Tailoring the microstructure and mechanical properties of the Cr–N modified layer on 316L steel via chromizing process parameters

Chromium–nitrogen (Cr–N) surface modification is an effective approach to enhancing the surface mechanical properties of structural materials. However, the influence of chromizing process parameters on the resultant microstructure and mechanical properties remains insufficiently understood, particularly for 316L stainless steel intended for nuclear applications. In this study, a combined chromizing and nitriding surface treatment was applied to 316L stainless steel. The effects of chromizing temperature (1000 °C, 1050 °C, and 1100 °C) and duration time (12 h, 15 h, and 18 h) on the microstructure, phase composition, crystallographic orientation, interfacial phase constitution, and mechanical properties of the Cr–N modified layer were systematically investigated. Under all processing conditions, the modified layer consists of both Cr 2 N and CrN phases and exhibits good metallurgical bonding with the substrate. Chromizing temperature is the dominant factor controlling layer thickness, which increases from 65.5 μm to 184.6 μm with rising temperature. A distinct grain-size gradient along the depth direction is observed. TEM analysis identifies a BCC-structured Fe-rich matrix adjacent to Cr 2 N precipitates, while EDS reveals local Cr depletion. Hardness peaked at approximately 1115 HV 0.1 at 1100 °C/12 h, but declined with extended holding duration time at high temperatures. Scratch tests indicate that the layers prepared at higher temperatures possessed better ploughing resistance. Overall, the chromizing temperature is the key parameter governing the thickness, grain size, compactness and hardness of the modified layer.