Microstructure, crystallography, mechanical and damping properties of a low-carbon bainitic steel austempered at various temperatures

Effects of austempering temperature on the microstructure, crystallographic characteristics, mechanical properties and damping capacity of a low-carbon bainitic steel were investigated in the present study. The results showed that a decrease in the austempering temperature from 380 to 320 degrees C led to an increase in both the fractions of bainitic ferrite and retained austenite, especially those with filmy morphology. Meanwhile, the thickness of bainitic ferrite laths was refined and the carbon concentration of retained austenite was increased, thereby significantly improving the yield strength, impact toughness and total elongation. Moreover, the variant selection effect became less important with decreasing austempering temperature, so that variants changed from belonging to the same Bain groups with low-angle grain boundaries to sharing the same parallel relation of close-packed groups with high-angle grain boundaries, which is an effective way to increase the impact toughness. The density of dislocation and the amounts of bainitic ferrite and retained austenite are the significant factors that enhance the damping capacity of austempered steel. An outstanding combination of both mechanical properties and damping capacity (925 +/- 10 MPa yield strength, 1440 +/- 2 MPa tensile strength, total elongation of 12.8 +/- 0.4%, and V-notch unit impact toughness at 20 and - 40 degrees C of 56 +/- 3 and 31 +/- 2 J/cm(2), as well as the maximum logarithmic decrement coefficient of 0.1164, respectively) was achieved by austempering at 320 degrees C.