Effects of Austempering Temperature on Strength, Ductility and Toughness of Low-C High-Al/Si Carbide-Free Bainitic Steel

The microstructure and the mechanical properties of a low-carbon Al/Si-alloyed carbide-free bainitic steel austempered at temperatures between 300 and 350 °C have been investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, x-ray diffraction analysis, and mechanical property tests. The results show that an excellent combination of tensile strength, ductility, and impact toughness is obtained at the austempering temperature of 320 °C—but neither at the lowest temperature of 300 °C nor at the highest temperature of 350 °C. These results are correlated with the features of the constituent phases of the microstructure, especially the amount and size of retained austenite, which are largely dependent on the austempering temperature. The observations are also due to the inconsistent effects of strain-induced martensitic transformation on strength, ductility, and toughness. The results of the present study suggest that an optimized isothermal temperature may also exist for any other bainitic steel, at which, if austempering treatment is carried out, an improved combination of tensile and impact properties can be obtained.