The tempering of a low-carbon internally twinned martensite

Abstract

The first stage of tempering of high-carbon martensites results in the precipitation of the metastable epsilon carbide; in low-carbon steels, it is generally considered that epsilon carbide does not form during tempering and only cementite is observed. Since high-carbon steels have an internally twinned martensite morphology, whereas low-carbon martensite (with the exception of high-nickel steels) consists of laths, it is not clear whether the first stage of tempering is controlled by carbon content or by martensite substructure. Therefore, a study was made of the tempering characteristics of a low-carbon steel in which the martensite was internally twinned. The steel chosen for this investigation contained 28 per cent nickel and 0.1 per cent carbon. Dark-field transmission electron microscopy revealed the presence of carbide precipitates in specimens that had been tempered for 48 hr at 100°C. Selected-area diffraction established that these carbides were the h.c.p. epsilon carbide. This indicates that the first stage of tempering of martensites containing carbon may be more directly related to the martensite morphology than to the carbon content alone.