Effects of Martensite Matrix Structure and Retained Austenite Distribution on Low-Temperature Toughness of 9 Pct Ni Steel with Intermediate Treatment

Abstract We subjected 9.2 mass pctNi steel with either a fine or coarse hierarchical martensite matrix, prepared by varying the austenitizing temperature, to intercritical annealing or full austenitization prior to tempering at various temperatures. Intercritical annealing produced a lamellar structure composed of fresh and tempered martensite ( α ′) with ferrite, leading to reversed austenite ( γ ) dispersion that remained as retained- γ ( γ R ) following tempering. Impact testing at 77 K revealed that specimens with high volume fractions of finely dispersed γ R obtained at relatively lower tempering temperatures, reaching a maximum at 873 K, exhibited stable plastic deformation and effective crack arrest, along with superior low-temperature toughness as a consequence. In contrast, further tempering at higher temperatures led to lower γ R volume fractions accompanied by significantly lower absorbed energies. This deterioration is ascribable to the extensive formation of reversed- γ at higher tempering temperatures that partially or fully retransforms into fresh- α ′ during subsequent cooling. This study suggests that this newly formed fresh- α ′ promotes crack propagation near the fracture surface, thereby degrading low-temperature toughness. Refinement of the α ′-matrix was found to facilitate a finer and more uniformly dispersed γ R that synergistically improved low-temperature toughness by suppressing quasi-cleavage fracture, which is another key finding. In contrast, full austenitization prior to tempering refined the α ′-matrix but reduced both the volume fraction and γ R dispersion, leading to preferential crack propagation along dense high-angle grain boundaries and poorer low-temperature toughness.