Influence of RT soaking on the stability of retained austenite in 72NiCrMo4 tool steel

Tool steels are used in a wide range of manufacturing applications including the automotive field. Their heat treatment implies a rigorous scheduling to avoid scarce properties and reliability related problems. Stabilization of retained austenite (RA) due to prolonged soaking between martensite start (Ms) and finish (Mf) leads to its more difficult transformation during tempering or even after subzero treatment. Aim of this research is to analyse the influence of different room temperature soaking periods (0, 5, 120h) on the stabilization of RA and the effect of an additional deep cryogenic treatment (DCT, -196°C, 30min) before tempering in a 72NiCrMo4-2 cold work steel. A set of samples has been isochronally tempered just after quenching, so to avoid RA stabilization. Dilatometry highlighted the three classical tempering stages, namely the precipitation of transition carbides (100-200°), the decomposition of retained austenite (250-300°C) and the precipitation of cementite from transition carbides and segregated carbon (200-450°C). DCT carried out just after quenching causes the almost complete transformation of RA, so that the expansion accompanying the II tempering stage was suppressed. The RT soaking stabilizes RA such that even DCT is no more sufficient to achieve a fully martensitic structure. Furthermore, the stabilization process also affects the early the tempering stages, in particular the precipitation of transition carbides.