Heat-Affected Zone Characteristics with Post-Weld Heat Treatments in Austenitic Fe–Mn–Al–C Lightweight Steels

This study investigates the relationship between the mechanical characteristics and microstructural transitions in heat-affected zone (HAZ) of various austenitic Fe–Mn–Al–C lightweight steels. The analysis was conducted on the simulated HAZ samples with various post-weld heat treatment (PWHT) conditions. The base steel was prepared via vacuum induction melting, hot rolling, and solution treatment prior to water quenching. Gleeble simulator was used to manufacture the simulated HAZs, and the samples were subjected to PWHT with various durations by electric furnace. To understand the effects of thermal history on the mechanical characteristics, ultimate tensile tests, micro-hardness tests, and cryogenic Charpy impact toughness tests were conducted, and microstructural analyses were conducted by optical microscopy, scanning electron microscopy, X-ray diffraction analysis, and transmission electron microscopy. The experimental results confirmed the influence of Al content on the austenitic Fe–Mn–Al–C lightweight steels, which promotes κ-carbide precipitation, both in the austenite matrix and grain boundaries, thereby increasing the strength and hardness while decreasing the ductility and toughness. Each mechanical property showed a linear relationship with the growth kinetics of the κ-carbide particles. However, abnormal coarsening of the inter-granular κ-carbide during welding and PWHT caused severe embrittlement, regardless of thermal history, by acting as inter-granular crack propagation channels. The experimental results suggest that Fe–Mn–Al–C lightweight steels should be developed with controlling the alloying elements to overcome HAZ discontinuity and mechanical property degradation occurring by intra- and inter-granular κ-carbide precipitation.