Continuous cooling transformation of super cooled austenite of a low-cost ultra-high-strength steel G31L

doi:10.13251/j.issn.0254-6051.2020.12.030

Abstract

Abstract: Thermal dilatometric curves of G31L steel were measured by Formastor-FⅡdilatometer, and the microstructure evolution and hardness at different cooling rates were studied, the supercooled austenite continuous cooling transformation(CCT) curves of the G31L steel were drawn, and the reliability of post-forging heat treatment process of real large deep blind hole forging was analyzed. The results show that the critical temperatures of the G31L steel as follows:Ac₁=740 ℃, Ac₃=816 ℃, Ms=315 ℃, and Mf=138 ℃. When the cooling rate is less than or equal to 0.028 ℃/ s, pearlite+ferrite+bainite mixed structure is obtained; when the cooling rate is 0.028-0.84 ℃/s, pearlite and ferrite disappear with the increase of cooling rate, and bainite decreases in content gradually until full martensite is obtained; when the cooling rate is greater than 0.84 ℃/s, full martensite is obtained. The large deep blind hole forgings actually produced are heat treated at 915 ℃ for 6 h and then oil quenched to room temperature and tempered. The solid head core of the forgings has a mixed structure of martensite and a small amount of bainite, while the R/2 and edge of the head and tail are all martensite, and the strength and plasticity of the forgings meet the product quality requirements.

Key words: CCT curves, martensite, ultra-high-strength steel, deep blind hole forgings

CLC Number:

TG161

Yang Peng, Ning Jing, Su Jie, Jiang Qingwei. Continuous cooling transformation of super cooled austenite of a low-cost ultra-high-strength steel G31L[J]. Heat Treatment of Metals, 2020, 45(12): 149-154.

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