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

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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.