PSI - Issue 81

Pavlo Prysyazhnyuk et al. / Procedia Structural Integrity 81 (2026) 552–557

554

Fig. 1. Calculated fragment of the isothermal section in the iron-rich corner for the Fe-Mo-C system at 1173 K (points indicate experimental data from (Sato et al., 1962)).

The thermodynamic modelling results, illustrated in Figure 2, provide a comprehensive insight into the phase evolution of the "high-manganese steel – Mo 2 C" system during solidification and subsequent cooling. The calculated pseudo-binary phase diagram (Fig. 2a) is of a eutectic type, featuring the reaction L → Austenite (A) + (Mo,M)₂C, where M=Fe, Mn, at approximately 1600 K. This transformation is critical as it yields a fine, intermixed microstructure known to enhance mechanical properties. For a representative alloy composition, the solidification pathway (Fig. 2b) commences at 1750 K with the primary crystallization of hard (Mo,M) 2 C carbides, which form a wear-resistant network. Following the eutectic transformation at ~1600 K, where the austenite matrix is formed, the microstructure continues to evolve in the solid state, with the partial decomposition of austenite into ferrite and M 6 C carbides at lower temperatures. This phase evolution is accompanied by the redistribution of alloying elements (Fig. 2c), a key phenomenon governing the final properties of the composite. As the alloy cools, the (Mo,M) 2 C carbide phase becomes heavily enriched in manganese, with its concentration nearly doubling from ~10 to 20 wt.%. Concurrently, the austenite matrix is progressively depleted of molybdenum, which is consumed in the carbide formation. Therefore, the analysis predicts the formation of an in-situ composite material, where the final properties are determined not only by the phase constituents but also by the profound chemical redistribution of alloying elements between the hard reinforcing phase and the metallic matrix.

Fig. 2. Thermodynamic modelling of the high-manganese steel – Mo 2 C system: (a) calculated pseudo-binary phase diagram; (b) phase fraction evolution as a function of temperature for an alloy containing 20 vol.% Mo 2 C; (c) temperature-dependent equilibrium concentrations of Mo in austenite and Mn in the (Mo,M) 2 C carbide.

Microstructural analysis (Fig. 3) reveals that the hardfaced layer exhibits a distinct composite structure. This structure is composed of bright, angular primary carbides, identified as (Mo,M )₂C, whose dendritic morphology indicates their crystallization directly from the liquid phase, embedded within a darker-contrast eutectic metallic matrix. It is important to emphasize that, unlike the continuous and brittle cementite networks found in hypereutectoid steels, this carbide framework is discontinuous, allowing the