Phase transformation mechanism and kinetics during step quenching of st37 low carbon steel

Abstract

Step quenching of st37 low carbon steel was studied for understanding the phase transformation kinetics. During step quenching, the amount of martensite (formerly austenite) decreased by increasing the holding time at the intercritical annealing temperature. Based on the Johnson–Mehl–Avrami–Kolmogorov (JMAK) analysis using the hardness data, Avrami exponents were determined in the range of 0.3 to 0.8, which indicate that the austenite to ferrite transformation in the intercritical temperature region is diffusion controlled. The activation energy of 84.2 kJ mol−1 was obtained, which is close to the activation energy for the diffusion of carbon in α-iron (87.4 kJ mol−1). Consequently, the diffusion of carbon in α-iron was found to be responsible for the microstructure formation via transformation of austenite to ferrite during step quenching. In fact, the grain-boundary ferrite phase grows into the surrounding austenite phase during step quenching while the austenite phase forms in place of pearlite and also nucleates on the ferrite grain boundaries during intercritical annealing of ferrite-pearlite microstructure. Based on these findings, the slower kinetics of the microstructure formation during step quenching (as compared to the conventional intercritical annealing) was related to the atomic diffusion mechanisms, where the diffusion of carbon in α-iron was calculated to be slower than that in γ-iron at 850 °C.