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In this report, the surface properties and microstructure of chromium-molybdenum steel samples prepared by two-stage gas nitriding with a short isothermal time in stage one are compared with those prepared by one-stage gas nitriding. Two-stage gas nitriding was performed with a short isothermal time in stage one followed by a second stage with lowered NH3 partial pressure, while one-stage gas nitriding was performed under conventional conditions. The variation in microstructure, compound layer thickness (CL), nitrided case depth (d) and surface hardness (HVs) was clarified. In one-stage gas nitriding, the CL, d and HVs increase with increasing isothermal time. In contrast, in two-stage gas nitriding, the CL decreases with isothermal time in the second stage, and surface microstructure observations show partial dissipation of the compound layer. The d and HVs increase at a lower rate of increase than the values observed in one-stage gas nitriding. However, when the second stage temperature was increased in two-stage gas nitriding, the CL decreases and partially dissipates in a shorter time, and d increases at a faster rate than those observed for one-stage gas nitriding. The HVs exhibits a faster rate of increase in the second stage when a higher temperature was used, but the rate is lower than that observed in one-stage gas nitriding. These experimental results are briefly discussed in relation to the microstructure. [doi:10.2320/matertrans.M2011397]

Surface Hardening and Microstructural Features of Chromium-Molybdenum Steel via Two-Stage Gas Nitriding with a Short Isothermal Time in Stage One