Strengthening and toughening mechanisms in Ni-alloyed steel: Enhancing the integral stability of retained austenite

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2022-09, Vol.852, p.143703, Article 143703
• Main Authors: Zhou, Cheng, Ye, Qi-Bin, Zhao, Tan, Hu, Jun, Gao, Xiu-Hua, Wang, Zhao-Dong
• Format: Article
• Language: English
• Subjects: Cryogenic toughness; Nano precipitates; Ni-alloyed steel; Retained austenite; TRIP effect
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2022.143703

In the present study, retained austenite (RA) characteristics of Ni-alloyed steel were tailored via a multi-stage intercritical heat treatment to evaluate the contribution of RA stability on yield strength, elongation, and cryogenic toughness. Two heat-treatment routes were used to tailor RA formation: first, stage Ⅰ intercritical annealing (IA) combined with stage Ⅱ high-temperature intercritical annealing (IA-HIA); and second, stage Ⅰ IA combined with stage Ⅱ low-temperature intercritical annealing (IA-LIA). Although a considerably higher amount of RA (10.6%) was obtained in the IA-HIA steel, its yield strength (770 MPa), elongation (17.1%), and cryogenic impact energy (49 J) were significantly lower than those of the IA-LIA steel. In the latter steel, a superior yield strength (920 MPa), elongation (19.1%), and cryogenic impact energy (95 J) were obtained at a much lower amount of RA (7.8%). Multi-alloy enrichment and thin-filmy morphology increased the thermal and mechanical stability of RA, which was further demonstrated via cryogenic holding and interrupted straining tests. The sustainable transformation-induced plasticity effect of stable filmy RA at larger strain under tensile and impact loads remarkably enhanced the elongation and cryogenic toughness. Nano-sized vanadium carbide precipitates greatly improved the yield strength without deteriorating the elongation and cryogenic toughness. It is concluded that postponing destabilization of RA is the key to improve the yield strength and cryogenic toughness. [Display omitted] •Amount and stability of retained austenite are optimized via IA-LIA heat treatment.•Retained austenite is further stabilized by multi-alloy enrichment and filmy shape.•Stabilized retained austenite enhances combination of strength-ductility-toughness.•Adequately stable retained austenite deflects crack propagation through TRIP effect.•Retained austenite cryo-stability is more crucial than amount for TRIP toughening.