Evolution of the Structure and Mechanical Properties of Medium-Carbon Microalloyed Steel during High-Temperature Tempering

The study investigates the evolution of the structure and mechanical properties of quenched 38G2F steel (0.42 C, 1.3 Mn, 0.09 wt% V) during tempering at 650°C. It has been shown that the microstructural change and strength reduction in steel during its isothermal tempering in the time (τ_temp) range from 2 to 3000 min resemble the results usually being attained in the course of two stages of actual procedure of tempering used to be employed: medium- (stage II) and high-temperature (stage III). The significant decrease in strength properties Δσ/Δτ_temp ~ 100 MPa/min during Stage II has been followed by minimal softening Δσ/Δτ_temp ~ 0.1 MPa/min during stage III. TEM, EBSD, and XRD techniques have been employed to examine the evolution of the microstructure and to determine the hardening components and their respective impact on yield strength during various “kinetic” stages of tempering. The tempering parameter has been used to compare the structural and phase state and strength data obtained from short-term heating at 650°C with the results of tempering at 500 and 550°C for different periods of time (stage II).