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Effect of welding processes on mechanical and microstructural characteristics of high strength low alloy naval grade steel joints
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ScienceDirect Defence Technology 11 (2015) 308e317 www.elsevier.com/locate/dt
Effect of welding processes on mechanical and microstructural characteristics of high strength low alloy naval grade steel joints S. RAGU NATHAN a,*, V. BALASUBRAMANIAN b,1, S. MALARVIZHI b,2, A.G. RAO c,3 b
a Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar, 608 002, India Centre for Materials Joining & Research (CEMAJOR), Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar, 608 002, Tamil Nadu, India c Naval Materials Research Laboratory (NMRL), Ambernath, Mumbai, 421 506, Maharastra, India
Received 19 May 2015; revised 8 June 2015; accepted 9 June 2015 Available online 30 June 2015
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. Copyright © 2015, China Ordnance Society. Production and hosting by Elsevier B.V. All rights reserved.
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 * Corresponding author. Tel.: þ91 04144 239734, þ91 97150 64464 (mobile); fax: þ91 04144 238275. E-mail addresses: [email protected] (S. RAGU NATHAN), [email protected] (V. BALASUBRAMANIAN), [email protected] (S. MALARVIZHI), [email protected] (A.G. RAO). Peer review under responsibility of China Ordnance Society. 1 Tel.: þ91 9443412249 (mobile). 2 Tel.: þ91 9487691742 (mobile). 3 Tel.: þ91 94215 40891 (mobile).
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 0.001e0.1wt% 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
http://dx.doi.org/10.1016/j.dt.2015.06.001 2214-9147/Copyright © 2015, China Ordnance Society. Production and hosting by Elsevier B.V. All rights reserved.
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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 [9e11]. 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 [12e14]. 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 C) 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 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.
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Fig. 1. Experimental details (unit:mm).
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. 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 mm) 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
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Table 1 Chemical composition (wt.%) of parent metal. C
Mn
N
Si
Mo
Ti
V
Nb
Ni
Cu
Al
W
0.08
1.42
0.015
0.19
0.02
0.016
0.032
0.035
0.69
0.126
0.032
0.04
Table 3 SMA and GMA welding conditions and parameters.
Fig. 2. Micrographs of parent metal.
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 Table 2 FSW conditions and welding parameters. Process parameters
Values
Rotational speed/rpm Welding speed/(mm$min1) Axial force/kN D/T ratio of tool Tool shoulder diameter/mm Pin length/mm Tool inclined angle/( ) Heat input/(kJ$mm1)
600 30 15 5 25 4 0 1.056
Parameter
SMAW
GMAW
Filler metal (electrode) Filler diameter/mm Current/A Voltage/V Welding speed/(mm$min1) Electrode Baking temperature/ C CO2 gas flow rate/(L$min1) Heat input/(kJ$mm1)
E 8018-C1 as per SFA 5.5 4 1.2 152 255 25 30 153 300 300 e e 12 1.489 1.530
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 joints, the joints 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. 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. 3.2. Macrostructure The cross-sectional macrographs of the weld joints are displayed in Fig. 4. All the three joints are free from macrolevel defects. The cross-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
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Table 4 Mechanical properties of the parent metal and welded joints. Joint
0.2% Offset yield strength/MPa
Tensile strength/MPa
Elongation in 25 mm gauge length/%
Impact toughness @ RT/J
Hardness of weld metal @ 0.5 N load/HV
Fracture location
PM FSW SMAW GMAW
438 502 473 485
610 664 578 580
29 19 19 22
78 48 62 69
270 410 315 304
Center ASHAZ PM PM
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)). 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
Fig. 3. Load vs displacement curves of PM and welded joints.
Fig. 4. Macrographs of weld cross-section 3.3 Microhardness.
Fig. 5. Microhardness survey across the cross-section of the welded joints. (LHDR: Lowest hardness distribution region).
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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). 3.3. 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.
3.4. 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 Fig. 8(a)e(l). No brittle cleavage fracture was found in any of the tensile tested fractographs presented at high magnification in the Fig. 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 (Fig. 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
Fig. 6. Optical micrographs of various regions of welded steel joints.
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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]. 4. Discussion 4.1. Effects of welding processes on microstructure
Fig. 7. SEM micrographs of weld metal region.
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. The scanned images and fractographs of fracture surface of charpy v-notch impact tested specimens are shown in Fig. 9(a)e(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
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 (Fig. 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
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Fig. 8. SEM fractographs of tensile specimens.
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
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Fig. 9. SEM fractographs of impact specimens.
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 loadedisplacement 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
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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 [2e4]. 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 joint due to slower cooling rate. The formation of acicular ferrite with high angle grain boundaries and the parallel stacking of ferrite and carbides (Fig. 7(c)) 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].
factors over the hardness values [28]. Also the SZ microstructure formed at peak temperature with highly straininduced 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 Fig. 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.
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
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.
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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. References
[1] Fujii Hidetoshi, Cui Ling, Tsuji Nobuhiro, Maeda Masakatsu, Nakata Kazuhiro, Nogi Kiyoshi. Friction stir welding of carbon steels. Mater Sci Eng A 2006;429:50e7. [2] Lee CH, Bhadeshia HKDH, Lee HC. Effect of plastic deformation on the formation of accicular ferrite. Mater Sci Eng A 2003;360:249e57. [3] Show BK, Veerababu R, Balamuralikrishnan R, Malakondaiah G. Effect of vanadium and titanium modification on the microstructure and mechanical properties of microalloyed HSLA steel. Mater Sci Eng A 2010;527:1595e604. [4] Yayla P, Kaluc E, Ural K. Effect of welding processes on the mechanical properties of HY 80 steel weldments. Mater Des 2007;28:1898e906. [5] Sun Weihua, Wang Guodong, Zhang Jiming, Xia Dianxiu, Sun Hao. Microstructural characterization of high heat input welding joint of HSLA steel plate for oil storage construction. J Mater Sci Technol 2009;25(6):857e60. [6] Magudeeswaran G, Balasubramanian V, Madhusudhan Reddy G, Balasubramaian TS. Effect of welding processes and consumables on tensile and impact properties of high strength quenched and tempered steel joints. J Iron Steel Res Inter 2008;15(6):87e94. [7] Kang J, Wang C, Wang GD. Microstructural characteristics and impact fracture behavior of a high-strength low-alloy steel treated by intercritical heat treatment. Mater Sci Eng A 2012;527:96e104. [8] Lambert A, Drillet J, Gourgues AF, Stuel T, Pineau A. Microstructure of martensite e austenite constituents in heat affected zones of high strength low alloy steel welds in relation to toughness properties. Sci Technol Weld Join 2000;5(3):168e73. [9] Lakshminarayanan AK, Balasubramanian V. An assessment of microstructure, hardness, tensile and impact strength of friction stir welded ferritic stainless steel joints. Mater Des 2010;31:4592e600. [10] Thompson B, Babu SS. Tool degradation characterization in the friction stir welding of hard metals. Weld Res 2010;89:256e61. [11] Galvao I, Leal RM, Loureiro A. Influence of tool shoulder geometry on properties of friction stir welds thin copper sheets. J Mater Process Technol 2013;213(2):129e35.
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[12] Zhu XK, Chao YJ. Numerical simulation of transient temperature and residual stresses in friction stir welding of 304L stainless steel. J Mater Process Technol 2004;146(2):263e72. [13] Park SHC, Sato YS, Kokawa H. Carbide formation induced by pcBN tool wear in friction-stir welded stainless. Metal Mate Trans A 2009;40A:625e36. [14] Gan W, Li ZT, Khurana S. Tool materials selection for friction stir welding of L80 steel. Sci Technol Weld Join 2007;12(7):610e3. [15] Lienert TJ, Stellwag JR, Grimmett BB, Warke RW. Friction stir welding studies on mild steel. Weld J 2003;89:1e9. [16] Lakshminarayanan AK, Balasubramanian V. Understanding the parameters controlling friction stir welding of AISI 409M ferritic stainless steel. Metal Mater Int 2011;17(6):969e81. [17] Konkol PK, Mruczek MF. Comparisons of friction stir weldments and submerged arc weldments in HSLA-65 steel. Weld J 2001;89:187e96. [18] Zhang Ying-qiao, Zhang Han-qian, LI Jin-fu, LIU Wei-ming. Effect of heat input on microstructure and toughness of coarse grain heat affected zone in Nb microalloyed HSLA steels. J Iron Steel Research Inter 2009;16(5):73e80. [19] Rajamanickam N, Balusamy V, Magudeeswaran G, Natarajan K. Effect of process parameters on thermal history and mechanical properties of friction stir welds. Mater Des 2009;30:2726e31. [20] Wan XL, Wei R, Wu KM. Effect of acicular ferrite formation on grain refinement in the coarse-grained region of heat affected zone. Mater Charact 2010;61:726e31. [21] Brown IH. The role of microsegregation in centerline cold cracking of high strength low alloy steel weldments. Scr Mater 2006;54:489e92. [22] Momeni A, Arabi H, Rezaei A, Badri H, Abbasi SM. Hot deformation behavior of austenite in HSLA-100 microalloyed steel. Mater Sci Eng A 2011;528:2158e63. [23] Yamamoto, Hasegawa T, Takamura. Effect of boron on intra-granular ferrite formation in Ti-oxide bearing steels. ISIJ Inter 1996;36:80e6. [24] Ghosh, Kumar, Mishra RS. Analysis of microstructural evolution during friction stir welding of ultrahigh-strength steel. Scr Mater 2010:851e85. [25] Dhua SK, Sen SK. Effect of direct quenching in the microstructure and mechanical properties of the lean chemistry HSLA-100 steel plates. Mater Sci Eng A 2011;52:6356e65. [26] Ghosh A, Das S, Chatterjee S, Ramchandra Rao P. Effect of cooling rate on structure and properties of an ultra-low carbon HSLA-100 grade steel. Mater Charact 2006;56:59e65. [27] Kenyon N. Effect of austenite on the toughness of maraging steel welds. Weld J 1968:193e8. [28] Aydin Hakan. Relationship between a bainitic structure and the hardness in the weld zone of the friction stir welded X80 API grade pipe-line steel. Mater Technol 2014;48(1):1580e2949.
Available online at www.sciencedirect.com
ScienceDirect Defence Technology 11 (2015) 308e317 www.elsevier.com/locate/dt
Effect of welding processes on mechanical and microstructural characteristics of high strength low alloy naval grade steel joints S. RAGU NATHAN a,*, V. BALASUBRAMANIAN b,1, S. MALARVIZHI b,2, A.G. RAO c,3 b
a Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar, 608 002, India Centre for Materials Joining & Research (CEMAJOR), Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar, 608 002, Tamil Nadu, India c Naval Materials Research Laboratory (NMRL), Ambernath, Mumbai, 421 506, Maharastra, India
Received 19 May 2015; revised 8 June 2015; accepted 9 June 2015 Available online 30 June 2015
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. Copyright © 2015, China Ordnance Society. Production and hosting by Elsevier B.V. All rights reserved.
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 * Corresponding author. Tel.: þ91 04144 239734, þ91 97150 64464 (mobile); fax: þ91 04144 238275. E-mail addresses: [email protected] (S. RAGU NATHAN), [email protected] (V. BALASUBRAMANIAN), [email protected] (S. MALARVIZHI), [email protected] (A.G. RAO). Peer review under responsibility of China Ordnance Society. 1 Tel.: þ91 9443412249 (mobile). 2 Tel.: þ91 9487691742 (mobile). 3 Tel.: þ91 94215 40891 (mobile).
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 0.001e0.1wt% 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
http://dx.doi.org/10.1016/j.dt.2015.06.001 2214-9147/Copyright © 2015, China Ordnance Society. Production and hosting by Elsevier B.V. All rights reserved.
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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 [9e11]. 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 [12e14]. 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 C) 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 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.
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Fig. 1. Experimental details (unit:mm).
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. 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 mm) 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
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Table 1 Chemical composition (wt.%) of parent metal. C
Mn
N
Si
Mo
Ti
V
Nb
Ni
Cu
Al
W
0.08
1.42
0.015
0.19
0.02
0.016
0.032
0.035
0.69
0.126
0.032
0.04
Table 3 SMA and GMA welding conditions and parameters.
Fig. 2. Micrographs of parent metal.
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 Table 2 FSW conditions and welding parameters. Process parameters
Values
Rotational speed/rpm Welding speed/(mm$min1) Axial force/kN D/T ratio of tool Tool shoulder diameter/mm Pin length/mm Tool inclined angle/( ) Heat input/(kJ$mm1)
600 30 15 5 25 4 0 1.056
Parameter
SMAW
GMAW
Filler metal (electrode) Filler diameter/mm Current/A Voltage/V Welding speed/(mm$min1) Electrode Baking temperature/ C CO2 gas flow rate/(L$min1) Heat input/(kJ$mm1)
E 8018-C1 as per SFA 5.5 4 1.2 152 255 25 30 153 300 300 e e 12 1.489 1.530
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 joints, the joints 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. 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. 3.2. Macrostructure The cross-sectional macrographs of the weld joints are displayed in Fig. 4. All the three joints are free from macrolevel defects. The cross-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
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Table 4 Mechanical properties of the parent metal and welded joints. Joint
0.2% Offset yield strength/MPa
Tensile strength/MPa
Elongation in 25 mm gauge length/%
Impact toughness @ RT/J
Hardness of weld metal @ 0.5 N load/HV
Fracture location
PM FSW SMAW GMAW
438 502 473 485
610 664 578 580
29 19 19 22
78 48 62 69
270 410 315 304
Center ASHAZ PM PM
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)). 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
Fig. 3. Load vs displacement curves of PM and welded joints.
Fig. 4. Macrographs of weld cross-section 3.3 Microhardness.
Fig. 5. Microhardness survey across the cross-section of the welded joints. (LHDR: Lowest hardness distribution region).
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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). 3.3. 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.
3.4. 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 Fig. 8(a)e(l). No brittle cleavage fracture was found in any of the tensile tested fractographs presented at high magnification in the Fig. 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 (Fig. 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
Fig. 6. Optical micrographs of various regions of welded steel joints.
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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]. 4. Discussion 4.1. Effects of welding processes on microstructure
Fig. 7. SEM micrographs of weld metal region.
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. The scanned images and fractographs of fracture surface of charpy v-notch impact tested specimens are shown in Fig. 9(a)e(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
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 (Fig. 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
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Fig. 8. SEM fractographs of tensile specimens.
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
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Fig. 9. SEM fractographs of impact specimens.
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 loadedisplacement 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
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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 [2e4]. 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 joint due to slower cooling rate. The formation of acicular ferrite with high angle grain boundaries and the parallel stacking of ferrite and carbides (Fig. 7(c)) 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].
factors over the hardness values [28]. Also the SZ microstructure formed at peak temperature with highly straininduced 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 Fig. 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.
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
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.
S. RAGU NATHAN et al. / Defence Technology 11 (2015) 308e317
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. References
[1] Fujii Hidetoshi, Cui Ling, Tsuji Nobuhiro, Maeda Masakatsu, Nakata Kazuhiro, Nogi Kiyoshi. Friction stir welding of carbon steels. Mater Sci Eng A 2006;429:50e7. [2] Lee CH, Bhadeshia HKDH, Lee HC. Effect of plastic deformation on the formation of accicular ferrite. Mater Sci Eng A 2003;360:249e57. [3] Show BK, Veerababu R, Balamuralikrishnan R, Malakondaiah G. Effect of vanadium and titanium modification on the microstructure and mechanical properties of microalloyed HSLA steel. Mater Sci Eng A 2010;527:1595e604. [4] Yayla P, Kaluc E, Ural K. Effect of welding processes on the mechanical properties of HY 80 steel weldments. Mater Des 2007;28:1898e906. [5] Sun Weihua, Wang Guodong, Zhang Jiming, Xia Dianxiu, Sun Hao. Microstructural characterization of high heat input welding joint of HSLA steel plate for oil storage construction. J Mater Sci Technol 2009;25(6):857e60. [6] Magudeeswaran G, Balasubramanian V, Madhusudhan Reddy G, Balasubramaian TS. Effect of welding processes and consumables on tensile and impact properties of high strength quenched and tempered steel joints. J Iron Steel Res Inter 2008;15(6):87e94. [7] Kang J, Wang C, Wang GD. Microstructural characteristics and impact fracture behavior of a high-strength low-alloy steel treated by intercritical heat treatment. Mater Sci Eng A 2012;527:96e104. [8] Lambert A, Drillet J, Gourgues AF, Stuel T, Pineau A. Microstructure of martensite e austenite constituents in heat affected zones of high strength low alloy steel welds in relation to toughness properties. Sci Technol Weld Join 2000;5(3):168e73. [9] Lakshminarayanan AK, Balasubramanian V. An assessment of microstructure, hardness, tensile and impact strength of friction stir welded ferritic stainless steel joints. Mater Des 2010;31:4592e600. [10] Thompson B, Babu SS. Tool degradation characterization in the friction stir welding of hard metals. Weld Res 2010;89:256e61. [11] Galvao I, Leal RM, Loureiro A. Influence of tool shoulder geometry on properties of friction stir welds thin copper sheets. J Mater Process Technol 2013;213(2):129e35.
317
[12] Zhu XK, Chao YJ. Numerical simulation of transient temperature and residual stresses in friction stir welding of 304L stainless steel. J Mater Process Technol 2004;146(2):263e72. [13] Park SHC, Sato YS, Kokawa H. Carbide formation induced by pcBN tool wear in friction-stir welded stainless. Metal Mate Trans A 2009;40A:625e36. [14] Gan W, Li ZT, Khurana S. Tool materials selection for friction stir welding of L80 steel. Sci Technol Weld Join 2007;12(7):610e3. [15] Lienert TJ, Stellwag JR, Grimmett BB, Warke RW. Friction stir welding studies on mild steel. Weld J 2003;89:1e9. [16] Lakshminarayanan AK, Balasubramanian V. Understanding the parameters controlling friction stir welding of AISI 409M ferritic stainless steel. Metal Mater Int 2011;17(6):969e81. [17] Konkol PK, Mruczek MF. Comparisons of friction stir weldments and submerged arc weldments in HSLA-65 steel. Weld J 2001;89:187e96. [18] Zhang Ying-qiao, Zhang Han-qian, LI Jin-fu, LIU Wei-ming. Effect of heat input on microstructure and toughness of coarse grain heat affected zone in Nb microalloyed HSLA steels. J Iron Steel Research Inter 2009;16(5):73e80. [19] Rajamanickam N, Balusamy V, Magudeeswaran G, Natarajan K. Effect of process parameters on thermal history and mechanical properties of friction stir welds. Mater Des 2009;30:2726e31. [20] Wan XL, Wei R, Wu KM. Effect of acicular ferrite formation on grain refinement in the coarse-grained region of heat affected zone. Mater Charact 2010;61:726e31. [21] Brown IH. The role of microsegregation in centerline cold cracking of high strength low alloy steel weldments. Scr Mater 2006;54:489e92. [22] Momeni A, Arabi H, Rezaei A, Badri H, Abbasi SM. Hot deformation behavior of austenite in HSLA-100 microalloyed steel. Mater Sci Eng A 2011;528:2158e63. [23] Yamamoto, Hasegawa T, Takamura. Effect of boron on intra-granular ferrite formation in Ti-oxide bearing steels. ISIJ Inter 1996;36:80e6. [24] Ghosh, Kumar, Mishra RS. Analysis of microstructural evolution during friction stir welding of ultrahigh-strength steel. Scr Mater 2010:851e85. [25] Dhua SK, Sen SK. Effect of direct quenching in the microstructure and mechanical properties of the lean chemistry HSLA-100 steel plates. Mater Sci Eng A 2011;52:6356e65. [26] Ghosh A, Das S, Chatterjee S, Ramchandra Rao P. Effect of cooling rate on structure and properties of an ultra-low carbon HSLA-100 grade steel. Mater Charact 2006;56:59e65. [27] Kenyon N. Effect of austenite on the toughness of maraging steel welds. Weld J 1968:193e8. [28] Aydin Hakan. Relationship between a bainitic structure and the hardness in the weld zone of the friction stir welded X80 API grade pipe-line steel. Mater Technol 2014;48(1):1580e2949.