Study of the Microstructure, Mechanical Properties, Residual Stresses, and Distortion in Type 316LN Stainless Steel Medium Thickness Plate Weld Joints

A nuclear reactor that utilizes fast neutrons to produce more fissile nuclear fuels than they consume while generating power is called Fast Breeder Reactor (FBR). The FBR components are currently welded by multi-pass Tungsten Inert Gas (TIG) welding process. There are limitations associated with multi-pass TIG welding of 316LN SS plates. This research work on 316LN stainless plates will be valuable in extending the utilization of the advanced welding process for fabricating structural parts in FBRs. In the present study, weld joints of 5.6 mm thick 316LN stainless steel plates have been made using multi-pass TIG welding process and by autogenous Activated-TIG (ATIG) welding process. An in-depth comparative study on the effect of the above welding processes on the weld attributes of the weld joints such as bead geometry, microstructures, ferrite content, hardness, tensile, impact toughness properties, residual stress and distortion has been carried out. ATIG joint manifested narrow weld bead when compared with that of the TIG weld joint. Though ferrite content was low, there was no hot cracking in the weld due to lower restraint and hence reduced shrinkage strains. TIG weld exhibited higher hardness and strength properties than the base metal and ATIG weld joint due to the existence of higher δ ferrite. However, at high temperature, both weld joints have exhibited similar values of ultimate tensile strength and percentage total elongation. Moreover, ATIG joint has higher impact toughness than TIG joint due to the very low δ ferrite content. The distortion is almost negligible in ATIG weld joint in comparison with that of the TIG joint. With significant advantages, autogenous ATIG welding is recommended for welding of medium thickness plates of 316LN stainless steel.