17-4PH不锈钢激光气体渗氮层显微组织与摩擦学性能

目的 提高17-4PH马氏体沉淀硬化不锈钢的表面硬度及耐磨性。方法 采用光纤激光器对17-4PH不锈钢进行激光气体氮化，采用不同激光功率在其表面制备渗氮层。利用光学显微镜(OM)、电子扫描显微镜(SEM)和X射线衍射仪(XRD)等设备分析渗氮层的显微组织和相组成;借助显微硬度仪测试渗氮层截面深度方向的硬度;采用多功能摩擦磨损试验机测试基体、渗氮层的摩擦学性能，并通过SEM分析磨痕形貌，揭示基体与渗氮层的磨损机制。结果 在渗氮前样品组织为回火马氏体，经激光渗氮后样品表面形成了由板条马氏体组成的熔化区和回火马氏体组成的热影响区构成的渗氮层。经渗氮后，样品的硬度均得到提高。在激光功率3 000 W下，渗氮层的表面硬度最高，达到了415HV0.2，约是基体硬度的1.2倍，渗氮层的硬度随着深度的增加呈下降趋势，在深度为2.6 mm处其硬度与基体一致。在回火马氏体向板条马氏体转变的相变强化，以及氮原子(以固溶方式进入基体)的固溶强化作用下，提高了渗氮层的硬度。经渗氮后，样品的摩擦因数均高于基体，但渗氮后其磨损量相较于基体有所减少，在激光功率3 000 W下，其磨损体积最小，相较于基体减少了62%。在激光功率2 500 W下马氏体转变不完全，在激光功率3 500 W下渗氮层出现了裂纹，都降低了渗氮层的硬度，其耐磨性也随之减小，且都略低于在3 000 W下。磨损机制由渗氮前的以黏着磨损为主，转变为渗氮后的以磨粒磨损为主。结论 在17-4PH马氏体沉淀硬化不锈钢表面进行激光渗氮后，其表面硬度和耐磨性均得到提高，在激光功率3 000 W下制备的渗氮层具有较高的表面硬度和优异的耐磨性。

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.