Scholarly article on topic 'Effect of welding processes on mechanical and microstructural characteristics of high strength low alloy naval grade steel joints'

Effect of welding processes on mechanical and microstructural characteristics of high strength low alloy naval grade steel joints Academic research paper on "Materials engineering"

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{"High strength low alloy steel" / "Friction stir welding" / "Shielded metal arc welding" / "Gas metal arc welding" / "Tensile properties" / "Impact toughness"}

Abstract of research paper on Materials engineering, author of scientific article — S. Ragu Nathan, V. Balasubramanian, S. Malarvizhi, A.G. Rao

Abstract Naval grade high strength low alloy (HSLA) steels can be easily welded by all types of fusion welding processes. However, fusion welding of these steels leads to the problems such as cold cracking, residual stress, distortion and fatigue damage. These problems can be eliminated by solid state welding process such as friction stir welding (FSW). In this investigation, a comparative evaluation of mechanical (tensile, impact, hardness) properties and microstructural features of shielded metal arc (SMA), gas metal arc (GMA) and friction stir welded (FSW) naval grade HSLA steel joints was carried out. It was found that the use of FSW process eliminated the problems related to fusion welding processes and also resulted in the superior mechanical properties compared to GMA and SMA welded joints.

Academic research paper on topic "Effect of welding processes on mechanical and microstructural characteristics of high strength low alloy naval grade steel joints"

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Effect of welding processes on mechanical and microstructural characteristics of high strength low alloy naval grade steel joints

S.Ragu Nathan, Research Scholar, V. Balasubramanian, Professor, S. Malarvizhi, Associate Professor, A.G. Rao, Scientist-"C"

PII: S2214-9147(15)00043-4

DOI: 10.1016/j.dt.2015.06.001

Reference: DT 139

To appear in: Defence Technology

Received Date: 19 May 2015 Revised Date: 8 June 2015 Accepted Date: 9 June 2015

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Defence Technology

Please cite this article as: Nathan SR, Balasubramanian V, Malarvizhi S, Rao AG, Effect of welding processes on mechanical and microstructural characteristics of high strength low alloy naval grade steel joints, Defence Technology (2015), doi: 10.1016/j.dt.2015.06.001.

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Effect of welding processes on mechanical and microstructural characteristics of high strength low alloy naval grade steel joints

*S. Ragu Nathan, (Corresponding Author) Research Scholar,

Department of Manufacturing Engineering, Annamalai University, Annamalainagar - 608 002. Tel: 04144-239734 Fax: 04144-238275

Mobile: +91 97150 64464

E-mail: ragucemajor@gmail.com V. Balasubramanian,

Professor, Centre for Materials Joining & Research (CEMAJOR),

Department of Manufacturing Engineering,

Annamalai University,

Annamalai Nagar - 608 002.

Tamil Nadu.

Mobile: +91 9443412249 E-mail:visvabalu@yahoo.com

3S. Malarvizhi,

Associate Professor, Centre for Materials Joining & Research (CEMAJOR),

Department of Manufacturing Engineering,

Annamalai University,

Annamalai Nagar - 608 002.

Tamil Nadu.

Mobile: +91 9487691742 E-mail: j eej oo@gmail. com

4A. G. Rao, Scientist-"C"

Naval Materials Research Laboratory (NMRL), Ambernath, Mumbai - 421 506. Maharastra.

Mobile: +91 94215 40891 E-mail: gouravdrdo@gmail.com

Abstract

Naval grade high strength low alloy (HSLA) steels can be easily welded by all types of fusion welding processes. However, fusion welding of these steels leads to the problems such as cold cracking, residual stress, distortion and fatigue damage. These problems can be eliminated by solid state welding process such as friction stir welding (FSW). In this investigation, a comparative evaluation of mechanical (tensile, impact, hardness) properties and microstructural features of shielded metal arc (SMA), gas metal arc (GMA) and friction stir welded (FSW) naval grade HSLA steel joints was carried out. It was found that the use of FSW process eliminated the problems related to fusion welding processes and also resulted in the superior mechanical properties compared to GMA and SMA welded joints.

Keywords: High strength low alloy steel; Friction stir welding; Shielded metal arc welding; Gas metal arc welding; Tensile properties; Impact toughness.

1. Introduction

High strength low alloy (HSLA) steels were primarily developed to replace low-carbon steels for the automotive industry in order to improve the strength-to-weight ratio and meet the need for higher-strength construction grade materials. When high strength steel is welded, non-uniform heating and cooling in weld metal and base metal generate harder heat affected zone (HAZ), cold crack susceptibility and residual stress in weldments. HSLA steels demonstrate unique properties, such as high strength, excellent ductility, and good weldability, and also exhibit outstanding low temperature impact toughness superior to that of high yield strength (HY) steels. HSLA steels have much improved weldability compared to HY steels [1]. Now-a-days, the micro-alloyed or HSLA steels become an indispensable class for different applications like construction of large ships, oil and gas transmission lines, offshore oil drilling platforms, pressure vessels, building construction, bridges, storage tanks.

DMR-249A is a low carbon micro-alloyed high strength low alloy (HSLA) steel, which is far superior grade compared to the numerous grades which have been in use for naval applications like construction of warships. Obviously, DMR249A demands weld metal with superior properties compatible with its own, i.e., a combination of high strength and high toughness. This is due to its composition that consists of 0.001 - 0.1wt% of alloying elements such as V or Ti [2]. An acicular ferrite in weld metals and wrought steels has predominant one owing to its combination of high strength and high toughness [2,4 , 5], thus this steel has designed to have a ferrite microstructure with small amount of pearlite less than 10% by volume [2,3].

In these grade steels, the heat affected zone (HAZ) is prone to failure due to the possibility of hydrogen induced cracking and only way to weld such steels is to use low hydrogen ferritic steel filler wire [6]. Charpy impact fracture of HSLA steel was improved by intercritical heat treatment which enhances the microstructure through the formation of ferrite microstructure with various morphologies, irregular martensite and 75% of microstructure with high angle grain boundaries [7]. The resistance to hydrogen-induced cracking and stress corrosion cracking was improved by coarse grain heat affected zone which consists of martensite-austenite constituents, thus showing the importance of reduction in carbon content of these steels [8].

Friction stir welding (FSW) is a novel solid state joining technique that is presently attracting significant attention on welding of hard metals such as steel and titanium [9-11]. FSW has appeared as an easy, ecological and promising productive welding method that reduces material waste and avoids radiation and harmful gas emissions, usually associated with the fusion welding processes. Mechanical action in the form of frictional stirring on the base material has modified the microstructure from the coarse grains to very fine grains due to plastic deformation and fast cooling rate [12-14]. Welding of steels is affected by both the temperature and composition which extensively affects the microstructure evolution. Friction stir welding enables us to control these factors and produce superior joint strength [1]. Much of the tool degradation may be attributed to the high heat (temperature around 1200 oC) and the stresses generated during friction stir welding of the high strength materials. However, the development of the wear resistant tool materials has benefited the FSW process and paved way for the rapid implementation of this process in the fabrication of high strength steel structures [15, 16]. The present investigation is to study the feasibility of friction stir welding of naval grade HSLA steel and compare the mechanical properties and metallurgical characteristics of FSW joints with the fusion welded (SMA and GMA welded) joints.

2. Experimental details

The rolled plates of naval grade HSLA steel with thickness of 5 mm were cut to the required dimensions (100 mm x 150 mm) by abrasive cutting to prepare the joint configurations, as shown in Fig.1( a) and (b). The chemical composition of parent metal is presented in Table 1. The microstructure of parent metal (Fig.2 (a) and (b)) is composed of ferrite with small amount of pearlite. A non-consumable rotating tool made of tungsten base alloy was used to fabricate FSW joints. The tool was manufactured through powder metallurgy route having a shoulder diameter of 25 mm and a tapered pin, tapering from 12 mm at the shoulder to 8 mm at the pin tip. The electrode for SMAW and filler wire for GMAW processes were supplied by Honavar Electrodes Pvt. Ltd. The basic classification of Mn and Ni base E 8018-C1 with nominal composition of 0.06% C, 1.8% Mn and 2.5% Ni electrode was used to weld the naval grade steels. The welding conditions and parameters used to fabricate the defect free joints are presented in Tables 2 and 3.

Table 1 Chemical composition (wt. %) of parent metal

C Mn N Si Mo Ti V Nb Ni Cu Al W

0.08 1.42 .015 0.19 0.02 0.016 0.032 0.035 0.69 0.126 .032 0.04

Table 2 FSW conditions and welding parameters

Process parameters Values

Rotational speed/rpm 600

Welding speed/(mm-min-1) 30

Axial force/kN 15

D/T ratio of tool 5

Tool shoulder diameter/mm 25

Pin length/mm 4

Tool inclined angle/(o) 0

Heat input/ (kJ-mm-1) 1.056

Table 3 SMA and GMA welding conditions and parameters

Parameter SMAW GMAW

Filler metal (electrode) E8018-C1 as per SFA 5.5

Filler diameter/mm 4 1.2

Current/A 152 255

Voltage/V 25 30

Welding speed/(mm-min-1) 153 300

Electrode Baking temperature/°C 300 -

CO2 gas flow rate/(L-min-1) - 12

Heat input /(kJ-mm-1) 1.489 1.530

(a) Joint configuration for FSW

(b) Joint configuration for SMA and GMAW Fig. 1. Experimental details (unit:mm)

(a) Optical micrograph

(b) SEM micrograph

Fig. 2. Micrographs of parent metal ASTM E8M-04 guidelines were followed for preparing the tensile test specimens. 100 kN electromechanical controlled universal testing machine (Make: FIE Bluestar, India; Model: UNITEK-94100) was used to evaluate the tensile properties. In each condition, three specimens were tested and the average value was presented. Charpy impact specimens were prepared to evaluate the impact toughness of the weld metal and hence the notch was placed (machined) in the weld metal (weld center). Since the plate thickness was small, the sub-size specimens were prepared. Impact testing was conducted on a pendulum-type impact testing machine (Enkay, India) at room temperature. The amount of energy absorbed in fracture was recorded. The absorbed energy is defined as the impact toughness of the material.

Vicker's microhardness tester (Make: Shimadzu, Japan and Model: HMV-2T) was used for measuring the hardness distribution across the welded joint along with mid thickness region with a load of 0.5 N. The specimen for metallographic examination was sectioned to the required size from the joint comprising weld metal, HAZ (heat-affected zone), and base metal regions, and polished using different grades of emery papers. Final polishing was done using the diamond compound (particle size of 1 ^m ) on the disc polishing machine. The specimens were etched with 2% of Nital solution to reveal the microstructural features of joints. Microstructural examination was carried out using an optical microscope (Make: MEJI, Japan; Model: MIL-7100) incorporated with an image analyzing software (Metal Vision). Field-emission scanning electron microscopy (FESEM, make: ZEISS) was also used to characterize the weld metal microstructure.

3. Results

3.1 Tensile and impact toughness properties

The tensile properties of the parent metal and welded joints are presented in Table 4. The yield strength and tensile strength of parent metal was 438 MPa and 610 MPa, respectively. But the yield strength and tensile strength of FSW joint are 502 MPa and 664 MPa, respectively. Similarly, the yield strength and tensile strength of SMAW joint are 473 MPa and 578 MPa, respectively, which are 5.2% lower than those of parent metal. However, the yield strength and tensile strength of GMAW joint are 485 MPa and 580 MPa, respectively, which are 5.1 % lower than those of parent metal. Of the three welded j oints, the j oints fabricated by FSW process exhibited higher strength values. Percentage of elongation of parent metal is 29%, whereas the percentage of elongation of GMAW joint is 22%. This suggests that there is a 24% decrement in ductility due to GMAW process. Similarly, the percentages of elongation of FSW and SMAW joints are 19%, which are 35% lower that those of the parent metal. Of the three types of welded joints, the joints fabricated by GMAW exhibited higher ductility values compared to FSW and SMAW joints. The stress (load) -displacement curves of parent metal and welded joints are shown in Fig.3. Table 4 Mechanical properties of the parent metal and welded joints

0.2% offset

yield strength/MPa

Elongation Tensile in 25 mm strength/MPa gauge ^■length/%

Impact toughness @ RT/J

Hardness of weld metal Fracture @ 0.5 N location load/HV

PM 438 610 29 78 270 Center

FSW 502 664 19 48 410 ASHAZ

SMAW 473 578 19 62 315 PM

GMAW 485 580 22 69 304 PM

Charpy impact toughness test results are presented in Table 4. The impact toughness of parent metal is 78 J at room temperature. When the weld metal is welded by FSW process, it exhibits 48 J, which is 38% lower than that of the parent metal. The lowest impact strength is showed by the joint fabricated by FSW process compared to SMAW (62 J) and GMAW (69 J) joints. The fusion welded joints (SMAW and GMAW) exhibits higher yield strength, lower tensile strength, and decreased elongation and reduced toughness t. FSW joint shows higher yield and tensile strength, lower elongation and toughness.

g 400 I 300 200

£ 600 1 500

FSW •••• SMAW---GMAW

2 4 6 8 10 12 Displacement/mm

Fig. 3. Load vs displacement curves of PM and welded joints

3.2 Macrostructure

The cross-sectional macrographs of the weld joints are displayed in Fig.4. All the three joints are free from macro-level defects. Thecross-section of FSW joint appears like 'basin-shape' (Fig.4(a)). It comprises of advancing side heat affected zone (ASHAZ), advancing side thermo-mechanically affected zone (ASTMAZ), stir zone (SZ), retreating side thermo-mechanically affected zone (RSTMAZ), retreating side heat affected zone (RSHAZ), weld metal (SZ), heat affected zone (HAZ) in the FSW macrostructures. The macrograph of SMAW and GMAW joints reveal weld center line (WCL), coarse grain heat affected zone (CGHAZ) and fine grain heat affected zone (FGHAZ). In SMAW process, the resultant bead structure was a coarser one due to high heat input leading to the slower cooling rate. The wider arc column is also a reason for this wide FZ (Fig.4(b)). In GMAW process, the energy density is comparatively higher than that in SMAW process. The high self-quenching rates that are associated with this process certainly promote the fast cooling rates. This could be attributed to this narrow FZ (Fig.4(c)).

(a) FSW

(b) SMAW

(c) GMAW

Fig. 4. Macrographs of weld cross-section3.3 Microhardness

Fig.5 shows the hardness variations across the weld. The hardness of the as-received parent metal is approximately 270 HV. The hardness of stir zone varies from 300 HV to 410 HV, depending on the grain size and phases sampled from each indentation. The microhardness of weld metal regions of SMAW and GMAW joints varies from 300 HV to 320 HV, depending on the grain size and phases sampled from each indentation. Hardness is found to be very high in HAZ of SMAW and GMAW joints. The failure location of the welded joints is consistent with hardness distribution profile. The failure occurred in all the joints along the lowest hardness distribution region (LHDR).

(a) FSW joint

Distance from the weld center line

(c) GMAW joint

Fig. 5. Microhardness survey across the cross-section of the welded joints (LHDR: Lowest Hardness Distribution Region)

3.4 Microstructure

Fig.6 shows the microstructures of the various regions of FSW, SMAW and GMAW joints. Since the intensities of heat and mechanical actions are more in TMAZ and SZ, both advancing side (AS) and retreating side (RS) of the FSW joint, the microstructural fruition in these zones is considered to be discussed initially. The SEM micrograph of weld metal region is shown in Fig.7. The high magnification SEM image clearly shows that FSW joint consists of upper bainite and acicular ferrite throughout the stir zone whereas the SMA and GMA weld metal consists of acicular ferrite and martensite. There is an appreciable morphology difference in the formation of acicular ferrite in both SMA and GMA weld metals.

(a) FSW:RSTMAZ

(b) SMAW:CGHAZ

(c) GMAW: C GHAZ

(e) SMAW:WM

(f) GMAW:WM

(g) FSW: ASTMAZ

(h) SMAW: FGHAZ

(i) GMAW: FGHAZ

Fig. 6. Optical micrographs of various regions of welded steel joints

(a) FSW joint

(b) SMAW joint

(c) GMAW joint

Fig. 7. SEM micrographs of weld metal region

3.5 Fracture surface analysis

The fractured surfaces of the tensile and impact tested specimens of parent metal and welded joints were analyzed using a scanning electron microscope. The fractographs of tensile and impact specimens are displayed in Figs. 8 and 9. The modes of failure of the tensile tested parent metal and welded joints are ductile with acceptable plastic deformation and are evident from the fracture location and fractured surface shown in the Figs.8 (a)-(l). No brittle cleavage fracture was found in any of the tensile tested fractographs presented at high magnification in the Figs.8 (c), (f),( i) and (l). However, an appreciable difference in fracture pattern was found. Fine and secondary dimples are the key features of superior tensile strength of FSW joint compared to other fusion weld joints and parent metal .A

substantial dissimilarity in size and alignment of the dimples in the fracture surface of FSW joint (Fig.8(f)) is observed with respect to parent metal and other joints (Figs.8(c),( i) and (l)). Variation in size of the dimples and presence of voids (Fig.8(i)) show that the ductile fracture occurred with considerable reduction in tensile strength of SMAW joint. Also the numerous micro-pores were observed, which may be the reason for attaining reduced tensile strength with respect to parent metal and other (FSW & GMAW) joints. In Fig.8(l), the entire surface shows a coarse and elongated dimple fracture which may be ruptured in ductile shear. The elongated cavities and coarse dimples suggesting localized slip which in turn results in more ductility without reducing the tensile strength of GMAW joint.

Joint Fractured specimen photograph

Scanned image of fracture surface

SEM fractograph

'•»y * A.? -y»'-1

■-w-^T.^Í;,'.....' - ■ ■ -

(j) (k) Fig. 8. SEM fractographs of tensile specimens

The scanned images and fractographs of fracture surface of charpy v-notch impact tested specimens are shown in Figs.9 (a)-(l). At higher magnification, few secondary dimples can be observed at the lip of the dimples. This could be the reason for high impact strength of the parent metal (Fig.9(c)). Scanned image of FSW joint depicts the type of brittle fracture. However, fracture morphology consists of cleavage facets which are characterized by the depth size of the dimple in the fracture surface can be clearly seen in Fig.9(f). The size variation of dimples and the presence of micro-pores and secondary cracks (Fig.9(i)) might be the reason for the drop of impact energy absorption of SMAW joint compared with the parent metal and GMAW joint. At higher magnification the presence of finer dimples and the secondary dimples are evidenced (Fig.9(l)). This might be the reason for the higher impact energy absorption for the GMAW joint. The dimple size exhibits a directly proportional relationship with the strength and ductility, i.e., if the dimple size is finer, the strength and ductility of the respective joint are higher and vice versa [6, 17]. Joint Fractured specimen Scanned image of

Photograph

fracture surface

SEM fractograph

(j) (k) (l) Fig. 9. SEM fractographs of impact specimens

4.0 Discussion

4.1 Effects of welding processes on microstructure

The weld metal microstructure of fusion welded joints is greatly influenced by the chemical composition of filler metal and the heat input of the process. In general, higher heat input leads to slower cooling rate which results in he coarse grains in weld metal [18]. However, lower heat input leads to fast cooling rate which results in fine microstructure.

Though the lower heat input can produce finer grains compared to higher heat input, the intrinsic nature of the process also plays major role in refining the weld metal microstructure. Of the three processes used in this investigation, the FSW process supplies lower heat input (1.05 kJ/mm) to the weld region compared to SMAW and GMAW processes which supply 1.493 kJ/mm and 1.530 kJ/mm, respectively. While the fast cooling rate (i.e., lower heat input) and mechanical action for refining microstructure process are coupled, the formation of upper bainite with small amount of acicular ferrite is possible. This could be one of the reasons that FSW joint consists of upper bainite grains in the stir zone (Fig.6(b) and Fig.7(a)). Due to low heat generation in SZ, TMAZ and HAZ (Fig.6 (a) and( g) ) the characteristics are also better than those of other fusion welded joints (Figs.6 (b), (h), (c) and (i)). This may be due to unique weld metal characteristics and similar thermal properties of SZ, TMAZ, HAZ and PM [19].

Owing to higher heat input, phase transformation occurred in the SMAW and GMAW weld metals. Parent metal microstructure (ferrite and small amount of pearlite) was transformed into acicular ferrite, small amount of retained austenite and martensite by heat input and chemical composition. Variations in filler metal and parent metal chemical composition lead to the thermal variations in weld metal and parent metal as well as the solidification of weld metal. Slow cooling rate may reduce the interfacial energy between the austenite and ferrite, which results in formation of acicular ferrite. An acicular ferrite microstructure has the potential of combining high strength and high toughness [2]. Microstructural stability is more in acicular ferrite compared to bainite in higher temperatures [20]. Due to high heat input and intensity of GMAW process, the coarse grains in weld metal and HAZ (Fig.7(c)) were formed. The relatively slow cooling rate of GMAW process led to the formation of martensite and acicular ferrite with some retained austenite in weld metal region. Moreover, filler metal addition also play vital role in weld metal microstructure. It is not possible to produce homogeneous weld metal by fusion welding processes.

4.2 Effects of welding processes on tensile properties

The weld metal is comparatively stronger , and the joint properties are controlled by weld metal chemical composition and microstructure. The strong carbide/nitride forming elements, like Nb, Ti, V, etc., have very limited solubility in ferrite and austenite, and normally the

precipitates act as fine dispersion of carbides, nitrides and/or carbonitrides and contribute to strength due to precipitation hardening [21]. This could be the reason that the yield strengths of all the joints increase compared to the yield strength of the parent metal. The yield point elongation is attributed to the interaction of solute atoms and moving dislocations [22].

Due to filler metal addition and high heat input, the quite increase in molybdenum and nickel may result in formation of ferrite with martensite and retained austenite. This indicates that there is a 5 % reduction in the strength values of fusion weld joints compared to parent metal due to an increase in prior-austenite grain size in reheated and isothermally held specimens [17]. Variation of ductility with microstructure and chemical composition is more complex. In general, all factors, except for grain size, which increase the strength would decrease the ductility. Also the ductility is severely affected by the presence of MnS inclusion in steels and varies with size, shape and volume fraction. The inclusions act as stress raisers, and the cracks easily initiate at the inclusion, either due to cracking of inclusions or decohession of inclusion and matrix [23]. Thus in all the cases, the nucleation of acicular ferrite is that a Mn depletion zone is formed around inclusions due to MnS precipitations on Ti2O3 which may lead to the reduction of the ductility of joint compared to parent metal.

In the case of FSW, the material is severely plastically deformed due to the stirring action of the pin tool, resulting in a fine grain structure. Dynamic recrystallization is a phenomena occurring during the material movement in FSW. Fig.3 illustrates the effects of different welding processes on strength and ductility. The load-displacement graph clearly indicates that the strength of FSW specimen is 8% more than that of base metal [24]. The increased strength is attributed to the fine grain microstructure consisting of acicular , upper bainite and small cluster of martensite regions. Although the formation of martensite increases the strength, it decreases the ductility and toughness. The fractograph in Fig.9 (e) shows the brittle cleavage fracture features, thereby confirming the presence of martensite in the FSW stir zone microstructure. Also, the elongated spherical inclusions of MnS may also be an important factor in reducing the ductility. High angle grain boundaries (>10°) result in enhancement of strength [2]. This is because the plates of acicular ferrite nucleate intragranularly on non-metallic inclusions within large austenite grains, and then radiate in many different orientations from those inclusions whilst maintaining an orientation relationship with the austenite [2, 3, 4]. However, the misorientation angle between the

grains could be the key feature in increasing the yield strength and tensile strength compared to parent metal.

4.3 Effects of welding processes on impact toughness properties

Charpy V-notch impact toughness of parent metal is 78 J at room temperature, whereas the impact toughness of FSW joint exhibits 48 J which is 38% lower than that of parent metal. The higher impact strength was achieved by joint fabricated by GMAW process compared to FSW and SMAW processes. The increased percentage of bainite leads to an enhancement of tensile properties, and the presence of small cluster of martensite is detrimental to impact toughness [25]. The presence of precipitate carbides raises the impact transition temperature and lowers the charpy shelf energy. The impact strength of upper bainite adversely affected by the presence of cementite as thin film at the lath boundaries of bainite increases the presence of martensite which does not contribute to the strength but lowers the toughness [26]. This could be the reason for reduction in toughness of FSW joint with respect to parent metal as well as fusion weld joints.

In the case of fusion welds, GMAW joint exhibited higher impact toughness than SMAW jointdue to slower cooling rate. The formation of acicular ferrite with high angle grain boundaries and the parallel stacking of ferrite and carbides (Fig.7c) arranged in the packets could make the propagation path of critical crack pass through an acicular ferrite microstructure, thereby leading to an improvement in toughness without compromising strength [2, 3, 10]. However, the reduced toughness of the fusion zone in SMAW joint compared to GMAW joint with respect to parent metal is attributed to the presence of retained austenite in the martensite microstructure which leads to void the formation in the austenite during deformation [27].

4.4 Effects of welding processes on hardness

The post weld microstructure of SZ consists of shear transformed bainitic ferrite with carbides and acicular ferrite, which could be the reason for higher hardness values. The microhardness values are less significant in affecting the mechanical properties because the inherent nature of the tool rotational speed has more influencing factors over the hardness

values [28]. Also the SZ microstructure formed at peak temperature with highly strain-induced area and localized fast cooling rate to room temperature occurred during FSW and subsequent transformation to a completely upper bainite and lath bainitic microstructure are the major contributors of the hardest stir zone in the FSW joint weld zone.

From Figs.5 (b) and (c) , it could be understood that the lower hardness of GMAW and SMAW joints were recorded, and the raising of hardness value towards the CGHAZ and FGHAZ is due to the presence of coarse distorted microstructure invariably in weld metal compared to fine and coarse grain heat affected zone. The hardness profiles (Fig.5(b) and ( c)) of the welded joints are in agreement with the resultant strength properties at the highest and intermediate strength levels, respectively. The higher hardness was found in the area where the prior-austenite grain size was relatively smaller than that closer to the fusion line [8] and also the higher hardness conforming the micro-segregation of the major alloying elements in these regions [21].

5. Conclusions

In this investigation, an attempt was made to study the effects of welding processes by evaluating the weld metal microstructure and mechanical properties of naval grade high strength low alloy (HSLA) steel joints. From this investigation, the following important conclusions are derived:

1) Of the three welded joints, the joint fabricated by FSW process exhibited higher strength values, and the enhancement in strength value is approximately 13% due to grain refinement in SZ, unique weld metal composition and strain-induced deformation during FSW.

2)Of the three joints, the joint fabricated using GMAW exhibited 28% and 10% higher impact toughness, respectively, compared to the joints fabricated by FSW and SMAW processes. The presence of martensite-austenite (M-A) constituents and the ferrite laths in bainitic matrix in the weld zone microstructure are the key reasons for enhancement of toughness properties.

3)Hardness of FSW joint(410 HV) is higher in the stir zone compared to the HAZ and BM regions . The lower hardnesses of the SMAW joints (315 HV) and GMAW joints (304 HV) were recorded. This is due to severe plastic deformation and continuous

dynamic recrystallization occurred in the stir zone and localized heating and fast cooling rate of the FSW process contributed to higher hardness in the stir zone.

Acknowledgements

The authors are grateful to The Director, Naval Material Research Laboratory (NMRL), Ambernath for financial support through CARS project No: G8/15250/2011 dated 29.02.2012 and providing base material for this investigation. Also the authors are grateful to Dr. A.K.Lakshminarayanan, Associate Professor, SSN College of Engineering, Chennai, India for his valuable suggestions, guidance and discussion to carry out this investigation.

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