ON THE AGING OF Fe-Ni-C AND Fe-Mn-C MARTENSITES

Abstract
Virgin and aged martensites were studied by dilatometry and Fe⁵⁷ Mössbauer spectroscopy. Dilatometric measurements were made between -80°C and +100°C with one high carbon manganese steel and three nickel steels with subzero M_s - temperatures. For nickel steels it was found that the aging curves, the change of length vs. log(t), were linear at temperatures below +40°C, but at higher aging temperatures the curves were found to consist of two linear parts. The changing of the slopes occurred at shorter times the higher was the aging temperature. In case of the manganese steel the aging curves were not linear at any temperature. Some proposals concerning clustering and precipitation processes are made. The Mössbauer spectra were measured at liquid nitrogen temperature by transmission method. A new fitting model was developed based on the utilization of Fe-Ni reference spectra. The fitting function consists of a great number of these patterns corresponding to every possible neighbour configuration of the iron atom up to the third neighbour shell. Each pattern is modified by the changes in hyperfine interactions due to the carbon atoms. In virgin martensite the carbon atoms could be explained to be distributed apart from each other rather than randomly and to cluster during the aging. The present fitting model of Mössbauer spectra produces explicit values for carbon contents and the fractions of the cluster phases. Carbon contents well correspond to the structures, which are expected to be found according to the aging temperature. From the fitting results the volume changes of the martensite during the aging were calculated. The volume changes were compared with the values measured by dilatometry. Agreement was promising.