Isothermal martensitic transformation in a tool steel and its effect on carbide formation under tempering

Abstract

Isothermal martensitic transformation in X153CrMoV12 and X220CrMoV13-4 tool steels and carbide formation under subsequent tempering are studied, using Mössbauer spectroscopy, X-ray analysis, mechanical spectroscopy, and transmission electron spectroscopy. As shown, the martensitic transformation is accompanied by plastic deformation, which is caused by the absence of ageing, if the martensite is formed at low temperatures. In contrast to the available hypotheses of the abnormally low tetragonality of isothermal martensite, it is attributed to the capture of carbon atoms by moving dislocations. Plastic deformation during isothermal martensitic transformation changes the carbide precipitation under subsequent tempering in the following way: (i) the precipitation of the intermediate ε (ε') carbide does not occur; (ii) precipitation of cementite particles and their growth are delayed; (iii) precipitation of special carbides is delayed or completely prevented. According to results obtained, the optimal temperature-time technology of deep cryogenic treatment (DCT) of the tool steels is proposed. The assumption is made that the increase in their wear resistance caused by DCT is caused by the strain-induced precipitation of special carbides under operation of a work tool.