محمدعلی مهتدی بناب

Stainless steel grade SS 304 and medium carbon steel EN 8 were welded with various combinations of weld parameters using tungsten inert gas (TIG) welding process. Significant differences in weld and interface microstructures were achieved under different weld process variables (current, voltage and speed). Microstructures of the weld region varied from large grains of γ-austenite to fine grain distribution of δ-ferrite and γ-austenite grains. Further to that, the phase boundaries between γ and δ phases showed presence of two dominant phase boundary relationships (PBR), 44°〈104〉 and 44°〈114〉. The weld metal (WM)-base metal (BM) interfaces also showed differences in the fusion zone (FZ) and heat affected zone (HAZ). Fusion zone at the interface of medium carbon steel and weld metal were characterized by presence of fine grains of ferrite which were identified as relatively strain free grains. These differences in microstructures were mainly due to the differences in heat inputs and solidification rates of the weld metal. Differences were also observed in the grain boundary fractions and local in-grain misorientation between different microstructures. The tensile strength of these welded joints were found to be clear functions of low angle grain boundary (LAGB) fractions, while impact energy was seen to be a function of austenite in-grain misorientation and δ-ferrite percentage. The pattern of fracture under tensile deformation displayed both mixed mode and ductile fracture. Differences in the fractured surfaces were seen in the distribution, size and shapes of dimples and microvoids. The present investigation attempts to address the pattern of microstructure development and the related mechanical behavior in dissimilar welded joints through a systematic approach.