Suppression of Austenite Grain Coarsening by Ferrite Pinning during Pseudo-carburizing Treatment

Abstract

Grain coarsening prevention while accelerating the carburizing process is critical for the manufacturer for case hardening components such as gears to improve efficiency. In this research, a novel method is proposed to prevent austenite grain growth by introducing the ferrite phase that could retard the austenite boundary movement. The addition of aluminum promotes the ferrite phase. Aluminum as strong ferrite former raises the A3 temperature so that the ferrite phase is present at high carburizing temperature. The effect of 1 wt.% aluminum on the microstructure evolution during pseudo-carburizing treatment in 20CrMn steel is investigated in details. It is shown that the formation of ferrite during pseudo-carburizing at 930 °C for 5h prevents austenite grain coarsening. The obtained mechanical properties are similar to non-aluminum added 20CrMn steel pseudo-carburized at 830 °C for 16h. When 20CrMn was pseudo-carburized at 930 °C for 5h, significant austenite grain growth was observed, which result in low impact toughness after the subsequent quenching and tempering treatment. The “ferrite pinning” effect effectively inhibits the grain growth at high temperature, revealed by metallography investigation.