CCT Curve And Deformation In Bainitic Transformation Region Of A Cr-Mo-V Steel

In recent years, the study demonstrates that the bainitic microstructure consisting of bainitic ferrite and carbide precipitates possesses excellent comprehensive mechanical properties. Numerous studies have been done on it. But few studies on the effects of deformation in bainitic transformation region on bainitic microstructure and transformation have been reported. In this paper, 3Cr-1Mo-0.25V steel was selected for experiments. The critical points, the CCT curve and the effects of deformation in bainitic transformation region on the microstructure and mechanical property of the steel were investigated by using of Gleeble-3500 thermal simulator, optical microscope, SEM, TEM and hardness measurement. It is observed that the critical points Ac₁, Ac₃, Ar₁and Ar₃ of the steel are 800℃, 890℃, 720℃ and 845℃ respectively, the CCT curve exists martensitic transformation region at high cooling rate and bainitic transformation region at lower cooling rate. Martensite can be obtained at the cooling rate higher than 30℃/s, mixed microstructure with martensite and bainnite can be obtained at the cooling rate in a range from 15℃/s to 30℃/s, bainite can be obtained at the cooling rate lower than 15℃/s. When deformation was performed after bainitic transformation, with the deformed temperature decreasing or the reduction increasing, the lath character of bainitic ferrite is evident and the hardness of the specimen increases. When deformation was performed during bainitic transformation, with the deformed temperature decreasing, the lath character of bainitic ferrite is evident, the dimension and dispersion of the precipitates decreases and the hardness of the specimen increases, and with the reduction increasing, the dimension of the precipitates decreases, the dispersion increases, the hardness of specimen increases. Deformation performed during bainitic transformation promotes carbide precipitate in bainitic ferrite laths. In the same deformation conditions, the hardness of the specimen deformed during bainitic transformation is lower than that deformed after bainitic transformation, because of soften due to precipitation.