Influence of nitrogen on the cavitation erosion resistance of martensitic stainless steel

ABSTRACT The addition of nitrogen on stainless steels has been studied as an alternative to the production of materials which combine good mechanical properties, corrosion and wear resistance. Some nitrogen enrichment methods are supported by diffusive processes such as low and high temperatures nitriding. Among these processes, it is worth mentioning the Solution Heat Treatment after Plasma Nitriding (SHTPN), which consists in performing plasma nitriding, with the purpose of adding nitrogen to the material, followed by a solubilization heat treatment, which dissolves the nitrates, causing the nitrogen to appear in interstitial solid solution. In this paper, the SHTPN technique was applied to ASTM CA-6NM martensitic stainless steel. Plasma nitriding was performed at 630 °C for 3 hours and three different solubilization conditions were used (1200 °C for 30 minutes, 1200 °C for 1 hour e 1100 °C for 1 hour). The samples were analyzed through x-ray diffraction, optical microscopy and microhardness measurements in the transverse section of the sample. All samples had an increase in surface hardness of ASTM CA-6NM steel, and no stabilization of the austenite phase was observed in the surface area richer in nitrogen. The cavitation experiment was conducted according to the ASTM G32/09 standard, using the indirect method, in which the sample is not directly fixed on the vibrating horn. The cavitated area of the samples was analyzed by scanning electron microscopy. The treatment increased cavitation erosion resistance of all samples and differences were observed among the various solubilization conditions.

摘要：针对不锈钢中添加氮元素的相关研究已较为广泛，其目标在于开发兼具优异力学性能、耐蚀性与耐磨性的替代材料。部分氮元素富集工艺依托扩散过程实现，例如低温与高温渗氮。其中，等离子渗氮后固溶处理（Solution Heat Treatment after Plasma Nitriding, SHTPN）工艺具有较高的应用与研究价值，该工艺先通过等离子渗氮为材料引入氮元素，随后进行固溶热处理，使氮化物溶解，让氮元素以间隙固溶体的形式存在于材料基体中。本研究将SHTPN工艺应用于ASTM CA-6NM马氏体不锈钢。实验参数设置如下：等离子渗氮工艺为630℃保温3小时，随后设置三种不同的固溶处理条件，分别为1200℃保温30分钟、1200℃保温1小时以及1100℃保温1小时。通过X射线衍射（X-ray diffraction）、光学显微镜（optical microscopy）以及试样横截面显微硬度测试（microhardness measurements）对试样进行表征分析。结果显示，所有试样的表面硬度均较原始ASTM CA-6NM钢有所提升，且在氮元素富集的表层区域未观察到奥氏体相的稳定存在。本研究参照ASTM G32/09标准采用间接法开展空蚀实验，该方法下试样不直接固定于振动变幅杆上。采用扫描电子显微镜（scanning electron microscopy）对试样的空蚀区域进行形貌分析。实验结果表明，该处理工艺可显著提升所有试样的抗空蚀性能，且不同固溶处理条件下的抗空蚀性能存在明显差异。