Austenitization and precipitate dissolution in high nitrogen steels: an in situ high temperature X-ray synchrotron diffraction analysis using the Rietveld method

Samples of a high nitrogen martensitic stainless steel (0.2 N and 0.4 C) were investigated by means of a monochromatic high energy X-ray synchrotron beam during the heat treatments up to 1050 °C and 1075 °C, with a heating rate of 700 °C/h, and an isothermal stage of 45 min. The diffraction patterns were analysed using the approach of multiphase Rietveld refinement. The results were correlated with dilatometric measurements. In annealed state, the steel is composed of a ferritic matrix with carbides M23C6 and nitrides Cr2N. The austenitic transformation occurs in two steps: a short one where 70% of the ferrite is transformed and a long one where the transformation finishes slowly. The dissolution of the carbides M23C6 begins prior to the austenitic transformation and stops at the beginning of the isothermal stage, whereas the dissolution of the nitrides Cr2N begins at the same time as the austenitic transformation and stops after 20 min of isothermal holding.