Microstructure and Tribological Properties of Laser Gas Nitriding Layers of 17-4PH Stainless Steel

17-4PH martensitic precipitation-hardening stainless steel has excellent comprehensive performance, high strength, toughness, and good corrosion resistance. As a result, it has wide application in industries such as aerospace, petrochemical, and nuclear. However, the poor wear resistance critically limits its application in friction conditions. Nitriding is a common surface protection process in which nitrogen atoms diffuse into the alloy matrix to harden the surface and improve wear resistance. This study aims to enhance the surface hardness and wear resistance of 17-4PH martensitic precipitation-hardening stainless steel through laser gas nitriding. Laser gas nitriding of 17-4PH stainless steel was carried out using a fiber laser in a nitrogen atmosphere. Nitrided layers were then formed on the steel surface using various laser powers. The microstructure and phase composition of the nitriding layer were analyzed by optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffractometer (XRD). The hardness of the nitriding layer was evaluated along the depth direction of the cross-section with a microhardness tester. The tribological properties of the substrate and the nitriding layer were tested with a multi-functional friction and wear tester. The wear scar morphology was analyzed by SEM to investigate the wear mechanism of both the substrate and the nitriding layer. The effect of laser gas nitriding on the substrate's microstructure, hardness, and wear resistance was systematically studied. Before nitriding, the sample tissue was tempered martensite. After laser nitriding, a nitrided layer was formed on the sample surface. This layer consisted of a melting zone of slate martensite and a heat-affected zone of tempered martensite. The hardness of the samples after nitriding was improved. The surface hardness of the nitrided layer was the highest at 3 000 W, reaching 415HV0.2, which was approximately 1.2 times higher than that of the substrate. The hardness within the nitrided layer decreased with depth. It was equal to the hardness of the substrate at a depth of 2.6 mm. The hardness of the nitrided layer was increased through two mechanisms: phase transformation strengthening, specifically the transformation of tempered martensite to slat martensite, and solid solution strengthening, where nitrogen atoms were incorporated into the matrix in a solid solution mode. The friction factors after nitriding were higher than those of the substrate, but the wear volume was reduced after nitriding compared with the substrate. The smallest wear volume was observed at 3 000 W laser power, which was 62% less than that of the substrate. Incomplete martensitic transformation at 2 500 W laser power and cracking of the nitrided layer at 3 500 W laser power both reduced the hardness of the nitrided layer so that the wear resistance at 2 500 W and 3 500 W laser power was less than that of the nitrided layer at 3 000 W laser power. The wear mechanism changed from being dominated by adhesive wear before nitriding to being dominated by abrasive wear after nitriding. The surface hardness and wear resistance of 17-4PH martensitic precipitation-hardened stainless steel surfaces are improved by laser nitriding. The nitrided layers prepared at 3 000 W power has high surface hardness and excellent wear resistance. This makes it suitable for a wide range of applications in frictional conditions. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.