Neutron Diffraction Study of Kinetics of Low Temperature Decomposition of Martensite in Medium Carbon Steel

It has been established that low temperature decomposition of martensite in quenched medium carbon steel takes place in two steps. At the first step, the decomposition intensity is higher than at the next step. The use of neutron diffraction made it possible to reveal two stages of transformation at the first step of martensite decomposition. It is demonstrated that the first stage is related mainly to carbon segregation on dislocations and the second stage is related to escape of carbon from oversaturated solid solution with formation of disperse particles of metastable carbides. The degree of tetragonality of the martensite lattice changes during aging and low tempering to a certain limit irrespective of cooling rate during quenching and tempering temperature, which is related to establishment of relative equilibrium between oversaturated solid solution and disperse particles of metastable iron carbide. It has been established that the determining process leading to changes in microhardness upon low temperature decomposition of martensite is escape of carbon from oversaturated solid solution.