Influence of heat treatment on the structure and mechanical properties of chrome steel with unstable austenite

Resumo

The structure and mechanical properties of 35Kh12G3MVFDR steel are investigated. After normalization or quenching, the steel contains up to 35 vol % austenite and may be assigned to the martensitic–austenitic class. On heat treatment—tempering, isothermal holding, or isothermal quenching—the austenite is converted to martensite within 2 h. The martensite in 35Kh12G3MVFDR steel is more thermally stable: the first signs of its conversion to sorbitic structure are observed after 25-h isothermal quenching at 640°C, and its complete decomposition requires 50 h. The breakdown of martensite is accompanied by decrease in the high-temperature strength and hardness. Aging of the quenched and tempered 35Kh12G3MVFDR steel at 670–720°C lowers the hardness from 61–65 HRA to 55–60 HRA after 1600–3200 h and the yield point at 20°C from 1350 MPa to 750–850 MPa. Likewise, the yield point at 720°C declines from 310 MPa to 160–230 MPa after 600 h and then stops. The state of the martensitic structure of 35Kh12G3MVFDR steel determines its creep resistance at 700°C. For example, the martensite remains in the steel structure after relatively brief isothermal quenching (up to 24 h at 640°C), and consequently the creep limit σ^700°C_0.1%/h is no lower than after simple quenching with subsequent high tempering: 86.2 ± 9.4 MPa and 89.3 ± 8.8 MPa, respectively. At the same time, in response to the decomposition of martensitic structure as a result of prolonged aging (1600 h at 670°C), σ^700°C_0.1%/h declines to 63.9 ± 7.1 MPa. In contrast to martensite, the austenite in 35Kh12G3MVFDR steel is thermally unstable and is converted to martensite after only 1–2 h of heating, depending on the temperature.