Abstract
The influence of isothermal treatments on microstructure, hardness, and tensile properties of a new designed medium carbon super-bainitic steel was studied by using the optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and mechanical property tests. The results show that the isothermal transformation temperature and holding time combine to determine the variations of microstructure and mechanical properties in the low temperature range of 140-260 °C. The size of bainitic laths is refined with the decrease in transformation temperature, but the bainitic transformation kinetic becomes lower. Longer treatment time can promote the transformation of complete bainite and also make the retained austenite more stable. The desired microstructure consisting of nanoscale carbide-free bainitic ferrite and thin film-like retained austenite located between the ferrite laths can be achieved at the temperature of 260 °C for 10 h, where the excellent combination of strength, ductility and hardness can be obtained.
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