Surface segregation effects in duplex stainless steel during plasma diffusion treatments using a solid carbon precursor

• Applicants: Dr.-Ing. Anke Dalke; Dr. Darina Manova
• Subject Area: Coating and Surface Technology ; Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 557879687

The project investigates the effects of a solid carbon precursor on the surface modifications of duplex steel for plasma nitrocarburizing processes. The use of a solid carbon precursor offers an alternative method for nitrocarburizing of stainless steel. This novel process is particularly suitable for duplex steels, as it offers the possibility of significant reduction of the thermal impact on the steel. Nitrocarburizing process and the modified surface properties of duplex steel are generally less studied compared to austenitic stainless steels. The research project aims at the comprehensive study of the interactions between the solid carbon precursor and the resulting process gas composition as well as the surface layer modfication of the duplex steel and the resulting properties with special emphasis on the corrosion behaviour. The project is based on three working hypotheses: (i) Different carbon materials lead to the formation of different reaction gas compositions. (ii) Nitrocarburizing in afterglow (without plasma on the steel substrate) sometimes leads to the formation of a closed carbon-rich layer and a near surface segregation of alloying elements inside the steel substrates, that can influence diffusion processes and consequently the expanded surface layer formation. (iii) The carbon-rich layer can effect the surface properties, especially the corrosion mechanism. The project is a collaboration between the Institute of Materials Engineering (IWT) at TU Bergakademie Freiberg and the Leibniz Institute of Surface Engineering (IOM) in Leipzig. Their competencies will provide synergy effects for evaluating the working hypotheses. The resulting gas composition for different carbon materials under varying plasma conditions is analysed using laser absorption spectroscopy (LAS). Material engineering analysis including cross-sectional light optical microscopy and SEM investigation, glow discharge optical spectroscopy (GDOS) and phase specific hardness measurement is done. In-situ XRD measurements are carried out to elucidate the dynamics during the diffusion processes and the resulting layer structures as observed during sputter depth profiling. Isotope markers are used to identify microscopic mobility and transport processes by means of ToF-SIMS measurements. Taking into account the 475 °C embrittlement typical for duplex steels the temperature dependence of the layer formation is investigated using tensile tests. Mechanism-based corrosion investigations are carried out to evaluate the influence of a carbon-rich layer and a changed nitrogen-carbon distribution in the expanded surface layer. The clarification of the process parameter-structure-property correlation using the complementary methods form both research groups should contribute to a fundamental knowledge regarding the interactions of a plasma-discharged carbon material and the surface properties of nitrocarburized duplex steels.

• DFG Programme: Research Grants