Response to thermal cycling of plasma nitrided hot work tool steel at elevated temperatures

Thermal fatigue performance of plasma nitrided hot work tool steel was investigated under conditions encountered by thixoforging dies in semi-solid processing of steels. Plasma nitriding does not offer any improvement in the thermal fatigue performance of hot work tool steels at elevated temperatures, due essentially to poor resistance to oxidation and to temper softening. Fe3O4 and Fe2O3 scales produced on the nitrided surface fail to sustain the thermal stresses produced by thermal cycling. They spall off, generating fresh surfaces for further oxidation. This sequence leads to substantial material loss and impairs the integrity of the surface beyond a quality level that would be tolerated in steel thixoforming. The surface hardening provided by plasma nitriding is also completely erased. The tempered martensitic structure is replaced by fine, equiaxed ferritic grains implying a dynamic recrystallization process during thermal cycling.