AUTHOR/S: H.K. Danielsen (DTU) P.E. Di Nunzio (CSM) J. Hald (DTU, DONG Energy)


The Z-phase nitride is seen as a detrimental phase in 9-12%Cr steels as it is in competition with beneficial MX particles. Two model steels, with 9%Cr and 12%Cr content respectively, were manufactured in order to follow the effect of Cr on Z-phase precipitation kinetics. No C, W or Mo content was included as to avoid other precipitate types than nitrides. The steels were annealed at 600°C, 650°C and 700°C for up to 30,000 h in order for Z-phase to replace MX. X-ray Diffraction (XRD) analysis of extracted precipitates was used to quantitatively follow the evolution of the nitrides population. It was found that the 12%Cr steel precipitated Z-phase 20-70 times faster than in the 9%Cr steel. Transmission Electron Microscopy (TEM) was applied to follow the Z-phase precipitation, using EDS line scans and atomic resolution imaging. It could be observed that the MX particles compositionally and crystallographically transform into Z-phase by Cr in diffusion from the matrix.

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