Pengcheng Zhu

The oxidation behavior of 316L stainless steel exposed at 400, 600 and 800 °C air for 100, 500 and 1000 h was investigated using different characterization techniques. Weight gain obeys a parabolic law, but the degree of deviation of n index is increasingly larger with the increase of temperature. A double oxide film, including Cr₂O₃ and Fe₂O₃ oxide particles in outer and FeCr₂O₄ oxides in inner, is observed at 400 °C. As regards to samples at 600 °C, a critical exposure period around 100 h exists in the oxidation process, at which a compact oxide film decorated with oxide particles transforms to a loose oxide layer with a pore-structure. In addition, an oxide film containing Fe-rich outer oxide layer and Cr-rich inner oxide layer is observed at 600 °C for 500 and 1000 h. Spallation of oxide scale is observed for all samples at 800 °C regardless of exposure periods, resulting in different oxidation morphologies, and the degree of spallation behavior is getting worse. A double oxide film with the same chemical composition as 600 °C is observed, and the thickness increases over exposure periods.

Precipitate phases in the P92 F/M steel after normalization at 1323 K (1050 °C) for 30 min followed by air cooling to room temperature have been investigated by transmission electron microscopy (TEM). Three types of phases consisting of M₃C, MX and M₂C have been identified. Needle-like Fe-rich M₃C carbide, (Fe_0.87Cr_0.13)₃C, has a simple orthorhombic lattice, and a size of 7-18 nm in short axis and 73-190 nm in long axis. Sphere-shaped Nb-rich MC carbide with a f.c.c. crystal structure has a chemical formula of about (Nb_0.69V_0.15Fe_0.1Mo_0.06)C and a size of 12 to 88 nm in diameter. Large-sized Fe-rich M₂C phase exhibits a block-like shape, and has a hexagonal crystal structure. TEM examination indicated that the number density of both the Fe-rich M₃C carbide and Nb-rich MX phase is higher than Fe-rich M₂C carbide phase.