Oxidation behaviors of the rotary friction welded joints of HT700 superalloy in saturated water steam at 700

•The main components of oxide films on the surface of the as-welded and PWHT joints are NiCr2O4 and Cr2O3.•At the early stage of oxidation, the oxidation resistance of the PWHT joint was better.•At the later stage of oxidation, the oxidation resistance of the as-welded joint was more remarkable.•The grain size determines the oxidation rate. The oxidation behavior of welded joints with or without heat-treatment of HT700 alloy was studied in 700 ℃ saturated water vapor with regards to oxidation kinetics, oxide film composition, and microstructure. The results show that the oxide film compositions of the as-welded joints and heat-treated welded joints were basically the same. The oxide film is composed of a small amount of iron oxide, NiCr2O4, and a large amount of Cr2O3 from the outer layer to the inner layer, with a small amount of internal oxidation products Al2O3. At the beginning of the oxidation process, the double layer oxide film grew at the same time, so that the as-welded joint had better performance in the later stage of oxidation. The formation of a continuous internal oxidation layer was later in heat-treated welded joints as compared to the as-welded joint, resulting in a larger thickness of the oxide film. The oxidation behavior of the welded seams in different regions of as-welded state and heat treatment state after welding displayed the following characteristics: small grain size, thin oxide film, and the weld fusion line with thin oxide film.