In situ ESEM investigations of the oxide growth on hot work tools steel:: effect of the water vapour

Tempered martensitic steel modified AISI H11 is used in forging processes where tool failure can result from the combination of thermo-mechanical and chemical damage. A better knowledge of the oxidation mechanisms in this material could be useful for a better appreciation of its service behaviour and lifetime. the low chromium content of this Fe-Cr type steel allows the development of mainly Fe2-xCrxO3 oxides with corundum structure and leads to enhanced oxidation in the presence of water vapour. In situ FEG-ESEM images show the scale microstructural modifications during high temperature exposure, as well as the lateral growth of oxide particles. Together with GIXRD, SEM/EDS and SIMS analysis. FEG-ESEM also allows assessment of the H2O effect on oxidation behaviour during high temperature exposures (600 and 700degreesC). Water vapour induces either pores or crystallites size increase, favours faceted oxides particles with enhanced density at the highest partial pressure. At this microscopic scale. anisotropic growth of crystallites is observed, and size expansion rates are found to be linear and characteristic of each individual particle. Temperature acts principally on oxide film microstructure. Whatever the environment, homogeneous scale growth is observed at 600degreesC whereas the steel surface is heterogeneously covered by oxides at 700degreesC.