Effect of annealing before cold forging on the behavior of abnormal grain growth during carburizing

Abstract

In order to clarify the mechanism of abnormal grain growth in steel for cold forging and carburizing, the effect of spheroidized annealing on the behavior of abnormal grain growth during carburizing was investigated. Abnormal grain growth was observed in annealed steel, whereas it was suppressed in normalized steel. However, both the normalized steel and annealed steel have almost the same size distribution of Nb (C,N) nano-precipitates. The effect of annealing on abnormal grain growth was not explained through the conventional theory based on pinning force by nano-precipitates. Spheroidized cementite was remained in annealed steel subsequently quenched from quasi-carburizing temperature 1203 K. Dissolution of spheroidized cementite and occurrence of abnormal grain growth took place simultaneously. Spheroidized cementite was thermally stabilized by Cr concentration. DICTRA simulation was carried out to discuss kinetics of cementite dissolution. Cr concentration affects dissolution rate of spheroidized cementite through the relationship between Cr concentration and cementite size. Cr concentration in cementite has wide distribution. Therefore, dissolution time is different in each cementite. This dissolution time lag among each cementite causes local and ununiform decreasing of pinning force by cementite. As a consequence, abnormal grain growth is likely to occur in annealed steel.