The article presents numerous experimental data of different researchers depending on the hardness of annealed steels hypoeutectoid on the percentage of carbon content. In addition to the classical Brinell hardness measurement, data of Jagar hardness, Shore, the width of lines on sclerometry Martens, weight loss during grinding on sandpaper are given. Selecting tendentiously experimental data we demonstrate the plateaus on the curves of hardness of annealed hypoeutectoid steels vs. % of C at ~0.5 % С. Similar plateaus were discovered by N.S.Kournakov in the curves of properties around FeAl3, Pb3Na, Cu3Zn, which enables one to follow the analogy and declare a ~ Fe42C phase in the Fe-C system. The anomalies of properties observed in the presence of interim phase at ~ 0.5% С are definitely established (such as magnetic susceptibility, electric resistance, density, ductility, etc.) for melts and austenite. Their derivative is a ferrite-cementite mixture, which – due to the experimentally established metallurgic heritage – may inherit the anomalies of properties of its parent phase. For the three states described (melt, austenite, mixed phase) the anomalies of properties at ~ 0.5 % С should be denoted in some compact way, ascribing them, for example the formula of interim phase of ~ Fe42C. The assumed ~ Fe42С phase has a specific analog in the In-Zn system consisting of eutectic mixture of pure In and Zn, which form an InZn8 phase with their crystal phase, found, similar to ~Fe42С phase under the liquidus bend.
