Effect of Aging on Transformation Behavior of Reverted Austenite and Toughness in Co-Free Maraging Stainless Steel

The effect of aging on transformation behavior of reverted austenite and impact toughness in Co-free maraging stainless steel were investigated via thermodynamic calculation, transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). Excluding the film-shaped austenite growing along the phase interface, other shapes of reverted austenite are evolved from the growth and aggregation of acicular austenite in the range of 300-600 °C. Under N-W orientation relationship, {111}_γ grows inside the martensite lath along <100>_α, and austenite merges in <100>_α and <110>_α simultaneously. Under K-S orientation relationship, the growth direction of {111}_γ is 60° or parallel to <112>_α. From 300 to 500 °C, Ni prefers to diffuse into η-Ni₃Ti and matrix. The precipitation of Ni₃Ti hinders the formation of reverted austenite and significantly deteriorates the toughness. Above 500 °C, due to the coarsening of Ni₃Ti and the recovery of matrix, the resistance to the formation of austenite is obviously weakened, and then austenite plays a leading role in the improvement of toughness. When the aging temperature reaches 600 °C, the dissolution of Ni₃Ti promotes the formation of austenite and η-Ni₃Ti changes to γ'-Ni₃Ti. The interaction between Ni₃Ti and reverted austenite essentially depends on the diffusion behavior of Ni.