Microstructure and cyclic oxidation behavior of modified Nb-alloyed A297 HH refractory austenitic stainless steel

Abstract The modified ASTM A297 Gr. HH austenitic stainless steels are a group of refractory alloys designed for high temperature applications that required elevated mechanical strength, such as those found in petrochemical furnace structures and in the steel industry. This study examines the overall microstructural features and cyclic oxidation behaviour of modified HH steels with 0.09%Nb and 0.77%Nb additions. The investigations involved thermodynamic simulations and microstructure analyses conducted by optical and scanning electron microscopy of the substrate materials in the as-cast and aged conditions. The cyclic oxidation tests were performed at 900 °C in atmospheric conditions with oxidation times up to 960h. The resulting oxide scales were analyzed by X-ray diffraction and scanning electron microscopy. The thermodynamic simulations and microstructure analyses of the as-cast steels revealed larger amounts of niobium carbides in the 0.77%Nb steel than in the 0.09%Nb steel, in connection with larger amounts of chromium in solution. Despite this precipitation difference, no significant changes were noticed in the long-term oxidation behaviour of the tested materials.