Uncovering the isothermal transformation dynamics and elemental diffusion behavior of reversed austenite in PH13-8Mo steel

Abstract

The isothermal transformation kinetics and microstructural evolution of the reversed austenite significantly influence the properties of PH13-8Mo steel. However, the microscopic phase transformation behavior related to elemental diffusion during isothermal processes for PH13-8Mo steel needs further examination. Herein, we systematically investigate the isothermal transformation and elemental diffusion behavior of reversed austenite in PH13-8Mo steel. Phase transformation kinetic measurements unravel that the complete elimination of residual austenite can be achieved. Only hierarchical martensitic microstructure without austenite is detected after the solution-cooling treatment at 927 °C with a cooling rate of 10 °C/s, determining the critical martensite transformation start and finish temperature to be 176.6 °C, 11.6 °C. The isothermal kinetics of reverted austenite satisfy the Johnson-Mehl-Avrami relationship. The fraction of reversed austenite increases from 0.14 % at 510 °C to 8.73 % at 593 °C. Lath martensite boundaries with large kernel average misorientation values promote elemental diffusion at the interface. Further, the underlying behavior of elemental diffusion is elucidated during the isothermal aging process. The interfacial model indicates the growing width of the reversed austenite up to 100 nm and the compositional gradient formed by the elemental diffusion during aging for 0–5 h at 593 °C. The interdiffusion coefficient calculations interpret the diffusion direction of alloying element Ni from martensite to reversed austenite; Cr and Mo may form alloying carbides at the austenite-martensite interface; Al possesses the fastest diffusivity in martensite and austenite phases. The present work unveils the phase transformation kinetics of PH13-8Mo steel, achieving quantitative regulation for maraging stainless steel.