PSI - Issue 81

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

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austenitic matrix (Jiang et al., (1998) ; Kryl’ and Prysyazhnyuk, (2013)). This morphology provides a good balance of hardness and crack resistance. Molybdenum carbide (Mo 2 C), a refractory compound with high hardness and thermal stability, is also a potent inhibitor of cementite phase formation during slow cooling or aging, making it an attractive alternative. (Yamasaki and Bhadeshia, (2003)). However, its interaction with an austenitic matrix, particularly one with high manganese content, has been the subject of limited investigation. Unlike the simple FCC structures of TiC and NbC, the Fe-Mo-C system involves more complex carbide phases and exhibits significantly different solubility characteristics in austenite (Shtansky and Inden, (1997)). Understanding these differences is crucial, as the resulting microstructure (whether it consists of discrete particles or an intercellular network) will fundamentally dictate the mechanical response of the coating, including its hardness and work-hardening behavior. The present work is dedicated to a comprehensive investigation of the structure and phase composition of FCAW coatings based on the high-manganese steel – Mo 2 C system. Utilizing a combined approach of thermodynamic modeling and experimental analysis (SEM/EDS), the phase equilibria, solidification pathway, and elemental distribution were analyzed. The hardness of the coatings was measured in both as-deposited and deformed states to evaluate the influence of Mo 2 C reinforcement on the material's initial hardness and its capacity for strain hardening. 2. Materials and methods Thermodynamic modelling was employed to predict the equilibrium phase composition in the Fe-Mo-C and Fe-Mn-Mo-C systems. Thermodynamic calculations were conducted using the Thermo-Calc software package, employing the thermodynamic database described in (Prysyazhnyuk and Di Tommaso, 2023). The nominal composition of the high-manganese steel matrix was defined as (in wt.%): Fe – 75.5, Mn – 19, Si – 4, C – 1.5. The core of the experimental flux-cored arc welding (FCAW) wires was prepared from a powder charge, the main components of which were: commercial-grade molybdenum carbide (Mo 2 C) powder (ТU 09.03.363 -75) with a particle size distribution of 1 – 100 μm, manganese powder (Grade MN997, GOST 6008 -90), and ferrosilicomanganese (Grade MNS17, GOST 4756-91). To ensure arc protection from the atmosphere and improve its stability, fluorite and rutile were incorporated into the mixture. Following homogenization, the resulting flux was drawn into a low-carbon steel sheath to produce the final FCAW wire. The coatings were deposited onto C45 steel (DIN 1.0503) substrates via manual flux-cored arc welding (FCAW). The process was performed using direct current, reverse polarity (DCEP) at 150 A with an arc voltage of 30 – 32 V to ensure a stable and energetic transfer of the electrode material. The deposited layers were subjected to microstructural and compositional examination using a ZEISS EVO 40XVP scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. The chemical composition was determined both over the area of the coating and along the fusion line of the coating with the base material. The phase constituents were identified through X-ray diffraction (XRD) analysis. Data acquisition was performed utilizing a Shimadzu XRD 7000 diffractometer, e mploying filtered CuKα radiation, and subsequent Rietveld refinement was applied to the collected diffractograms. Bulk hardness of the coating surface was determined via the Rockwell C scale (HRC), with the final value representing an average of three indentations. The strain-hardening behaviour of the coating was evaluated through indentation tests, wherein the surface was deformed under a 1000 N load using a 5 mm tungsten carbide ball actuated by a Brinell press. 3. Results and discussion At the initial stage of analysing the structure formation process, the correctness of the selected thermodynamic parameters was assessed by comparing the calculated phase equilibrium regions for the iron-rich corner of the Fe-Mo-C system with experimental data (Sato et al., 1962). The results of this comparison (Fig. 1) show a good correspondence for the primary regions of coexistence between austenite, ferrite, and the carbide phases that form in high-molybdenum steels. It should be noted that, unlike Fe (steel) systems with TiC and NbC (Kryl’ and Prysyazhnyuk, 2013; Shihab et al., 2020), this system does not form carbide phases with a n FCC structure. Instead, the stable phases in the system are the complex M 6 C carbide (Fe 3 Mo 3 C), the ξ -carbide (FeMo 2 C), Mo 2 C, and a Mo-alloyed cementite phase. Alongside this, a relatively high solubility of Mo 2 C in γ -Fe is observed, leading to the formation of a wide single-phase austenitic region.