Effect of cooling rate and isothermal temperature on the phase transformation and microstructure evolution in SWRH82B high-carbon steel

To study the effect of isothermal temperature and cooling rate on the phase transformation and microstructure evolution in SWRH82B steels, a thermal dilatometer was used in this study to generate the static continuous cooling transformation (CCT) and isothermal-transformation thermal-expansion diagrams. In addition, scanning electron microscopy was used to analyze the effect of process parameters on microstructural transformation in the steel specimens. The results suggest that with a decrease in the isothermal temperature, the incubation time and duration of phase transformation gradually decreased; further, the content of non-lamellar pearlite in the microstructure gradually increased at room temperature. Compared with the time-temperature-transformation (TTT) diagram of SWRH82B steel subjected to conventional process conditions (rapid cooling before isothermal treatment), the incubation periods observed in this study were significantly shorter. When the loop-laying temperature was 850 °C, austempering temperature was 630 °C, and cooling rate was 5 °C s−1, the incubation time and phase-transformation duration of the experimental steel were 9.60 and 28.52 s, respectively. With an increase in the cooling rate, the incubation period gradually increased, while the phase-transformation duration initially increased and then decreased. The room-temperature microstructure, obtained by austempering at 630 °C for 300 s, was mainly composed of well-distributed lamellar pearlite. With an increase in the cooling rate, the pearlite interlaminar spacing gradually reduced. However, the room-temperature microstructure, obtained by austempering at 550 °C for 300 s contained a large proportion of non-lamellar structures and the distribution of pearlite became disorganized.