Conversion of MX nitrides to Z-phase in a martensitic 12% Cr steel |
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Authors: | Leonardo Cipolla Hilmar K. Danielsen Dario Venditti Paolo Emilio Di Nunzio John Hald Marcel A.J. Somers |
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Affiliation: | 1. Centro Sviluppo Materiali SpA, Rome, Italy;2. Department of Mechanical Engineering, Technical University of Denmark, DTU, Lyngby, Denmark;3. DONG Energy, Fredericia, Denmark |
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Abstract: | A 12% Cr model steel was designed with the purpose of studying the nucleation and growth of modified Z-phase, Cr(V,Nb)N. The model alloy develops Z-phase after relatively short ageing times and contains only nitrides of Cr, V and Nb. Interferences from the presence of carbides and the development of Laves phase were avoided by keeping the C, W and Mo contents as low as possible. Transmission electron microscopy and X-ray diffraction analysis of extracted particles were used to follow the evolutions of phase composition, phase morphology and phase fraction, particularly of the precipitation of Z-phase, during ageing at 600, 650 and 700 °C for up to 103 h. The development of Z-phase appears to be accomplished by the diffusion of Cr atoms into (V,Nb)N particles and their subsequent conversion into cubic or tetragonal Z-phase. Studies at various temperatures indicate that Z-phase development proceeds fastest at 650 °C and that Z-phase forms faster at prior austenite grain boundaries. |
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