Characterization of Microstructure and Mechanical Properties of Friction Stir Welded Reduced Activation Ferritic–Martensitic Steel in As-Received, As-Welded and Post-weld Heat-Treated States

Abstract

In this paper, an attempt has been made to explore the feasibility of employing friction stir welding (FSW) of 12-mm-thick reduced activation ferritic–martensitic (RAFM) steel plates using a polycrystalline cubic boron nitride (PCBN) tool at a rotation speed of 200 rpm. Deep penetration bead-on-plate welds were successfully produced. Microstructures evolved in stir zone (SZ), thermomechanically affected zone (TMAZ), heat-affected zone (HAZ) and unaltered base metal zones, and their impact on microhardness was assessed. The SZ and TMAZ revealed high hardness, while HAZ became much softer than the base metal. Several grain boundaries in SZ and TMAZ were found to be free from carbides, and SZ contained martensite laths, Fe3C needles and very high dislocation density. HAZ displayed coarse and coagulated carbides. The inhomogeneous microstructure and hardness variation across the weld joint have been rectified by appropriate post-weld normalising and tempering (PWNT) treatment. Tensile tests were conducted at room and elevated temperatures (450 °C, 500 °C, 550 °C) on transverse samples profiled from as-welded and PWNT states; and the displayed stress–strain behaviour and mechanical properties were compared with those obtained concurrently on base metal at various temperatures. The observed tensile hardening/softening as a function of temperature has been explained based on the substructure evolution. It has been noticed that the variation of the fraction of low-angle grain boundaries and high-angle grain boundaries played a significant role in influencing the tensile deformation behaviour in the as-welded and PWNT states. The low ductility observed at 450 °C has been ascribed to dynamic strain ageing. Alloy displayed ductile fracture, in spite of variation in tensile properties depending upon the state of the material and tensile test temperature. This is the first ever investigation dealing with the tensile behaviour of friction stir welded RAFM steel under different states and conditions.