Oxidation and oxide spallation of heat resistant cast steels for superplastic forming dies

The oxidation and oxide spallation behaviour of austenitic and ferritic heat resistant cast steels used for manufacturing superplastic forming dies is investigated in isothermal and cyclic conditions. Gravimetric, microstructural and mechanical approaches are utilised to address the oxidation and spallation kinetics, to determine the nature of oxides grown at the surface of the materials and to discriminate the different routes to spallation depending on the substrate metallurgical structure and the oxidation conditions. It is shown that both types of material develop similar oxide scales with similar kinetics. The ferritic alloys appear to be more resistant to oxide spallation than the austenic alloys. The decrease of the fracture energy at the interface substrate/oxide during oxidation resulting from an aggregation of interfacial defects, causes spallation by wedging and buckling for thin and thick oxide layers, respectively.