The Effects of Thermomechanical Pretreatment on Abnormal Grain Growth of AISI 4121 Steel During Carburization
The Effects of Thermomechanical Pretreatment on Abnormal Grain Growth of AISI 4121 Steel During Carburization
Tuesday, October 17, 2023: 11:10 AM
313 AB (Huntington Convention Center)
Carburization is 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
significantly 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 both warm work and cold work on AGG were investigated. AISI 4121 and a modified AISI 4121 that utilizes Nb and Mo microalloying additions 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) generally decreases as the degree of warm work increases, with the NbMo-modified alloy having a finer PAGS at all percentages of warm reduction and 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, in the Nb-modified alloy, the PAGS increased greatly with CR compared to the standard alloy, reflecting more AGG.
AGG, the microstructure undergoes bimodal grain growth with some grains growing
significantly 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 both warm work and cold work on AGG were investigated. AISI 4121 and a modified AISI 4121 that utilizes Nb and Mo microalloying additions 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) generally decreases as the degree of warm work increases, with the NbMo-modified alloy having a finer PAGS at all percentages of warm reduction and 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, in the Nb-modified alloy, the PAGS increased greatly with CR compared to the standard alloy, reflecting more AGG.