Study on Welding Process and PWHT Parameters of Thin Walled P91 Steel Sheet to SS316LN Steel Plate of Hexcan with Foot Dissimilar Weld Joints

The metal fuel irradiation program at the Indira Gandhi Centre for Atomic Research (IGCAR) will focus on creating a test sub-assembly (TSA) that contains metal fuel pins. Each TSA is comprised of a hexagonal wrapper tube that houses 37 sodium-bonded fuel pins arranged in a triangular configuration. Within the TSA, thin hexcan sheets made from P91 steel are connected to thicker header portions made from AISI 316LN steel at the top and footers at the bottom (also AISI 316LN), forming a dissimilar weld joint setup. The welding of hexcan and foot joints was modeled by joining 1 mm thick P91 steel with 8 mm thick AISI 316LN using a 500 W Nd:YAG laser. Welding parameters, such as peak power, pulse duration, frequency, and welding speed, were optimized to achieve the desired depth of penetration. An experimental study was conducted to investigate how post-heat treatment conditions, including soaking temperature and dwell time, affect the mechanical properties of dissimilar laser-welded joints. Weld penetration, microstructures, and micro hardness were analyzed before and after heat treatment. The microstructural analysis showed the presence of martensite and ferrite in the weld and heat affected zone (HAZ) on the ferritic steel side, with a predominantly dendritic austenitic phase found along the SS316LN. A type-II boundary was also noted at the junction between the fusion zone (FZ) and the SS 316LN HAZ. Before post-weld heat treatment (PWHT), the maximum hardness recorded in the weld zone was 432 HV. However, after the welded sample underwent PWHT at 760 °C for a dwell time of 30 min, the hardness decreased to 390 HV. The elevated hardness observed before PWHT is attributed to the formation of a partially tempered martensitic structure.