Microstructure of horizontally continuously cast Stellite rods

This study investigates the microstructural and phase characteristics of Stellite 6, Stellite 12, and Stellite 21 alloys produced by horizontal continuous casting. The alloys were synthesized from pure metallic elements and ferroalloy (FeW) under an argon atmosphere and solidified in a water-cooled copper mould (Ø3.3 mm–Stellite 6 and Stellite 21, and Ø6.1 mm–Stellite 12), resulting in extremely high cooling rates and fine-grained microstructures. Thermodynamic calculations were performed to predict equilibrium solidification, which was compared with experimental microstructural observations. Optical and scanning electron microscopy (SEM-EDS) analyses revealed Co-rich dendritic grains surrounded by eutectic regions containing carbides of the M₇C₃, M₂₃C₆, and M₆C types. X-ray diffraction (XRD) confirmed the presence of an FCC Co matrix without HCP phase formation, attributed to rapid solidification. The volume fraction of carbides increased with carbon content, influencing hardness. Vickers hardness measurements showed that Stellite 6 exhibited the highest hardness (508 HV10), followed by Stellite 12 (470 HV10) and Stellite 21 (345 HV10). The finer grain size and higher carbide fraction in alloys with greater carbon content contributed to enhanced hardness and structural uniformity. These findings demonstrate that horizontal continuous casting enables the production of fine-grained Stellite rods with a homogeneous composition. The cast rods show excellent structural integrity and are well-suited for subsequent hard-facing applications in wear-resistant environments.

1 Introduction

Stellite alloys belong to the group of cobalt-based superalloys, known for their excellent oxidation and wear resistance (Davis, 2000). These properties, combined with good biocompatibility, make Stellite suitable for medical applications, such as implants (e.g., hip, knee, and dental replacements) (Saldívar-García and López, 2005; Kurosu et al., 2010; Hu et al., 2014; Narushima and Ueda, 2015; Lu et al., 2016; Nová et al., 2017; Tunthawiroon and Chiba, 2019; Li et al., 2025). Stellite is commonly welded onto mechanical components to enhance their performance and extend their service life. These alloys are widely used in the energy, oil, wood, automotive, and aerospace industries. Thin deposited layers on component surfaces provide outstanding wear, oxidation, and corrosion resistance, even at elevated temperatures (Frenk and Kurz, 1994; Davis, 2000; Aoh and Chen, 2001; Radu et al., 2004; Radu and Li, 2005; Kapoor et al., 2012; Liu et al., 2015; Roy et al., 2021; Smolina and Kobiela, 2021).

In addition to cobalt, Stellites contain a substantial amount of chromium (typically 25–33 wt%), which imparts exceptional oxidation resistance both at room and elevated temperatures. The chromium content is sufficiently high to form a compact Cr₂O₃ oxide film on the surface, which prevents further oxidation and degradation of the material (Phalnikar et al., 1956; Kofstad and Hed, 1969a; Kofstad and Hed, 1969b; Codaro et al., 2000; Davis, 2000; Kapoor et al., 2012; Li et al., 2013; Hu et al., 2014; Tunthawiroon et al., 2020). Apart from this protective effect, chromium also contributes to the solid solution strengthening of the matrix (Shin et al., 2003; Pala et al., 2017). Other key elements that enhance solid solution strengthening include tungsten (W) and molybdenum (Mo), which are typically added separately to the alloy. Stellites are generally categorised into Co-Cr-W alloys, which offer superior wear resistance, and Co-Cr-Mo alloys, which provide better corrosion resistance (Kuzucu et al., 1997; Davis, 2000; Shin et al., 2003; Huang et al., 2007; Kapoor et al., 2012; Hu et al., 2014; Motallebzadeh et al., 2015; Narushima and Ueda, 2015; Liu et al., 2015).

Despite solid solution strengthening, the matrix of Stellite alloys remains relatively soft and not inherently wear-resistant. Their wear resistance is primarily provided by the presence of carbides, which are crucial in these alloys (Shin et al., 2003; Huang et al., 2007; Kapoor et al., 2012; Nová et al., 2017; Ravi Kumar et al., 2020; Ahmed, De Villiers Lovelock and Davies, 2021; Chen et al., 2024; Khan et al., 2025). The carbon content typically ranges from 0.2 to 3.5 wt%, directly influencing the alloy’s wear resistance (Krell et al., 2020). Stellites with lower carbon content are used in more corrosive environments–for instance, Stellite 21, which, in addition to good corrosion resistance, also offers excellent creep resistance (Davis, 2000; Huang et al., 2007; Liu et al., 2015; Zhao et al., 2024). Alloys with higher carbon content, such as Stellite 12 and Stellite 6, contain a larger volume fraction of carbides and are thus preferred for wear-resistant applications (Kuzucu et al., 1997; Shin et al., 2003; Hou et al., 2005; Kapoor et al., 2012; Motallebzadeh et al., 2015; Liu et al., 2015; Smolina and Kobiela, 2021; Karthik et al., 2022; Chen et al., 2024).

Carbides in Stellites form during the final stages of solidification, resulting in a eutectic structure with the Co-FCC phase. The proportion of eutectic and therefore carbides is relatively high, and the carbide network within the interdendritic regions contributes significantly to the wear resistance of these alloys. The most common carbides are chromium-rich M₇C₃ and M₂₃C₆ types (where M = Cr, Co). M₇C₃ carbides form during eutectic solidification and are likely present at room temperature in Stellite 6 and Stellite 12. These M₇C₃ carbides are metastable and may transform into M₂₃C₆ carbides during cooling (Kuzucu et al., 1997; Davis, 2000; Kapoor et al., 2012; Wang et al., 2021; Xiong et al., 2023; Yao et al., 2023; Ahmed et al., 2024; Chen et al., 2024). The addition of Mo to Stellite 6 and 12 suppresses the formation of M₇C₃ carbides, thereby favouring the formation of M₂₃C₆ eutectic carbides. (Kuzucu et al., 1997; Shin et al., 2003). The addition of W and Mo promotes the formation of M₆C-type carbides (where M = W, Mo, Co), which are harder and enhance the wear resistance of the alloy (Shin et al., 2003; Chen et al., 2020; Karthik et al., 2022).

In certain alloys, such as Stellite 21, intermetallic phases, most commonly Laves phases, also form and contribute to improved wear resistance and creep performance at elevated temperatures. Examples of intermetallic phases identified in Stellite 21 include Co₃Mo, Co₃Mo₂(Cr,Si), and Co₂MoCr (Radu et al., 2004; Huang et al., 2007; R. Liu et al., 2015; Liu et al., 2015; Li et al., 2025).

At high temperatures, cobalt atoms adopt a face-centred cubic (FCC) crystal structure, which transforms to a hexagonal close-packed (HCP) structure below 427 °C under equilibrium conditions(Davis, 2000). The addition of alloying elements alters the transformation temperature between the FCC and HCP phases, while significantly reducing the transformation kinetics (Kuzucu et al., 1997; Davis, 2000; Kapoor et al., 2012; Narushima and Ueda, 2015; Chen et al., 2024; Li et al., 2025). Due to the sluggish transformation kinetics, both metastable FCC and HCP phases coexist in the cast microstructure at room temperature. Since the HCP phase has fewer slip systems, it is less deformable; therefore, Stellites with a higher fraction of HCP phase exhibit higher hardness and improved wear resistance. The FCC → HCP transformation in Stellites is martensitic and can occur during rapid cooling from the temperature range where the FCC phase is stable (athermal martensitic transformation). It can also occur upon annealing at elevated temperatures within the HCP stability region. Owing to the low stacking fault energy, strain-induced transformation to the HCP phase is also possible (Frenk and Kurz, 1994; Radu et al., 2004; Radu and Li, 2005; Saldívar-García and López, 2005; Huang et al., 2007; Narushima and Ueda, 2015; Chen et al., 2024). The mechanical properties of the matrix are strongly influenced by the ratio between the FCC and HCP phases, a factor particularly significant in Stellites with low carbide or intermetallic content, such as Stellite 21.

In this research, a characterisation of key representatives of the Stellite alloys was carried out. The alloys were produced using a horizontal continuous casting process, during which solidification of the melt occurs rapidly due to intensive heat transfer. The cast microstructure of Stellite rods exhibits grain sizes comparable to those found in hard-faced layers, as typically used for Stellites. This study focused on the characterisation of the fine-grained cast microstructure of cast rods. A detailed characterisation was performed on Stellite 6, 12, and 21 alloys.

2 Materials and methods

The alloys with compositions specified in Table 1 were produced using an induction furnace. Metallurgically pure elements and ferro-alloy (Co, Cr, C, Si, Mn, Mo, Ni, W, and FeW) were used to prepare each alloy. The melting of the materials was carried out under a protective Ar atmosphere. Once fully molten, the alloy was cast into rods using a horizontal continuous casting process. Stellite 6 and Stellite 21 were cast into rods with a diameter Ø3.3 mm, while Stellite 12 was cast into rods with a diameter of Ø6.1 mm. Figure 1 shows an induction furnace with a horizontal continuous caster for metallic materials. A water-cooled copper mould (crystalliser) is positioned inside the induction furnace. Before melting, the opening of the copper mould is sealed with a cold starter bar. When the melt is ready for casting, an electric motor periodically pulls the starter bar (approx. 3.5 Hz frequency) in 15 mm increments, which makes the effective cooling speed 52.5 mm/s. The molten metal fills the copper mould, where it rapidly cools and solidifies. This rapid cooling ensures directional solidification. Each newly solidified segment is welded to the preceding section of the rod. The chemical composition of the samples was determined from a section of the rod taken approximately midway through the casting process. The carbon content was determined using an ELTRA SC-800 combustion mass spectrometer (ELTRA), and the content of other elements was determined by XRF Thermo Scientific Niton XL3t GOLDD+ (Thermo Fisher Scientific, Waltham, MA, United States).

Table 1

Table 1. Chemical composition of cast Stellite alloys, values in wt%.

Figure 1

Figure 1. Induction furnace with horizontal continuous caster.

Thermodynamic calculations of the equilibrium phase composition were performed using the JMatPro9.0 software package (Sente Software Ltd., Guildford, United Kingdom). The chemical compositions used for the thermodynamic calculations are listed in Table 1. The program was employed to calculate the formation of phases during equilibrium solidification and cooling within the temperature range from 1500 °C to room temperature (RT).

In horizontal continuous casting, the solidification rate is relatively high, with cooling rates ranging between 200 and 2000 K/s. As a result, the crystal grains are extremely fine, and the phases form far from equilibrium conditions.

Samples for microstructural characterisation were sectioned transversely from the cast rods with respect to the casting direction. The specimens, taken from the edge and the centre as shown in Figure 2, were mounted in a bakelite resin and subsequently ground and polished. Samples were sequentially ground under water lubrication using SiC papers from 120 to 4000 grit to remove sectioning damage and achieve a uniform surface. This was followed by polishing with 6 μm, 3 μm, and 1 µm diamond suspensions on cloths, and a final chemical–mechanical polish using 0.05 µm colloidal silica to obtain a deformation-free finish. After polishing, specimens were rinsed, ultrasonically cleaned, and dried. The microstructure of the Stellite alloys was examined on ion-etched samples (10 keV, 380μA, 5 min). The metallographic analysis was performed using an Axio Imager Z2 m optical microscope (Carl Zeiss AG, Oberkochen, Germany). Additional microstructural characterization was carried out using a Thermo Fisher Scientific Apreo 2S scanning electron microscope (SEM) equipped with EDS detectors (Thermo Fisher Scientific, Waltham, MA, United States). The working parameters were 15 kV accelerating voltage, 8–15 mm working distance.

Figure 2

Figure 2. Schematic representation of the sampling procedure from the cast rods.

The crystalline phases present in the cast microstructures were analysed by X-ray diffraction (XRD) using a Panalytical XPert Pro PW3040/60 diffractometer (Malvern, United Kingdom). The diffraction patterns were collected from ground and polished bulk specimens over a 2Θ range of 30°–90°, with a step size of 0.002° and a time per step of 60 s. The XRD spectra were recorded using a Cu anode operated at 45 kV and 40 mA, providing K_α1 radiation (λ = 1.5406 Å) and K_α2 radiation (λ = 1.54443 Å) with a K_α1/K_α2 intensity ratio of 0.5.

Vickers hardness and microhardness of individual samples were measured using a FALCON 800G2/AO3 (INNOVATEST, Maastricht, Netherlands), in accordance with HV10 and HV0.02 standards. All three sample had at least five indentations made for HV10 and at least fifteen measurements for HV0.02. The measurements were performed in an accredited laboratory for Vickers testing SIST EN ISO 6507-1.

3 Results and discussion

3.1 Thermodynamic calculations

Figures 3–5 present the equilibrium phase diagrams for the investigated Stellite alloys. Equilibrium solidification of the S6 sample begins at a temperature of 1338 °C with the formation of primary FCC grains of the FCC phase (GAMMA). This is followed by the eutectic solidification of a cobalt-rich phase and carbides (GAMMA + M₇C₃) at 1278 °C. The solidification concludes at 1269 °C. During cooling, a transformation of M₇C₃ carbides into M₂₃C₆ occurs. At room temperature, the carbide content amounts to approximately 15 wt%. At 820 °C, atomic redistribution within the matrix leads to the formation of the HCP phase, named (COBALT_HCP). Under equilibrium conditions, this phase is stable within a narrow temperature range, after which the atoms preferentially rearrange into the fcc crystal structure. Further cooling results in matrix saturation, which promotes the formation of M₆C-type carbides, rich in tungsten. Under equilibrium cooling, the intermetallic phases µ (MU) (Co_xW_y(NiCr)_z) and G_phase (Ni_xSi_y(CrMn)_z) are formed. At lower temperatures, matrix HCP (COBALT_HCP) decomposition occurs, leading to the formation of Co-rich FCC and Cr-rich (SIGMA) phases (Karaali et al., 2005).

Figure 3

Figure 3. Equilibrium phase composition of S6.

Figure 4

Figure 4. Equilibrium phase composition of S12.

Figure 5

Figure 5. Equilibrium phase composition of S21.

The microstructural phases observed in the S12 alloy are similar to those identified in S6. Equilibrium solidification begins at a slightly lower temperature of 1308 °C, which can be attributed to the higher concentrations of alloying elements such as carbon and tungsten. Eutectic solidification starts at approximately 1270 °C, leading to the formation of a eutectic structure consisting of γ (GAMMA) + M₇C₃ carbides, which continues until about 1254 °C. Solidification is completed at this temperature. Upon cooling, M₇C₃ carbides transform into M₂₃C₆; at room temperature, their proportion slightly exceeds 20 wt%. At 1050 °C, M₆C carbides rich in tungsten begin to precipitate. At 750 °C, the HCP phase (COBALT_HCP) forms. However, due to the high content of Fe, Ni, and C, the stability of the HCP phase is limited. At lower temperatures, the matrix decomposes, and atoms are again arranged in the FCC crystal structure. In this alloy, intermetallic phases and matrix decomposition at lower temperatures also occur.

The S21 alloy contains a low carbon content, which contributes to a higher initial solidification temperature. Formation of the FCC phase begins at 1383 °C, predominantly resulting in FCC grains. At the end of solidification, at 1287 °C, a small amount of eutectic (GAMMA + M₂₃C₆) is formed. The content of M₂₃C₆ carbides at room temperature is approximately 5 wt%. The HCP (COBALT_HCP) phase is stable over a broader temperature range, with equilibrium transformation occurring at around 920 °C. Below 400 °C, the FCC and σ phases form as a consequence of matrix decomposition. Upon cooling below 830 °C, the intermetallic µ phase (Co_xMo_y(CrSi)_z) begins to form; at room temperature, its proportion exceeds that of the M₂₃C₆ carbides.

3.2 Metallography

Figures 6–8 show the microstructures of the cast Stellite samples produced by horizontal continuous casting. The figures present cross-sections taken perpendicular to the casting direction. Microstructural observations were performed both at the centre and at the outer edge of the rods. All samples exhibit shrinkage porosity in the centre of the cast rods, which results from the rapid cooling and solidification during horizontal continuous casting. Due to the high solidification rates in the water-cooled copper mould, solidification occurs separately in each stroke. The crystal grains are fine and the phases are uniformly distributed throughout the material, which is also a consequence of the extremely high cooling rates. The grains near the edges are finer compared to those in the central region of the rods. Sample S12 exhibits slightly larger grains due to the larger diameter of the cast rods (Ø6.1 mm) compared to samples S6 and S21 (Ø3.3 mm). In all samples, solidification began with the formation of Co-rich dendrites (Co matrix FCC), followed by the solidification of a eutectic in the interdendritic regions. The eutectic consists of a Co-rich phase and carbides (FCC + Carbides), and its volume fraction increases with the carbon content in the alloy.

Figure 6

Figure 6. Optical microscopy of cast sample S6.

Figure 7

Figure 7. Optical microscopy of cast sample S12.

Figure 8

Figure 8. Optical microscopy of cast sample S21.

Studies on Cu and Fe alloys show that HCC, with its rapid cooling in water-cooled moulds, produces a transition from coarse columnar grains to finer equiaxed grains. This refinement is attributed to the higher cooling rates and solidification speeds (Bockus, 2006; Strzępek et al., 2025). HCC is promising for cobalt-based alloys, especially for high-performance applications such as medical implants and wear-resistant components. Recent research highlights the process’s ability to produce high-quality Co-based alloy rods with desirable microstructural and mechanical properties.(Ahmed et al., 2024).

Secondary dendrite arm spacing (SDAS) measurements were extracted from micrographs, giving values of 2.6–3.0 μm for S6, 2.9–5.7 μm for S12, and 2.7–3.4 μm for S21 (as shown in Table 2). Using empirical correlations of Equation 1.

$\text{SDAS}=\mathrm{B}·{\mathrm{T}}^{\left(-\mathrm{n}\right)}(1)$

where B is 29.2⋅10^–6 with n = 0.32 for Co-based alloys (Frenk and Kurz, 1994). The corresponding cooling rates were estimated as 1200–1900 K/s (S6), 200–1400 K/s (S12), and 800–1700 K/s (S21). These values agree well with cooling rates reported for HCC rods of similar diameters. This supports our interpretation that suppressed FCC to HCP transformation and limited M₇C₃ to M₂₃C₆ conversion result from rapid solidification.

Table 2

Table 2. SDAS of the S6, S12 and S21 samples.

3.3 Hardness and microhardness

Table 3 presents the measured hardness values of the S6, S12, and S21 in the as-cast state. The samples exhibit high hardness due to their finer grain size. The S21 sample shows the lowest hardness as a result of its low carbon content and, consequently, a lower carbide fraction. The S6 alloy was cast into rods with a diameter of Ø3.3 mm, while the S12 alloy was cast into rods with a diameter of Ø6.1 mm. The solidification of the S6 alloy sample was significantly faster, resulting in smaller crystal grains and, consequently, higher hardness compared with the S12 sample. The hardness trend (S6 > S12 > S21) correlates strongly with differences in carbide fraction, grain size, and solid-solution strengthening. Under equilibrium expectations based solely on nominal compositions, S12—with the highest carbon and tungsten content—would be expected to exhibit the highest hardness. However, the higher cooling rate in the smaller-diameter S6 rods (Ø3.3 mm) produced significantly finer dendritic grains than in S12 (Ø6.1 mm), resulting in stronger Hall–Petch grain boundary strengthening. Moreover, SEM/EDS analysis shows that S6 develops a highly refined eutectic network with uniformly dispersed M₇C₃/M₂₃C₆ carbides, while S12 exhibits coarser eutectic regions due to slightly slower solidification. The carbide morphology in S6 (fine, continuous interdendritic network) contributes more effectively to strengthening than the larger, more heterogeneously distributed carbides in S12. In contrast, S21 contains only about 5 wt% carbide and displays a predominantly solid-solution–strengthened FCC Co matrix, resulting in the lowest hardness. Additionally, differences in porosity between alloys and diameters may contribute locally to hardness scatter. Taken together, the hardness hierarchy arises not only from carbon content but from the combined effects of cooling rate, eutectic refinement, carbide morphology, and grain size.

Table 3

Table 3. Hardness of the S6, S12 and S21 samples.

Figures 9–11 show the microstructural images of the samples. The hardness values (HV10) are written on the upper edge of the figures, while the microhardness values (HV0.02) are shown below the individual phases. The measured HV0.02 values were slightly higher than those obtained under the higher load (HV10), which is consistent with previously reported data, where the apparent microhardness increases with decreasing applied load (Petrík and Palfy, 2011). With increasing amounts of dissolved carbon and other alloying elements, the hardness of the matrix phase increases. However, the determination of hardness for individual phases was limited, due to the fine grain size.

Figure 9

Figure 9. Microhardness measurements of as-cast sample S6.

Figure 10

Figure 10. Microhardness measurements of as-cast sample S12.

Figure 11

Figure 11. Microhardness measurements of as-cast sample S21.

3.4 XRD analysis

Figure 12 shows the XRD patterns of the analysed samples. All samples exhibit an FCC Co matrix with carbides of the M₇C₃ and M₂₃C₆ types. No HCP Co phase was detected, indicating that the transformation to the ε phase did not occur due to the homogeneous chemical composition and the rapid cooling during solidification. In samples S6 and S12, M₇C₃ carbides predominate, formed during eutectic solidification. The rapid cooling prevented their transformation into M₂₃C₆ carbides. Sample S21 contains only M₂₃C₆ carbides, with no other intermetallic phases detected. The short cooling times hindered phase transformations in the solid state.

Figure 12

Figure 12. XRD patterns of the samples S6, S12, and S21.

3.5 SEM microstructure characterization

Figures 13–15 show SEM images of samples S6, S12, and S21, along with their corresponding EDS elemental mapping, EDS point analysis results are presented in Table 4. In sample S6, dendritic grains of the Co matrix exhibit an increased cobalt concentration. A fine eutectic structure, consisting of the Co matrix and carbides, is observed around the dendritic grains. The carbides are rich in Cr and W. Regions enriched with Mo are also detected between the dendritic grains, indicating the formation of Mo-rich carbides. Carbide types are confirmed using quantitative EDS point analyses (Table 4). M₇C₃ carbides show high Cr/Co ratios, M₂₃C₆ remain Cr-rich with lower Cr/Co ratios, whereas M₆C carbides exhibit strong W enrichment. Submicron-sized non-metallic inclusions of SiO₂ are present in the material. These inclusions are spherical in shape, suggesting that they formed in the melt and were pushed into the final solidification zone, where the eutectic developed. In sample S12, dendritic grains of the FCC Co matrix are present, with eutectic regions containing various carbides formed in the interdendritic areas. The eutectic grains in these regions are coarser in sample S12 due to slower solidification. Carbides rich in chromium (M₇C₃) are predominant, while W- and Co-rich carbides of the M₆C type are also observed. Sample S21 exhibits a homogeneous microstructure in which almost all alloying elements are dissolved in the Co matrix. Spherical non-metallic SiO₂ inclusions, formed in the melt, are also present in the microstructure. SiO₂ inclusions likely originate from the ferroalloy and minor oxidation in the melt. Their spherical morphology suggests formation in the liquid melt. Their size (<1 μm) and distribution do not significantly affect hardness or eutectic uniformity.

Figure 13

Figure 13. Backscattered SEM image of S6 samples and EDS elemental mapping of C, Cr, W, Co, Mo and Si.

Figure 14

Figure 14. Backscattered SEM image of S12 samples and EDS elemental mapping of C, Cr, W, Co, O and Si.

Figure 15

Figure 15. Backscattered SEM image of S21 samples and EDS elemental mapping of Co, Cr, and Mo.

Table 4

Table 4. EDS microchemical composition of cast Stellite alloys, values in wt%.

Although equilibrium thermodynamic calculations predict the formation of the HCP phase, σ, μ, and M₆C carbides, these phases were not detected in any of the examined rods. This can be attributed to the non-equilibrium cooling conditions characteristic of horizontal continuous casting, where the actual cooling rates are significantly higher than those assumed under equilibrium conditions. The FCC to HCP transformation in Co-based alloys is known to be kinetically sluggish, requiring long-range diffusion and sufficiently slow cooling (Kuzucu et al., 1997; Davis, 2000; Kapoor et al., 2012; Narushima and Ueda, 2015; Chen et al., 2024; Li et al., 2025). Because the HCP phase is thermodynamically stable only within a narrow temperature window, the rapid extraction of heat suppresses atomic rearrangement and stabilizes the high-temperature FCC Co phase down to room temperature.While equilibrium thermodynamic calculations predict the formation of M₆C, σ, and μ phases, none of these were detected by XRD. This discrepancy is expected due to the extremely fast cooling in HCC, which suppresses solid-state diffusion required for their nucleation. The absence of σ and μ phases in XRD is therefore consistent with diffusion-limited transformation kinetics. This supports our conclusion that rapidly solidified HCC rods capture a metastable solidification microstructure.

Similarly, the precipitation of σ and μ intermetallic phases and late-stage M₆C carbides depend on extensive solid-state diffusion of Cr, Mo, and W, which is strongly limited at lower temperatures. The fine dendritic structure and short diffusion distances produced by rapid solidification prevent the chemical partitioning required for these phases to nucleate and grow (Bockus, 2006; Strzępek et al., 2025).

4 Conclusion

Based on the detailed microstructural characterisation of Stellite 6, 12, and 21 alloys produced by horizontal continuous casting (HCC), the following conclusions are established:

All alloys solidified into a primary FCC Co dendritic matrix with interdendritic carbides, and fine grains across the rod cross-sections, due to high cooling rates of 200–500 K/s.

There are no solid-state transformations as confirmed by the X-ray Diffraction (XRD) analysis. More specifically, the suppression of solid-state phase transformations of both the matrix FCC to HCP and carbide M₇C₃ to M₂₃C₆, nor intermetallic phase formation despite thermodynamic predictions.

The results show that fine grains and homogeneous carbide distribution are key to high hardness, as alloys with slightly lower carbon achieved higher hardness values, due to finer microstructure. However, when the carbide content is too low, hardness decreases significantly. Solid solution strengthening of the Co matrix is of secondary meaning when aiming at high hardness values. The refining of the eutectic carbide network with high solidification and cooling rates is an effective way to increase hardness.

The fine and homogeneous microstructures make the HCC rods well suited for hard-facing feedstock, where chemical uniformity is especially beneficial for forming wear-resistant layers.

The present study does not include wear testing or high-temperature evaluations, which are essential for linking microstructure to functional performance. Future work should also evaluate scale-up effects for larger diameters.

Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions

AB: Data curation, Formal Analysis, Visualization, Methodology, Conceptualization, Writing – original draft, Investigation, Resources. JB: Conceptualization, Investigation, Resources, Supervision, Writing – review and editing, Methodology.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This research was funded by ARIS research program P2-0050.

Conflict of interest

Author AB was employed by Metal Tech Solutions.

The remaining author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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Footnotes

^{Abbreviations:}FCC, face-centred cubic; HCP, hexagonal close-packed; HCC, horizontal continuous casting; XRD, X-ray diffraction; SEM, scanning electron microscopy; EDS, energy-dispersive X-ray spectroscopy; HV, Vickers hardness; HV10, Vickers hardness at 10 kgf; HV0.02, Vickers microhardness at 0.02 kgf; RT, room temperature.

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