Effect of vanadium addition on the creep resistance of 18Cr9Ni3CuNbN austenitic stainless heat resistant steel

•The first observation of V-rich Z-phase in austenitic stainless steel.•Separate precipitation of Nb-rich MX and V-rich Z-phase in Nb and V contained steel.•The effect of nanometer sized Z-phase on the improvement of creep strength.•The predicted size of MX precipitates by MatCalc agreed well with measured size. The effect of vanadium addition on the creep property of niobium-containing 18Cr–9Ni austenitic heat-resistant steel was studied. After solution treatment, the MX precipitates of vanadium-free steel contained Nb only. On the contrary, high Nb and low V contents were detected from the MX precipitates in the steel with 0.3wt% of vanadium. Vanadium-rich MX carbo-nitride was not observed in the matrix and at the grain boundaries after solution treatment. The vanadium precipitated as a form of Z-phase during early-stage creep deformation was attributed to the improvement in creep strength at 700°C with applied stress higher than 150MPa. And metallic Cu precipitates were confirmed in the nanometer scale using TEM and EDS technique. The Cu precipitates are believed to contribute to the strengthening of austenitic heat resistant steel independently. The precipitation behavior is discussed using transmission electron microscopy and thermo-kinetics simulation technique.