Interaction between the microstructure and the properties of plasma nitrided austenitic stainless steels

本研究项目致力于综合研究离子渗氮工艺、材料组织和所产生的性能之间的相互作用，特别是关于奥氏体不锈钢的腐蚀行为。初步结果表明，S相的厚度是决定耐磨性的主要因素。结果表明，在渗氮条件相同的情况下，随着冷轧或渗氮前喷丸变形量的增加，S相厚度增加，材料组织和表面状态对腐蚀性能有显著影响。特别是，塑性变形导致的缺陷密度增加降低了离子渗氮钢在盐雾试验中的耐蚀性。与普遍的现象学假设相反，降低处理温度可以改善腐蚀行为，研究发现，低温处理(370℃，960分钟)在盐雾试验中导致的腐蚀行为明显比高温处理(420℃，360分钟)差得多。在这两组参数下，S相的厚度都得到了很好的匹配。这一行为的原因在于沉淀的形成，导致基质中铬的耗尽。为了建立一个基于机制的描述析出机制的初始组织和处理参数，将进行系统的透射电子显微镜研究。此外，在H_2SO_4中的动电位极化测试表明，所有的材料状态(氮化和非氮化)都具有耐蚀性。另一方面，渗氮样品在氯化钠溶液中的动电位极化测试结果与盐雾测试结果在腐蚀行为方面是一致的。然而，所有未渗氮的样品都表现出比渗氮的样品更高的腐蚀电流。基于这些结果和所得出的假设，本研究的重点是通过微观组织和微观分析的方法，对微观组织的变化及其随组织和渗氮参数的变化的动力学进行基本的了解。在此基础上，另一个重点是确定标准化分析方法，以便在微观结构和由此产生的腐蚀性能之间建立关联。该项目的总体目标是制定一个概念性模型，描述微观结构、化学成分和渗氮过程之间的关系，以及由此产生的腐蚀和磨损行为。

The research project focuses on a comprehensive study of the interactions between the plasma nitriding process, the material microstructure and the resulting properties, especially concerning the corrosion behavior, of austenitic stainless steels. The preliminary results show that the wear resistance is dominated by the thickness of the S-phase. It was found, that with increasing degree of deformation due to cold rolling or shot peening prior to nitriding, the thickness of the S- phase increases with otherwise identical nitriding conditions .There were significant influences of the materials microstructure and the surface condition on the corrosion properties. In particular, the increase in defect density due to plastic deformation reduces the corrosion resistance of plasma nitrided steels in the salt spray test. Contrary to the prevailing phenomenological assumption, that a reduction of the treatment temperature improves the corrosion behavior, is was found that a low temperature process (370 °C, 960 min) causes significantly worse corrosion behavior in salt spray test than a high temperature process (420 ° C, 360 min). With both parameter sets good matching thicknesses of the S phase were obtained. The reason for this behavior is seen in the formation of precipitates, resulting in a chromium depletion of the matrix. In order to formulate a mechanism based description of the precipitation mechanism in dependence of initial microstructure and treatment parameters, systematic TEM investigations will be carried out. Furthermore, potentiodynamic polarization tests in H2SO4 reveal, that all material states (nitrided and unnitrided ) were resistant to corrosion. Potentiodynamic polarization tests of nitrided sampels in NaCl, on the other hand, are in good accordance with the salt-spray test results in terms of the corrosion behavior. However, all unnitrided samples showed systematically a higher corrosion current than the nitrided samples.Based on the results and the derived hypotheses, the focus of the proposal is to gain a fundamental understanding by means of microstructural and microanalytical studies concerning the microstructural changes and their kinetics as function of microstructure and nitriding parameters. Based on this, another focus is the identification of standardized analysis methods which allow for a correlation between the microstructure and the resulting corrosion properties. The overall objective of the project is to formulate a conceptual model, which describes the relationship between microstructure, chemical composition and the nitriding process on the resulting corrosion and wear behavior.