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

该研究项目的重点是全面研究奥氏体不锈钢的等离子渗氮工艺、材料微观结构和由此产生的性能之间的相互作用，特别是关于腐蚀行为。初步结果表明，耐磨性主要由S相的厚度决定。结果表明，在相同的渗氮条件下，随着渗氮前冷轧或喷丸变形量的增加，S相的厚度增加，材料的显微组织和表面状态对腐蚀性能有显著影响。特别地，由于塑性变形导致的缺陷密度的增加降低了盐雾试验中等离子体渗氮钢的耐腐蚀性。与普遍的现象学假设相反，即降低处理温度改善腐蚀行为，发现低温过程（370 °C，960 min）在盐雾试验中比高温过程（420 ° C，360 min）导致显著更差的腐蚀行为。与这两个参数集得到良好的匹配厚度的S相。这种行为的原因在于沉淀物的形成，导致基质的铬耗尽。为了制定一个机制为基础的描述依赖于初始微观结构和处理参数的沉淀机制，将进行系统的TEM调查。此外，在H2SO4中的动电位极化测试表明，所有材料状态（氮化和未氮化）都耐腐蚀。另一方面，氮化试样在NaCl中的动电位极化测试与盐雾试验结果在腐蚀行为方面是一致的。然而，所有未氮化的样品表现出系统的更高的腐蚀电流比氮化samples.Based的结果和推导的假设，该建议的重点是获得一个基本的理解，通过显微组织和微观分析研究有关的显微组织的变化及其动力学的显微组织和氮化参数的函数。在此基础上，另一个重点是确定标准化的分析方法，允许微观结构和所得腐蚀性能之间的相关性。该项目的总体目标是制定一个概念模型，该模型描述了微观结构、化学成分和氮化工艺对腐蚀和磨损行为的影响。

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.