Bryansk State Technical University (Bryansk, Russia):

S. V. Davydov, Dr. Eng., Prof., Dept. of Tribotechnical Materials Science and Technology of Materials, E-mail: fulleren_grafen@mail.ru

In the present work low-temperature carbide phase transformations in the system of Fe-C alloys based on ε-сarbide Fe₂C with consideration of identification of θ-Fe₃C cement as a solid solution were studied. It has been proved that the θ-Fe₃C cement slurry is colourfastonide, and the ε-Fe₂C carbide slurry is bertollide. When tempering hardened steels, ε-сarbide Fe2C is emitted in the structure of hardened martensite, which is absent in the phase diagram of iron-carbon system alloys. It is believed that ε-сarbide Fe₂C is not a stable phase, and since it is metastable, it is formed only in quenched steels under non-equilibrium conditions. The isolation and dissolution of ε-сarbide Fe₂C is a classic phase transformation and the absence of this transformation on the diagram is not caused by the metastable phase of ε-сarbide Fe₂C, but by the incomplete iron-carbon diagram. The martensite decomposition phase transformation is based on the formation of carbon enriched zones. The processes of carbon segregation on dislocation structures and grid planes of martensite create zones with excess energy. Beginning approximately with temperature 100 °С in structure of martensite begins to allocate ε-сarbide Fe₂C, finishing a stage of two-phase segregational disintegration of martensite. At rather small concentrations of carbon in cluster zones the fastest and most effective way of relaxation of redundant energy in these zones, as well as in the tetragonal lattice of martensite is the formation of phases with low value of work of nucleation, first of all ε-сarbide Fe₂C and α-Fe(C) or ferrite. The main stages of phase transformations in the peritectoid reaction of martensite decomposition are considered. It is proposed to introduce the peritectoid transformation horizontal at 382 °C and the peritectic transformation horizontal of cement at 1650 °C into the Fe-C alloy state diagram.

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