In 2018 it is 150 years since D.K. Chernov, the metallurgical science expert, made the report on the results of the research of the Fe-C alloy samples and ingots and for the first time ascertained the presence of the critical points, i.e. phase transitions. The work of D.K. Chernov served as the beginning of the construction of the iron-carbon diagram. A great amount of research on the construction and specification of the iron-carbon phase diagram has been carried out during the past period.In the present work a critical analysis of the generally recognized combined iron-carbon and iron-cementite diagram is given, theoretical and experimental investigations by studying the microstructures of the iron-carbon alloy samples quenched from various temperatures have been carried out. Based on the research, a full equilibrium phase diagram of the iron-carbon system has been constructed, in which the non-variant three-phase equilibrium ferrite+graphite ⇔ cementite at the temperature 727 °C, two-phase equilibrium ferrite+graphite in the temperature range 738 – 727 °C and two two-phase regions ferrite+cementite (up to 6.67 wt.% C) and cementite+graphite (more than 6.67 wt.% C) at the temperatures below 727 °C are present. It is shown that cementite cannot be formed in the iron-carbon alloys at the temperatures exceeding 738 °C at high cooling rates. The author considers expedient not to use the version of the metastable iron-cementite phase diagram and to exclude the designation of the alloy microstructures (perlite and ledeburite) in the phase diagram.Keywords: Phase diagram, iron-carbon system, microstructure, equilibrium, metastable system
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