Effect of post-oxidation treatment on thermal fatigue behaviour of plasma nitrided hot work tool steel at elevated temperatures

The post-oxidation treatment at 500 °C produced a dense, adherent oxide film on the compound layer. This oxide layer, however, failed to improve the oxidation resistance and the plasma nitrided tool steel subsequently given a post-oxidation treatment failed in nearly the same fashion as the tool steel thermally cycled without the post-oxidation treatment. The thermal stresses acting on the surface during much of the thermal cycle lead to the spallation of the oxide scales, generating fresh surfaces for further oxidation. This sequence is repeated throughout thermal cycling leading to substantial material loss from the centre of the front face of the thermally cycled samples. The thermal fatigue test was terminated after only 500 cycles due to extensive surface damage before there was any evidence for thermal fatigue cracking. Much of the section has suffered considerable loss of mechanical strength due to thermal exposure. The tempered martensitic structure was replaced by fine, equiaxed ferritic grains near the surface. The martensite to ferrite transformation was accompanied by dynamic recrystallization. The selective recrystallization of the nitrided surface layer evidences the relatively higher stresses encountered at the surface.