Abstract

This study investigates the microstructural evolution and mechanical response of laser-clad IN625 on 3.5% NiCrMoV steel under as-welded and post-weld heat-treated (PWHT) conditions. Optical microscopy and microhardness mapping characterized regional transformations, while Charpy V-notch testing was conducted from 25 to −196 °C. Additionally, SEM fractography correlated mechanical performance with operative fracture mechanisms across the weld metal (WM), heat-affected zone (HAZ), and base metal (BM). The as-welded WM exhibited a dendritic austenitic structure, whereas the CGHAZ consisted of martensitic and bainitic constituents. Following PWHT at 620 °C for 1 h, the WM retained its austenitic morphology with increased hardness, while the CGHAZ transformed into tempered martensite and bainite, resulting in moderate softening. Impact testing revealed stable cryogenic performance in WM, while the HAZ showed improved room-temperature impact energy after PWHT. Fractographic analysis confirmed a transition from ductile micro-void coalescence (dimples) at 25 °C to brittle transgranular cleavage at −196 °C, with PWHT promoting a more uniform dimple distribution in the HAZ. These combined results indicate that controlled PWHT effectively redistributes strength and toughness across the joint, clarifying the microstructural mechanisms governing the laser-based repair of NiCrMoV rotor components.