Effects of Martensite Matrix Structure and Retained Austenite Distribution on Low-Temperature Toughness of 9 Pct Ni Steel with Intermediate Treatment
摘要
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 (γR) following tempering. Impact testing at 77 K revealed that specimens with high volume fractions of finely dispersed γR 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 γR 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 γR 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 γR dispersion, leading to preferential crack propagation along dense high-angle grain boundaries and poorer low-temperature toughness.