Effect of interstitial solutes on precipitation behavior of 9–12% chromium ferritic steels

Effects of nitrogen and carbon content on precipitation behavior in two 9–12% Cr ferritic heat-resistant steels during isothermal transformation were investigated. Isothermal aging treatments at 700 °C after solution annealing were carried out for the different periods up to over 220 h. Microstructure characterization was performed by many characterization methods, such as physicochemical phase analysis, X-ray diffraction and scanning electron microscope etc. It was found that Cr-rich M2N nitride was formed as the main precipitate in the alloy A with content of 0.15% N and 0.03% C in wt%, together with some Cr-rich M23C6 carbide and Nb-rich MN nitride. Comparatively, the aged alloy B with 0.05% N and 0.11% C, predominantly contains Cr-rich M23C6 carbides. Besides, both alloys showed the similar microstructure evolution process: The precipitates were found to be formed initially along prior austenite grain boundaries, then grew toward interior of grain in the form of cell with the increasing aging time. Meanwhile, the cell growth feature was also discussed based on the experimental observation in well-controlled specimens.