Cyclic oxidation of P91 at 1073, 1123 and 1173 K

Abstract

Cyclic oxidation behaviour of modified 9Cr-1Mo steel has been investigated under three temperatures of 1073, 1123 and 1173 K. The adherence characteristics of the scale were studied by the 'transient mass gain' method by employing a thermogravimetric balance. A cycle time of 7.2 ks (2 h) was employed and at each temperature the specimen was oxidised for 360 ks (100 h, 50 cycles). Experiments revealed that the scale is mostly adherent in an isothermal condition. However, on cooling, the thermal expansion differential between the oxide scale and the parent alloy caused the development of cracks leading to spallation. The threshold thickness of the scale for the initiation of cracks and spallation was evaluated. Post-oxidation examinations were carried out by using XRD, SEM and EDS. Oxides of the type Fe2O3, FeCr2O4 were predominant at the surface at 1123 and 1173 K. At the relatively lower temperature of 1073 K, in addition to the above oxides, the scale also contained Cr2O3. The interspace of the oxide/alloy also contained significant amounts of oxide of chromium. The reason for such distribution of oxide phases has been explained by invoking thermodynamic and diffusion concepts.