Isothermal oxidation of plasma-nitrided hot-work tool steel at 750 degrees C

Isothermal oxidation of plasma-nitrided tool steel at 750 degrees C, typical of steel thixoforming process, was investigated. A multi-layered oxide forms on the surface of plasma nitrided X32CrMoV33 hot work tool steel when held at 750 degrees C in an air furnace for 6 hours. The hematite layer at the surface is replaced by magnetite in the middle. The innermost layer is an iron chromium spinel with approximately 4 at% Cr. These features are typical of oxidised low-Cr steels and suggest that nitriding does not affect the response to thermal exposure of hot work tool steels at elevated temperatures. It is thus concluded that hot-work tool steels are inadequate for tooling in the steel thixoforming process even when plasma-nitrided. The substrate tool material must contain higher levels of Cr to assure a slowly growing, adherent Cr2O3 layer to reduce the overall oxidation rate.