Surface and image analysis of oxides grown and spalled on heat resistant cast steels exposed to thermal cycles

Heat resistant cast steels, used as tools for sheet superplastic forming, are oxidized in near-service thermal cycling conditions. Data from 3D roughness measurements and scanning electron microscope (SEM) micrographs are processed using image analysis software. The surface spacing parameters, the distribution and nature of the various spalled oxides as well as the surface recession of the metal substrate are investigated with respect to high temperature exposure time and related number of cycles. Based on this analysis, the nucleation, growth and spallation of oxides due to thermal stresses occurring upon cooling is discussed. It is shown that the amount of spalled oxide increases with the exposure time. Spallation results either from adhesive or cohesive rupture of the oxide. Texture of the spalled cells, strong for short exposure time, tends to vanish when the exposure time increases. Surface recession of the substrate is successfully calculated.