Martensite Reverse Transformation Mechanism and Its Effects in Crack Propagation of Plasma Arc Additive Manufactured 300 M Steel

The complex thermal cycles in the plasma arc additive manufacturing process can induce the transformation of martensite to γ‐austenite, which significantly influences mechanical properties. The study explores the reversed austenite evolution and its impact on crack propagation. Results reveal a nonmonotonic distribution of γ‐austenite content from the top to bottom area of additively manufactured components, with the top area containing fine residual austenite and the bottom region dominated by coarse reversed austenite along high‐angle grain boundaries. Martensite and austenite follow the Nishiyama–Wassermann (N–W) orientation relationship with 12 variants, though the variant count decreases in the bottom region due to lower transformation temperatures. The reverse transformation is primarily diffusion based, with preferential nucleation at high‐angle grain boundaries. Repeated phase transformation under thermal cycling refines the microstructure, reducing grain size in the bottom region by 38.3%. The fine and dispersed γ‐austenite in the top region forms a “hard shell/soft core” structure and activates the transformation‐induced plasticity effect, enhancing crack resistance. In contrast, coarse reversed austenite in the bottom region promotes crack propagation due to its stable blocky morphology and high dislocation density. This study enables better control of mechanical properties in additively manufactured components.