325 Air-cooled Carbide Free Bainite-Martensitic Steel and Its Properties

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Effect of tempering temperature on the combination of strength and toughness and susceptibility to hydrogen embrittlement for a low carbon Mn-Si-Cr alloy steel with the microstructure of carbide free bainite-martensite (CFB/M) has been experimentally investigated. Filmy, but not blocky, retained austenite exists in the air cooled CFB/M microstructure. The retained austenite films in CFB/M possess a good thermal and mechanical stability. The experiment results also showed that the air-cooled CFB/M has a better combination of strength and toughness compared with water quenched full martensite microstructure after intermediate temperature tempering. After tempering at 360℃, the combination of strength and toughness of the steel with CFB/M structure is σ_<0.2> : 1355MPa, σ_b : 1600MPa, δ_5 : 13.5%, ψ : 56.2%, A_k : 81J, while that of the same steel with full martensite microstructure is σ_<0.2> : 1350MPa, σ_b : 1617MPa, δ_5 : 14.1%, Ψ : 59.5,A_k : 67.5J. With the tempering temperature increase, the susceptibility to hydrogen embrittlement decreases. The cause might be that the carbide free bainite has a higher temper resistance than the martensite in the same steel. And carbon enriched retained austenite film in CFB with a good thermal stability might improve the toughness of the steel after tempering at intermediate temperature.
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