The Influence of the Chemical Composition of Steel on the Limitation of Austenite Grain Growth in the High-Temperature, Low-Pressure Carburizing Process

High-temperature low-pressure carburizing significantly reduces the time required to produce carburized layers. However, its application promotes austenite grain growth and, consequently, the formation of coarse acicular martensite. In this study, the possibility of limiting this phenomenon in AMS 6265 and 18CrNiMo7-6 steels using the PreNitLPC® technology was evaluated. The process was carried out at 1050 °C, with pre-nitriding applied during charge heating. In both steels, comparable carbon concentration profiles and carburized layers with an effective case depth of approximately 1.0 mm were obtained. The introduction of nitrogen into the surface layer resulted in a local reduction in austenite grain growth compared with the core. The average grain size in the surface layer was approximately 14.5 µm for AMS 6265 steel and 12.5 µm for 18CrNiMo7-6 steel, whereas in the core it increased to approximately 25.1 µm and 24.1 µm, respectively. At the same time, AMS 6265 steel exhibited a higher fraction of retained austenite, approximately 20%, compared with approximately 15% for 18CrNiMo7-6 steel. This resulted in a lower near-surface hardness of AMS 6265 steel, approximately 750 HV0.1, compared with approximately 800 HV0.1 for 18CrNiMo7-6 steel, corresponding to a hardness difference of about 50 HV0.1. TEM/NBD/EDS investigations showed that nanoscale AlN precipitates formed in both steels and acted as the main factor inhibiting austenite grain-boundary migration. The results confirm that the PreNitLPC® technology enables high-temperature low-pressure carburizing without detrimental grain growth in the surface layer.