Junyu Tian

In Situ Observation of the Lengthening Rate of Bainite Sheaves During Continuous Cooling Process in a Fe–C–Mn–Si Superbainitic Steel

The evolution of lengthening rate of bainite sheaves during continuous cooling process in a Fe–C–Mn–Si superbainitic steel was investigated by in situ observation on high-temperature laser scanning confocal microscope. The lengthening rates of bainite sheaves in three temperature ranges were calculated. The results indicate that the lengthening rate of bainite sheaves continuously decreases with the decrease of temperature during continuous cooling process. The lengthening rate of bainite sheaf depends on undercooling, transformation temperature, the diffusion of carbon atoms and the carbon content in parent austenite etc. The lengthening rate at high temperature is large due to the favorable carbon diffusion, smaller carbon content and less plastic deformation in untransformed austenite. Additionally, the microstructures after different isothermal holding temperatures were analyzed, indicating that the larger lengthening rate of bainite sheaves due to the high isothermal transformation temperature does not mean more amount of bainite transformation. Lower bainitic transformation temperature results in more and finer bainite plates.

Kinetics model of bainitic transformation with stress

Thermal simulations were conducted on a Gleeble 3800 simulator. The main purpose is to investigate the effects of stress on the kinetics of bainitic transformation in a Fe-C-Mn-Si advanced high strength bainitic steel. Previous studies on modeling the kinetics of stress affected bainitic transformation only considered the stress below the yield strength of prior austenite. In the present study, the stress above the yield strength of prior austenite is taken into account. A new kinetics model of bainitic transformation dependent on the stress (including the stresses below and above the yield strength of prior austenite) and the transformation temperature is proposed. The new model presents a good agreement with experimental results. In addition, it is found that the acceleration degree of stress on bainitic transformation increases with the stress whether its magnitude is below or above the yield strength of austenite, but the increasing rate gradually slows down when the stress is above the yield strength of austenite.