consisted of finer equiaxed grains (Fig. 6f).

3.3. Tensile and bend properties

Transverse tensile properties such as yield strength, tensilestrength and percentage of elongation of the joints were evaluatedand are presented in Table 3. Friction stir welded FSS joints areovermatched relative to the base metal. The ultimate tensile

strength of base metal and friction stir welded joint are 536 MPaand 574 MPa respectively. Notch tensile strength of the friction stirwelded joint is 1030 MPa with the failure taking place in the basemetal region. Since the tensile specimens involving the entire jointshown in Fig. 7a fractured in the base metal region, small tensilespecimens of which the gauge length was covered by the weldnugget shown in Fig. 7b were used to characterize the tensile prop-erties of the stir zone. In this case, the 0.2% offset yield strength andultimate tensile strength are 702 MPa and 906 MPa respectively.The ductility (percentage of elongation) of weld nugget of frictionstir welded 409M joint is 19% lesser compared with the base metal.Fig. 8a illustrates the three-point bend test configuration and

(a) Base metal (b) Stir zone – Shoulder Influenced Region

(c) Stir zone – Pin influenced region

(d) Retreating side (e) Advancing side (f) Low Temperature Heat Affected Zone

Fig. 6. Optical micrographs of friction stir welded 409M ferritic stainless steel joint.

Table 3Tensile and impact property of FSW joints in comparison with the base metal.

Joint Yield strength(MPa)

Tensile strength(MPa)

Elongation(%)

Notch tensile strength(MPa)

Notch strength ratio(NSR)

Impacttoughness (J)

Jointefficiency (%)

WM HAZ

BM 364 (4.32) 536 (5.12) 31 (1.15) 662 (3.54) 1.24 34 (0.96) –FSW joint (failure in BM

region)382 (5.18) 574 (6.42) 33 (0.931) 1030 (1.10) 1.79 28

(1.51)30(0.81)

107

FSW (all weld) 610 (7.14) 906 (3.81) 25 (1.41) – – – – –

Values given in the brackets are standard deviation of experimental results.

4596 A.K. Lakshminarayanan, V. Balasubramanian / Materials and Design 31 (2010) 4592–4600

Fig. 8b shows the root and face bent specimens of the as-weldedjoint indicates that the FSW joints exhibited acceptable ductility.There are no visible cracks in the cross-section of weld region(Fig. 8b).

3.4. Hardness

The microhardness across the mid thickness of the welded jointwas measured and presented in Fig. 9. The hardness of the as-re-ceived base metal is approximately 170 HV. The hardness of thestir zone varies from 320 to 382 HV, depending on the grain sizeand phases sampled by each indentation.

3.5. Impact toughness

Charpy impact toughness of the FSW joint was evaluated andpresented in Table 3. The impact toughness of unwelded base me-tal is 34 J, However, the impact toughness of FSW joint with notchplaced at the weld centerline and HAZ region are 28 J and 30 Jrespectively.

3.6. Fracture surface

Fig. 10 displays the fractographs of tensile and impact testedspecimens of basemetal and friction stir welded FSS joints. The dis-

played fractographs invariably consist of dimples, which are anindication that most of the tensile test specimens failed in a ductilemanner under the action of tensile loading. However, finer dimpleswere observed in FSW joints compared with the base metal.

3.7. X-ray diffraction analysis

Fig. 11a and b shows the X-ray diffraction pattern for base metaland weld metal of FSW joint respectively. The presence of largerpeaks corresponding to ferrite (a) and peaks of smaller intensity,which correspond to the precipitates, can be seen. The XRD resultsof base metal indicated the presence of ferrite (a) and Cr23C6 car-bides. However, the diffractograms of weld metal region indicatesthe presence of ferrite (a), martensite and carbides. Through thediffractograms it is possible to verify that, for the sample weldedwith FSW process, the carbides present were of the Cr23C6 andFe6W6C types.

3.8. Load/torque characteristics

Knowledge of tool load and torque, during FSW is very usefulfrom the standpoint of process development since the load actingon the tool decides the quality of the weld as well as tool life[21]. During each stages of friction stir welding, longitudinal force(the force resisting the motion of the tool in the direction of weld-

(a) Smooth Tensile specimens (b) Small tensile specimens

Fig. 7. Fracture location of tensile specimens.

(a) Set-up for three point bend test (b) root and face bent specimens

Fig. 8. Bend test results (Radius = 3 x Thickness).

Fig. 9. Hardness profile of friction stir welded 409M ferritic stainless steel.

A.K. Lakshminarayanan, V. Balasubramanian / Materials and Design 31 (2010) 4592–4600 4597

ing), and axial force (the forge force perpendicular to the work-piece top surface) were continuously recorded and presented inFig. 12. In the present study, a very slow plunging rate of 1 mm/min was used in order to reduce the forces acting on the pin andinitial tool wear. During plunging of the tool, the axial force in-creased sharply to about 16 kN as the pin contacts the workpiecematerial and slightly dropped (approximately 4 kN) as the materialis locally softened. Further a steep increase (38 kN) was recordedas the shoulder touches the workpiece material. A marginal de-crease in the axial force was then measured as the material belowthe shoulder softened during the frictional heating and it reachesthe steady state (33 kN). The torque value fluctuates during theplunge stage and reached steady state only after the shoulder

touches the workpiece material. Average axial load and longitudi-nal in the range of approximately 33.5 kN and 14.7 kN were re-corded respectively during the welding period.

4. Discussion

Transverse tensile properties of the base metal and weld arepresented in Table 3. The weld metal region has the higher ulti-mate tensile strength (UTS) and 0.2% offset yield strength (YS),and lower elongation than the BM. The tensile specimen of theweld fractured in the BM region, because the BM had the lowesthardness in the present weld, as shown in Fig. 9. The strength is

Base metal - Tensile FSW Joint - Tensile

Base metal - Impact FSW Joint - Impact

Fig. 10. SEM fractographs of tensile and impact specimens.

4598 A.K. Lakshminarayanan, V. Balasubramanian / Materials and Design 31 (2010) 4592–4600

roughly proportional to hardness, so that the BM region wouldpreferentially yield and then fail during transverse tensile test. Thiscan be explained by the microstructure obtained in the various re-gions of FSW joint. The top region of the stir zone (Fig. 6b) showsvery fine equiaxed grains of ferrite and few grains of martensite.The gradient in microstructural refinement seen here is consistentwith the development of a strain gradient as a function of distancefrom the tool shoulder [22]. Workpiece material closest to the toolshoulder experiences greater strain relative to material away fromthe shoulder, resulting in refinement in the top of the stirzone.

The grain refinement mechanism by friction stir welding can besimply explained on the basis of plastic flow and dynamic recrys-tallization. It is well accepted that generation of fine and equiaxedgrains in the SZ is a result of dynamic recrystallization due to thefriction heat between the tool and workpiece during friction stirwelding [23,24]. According to the general principles for recrystalli-zation, dynamic recrystallization may occur discontinuously bynucleation of new grains and growth by motion of high angle grainboundaries or continuously by increasing misorientation of exist-ing subgrain boundaries due to absorption of dislocations [25].Materials with low stacking fault energy (SFE) such as austeniticstainless steels generally undergo discontinuous dynamic recrys-tallization. Whereas, materials with high SFE such as ferritic stain-less steels tends to undergo continuous dynamic recrystallization[26]. Continuous dynamic recrystallization has been widely stud-ied in commercial aluminum alloys and dual phase stainless steel[27–29].

The bottom of the stirzone is characterized by banded appear-ance often referred to as the ‘‘onion ring” structure (Fig. 6c). Theformation of the onion rings has been explained by the geometricaleffect [30], the variations in grain size [31] and the particle-richbands [32]. Krishnan [30] reported that the spacing of the onionrings was equal to the forward movement of the tool in one rota-

tion. Xie et al., [31] found that the onion rings are due to formationof coarse grain boundary and fine grain boundary might associatedwith insufficient stirring and lower heat input at lower rotationrate. Sutton et al. [32] reported that the onion rings were charac-terized by a segregated banded microstructure consisting of alter-nating hard particle-rich and hard particle-poor regions.Microstructural observations indicated that the onion rings con-sists fine duplex structure of ferrite and martensite.

The tendency for martensite formation in weld metal can bedetermined by the Kaltenhauser Ferrrite Factor (KFF). Specifically,a low KFF below 7.5 indicates that ferrite formation is neglible,while high values above 9.5 indicate that substantial ferrite willbe retained [33]. At the time when the KFF was proposed, the pres-ence of hard untempered martensite would reduce the ductility ofthe weld and could act as stress raisers and crack initiation points[34]. However, one of problems plaguing ferritic stainless steels israpid grain growth at high temperature with subsequent brittle-ness, and substantial transformation of austenite and martensiteduring cooling will result in significant grain refinement andimprovement in toughness [35]. Since weld microstructure isgreatly influenced by chemical composition, the chromium equiv-alent and nickel equivalent based on Balmforth and Lippold consti-tutional diagram [36] for ferritic–martensitic stainless steel arecalculated by using the Eqs. (2) and (3).

CrEq ¼ Crþ 2Moþ 10ðAlþ TiÞ ð2Þ

NiEq ¼ Niþ 35Cþ 20N ð3Þ

By plotting CrEq and NiEq values on the constitution diagram,approximately 80% ferrite and 20% martensite is expected in theweld metal. Microstructural observation suggests that during fric-tion stir welding, the material undergoes phase change from fullyferritic structure to ferritic plus austenitic structure and upon cool-

(a) Basemetal

(b) FSW Joint

Fig. 11. X-ray diffraction pattern for base metal and friction stir weldment.

A.K. Lakshminarayanan, V. Balasubramanian / Materials and Design 31 (2010) 4592–4600 4599

ing the austenite transforms to martensite in the ferrite matrix dueto the rapid cooling rate and high strain induced by severe plasticdeformation caused by frictional stirring.

The notch effect on tensile specimens of the friction stir weldedjoint is less significant in comparison to the annealed base metaland the reason for this phenomenon could be attributed to the veryfine dual-phase microstructure which is characterized by the pres-

Fig. 12. Load/torque characteristics during

ence of a second phase (relatively harder martensite phase and car-bides) in a ferritic soft matrix [37]. This microstructuralconfiguration influences the stress/strain behavior of the materialby work hardening due to the plastic deformation. The yieldstrength is determined by the onset of plastic flow in the softphase, i.e., ferrite. At this stage the hard phase is still in the elasticregion [38]. Also, a dual-phase microstructure acts beneficially inimproving the impact toughness. It is interesting to note that Hay-den and Floreen [39] and Wright and Wood [40] has shown thatduplex ferritie-martensitic microstructures based Fe–Cr–Ni andFe–Cr–Mn systems, respectively, have superior impact toughnessproperties relative to either full ferritic or fully martensitic steelsof similar composition. However, the XRD results (Fig. 11b) indi-cates the presence of Fe6W6C carbides which essentially precipi-tated due the tool wear caused by high temperature and forcesacting on the tungsten alloy tool during friction stirring. The tooldebris in the pin influenced region resulted in reduction of impacttoughness of friction stir welded joint compared to the base metal.

5. Conclusions

The microstructure and mechanical properties of friction stirwelded AISI 409M ferritic stainless steel joints were investigated.From this investigation the following important conclusions arederived:

� The coarse ferrite grains in the base material are changed tovery fine duplex structure of ferrite and martensite due to therapid cooling rate and high strain induced by severe plasticdeformation caused by frictional stirring.� Tensile specimen of which, the gauge length covered by the

weld nugget was used to characterize the transverse tensilestrength of weld metal and it was found to be 57% higher com-pared to the base material. This is mainly due to very fineduplex ferritic martensitic structure of the weld metal causedby friction stirring.� The hardness of the stir zone was varying from 320 to 382 HV,

which was higher than the base material (170 HV).� Inclusion of tungsten in the pin influenced region resulted in

the reduction of ductility and impact toughness of the weldmetal. However, the bend test results indicate that the frictionstir welded joint exhibited acceptable ductility.

FSW of 409M ferritic stainless steel.

4600 A.K. Lakshminarayanan, V. Balasubramanian / Materials and Design 31 (2010) 4592–4600

Acknowledgement

The authors wish to place their sincere thanks to the Depart-ment of Science and Technology, Government of India for thefinancial support through Fast Track Scheme for Young ScientistsR&D project (SR/FTP/ETA043/2009) to carry out this investigation.Also the authors are to grateful to Dr. G. Madhusudhan Reddy, Sci-entist-‘G’ and Head, Metal Joining Group, Defence Metallurgical Re-search Laboratory (DMRL), Hyderabad, for his support andguidance to carry out this investigation.

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