Materials and Design 32 (2011) 3617–3623

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Materials and Design

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Technical Report

Effect of heat input on the microstructure and mechanical properties of gastungsten arc welded AISI 304 stainless steel joints

Subodh Kumar, A.S. Shahi ⇑Department of Mechanical Engineering, Sant Longowal Institute of Engineering & Technology, Longowal, Sangrur, Punjab 148 106, India

a r t i c l e i n f o

Article history:Received 20 October 2010Accepted 7 February 2011Available online 3 March 2011

0261-3069/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.matdes.2011.02.017

⇑ Corresponding author. Tel.: +91 1672 253272; faxE-mail address: ashahisliet@yahoo.co.in (A.S. Shah

a b s t r a c t

Influence of heat input on the microstructure and mechanical properties of gas tungsten arc welded 304stainless steel (SS) joints was studied. Three heat input combinations designated as low heat (2.563 kJ/mm), medium heat (2.784 kJ/mm) and high heat (3.017 kJ/mm) were selected from the operating win-dow of the gas tungsten arc welding process (GTAW) and weld joints made using these combinationswere subjected to microstructural evaluations and tensile testing so as to analyze the effect of thermalarc energy on the microstructure and mechanical properties of these joints. The results of this investiga-tion indicate that the joints made using low heat input exhibited higher ultimate tensile strength (UTS)than those welded with medium and high heat input. Significant grain coarsening was observed in theheat affected zone (HAZ) of all the joints and it was found that the extent of grain coarsening in the heataffected zone increased with increase in the heat input. For the joints investigated in this study it was alsofound that average dendrite length and inter-dendritic spacing in the weld zone increases with increasein the heat input which is the main reason for the observable changes in the tensile properties of the weldjoints welded with different arc energy inputs.

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1. Introduction

Austenitic stainless steels have been used widely by the fabrica-tion industry owing to their excellent high temperature and corro-sion resistance properties. Some of the typical applications of thesesteel include their use as nuclear structural material for reactorcoolant piping, valve bodies, vessel internals, chemical and processindustries, dairy industries, petrochemical industries etc. Out of300 series grade of these steels type 304 SS is extensively used inindustries due to its superior low temperature toughness and cor-rosion resistance. One of the typical applications of type 304 SS in-clude storing and transportation of liquefied natural gas (LNG),whose boiling point is �162 �C under 1 atmosphere.

A study on fatigue crack growth rate for type 304 SS over a tem-perature range from room to �162 �C has shown that base metalpossesses superior resistance to crack growth relative to weld met-als over the entire temperature range [1]. Another typical applica-tion of this material includes its use as bellows used as conduit forliquid fuel and oxidizer in propellant tank of satellite launch vehi-cle [2].

Chen et al. [3] found that when Cu–Si enriched type 304 SS(containing 2–2.5 wt.% copper and 1–1.5 wt.% silicon) and a con-ventional type 304 SS was welded using gas metal arc welding(GMAW), process ductility decreased and ferrite levels increased

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in both weldments, as the heat input was increased. A comparativestudy by Yan et al. [4] on the microstructure and mechanical prop-erties of 304 SS joints by tungsten inert gas (TIG) welding, laserwelding and laser-TIG hybrid welding showed that laser weldingcould give highest tensile strength and smallest dendrite size inall joints whereas TIG welding gave lowest tensile strength andbiggest dendrite size. Work reported by Muthupandi et al. [5] onthe effect of weld chemistry and heat input on the structure andproperties of duplex stainless steel welds using autogenous-TIGand electron beam welding process shows that chemical composi-tion exerts a greater influence on the ferrite–austenite ratio thanthe cooling rate. Jana [6] has reported the effect of varying heat in-puts on the properties of the HAZ of two different duplex steels andfound that as arc energy increased hardness of both weld metaland the HAZ decreased, whereas width of the HAZ increased withincreased arc energies. Study on the influence of welding heatinput on submerged arc welding (SAW) welded duplex steel jointsimperfections has been reported by Nowacki et al. [7] where heatinput from 2.5 to 4.0 kJ/mm was used for plate thickness of10–23 mm and it was concluded that usage of larger welding heatinput provided the best joints quality.

Zumelzu et al. [8] studied the mechanical behaviour of AISI316L welded joints using shielded metal arc welding (SMAW)and GMAW process with different electrodes types. Their workconcludes that a direct correlation exists between the thermal con-tribution and tensile strength for the materials studied. The effectsof minor elements and shielding gas on the penetration of TIG

3618 S. Kumar, A.S. Shahi / Materials and Design 32 (2011) 3617–3623

welding in type 304 SS have been studied using bead on plateexperimentation technique and it is concluded that minor ele-ments such as oxygen, aluminium and sulfur have a significant ef-fect on the weld depth to width ratio [9]. Experimentalinvestigations on the effect of hydrogen in argon as a shieldinggas in TIG welding of austenitic stainless steel show that meangrain size in the weld metal increases with increasing hydrogencontent besides increasing the weld metal penetration depth andits width [10]. Lu et al. [11] have reported in their experimental re-sults that small addition of oxygen content to the He–Ar mixedshielding can significantly change the weld shape from a wideshallow type to a narrow deep one.

Lee et al. [12] have reported in their studies on effects of strainrate and failure behaviour of 304L SS SMAW weldments and findthat as the strain rate increases, the flow stress increases and thefracture strain decreases. Korino et al. [13] have reviewed the con-siderations for weldability of 304L SS and recommend Creq to Nieq

ratio of 1.52–1.9 to control the primary mode of solidification. Leeet al. [14] while investigating the pitting corrosion behaviour ofwelded joints of AISI 304L using flux cored arc welding (FCAW)process found that tensile and yield strengths were increased withincreasing equivalent ratio of Creq/Nieq. Milad et al. [15] found thatyield and tensile strengths of 304 SS increased gradually at thesame rate with increasing degree of cold work. Shyu et al. [16] haveinvestigated the effect of oxide fluxes on weld morphology, arcvoltage, mechanical properties, angular distortion and hot crackingsusceptibility of autogenous TIG bead on plate welds. Their resultsindicate that penetration is significantly increased which in turnincreases depth to bead-width ratio and tends to reduce angulardistortion.

Other studies which show that 304 SS and 304L SS grade hasbeen the topic of research of many researchers include variousstudies like experimental determination of grain boundary compo-sition of 304 SS in low temperature sensitization condition using ascanning Auger microprobe [17], measuring chromium depletionafter various thermal heat treatments [18], modelling of low tem-perature sensitization of austenitic stainless steel [19], studyingsensitization behaviour of grain boundary engineered austeniticstainless steel [20], arresting weld decay in 304 SS by twin-inducedgrain boundary engineering [21] etc.

From the literature reviewed on the material processing of 304SS it is observed that no systematic work on the effect of heat inputon microstructure and tensile properties of gas tungsten arc (GTA)welded has been reported. In view of the fact that arc welding pro-cesses like GTAW offer a wide spectrum of thermal energy for join-ing different thicknesses of steels it was considered important thatundertaking the present study would be beneficial in gaining an

Table 1Chemical composition (wt.%) of the base metal and filler used.

Alloy element C Si Mn P

Base (304 SS) 0.06 0.42 1.89 0.0Filler (ER 308 SS) 0.08 1.0 1.59 0.0

Table 2Process parameters used for fabricating butt welded joints.

Specimen no. Pass Current (A) Voltage (V) Average wspeed (mm

A (low heat) First 120 30 2.252Second 120 30 2.243

B (medium heat) First 150 35 3.030Second 150 35 3.003

C (high heat) First 180 40 3.846Second 180 40 3.787

understanding about the metallurgical aspects that affect the ser-vice performance of these welded joints made using different heatinput combinations.

2. Experimental details

2.1. Base and filler material combination

The base material used in the present investigation was in theform of AISI 304 SS plates of sizes 200 mm � 100 mm � 6 mmwhich were cut from a rolled sheet and the filler was 308 SS solidelectrode of 3.15 mm diameter. Table 1 shows the chemical com-position of the base and the filler used.

2.2. Welding procedure

In the present work double V-groove design was used so thatwelding could be accomplished in two numbers of passes ensuringfull penetration. Before welding all the edges were thoroughlycleaned mechanically and chemically in order to avoid any sourceof contamination like rust, scale, dust, oil, moisture etc. that couldcreep into the weld metal and later on, could result possibly into aweld defect. After tacking the plates together the first weld passwas given using GTAW process with welding conditions as men-tioned in Table 2 and prior to giving of second pass an interpasstemperature of around 150 �C was maintained. No preheat or postheat treatment was given to the specimens. Although GTAW pro-cess was used in the manual mode, still utmost care was taken dur-ing recording of the arc on time so as to facilitate calculations ofwelding speed for heat input calculations. It is worth mentioninghere that the best welding practice available in the fabricationindustry was used in the present work.

It is a well established fact that among all the welding variablesin arc welding processes welding current is the most influentialvariable since it affects the current density and thus the meltingrate of the filler as well as the base material. So in accordance withthis fundamental fact three different heat input combinations cor-responding to different welding currents i.e. 120 A (low heat in-put), 150 A (medium heat input) and 180 A (high heat input)combinations were selected for the present study. The reason forusing these specific welding current values was twofold firstly, thisspectrum of heat input combinations results in arc energies whichare sufficient to cause adequate fusion of the base and weld metalselected for the present study and secondly, a step increase of 30Awas anticipated to be sufficient enough to cause a direct and signif-icant influence on the microstructure and tensile properties of the

S Cr Ni Fe

32 0.014 18.67 8.53 Balance45 0.03 18.15 10.02 Balance

elding/s)

Average heat input per unitlength per pass (kJ/mm)

Total heat input per unit lengthof the weld (kJ/mm)

1.280 2.5631.2831.386 2.7841.3981.497 3.0171.520

Fig. 1. Photograph showing the base plates in the as welded condition at differentheat inputs.

S. Kumar, A.S. Shahi / Materials and Design 32 (2011) 3617–3623 3619

welded joints. During and after welding the joints were visually in-spected for their quality and it was ensured that all weld beadspossessed good geometrical consistency and were free from visibledefects like surface porosity, blow holes etc. Fig. 1 shows the platesin the as welded condition using different heat inputs. Other de-tails related to the process and procedures used in the presentwork include:-

Type and size of the non-consumable for the joints investigatedin this study tungsten electrode = EW-Th-2 (Thoriated tungsten) of3 mm diameter, Shielding gas flow rate of industrially pure Ar-gon = 15 L/min, Electrode to work angle = 45�, Polarity = DC elec-trode positive.

2.3. Specimen sampling

The specimens for tensile testing, micro hardness testing andmicrostructural studies were taken from the weld pads as sche-matically illustrated in Fig. 2.

2.4. Tensile test

Three specimens per heat input combinations, were machinedout from the weld pads as mentioned in Fig. 2. Each tensile speci-men size was prepared in accordance with ASTM E08 standards[22] as illustrated schematically in Fig. 3. The specimens were

Fig. 2. Schematic illustration of the spec

Fig. 3. Specifications of the tensile spe

tested on a servo hydraulically controlled digital tensile testingmachine of 400 kN capacity.

2.5. Metallography

In order to observe the microstructural changes that take placeduring welding, corresponding to each heat input combination,specimens were machined out from the weld pads as shown inFig. 2. After polishing and macroetching the cross sections of thejoints were captured with the help of Image analysis software cou-pled with a stereozoom microscope at a magnification of 10� tofacilitate measuring of the details like cross sectional areas of thefusion zone and HAZ. Standard polishing procedures were usedfor general microstructural observations [23]. An electrolytic oxalicacid etch was used with the conditions (Electrolyte used: Oxalicacid (10 g) + distilled water (100 mL), Cell voltage: 6 V, Etchingtime: 1 min).

Microstructures of different zones of interest like weld metal,HAZ and fusion boundary under different heat input combinationswere viewed and captured with an optical microscope coupledwith an image analyzing software. Microhardness of differentzones of the weldments was measured using Vickers’s micro hard-ness testing machine with a load of 0.5 kg. Fractured ends of thetensile tested specimens were analyzed using Scanning electronmicroscopy (SEM) to assess the nature of the fracture mode.

3. Results and discussion

3.1. Metallographic studies

Full penetration welds were obtained in all the three combina-tions of heat input as shown in Fig. 4. Measured areas of fusionzone and HAZ of different weldments are shown in Table 3. As indi-cated by these values it is found that as heat input increases the fu-sion areas of the joints also increase proportionately. The sametrend is followed for the HAZ area associated with each of thesejoints. Yan [4] and Jana [6] have reported similar trends whilestudying TIG welded 304 SS and SMAW welded duplex SS respec-

imen sampling from the weld pads.

cimen used in the present work.

Fig. 4. Stereozoom images showing the cross sections of the weld joints at different heat inputs (a) low heat (b) medium heat (c) high heat (10�).

Table 3Macro and microstructural details of the weld joints.

Heat input Tensile properties Macrostructural details (Cross sectional area) Microstructural details Location offracture

Jointefficiency (%)

Ultimatetensilestrength (MPa)

Percentageelongation(%)

Fusion zone withreinforcement(mm2)

Fusion zone withoutreinforcement(mm2)

HAZ area(mm2)

Dendrite length inthe weld zone (lm)

Interdendritespacing (lm)

Low 657.32 24.28 36.74 21.68 12.83 111.10 10.29 Base metal 107.61Medium 639.45 22.85 38.86 23.57 14.79 151.75 15.42 Base metal 104.69High 622.8 21.42 43.02 26.29 16.24 201.14 22.87 Base metal 101.96Base metal 610.8 38.57 – – – – – – –

3620 S. Kumar, A.S. Shahi / Materials and Design 32 (2011) 3617–3623

tively, that fusion zone and HAZ area increase with increase in heatinput.

Optical micrographs showing the microstructures of weld zone,fusion boundary and HAZ for different heat input combinations arepresented from Figs. 5–7. The measured values of dendrite lengthsand inter-dendritic spacings for these joints are mentioned in Table

ba

Fig. 5. Optical micrograph showing the microstructure of (a) we

a

Fig. 6. Optical micrograph showing the microstructure of (a) weld

3. It is observed from these optical micrographs that as heat inputincreases the dendrite size and inter-dendritic spacing in the weldmetal also increase. This dendrite size variation can be attributedto the fact that at low heat input, cooling rate is relatively higherdue to which steep thermal gradients are established in the weldmetal, which in turn allow lesser time for the dendrites to grow,

HAZ

FB

ld metal (b) fusion boundary and HAZ (low heat, at 100�).

FB

HAZ

b

metal (b) fusion boundary and HAZ (medium heat, at 100�).

FB

HAZ

a b

Fig. 7. Optical micrograph showing the microstructure of (a) weld metal (b) fusion boundary and HAZ (high heat, at 100�).

Low heat

S. Kumar, A.S. Shahi / Materials and Design 32 (2011) 3617–3623 3621

whereas at high heat input, cooling rate is slow which providesample time for the dendrites to grow farther into the fusion zone.

-10 -8 -6 -4 -2 0 2 4 6 8 10150160170180190200210220230240250260270280

Vic

ker

hard

ness

(H

V0.

5)

Distance from the weld centre (mm)

Medium heat High heat

Fig. 9. Microhardness profile showing micro hardness of different zones of theweldments at different heat inputs.

3.2. Microhardness

Microhardness measurements were taken in two directionsfirstly in the transverse direction i.e. perpendicular to the baseplate surface and secondly, in the longitudinal direction i.e. parallelto the base plate surface and the same are shown in Figs. 8 and 9respectively. Fig. 8 shows that the micro hardness near the top ofthe weld bead surface is high and as the centre of the fusion/weldzone is approached by the indentor it gradually reduces, which isdue to the fact that cooling rate is relatively higher at the top ofthe weld bead surface than at the centre of the weld metal. FromFig. 9 it is observed that as the indentor traverses outwards (paral-lel to the base plate surface) from the centre of the weld/fusionzone towards the fusion boundary, micro hardness increases from205.5 to 228.8 VHN for low heat input, 194.0–210.2 VHN for med-ium heat and 181.1–197.4 VHN for high heat input welded joint.Fusion boundary or transition zone encountered while traversingin this direction is indicated by a steep rise in the micro hardnesswith value of 272.4 VHN, 262.6 VHN and 251.6 VHN respectivelyfor low, medium and high heat input respectively. High hardnessas possessed by the fusion boundary zone (FBZ) in all the jointscan be attributed to the presence of partially unmelted grains atthe fusion boundary which are partially adopted as nuclei by thenew precipitating phase of the weld metal during the solidification

-4 -3 -2 -1 0 1 2 3 4160

170

180

190

200

210

220

230

240

250

Vic

ker

hard

ness

(H

V0.

5)

Distance from the weld centre (mm)

Low heat Medium heat High heat

Fig. 8. Microhardness profile showing micro hardness at different points in theweld metal at different heat inputs.

stage. After reaching this peak value micro hardness shows adecreasing trend in the HAZ. In all the joints, HAZ area adjacentto the fusion boundary was coarse grained HAZ (CGHAZ) whichpossessed low hardness whereas the HAZ area adjacent to the basemetal was fine grained HAZ (FGHAZ) which possessed high hard-ness. The reason for this trend of micro hardness in the HAZ ofall the joints is that the area adjacent to the weld/fusion zone expe-riences relatively slow cooling rate and hence has coarse grainedmicrostructure, whereas the area adjoining the base metal under-goes high cooling rate due to steeper thermal gradients and conse-quently has fine grained microstructure. This is evident from thetrend depicted by the micro hardness profile within the HAZ ofeach of these joints.

In general it is observed from these micro hardness studies thathardness follows an increasing trend in the order of weld metal,HAZ, unaffected base metal and fusion boundary for all the jointsmade at different heat inputs. It is also observed that there is sig-nificant grain coarsening in the HAZs of all the joints. Further itis observed from the optical micrographs shown from Fig. 5b–7bthat the extent of grain coarsening in the HAZ increases with in-crease in heat input.

3.3. Tensile properties

The transverse tensile strength of all the joints made using dif-ferent heat input conditions has been evaluated. In each condition

3622 S. Kumar, A.S. Shahi / Materials and Design 32 (2011) 3617–3623

three specimens were tested and the average tensile strength ofthree specimens per heat input and their corresponding percent-age elongations thus obtained is mentioned in Table 3. The tensileresults so obtained show that maximum tensile strength of657.32 MPa is possessed by the specimens made using low heat in-put combination followed by 639.45 MPa using medium heat inputand 622.8 MPa using high heat input combination. Table 3 showsthe microstructural details of the weld metal in terms of dendritesize and cell spacing, which indicates that high tensile strengthand ductility is possessed by the joints at low heat input, whichcan be attributed to smaller dendrite sizes and lesser inter-den-dritic spacing in the fusion zone. Relatively lower tensile strengthand ductility is possessed by the joints with long dendrite sizesand large inter-dendritic spacing in the fusion zone of the jointwelded using high heat input. Further it is found that all the tensile

Fig. 10. Photograph of the tensile tested specimens showing the location of fra

Fig. 11. SEM fractograph of the tensile specimen wel

Fig. 12. SEM fractograph of the tensile specimen welde

specimens fractured in the base metal as shown in Fig. 10which indicates that weld metal in all the joints possessed highertensile strength than the base metal and thus joint efficiencies [de-fined as (UTSweldjoint)/(UTSbasemetal) � 100] of 107.61%, 104.69% and101.96% were achieved for low, medium and high heat input com-bination respectively.

The fractured surfaces of the tensile specimens were analyzedusing SEM. Figs. 11–13 show the SEM fractographs of all the jointstensile tested. Dimples of varying size and shape were observed inall the fractured surfaces which indicate that major fracturingmechanism was ductile. From Fig. 11 it is observed that fracturedsurface of the specimen at low heat input contains a large popula-tion of small and shallow dimples which is indicative of its rela-tively high tensile strength and ductility. From Figs. 12 and 13 itis observed that as heat input increases coarse and elongated dim-

cture in the base metal (a) low heat (b) medium heat (c) high heat input.

ded at low heat input (a) at 1000� (b) at 2000�.

d at medium heat input (a) at 1000� (b) at 2000�.

Fig. 13. SEM fractograph of the tensile specimen welded at high heat input (a) at 1000� (b) at 2000�.

S. Kumar, A.S. Shahi / Materials and Design 32 (2011) 3617–3623 3623

ples are observed. It is also observed that small dimples are sur-rounded by the large ones in all the specimens and a small quantityof tearing ridge is also present. A similar fractograph observationhas been reported for 3 mm thick TIG welded 304 SS where rela-tively minor size dimples surround coarse dimples besides thepresence of small quantity of tearing ridge [4].

4. Conclusions

The following conclusions can be drawn from the presentwork:-

� Good joint strength is exhibited by all the joints which showthat for welding 6 mm thick AISI 304 SS the operating envelopeof GTAW process offers a wide range of parameters to thefabricator.� As the dendrite size in the fusion zone is smaller in low heat

input joints than the dendrites in medium and high heat inputjoints, it is found that maximum tensile strength and ductility ispossessed by the weld joints made using low heat input.� As heat input increases, the fusion zone and HAZ area also

increase. Significant grain coarsening is found in the HAZs ofall the joints. It is also observed that the extent of grain coars-ening increases with increasing heat input.� Near to the fusion boundary the size of the grains in the HAZ of

the joints is found to be relatively coarser at high heat input andfiner at low heat input.

Based upon the present study it is recommended that low heatinput should be preferred when welding AISI 304SS using GTAWprocess because of the reason that besides giving good tensilestrength and ductility, the size of the HAZ and the extent of graincoarsening obtained in these weld joints is less.

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