Sirosh V. Formation and decomposition mechanism of isothermal martensite in Fe-C alloys

Using dilatometry, Mossbauer spectroscopy, X-ray diffraction, mechanical spectroscopy, transmission electron microscopy, scanning electron microscopy for the integrated studies of carbon steels, it is established that isothermal martensitic transformation at low temperatures decisively affects decomposition of martensite during subsequent heating. It was found that isothermal martensitic transformation occurs in carbon steels in the temperature range from -100 °C to -170 °C with a maximum intensity at -150 °C. It is established that isothermal martensitic transformation in carbon steels is accompanied by plastic deformation because of the low strength (hardness) of virgin low temperature martensite. Abnormally low tetragonality of the low temperature martensite obtained in the course of isothermal martensitic transformation is attributed to the binding of carbon atoms by dislocations moving during plastic deformation. It is established that plastic deformation in the course of isothermal martensitic transformation suppresses the precipitation of intermediate "epsilon"-carbides and shifts precipitation of cementite and special carbides during subsequent heating of martensite to higher temperatures. It is established that, after isothermal martensitic transformation and tempering in the range of secondary hardness, the concentrated state of the "alpha"-solid solution is characterized by clusters of carbon atoms with carbide-forming elements. This solid solution possesses a significant potential for precipitation of special carbides. Based on the studies of the effect of isothermal martensitic transformation on the precipitation of special carbides and the concentration state of the -solid solution, it is assumed that the stress-induced precipitation of special carbides takes place in the course of the service of steel tools, which constitutes the essence of the positive impact of deep cryogenic treatment of tool steels on their wear resistance.