Effect of Cooling Rate on Phase Transformation Behavior during Isothermal Annealing of SCr420 Steel

Herein, the effect of cooling rate on microstructure formation and phase transformation behaviors during isothermal annealing of the SCr420 steel is investigated. During isothermal annealing, tempering is performed after normalizing, and the effect of cooling conditions from normalizing to tempering is analyzed. As the cooling rate increases, the nonuniform microstructure with the band structure formed after hot forging tends to be homogenized. When the temperature reached during cooling is lowered, the phase transformation is rapidly completed during the cooling process, and the microstructure becomes more homogeneous. Phase transformation behavior during cooling from the normalizing temperature is analyzed through differential scanning calorimetry. Two exothermic reaction peaks are detected, and those peaks are related with the γ → γ + α → α + θ phase transformation. The higher the cooling rate, the lower the phase transformation starting temperature and the higher the phase transformation rate. In addition, from the thermal analysis data, the phase transformation kinetics and activation energy during cooling are analyzed by the Friedman method and the additivity rule. Through this analysis, the Avrami exponent and the activation energy for the phase transformation are derived. As the cooling increased, the microstructure tended to be homogenized. However, when the samples are cooled to the tempering temperature of 700 °C, a non‐uniform microstructure was formed. To complete the phase transformation during cooling, the samples are cooled to 600 °C after normalizing and then reheated to 700 °C for tempering. The more uniform microstructure is obtained even at the same cooling rate.