Comparison of the performance properties of commercially produced roller cone bit coatings

The tests were carried out on samples provided by Glinik Drilling Tools. The rectangular samples (Figure 3) were prepared using commercial materials by gas welding and plasma transferred arc (PTA) welding methods. Depending on the characteristics of the method, rod and powder were used to produce the overlay, respectively. The exact chemical composition and manufacturing parameters are company “know-how” and therefore proprietary. All samples were heat treated after welding, which included carburizing, hardening, and tempering. The exact heat treatment parameters are proprietary information. On the delivered samples, metallographic sections were made and subjected to microstructural analysis. Mechanical properties were evaluated.

Fig. 3.

The cross-section of the overlay made by gas welding is shown in Figure 4. The overlay thickness was from 22 to 25 mm. The consistent and uniform microstructure of the layer proves the good weldability of the material. Microscopic observations did not reveal the presence of discontinuities such as pores or cracks throughout the overlay. However, the dissolution of tungsten carbide particles is observed in the bottom part of the overlay, which is clearly visible during mapping analyses (Figure 5). The basic material used by the Glinik Drilling Tools company is a gas surfacing rod, which, as the manufacturer declares, consists of a steel tube filled with a mixture of WC and Si-Mn alloy. This information is consistent with the chemical composition analyses performed.

Fig. 4.

Fig. 5.

Spherical and polyhedral particles are visible inside, with a 50.7% and 4.7% volume fraction, respectively. The diameter of the spherical particles was in the range of 400 to 600 μm. The different microstructures of particles are demonstrated in Figure 6. The spherical particles are characterized by typical features of cermet with well-bonded prismatic WC grains and Co matrix [18, 19].

Fig. 6.

EDS analysis (Figure 7, Table 1) was used to characterize the particles observed. It showed that both contain predominant amounts of tungsten, iron, and carbon. Polyhedral and spherical particles differ in that the first also contains nickel and is richer in iron (spots 1 and 3), and in the second (spots 2 and 4), cobalt is also detected.

Fig. 7.

The cross-section of the overlay made by PTA is shown in Figure 8. The PTA overlay’s thickness was lower than that made by gas welding and amounted to 15–17 mm. Similarly, the coating was well bonded with the steel base and uniform without defects like microcracks or pores. The main particles had spherical shapes; however, their concentration was lower in some parts of the overlay. Comparably to the previous overlay, the volume fraction of the strengthening phase was 55%. The particles were finer, however, and their diameter was about 120 μm. The distribution of primary elements in the whole was similar to the gas-welded overlay (Figure 9). According to the company, the powder used to make overlay by PTA methods contains WC and nickel alloy as a matrix, which is in line with mapping and EDS results (Figure 9). The enrichment of iron observed in the fusion zone may be caused by the diffusion of the element during the surfacing process; similar results were observed by Li et al. [23].

Fig. 8.

Fig. 9.

Microstructure analysis using a scanning electron microscope revealed two morphological zones in the padding weld, differing microstructure elements caused by the coating application process, and the parameters used (Figure 10). The first zone is the fusion zone of the surfacing with the base material. The surfacing adheres tightly to the base material. Visible deformations can be caused by the impact of hard particles on the substrate material during the process. In the matrix, the eutectics are clearly visible. In zone two, carbides are seen in the dendrites’ nickel matrix. In each zone, secondary carbides are visible, mainly formed around the primary ones. Other authors observed similar microstructures in a study of materials strengthened with WC particles [24–26]. The dendritic structures with motile, floral, star-shaped, and blockshaped carbides were formed by the melting of tungsten carbide particles during the surfacing process. The formation or not of dendritic structures in the matrix depended on the parameters of the layering process [23]. A detailed study on the microstructure analysis of Ni-WC surfaced layers using transmission electron microscopy (TEM) was carried out by Liyanage and coauthors [26]. A Ni-base matrix composed of a eutectic mixture of dendrites and interdendritic, identified as γ-Ni dendrite eutectic and γ-Ni+Ni₃B eutectic, respectively.

Fig. 10.

EDS analyses of blocky-shaped structures (Figure 11, Table 2) also confirmed the partial dissolution of the tungsten carbides and formation of transition phases between the matrix and the reinforcing particle, in agreement with the literature [16, 18, 20, 26]. Inside the spherical particles, only tungsten and carbide were detected. The η carbides also consist of various chromium, iron, and nickel quantities. Figures 12 and 13 show the phase composition of the samples made using gas and plasma methods. In the first one, only WC tungsten carbide occurs; in the second, two types occur: WC and W₂C. Both overlays contain nickel tungsten of the formula Ni₁₇W₃. Gas-welded overlay also has phases, such as iron, carbon, and iron tungsten carbide (Fe₃W₃C) and overlay composed of PTA nickel and carbon. There are also differences regarding the peaks’ half-width and intensity. The broadening of the peaks from tungsten carbide in the PTA-generated overlay may indicate the state of dispersion of this phase and the presence of residual stresses in the coating. The differences in peak intensities from the same phases in different coatings may be due to the different textures of these phases caused by different manufacturing methods. The available data in the literature show that complex secondary carbides form at the interface between the primary carbide (WC) and the matrix, formed from a solution supersaturated with carbon and tungsten. The higher diffusion coefficient of carbon in the matrix than in the carbide-forming metal contributes to faster diffusion out of the carbide, allowing the creation of some forms of mixed carbides [27]. The tungsten carbide is sensitive to high temperatures despite the high melting points of 2870°C and 2730°C for WC and W2C, respectively. The self-fluxed nickel alloys, with melting temperatures between 1000 and 1100°C, are used for hardfacing to avoid degradation of tungsten carbides. Adding boron and silicon is intended to lower the melting point and reduce the heat impact. The addition of chromium to nickel alloys is questionable. On the one s-hand, its presence increases the hardness of the matrix, improves corrosion resistance, and prevents porosity through carbon capture. On the other hand, it facilitates carbide dissolution, increasing matrix brittleness and susceptibility to cracking. For this reason, nickel alloys with chromium are recommended in processes with less heat input and a slow solidification rate, like manual gas welding but not PTA. The dissolution of carbide is also influenced by its type and shape. Macro crystalline tungsten carbide is more stable and less prone to dissolve than the eutectic WC/W₂C carbides. Irregularly shaped carbides tend to dissolve faster because they tend to heat up locally. Additionally, fine carbides are more prone to dissolving than coarse ones [18, 20, 28, 29]. WC is a line compound with insignificant deviation from stoichiometric composition. The dissolved WC particles will participate in two ways depending on the carbon concentration in the base alloy. If it is high, smaller WC particles can be formed. In the opposite situation, W₂C and η phase complex carbides can occur. W₂C has three types—α-W₂C, β-W₂C, and γ-W₂C— which can undergo polymorphic transformation at high temperature (2000–2776°C). η phase complex carbides are formed as a result of dissolved tungsten and carbon reacting with the matrix elements, like nickel or chromium. They exist in wide stoichiometries, but the most common are M₆C and M₁₂C. Carbides like M₇C₃ and M₂₃C₆ occur less frequently because they are better stabilized by the Cr content in the alloy [23, 26, 30]. Hamar-Thibault et al. [31] have also identified the occurrence of more complex forms with the chemical formula (NiSi)_A(CrW)_B. This information is consistent with the identification of phase Ni₁₇W₃ present in both tested surfacings (Figure 12). Either W₂C and η phase carbides, like those present in the analyzed samples Fe₃W₃C, are brittle and have poor chemical and wear resistance compared to the WC.

Fig. 11.

Fig. 12.

Fig. 13.

The results of the microhardness tests are shown in Table 3 and Figures 14 and 15. The impressions were made in both the base and the surfacing, taking 0 points as the joint. In the sample made by gas welding, both the base and matrix of the surfacing exhibited higher hardness compared to the sample made using the PTA method. An inverse correlation was observed for the strengthening phase. In both cases, the hardness of the coating matrix was about twice that of the steel base material. Changes in matrix microhardness are related to the dissolution of the tungsten carbides and the formation of secondary carbides during surfacing. Similar results were observed by Maroli and Dizdar [20] and Czupryński [21]. The microhardness tests showed that structures identified as secondary carbides show lower hardness than tungsten carbides, but higher hardness than the matrix. It confirms the data in the literature about the hardness of secondary eta carbides [18]. The tungsten carbides also showed a high range of variability. Garcia-Ayala et al. [19] pointed out that the hardness, and consequently the toughness of tungsten carbides, is a function of the average WC grain size and content of Co. The finer grains and lower Co content increase hardness. The highest hardness shows carbides with ultrafine grains amounted to 0.2–0.5 μm. On this basis, it can be concluded that the strengthening phase used in plasma surfacing has a finer structure and/or lower Co content. Yang et al. [32] pointed out that an increase in microhardness positively correlates with the material’s wear resistance and negatively with the resistance to brittle fracture.

Fig. 14.

Fig. 15.

The impact test results are shown in Figure 16. It is worth noting that these results can only be used to compare series (explanation in section 2.2.6. Impact test). Fracture toughness mainly depends on the bonding phase type and the absence of destructive brittle phases like η carbides, which can lead to the initiation and propagation of the cracks [33]. The samples made by the PTA method with the Ni matrix showed higher resistance to dynamic loads. However, it can be speculated that the presence of secondary carbides reduces the strength of the fracture toughness.

Fig. 16.

The comparison of relative abrasion resistance of the analysis series is shown in Figure 17. Overlay made by the PTA method characterized two times better abrasive wear resistance than series produced by gas welding. The abrasive wear of welding overlays depends on several structural factors, for example, the tungsten carbides’ amount, shape, and distribution. However, there is a maximum content for the reinforcement phase (depending on the welding methods and matrix phase), above which there are problems with aggregation and bonding with the matrix. There is also a significant influence on the degree of dissolution of the WC, which depends on the matrix’s chemical composition and the welding process parameters [12, 16, 20, 33, 34]. The higher wear resistance correlates with higher WC content and finer particles. As with impact strength, the presence of the eta phase contributes to the deterioration of wear resistance.

Fig. 17.