Mechanism of Austenite Grain Coarsening during Carburizing of Cold-forged Steel Parts

It is known that austenite grain coarsening during carburizing is likely to occur in cold-forged steel parts. For efficient use of this production process, it is important to clarify the mechanism of grain coarsening. In this study, mechanism of austenite grain coarsening of the simulated cold-forged and carburized low-alloy steel was investigated by microstructural observation, measurement of crystal orientation by using EBSP (Electron backscatter diffraction pattern) and CAE analysis. Austenite grain coarsening occurred at highly shear-strained region of the cold-upset specimen. In the highly shear-strained region, recrystallized ferrite grains with relatively-randomized crystal orientation are formed during heating up to carburizing temperature. Because these recrystallized ferrite grains transform to fine austenite grains, the accelerated austenite grain coarsening occurs at the highly shear-strained region. It could be suggested that reduction of shear strain during cold forging prevent austenite grain coarsening.