Effect of heat input on the metallurgical properties of HSLA steel in multi-pass MIG welding

Abstract

A special class of high strength low alloy (HSLA) steel developed for armour application is presently welded by SMAW. Here its weldability by AutoMIG welding using 309L electrode wire has been studied: this results in higher welding speeds and deposition rates and deeper penetration. HSLA steels are difficult to weld due to their higher carbon equivalent and consequent susceptibility to cold- and hot-cracking. The cooling rates are higher compared to SMAW, where the slag blanket produces low cooling rates of the weld beads. Similarly, the high welding speeds of AutoMIG welding causes airdrift into the weld zone, leading to its contamination. However, the multi-pass mode of welding produces bead tempering and lower residual stresses. The welding speeds can also be suitably selected to control the heat input, which is a major parameter affecting the microstructure and mechanical properties of the fusion zone. The heat input is defined by the expression: IV/1000 S in kJ/mm, where I is the welding current, V is the voltage and S is the welding speed in mm/s. Thus the heat-input rate can be varied by changing the voltage and current setting also, but this is not preferable. Higher voltages alter the bead geometry and fusion area, which affects the resultant microstructure, and can also destabilise the arc and produce spatter. Current variation is not adopted since for a particular wire the recommended range of current setting is small. In the present studies, therefore, the heat-input rate is varied by employing different welding speeds: these were selected in 6 steps between 250 to 600 mm/min, which resulted in the corresponding heat input range of 1.9 to 0.8 kJ/mm.