Surface Analysis of Austenitic Fe-18Cr-19Mn-C-N
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The work deals with an analysis of recently developed high strength austenitic stainless Cr-Mn-steel which uses (C+N) alloy concept. The Fe-18Cr-19-Mn-C-N steel powder was produced as a steel powder by the gas atomization technique. The advantages of an austenitic matrix are the very good persistence against erosive corrosion, good toughness, cold work hardening, high temperature and creep strength. However improved strength of the stainless austenitic steel is often compromised by worsening of its corrosion resistance. One way to increase the strength is by solid solution hardening with interstitial elements like carbon and/or nitrogen. The development of stainless steels has concentrated on nitrogen as alloying element as many positive effects have been observed. Nitrogen stabilizes the austenite, forms less precipitation and increases the persistence against corrosion. In the field of high strength stainless steels with high interstitial elements content it is the state of the art to combine carbon and nitrogen as alloying elements, which is called (C+N) alloying concept. Attempts to produce Fe-18Cr-19Mn-C-N steel parts out of the powder steel did not succeed until now. The high affinity of chromium and manganese to oxygen causes a layer of oxides on the powder surface that builds a diffusion barrier which slows down sintering. After sintering surface oxides remain in the product which also extremely decrease the performance of the steel. The aim of this work was to define the process that takes place during vacuum sintering of such materials. During the first step the powder in as received state was examined by surface sensitive analysis techniques. X-ray photoelectron spectroscopy and high resolution electron microscopy combined with EDX analysis were applied to characterize the composition of the powder surface and the morphology, distribution and composition of the contaminants. The results show an intense enrichment in manganese and oxygen on the as received surface indicating high surface coverage by manganese oxide. Thorough powder observation by high resolution SEM reveals a high surface coverage by fine particulate oxides with a size of about 50 nm that often form agglomerates with a size up to hundreds of nanometers. The next step was to trace changes in the powder surface composition by in situ heat treatment in a high vacuum furnace attached to XPS analysis chamber at temperatures between 500°C and 1000°C. Under the applied conditions iron reduction was already measurable after heat treatment at 500°C, chromium reduction at around 600°C and manganese reduction at around 700°C. After manganese oxide reduction significant manganese evaporation was detected after 700°C, which leads to a considerable depletion of the outer surface in manganese at high temperatures.
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