Oxide scale damage and spallation in P92 martensitic steel

Abstract

9 Cr martensitic steels are widely used in high temperature steam environments for their combination of creep strength and oxidation resistance. These materials are pushed to their limit in the quest for higher efficiencies in boilers and steam turbines and loss of the protective oxide scale may result in premature failure of critical components.The major cause of oxide scale spallation is a temperature drop. The mechanism behind the spallation process is the generation of internal stresses in the oxide scale due to the mismatch of thermal expansion coefficients between the substrate and the haematite, magnetite and spinel layers in the oxide scale. These scales can be partially relaxed due to creep processes if the cooling rate is sufficiently slow.Specimens of P92 have been oxidised in flowing steam for periods up to 2000 h and cooled at a linear rate of 100°C h–1. Acoustic emission (AE) of the samples was monitored during the oxidation and cooling periods. Specimens were sectioned and prepared metallographically and the damage in each layer of the oxide scale was examined. The spall from the specimens was also collected and characterised.The damage observed in the oxide scale and the AE signals have been explained by comparison with a simple stress analysis arising from thermal expansion mismatch. Future work will extend the modelling approach to incorporate creep relaxation effects.