Alloying and Microstructure Design for Nitriding Steels

The influences of Si and V contents on the nitrided case properties were evaluated for 0.4% C steels with martensite and bainite starting microstructures. Alloys were designed based on two levels of Si content (0.73 and 1.53 wt pct) and two levels of V content (0.1 and 0.23 wt pct) in a full factorial matrix. In order to form martensite, samples of each alloy were austenitized at 1050 °C for 30 min and oil quenched. In order to form bainite, samples of each alloy were austenitized at 1050 °C for 30 min and quenched into molten salt at a temperature just above the martensite start temperature. Each alloy and starting microstructure was tempered at 500 °C for 1 h prior to nitriding. Gas nitriding was conducted at 525 °C for 18 h. Multiple characterization techniques such as transmission electron microscopy, scanning electron microscopy, electron backscatter diffraction, and x-ray diffraction were employed to elucidate the effect of Si and V on strengthening mechanisms in the nitrided case. V increased the hardness of the case primarily through precipitation hardening, while Si increased the case hardness through a combination of solid solution strengthening and numerous other changes to the microstructure.