Cover Picture: Materials and Corrosion. 4/2015

Abstract

Cover:The composition of the high strength austenitic steel Sanicro 25 is characterized by a high chromium content and additions of tungsten, niobium, copper as well as significant levels of nitrogen. The steel microstructures in the bulk of the sample and the sub surface zones after exposure times of 1000, 3000 and 10000 hours in steam at temperatures in the range 600–750 °C were analysed by scanning and transmission electron microscopy as well as X‐ray diffraction. Additionally, the surface oxide scale formation as well as the oxidation induced sub surface microstructural changes were studied using depth profiling by secondary neutrals mass spectrometry. In the as‐received condition the only detectable precipitate was (Nb,Cr)N‐base Z‐phase. The oxidation rates were found to be only slightly higher than those of a typical nickel base wrought material such as alloy 617. In the subsurface zones depletion of scale forming elements was accompanied by enrichment of tungsten and copper rich phases. In spite of the fact that precipitate sizes were, especially after exposure at lower temperatures, quite small unequivocal identification of the phases present in the alloy microstructure was possible by combining EDX and WDX data with results from EBSD analyses. The cover picture illustrates how information about elements present in the various precipitates as analysed by EDX and WDX can be compared with compounds prevailing in the structure database ICSD. Subsequently, an EBSD mapping of the respective best fitting compounds was made. The EBSD mapping reveals precipitates of (Cr,Nb)N‐base Z‐phase, Cr2N, tungsten rich µ‐phase and M23C6 carbides of various composition.More information can be found in: J. Zurek, S.‐M. Yang, D.‐Y. Lin, T. Hüttel, L. Singheiser, W. J. Quadakkers, Microstructural stability and oxidation behaviour of Sanicro 25 during long term steam exposure in the temperature range 600–750 °C, Materials and Corrosion, 2015, 66, 315.