PSI - Issue 14

P D Gosavi et al. / Procedia Structural Integrity 14 (2019) 304–313

305

P D Gosavi et al./ Structural Integrity Procedia 00 (2018) 000 – 000

1. Introduction

High-strength low-alloy (HSLA) steels are structural steels having good strength, toughness and weldability. This combination of properties have led to their varied applications in the automotive industry, manufacturing of large diameter pipes for gas and oil transportation in the areas of low temperature and fabrication of plates for naval ship’s construction (Khaki DM et al.,2013). HSLA steel has greater resistance to atmospheric corrosion than conventional carbon steels. The chemical composition of HSLA steel may vary from different thickness to meet required mechanical properties and high yield strength (greater than 275MPa). The HSLA steel has low carbon content (0.05 to 0.25% C) in order to possess adequate formability and weldability. Welding of thicker HSLA steel (>20 mm) is challenging task for naval applications. Welding process parameters are important such as heat input for sound weld. Heat input and weld metal chemical composition are the two important factor which governs the weld microstructure. Generally, for welding of HSLA steel higher heat input is used in SAW process as compared to other processes. Viano et al.(2000) investigated the effect of heat input and travel speed on microstructural characteristics and mechanical properties of welds. Dhau et al.(2002) found that the mechanical properties in a high strength steel weld primarily depend on the final microstructure obtained. Babu (2004) demonstrated that an optimum combination of strength and toughness in HSLA steel weld metal was obtained by the formation of acicular ferrite microstructure. The author reported that the nucleation and growth of acicular ferrite takes place during decomposition of austenite structure. Loder et al.(2016) find out that the medium cooling rate can produced more area percentage of acicular ferrite in weld metal after welding and stated that the increase in percentage acicular ferrite in weld region can enhance the toughness property of HSLA weld.In recent years, acicular ferrite has also been of increasing interest to steel producers; the excellent combination of toughness and strength makes acicular ferrite favourable for HSLA (high-strength low-alloy) steels (Bhadeshia, 1992; Sarma, Karasev, & Jönsson, 2009). As welding of thicker steel with expected properties especially for naval application is main objective and whether microstructure of weld impacts on mechanical properties or not specially percentage of acicular ferrite in weld microstructure. In this study, percentage of acicular ferrite in weld microstructure of different weld joint of various welding process was analyzed to correlate mechanical properties with weld microstructure 2. Experimental Procedure 2.1. Materials In this study low Carbon Ni-Cr-Mo HSLA steel was used as the base material having carbon equivalent (P cm ) in the range of 0.32 to 0.38.The steel has low carbon content to improve the weldability and toughness. 2.2. Welding & Weld Joint Preparation The different processes used for welding of the steel were SMAW, GMAW, SAW and GTAW. The size of coupons was 550 x 420 x 40-70 mm with 550 mm along rolling direction as well as in welding direction. The root gap (RG) used was 3-6 mm and root face (RF) was 1-2 mm with included angle (IA) at 60°.Welding was carried out at constant potential mode for SAW and GMAW process while it was constant current for SMAW and GTAW processes. Since the carbon equivalent P cm of the steel was on the higher side the weld joints are more susceptible for hydrogen induced cold cracking. Therefore, the weld consumables like electrodes and flux were baked, at temperature of 460° C for 3 hours and 650° C for 4 hours, respectively, so as to avoid hydrogen induced cold cracking. Preheating requirement was up to 50-70° C for SMAW process while for GTAW preheating was up to 20° C and the inter-pass temperature 250° C was maintained in all the processes. The welding parameters used during welding of the coupons are provided in table 1and Joint configuration in fig.1. Dye penetrate test (DPT) was carried out after grinding of root pass from chip back side until metal finish was obtained. Weld coupon was turned upside down in between the layers to prevent chances of welding distortion. Back side of weld was defined as the side in which last pass was laid. Then after successful Radiography Testing (RT), the weld coupons were taken up for