Effect of Spheroidised Annealing Before Cold Forging on the Behavior of Austenite Grain Coarsening during Carburizing

In order to clarify the mechanism of austenite grain coarsening in steel for cold forging and carburizing, the effect of spheroidizing annealing on the behavior of grain coarsening during carburizing was investigated. Coarsening of the austenite grain after quenching was observed in previously annealed steel, but not in non-annealed steel. However, since both the annealed steel and the non-annealed steel have almost the same size distribution of Nb(C,N) nano-precipitates, the effect of annealing on austenite grain coarsening was not explained by the conventional theory based on the pinning force by nano-precipitates. Spheroidized cementite was remained in the annealed steel which was immediately quenched from the carburizing temperature of 1203 K. Dissolution of spheroidized cementite and austenite grain coarsening took place simultaneously. Spheroidized cementite was thermally stabilized by Cr concentration. A DICTRA simulation was carried out to discuss the kinetics of cementite dissolution. Cr concentration affects the dissolution rate of spheroidized cementite through the relationship between the Cr content and the cementite size. The Cr content in cementite has a wide distribution, and the dissolution time is different in each cementite. This dissolution time lag among cementites causes a local and non-uniform decrease of the pinning force by cementite. As a consequence, austenite grain coarsening is likely to occur in annealed steel.