The Effects of Thermomechanical Pretreatment on Abnormal Grain Growth of AISI 4121 Steel During Carburization

Carburization 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 carburization 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 carburization. Warm work is also sometimes utilized in part forming prior to carburization. In this study, the effects of warm work on AGG were investigated. AISI 4121 and a modified AISI 4121 that utilizes 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) tends to decrease as the degree of warm work increases, 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. Compared to cold reductions (CR) of the same percentages, the average PAGS was larger in these samples. The 50% warm reduced condition was also cold rolled at 5, 10, and 25% reductions prior to simulated carburization. The average PAGS of these CR samples were lower than their 0% CR counterparts and demonstrated a high correlation in the modified alloy between CR and average PAGS, increasing proportionally.