M2N nitride phases of 9% chromium steels for nuclear applications

M₂N nitride phases of 9% chromium steels with an extra-low carbon content have been investigated using a transmission electron microscope and an energy-dispersive X-ray (EDX) spectroscopy. The steel samples were normalized for 1 h at 1050 °C and then tempered at 600-780 °C for 30 min to 5 h followed by an air cooling. Through the analyses of the electron micro-diffraction patterns and EDX data for the precipitate particles on the extracted carbon replica, two types of Cr-rich M₂N nitride phases with the same hexagonal structure but totally different lattice parameters, a = 2.80 Å/c = 4.45 Å and a = 7.76 Å/c = 4.438 Å, were determined in the steels. Four types of Cr-rich M₂N phases with different lattice parameters probably existed in the steels. The M₂N phase revealed a decrease in its Cr content, an increase in its V content as the tempering temperature was increased, and no obvious change in its content for the metal fraction with an increasing tempering time.