Abstract
Carburizing is frequently utilized in the automotive industry in order to increase the surface hardness of a steel alloy while retaining toughness and ductility in the core. At elevated temperatures where some carburizing processes are performed, abnormal grain growth (AGG) can occur. During AGG, the microstructure undergoes bimodal grain growth with some grains growing exponentially faster than others. The growth of large austenite grains through AGG compromises the fatigue performance of carburized steels. AGG is further exacerbated by cold work introduced into the alloy prior to carburizing. Warm work is also sometimes utilized in part forming prior to carburizing. In this study, the effects of warm work on AGG were investigated. AISI 4121 and a modified AISI 4121 that contains Nb and Mo microalloying additions rather than Al for grain size control were warm worked in a range of 0-50% at a temperature of 900°C and then heated in a furnace for various lengths of time at a temperature of 930 °C to simulate a carburizing thermal history. The average prior austenite grain size (PAGS) tended to decrease as the degree of warm work increased, with the NbMo-modified alloy presenting a finer PAGS at all percentages of warm reduction and different lengths of time at the simulated carburization temperature. Specimens of the 50% warm reduced condition were also cold rolled at 5, 10, and 25% reductions, typical of cold sizing, prior to simulated carburization. The average PAGS of these CR samples was finer than their 0% CR counterparts, but the PAGS increased with CR in the modified alloy after 328 minutes of simulated carburization.
