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

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.