Vanadium nitride precipitate phase in a 9% chromium steel for nuclear power plant applications

Vanadium nitride precipitate phase in a 9% Cr steel was observed and analyzed using transmission electron microscopy and energy dispersive spectroscopy. The steel samples were normalized at 1050 and 1100 °C for 1 h and then tempered at 750 °C for 30 min to 5 h followed by an air cooling. Through the microdiffraction pattern analyses and energy dispersive X-ray data, two kinds of vanadium nitride precipitates were determined to be (V_0.6Nb_0.2Cr_0.2)N and (V_0.45Nb_0.45Cr_0.1)N with the same fcc crystal structure and different lattice parameters ā = 4.070 and 4.232 Å, respectively. Lattice parameters estimated for the precipitates regarding the VN phase agree well with the present data from the microdiffraction patterns, indicating that the precipitates do not belong to the VC phase. Observed (V_0.45Nb_0.45Cr_0.1)N precipitates consisted of undissolved particles remaining after a normalizing and the particles newly precipitated during a tempering, whilst, the observed (V_0.6Nb_0.2Cr_0.2)N precipitates were formed during a tempering. These two vanadium nitrides seem to be a stable phase, and not an intermediary phase.