New strategy to coordinately control microstructure-toughness in the heat-affected zone of cryogenic high-Mn steel by isothermal aging

• Developing a novel PWHT process: Isothermal aging + Water quenching. • Achieves significant optimization of grain boundary structure via the novel process. • Charpy tests demonstrate isothermal aging enhances toughness in HAZ of cryogenic high-Mn steel. This study introduces a novel post-weld heat treatment process designed to improve the toughness in welding heat-affected zone (HAZ) of cryogenic high-Mn steel, namely, isothermal aging at 500 °C/30 min followed by water quenching. Through comparative analysis of the microstructure and impact toughness (at room temperature and −196 °C) in HAZ before and after isothermal aging, it was demonstrated that isothermal aging can alleviate the segregation of carbon at the grain boundary, reduce the fraction of low-angle grain boundaries, and concurrently increase the proportion of high-angle grain boundaries and special grain boundaries (Σ3 n ), thereby improving the crack formation energy and crack propagation energy during impact testing. This is mainly attributed to atomic diffusion homogenization and recrystallization during isothermal aging. Consequently, the toughness in HAZ of high-Mn steel welded joint exhibits a minimum increase of 27.2 %.