PSI - Issue 81

Pavlo Prysyazhnyuk et al. / Procedia Structural Integrity 81 (2026) 216–220

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such as tungsten carbide (WC) and titanium carbide (TiC), are commonly used as reinforcements (Fan et al., (2025)). However, NbC presents a particularly attractive alternative, possessing a unique combination of high hardness (over 2400 HV), a high melting point, and excellent chemical stability (Pan, (2024)). Furthermore, in contrast to many other refractory compounds, NbC exhibits very good wettability by molten iron (Li et al., (2017)), which is crucial for forming a strong interfacial bond and a coherent composite structure. The formation of a composite coating combining a ductile iron-based matrix with hard, well-distributed NbC particles could therefore yield a material with an optimal balance of abrasive wear resistance and impact toughness. Despite the potential advantages, the performance of such coatings is highly dependent on their microstructure, including the size, morphology, and distribution of the carbide phase, as well as the integrity of the interface between the coating and the substrate. The processing conditions play a crucial role in determining this microstructure and the resulting phase composition. Therefore, a detailed investigation into the formation of Fe-NbC coatings is essential to understand the relationships between processing, structure, and properties. This study focuses on the characterization of the phase composition and microstructure of a FCAW hardfacing developed within the Fe-NbC system. By employing thermodynamic calculations, X-ray diffraction (XRD), and electron microscopy (SEM/EDS), the distribution of carbide phases within the steel matrix, the nature of the fusion zone, and the elemental partitioning between phases were analyzed. The objective is to provide a understanding of the coating's structural formation and to evaluate its potential for applications requiring high wear and impact resistance. 2. Materials and methods To predict the equilibrium phase composition of the Fe-Nb-C hardfacing alloys, a thermodynamic modelling approach was utilized. All calculations were conducted with the Thermo-Calc software suite (v. 2022a), supplemented by a specialized materials database (Prysyazhnyuk and Di Tommaso (2023)). The core of the flux mixture consisted of a commercial fine- grade NbC powder (ТU 6.09.03.6 -75), characterized by a particle size distribution of 1 – 5 μm, selected to facilitate rapid dissolution and precipitation during the hardfacing thermal cycle. Arc stabilization and atmospheric shielding were achieved by incorporating fluorite and rutile into the powder blend. To eliminate hygroscopic moisture, all powder components underwent a preliminary drying stage at 120 °C for 1.2 hours in a SNOL-type furnace. Following this, the constituents were homogenized for 8 hours in a laboratory-scale gravity tumble mixer. A final post mixing drying cycle at 120 °C for 0.5 hours was implemented to ensure minimal moisture content prior to wire fabrication. The homogenized powder mixture was encapsulated within a low-carbon 08kp steel wire (DSTU EN 10139:2018), which was then drawn to form a seamless flux-cored wire. Hardfacing layers were deposited onto a substrate using the FCAW technique with a VDU-506 rectifier. The process was conducted under direct current, reverse polarity (DCEP) conditions, with parameters maintained at a current of 170 A and an arc voltage of 30 – 32 V to ensure a stable and energetic material transfer. Phase composition was determined by X-ray diffraction (XRD) using a Shimadzu XRD-7000 diffractometer with filtered CuK α radiation, followed by Rietveld refinement of the diffractograms. Microstructural and compositional analyses of the deposited layers were conducted using a ZEISS EVO 40XVP SEM equipped with an EDS detector. The mechanical properties were evaluated at both micro and macro scales. Microhardness profiles across the coating-substrate interface were generated using a PMT-3 tester with a Vickers indenter, applying a load of 0.1 N (HV 0.1 ). The bulk hardness of the coating surface was assessed via the Rockwell C scale (HRC), with the final value representing the average of three indentations performed on a modernized TS-BRV hardness tester. 3. Results and discussion XRD analysis of the deposited layer, the pattern of which is shown in Fig. 1, revealed the formation of a two-phase structure consisting NbC and ferrite. A detailed analysis of the niobium carbide phase confirmed the presence of a face-centered cubic (FCC) lattice (space group Fm-3m). The calculated lattice parameter was a=4.376±0.0026 Å, which is significantly smaller (by approximately 0.1 Å) than the reference value for stoichiometric NbC. Such a substantial lattice contraction is likely the result of the synergistic effect of two parallel mechanisms. A non-stoichiometric carbide (NbC x ) is formed, as the partial dissolution of the carbide phase is accompanied by the diffusion of carbon atoms from its lattice into the iron matrix, leading to the formation of vacancies in the carbon sublattice and, consequently, to its contraction. Concurrently, a counter-diffusion of iron atoms occurs from the matrix into the carbide lattice, where they substitute niobium atoms, forming a complex carbide of the (Nb,Fe)C type. Since the atomic radius of iron (~126 pm) is smaller than that of niobium (~146 pm), this substitution also contributes to the decrease in the lattice parameter. On the other hand, the ferrite matrix exhibits a body-centered cubic (BCC) structure with a lattice parameter of a = 2.870±0.00044 Å. This value is slightly larger (by 0.01 Å) than the parameter of pure Fe α , indicating an expansion of its lattice. This expansion is a direct consequence of the dissolution of niobium and carbon atoms, which were released from the carbide phase, into the iron.