In order to study the inhibitory effect of Cr3C2 on the particle size of in-situ generated WC reinforcement phase and its influence on the coating performance, WC reinforced nickel-based coatings with Cr3C2 contents of 0, 1%, 1.5%, 2%, and 2.5% were prepared on the surface of H13 steel. The microstructure of the coating was analyzed by scanning electron microscopy (SEM), and the changes in WC particle size and the element distribution in different morphological regions of the coating were compared; the phase composition of the coating was analyzed by X-ray diffractometer (XRD); the hardness of the coating was analyzed by microhardness tester; and the friction and wear properties of the coating were tested by friction and wear tester. The results show that Cr3C2 can significantly inhibit the growth of WC particles, but there is an optimal value. The main reason why Cr3C2 inhibits the growth of WC grains is that Cr3C2 can reduce the solubility of WC in the bonding phase. With the addition of Cr3C2, the chromium content in the coating increases, and the microhardness of the coating also increases. When the Cr3C2 content is 2%, the microhardness of the coating is the largest. The friction and wear performance of the coating with Cr3C2 inhibitor is also better, and the wear mechanism is mainly adhesive wear.
H13 steel is widely used in the hot working mold industry due to its superior mechanical properties. However, since the mold is in service for a long time under high temperature and severe friction, it is easy to fail due to fracture, wear and other failures, so it is often necessary to repair the failed mold. Using laser cladding technology to repair failed molds or directly strengthen the surface of mold steel is one of the most commonly used technical means. Compared with traditional welding technology, it has small deformation of the workpiece during the strengthening process, low dilution rate and easy control of the thickness of the strengthening layer. Therefore, it has been more and more widely used in various industries.
When repairing the failed area of the mold or cladding strengthening the surface of the mold steel, in order to further improve the performance, a ceramic metal-based material coating is often prepared on its surface, but the distribution of ceramic reinforcement particles in the coating and the size of the particles have a significant effect on the performance of the cladding layer. Studies have shown that when the reinforcement phase of the coating can be dispersed and the degree of refinement of the reinforcement phase particles is high, the performance of the coating can be significantly improved. Fine grain strengthening is an important means to reduce the particle size of the reinforcement phase and disperse it. At present, Cr3C2 is widely used as a grain growth inhibitor in the sintering of cemented carbide. Liu Shourong et al. [15] studied the effect of adding Cr3C2 on the microstructure and properties of WC-10Co alloy. The results showed that a small amount of Cr3C2 can significantly refine the WC grains and improve the compressive strength, oxidation resistance and corrosion resistance of cemented carbide. Li Ning et al. [16] studied the role of Cr3C2: adding Cr3C2 inhibitor can effectively inhibit grain growth, and the performance of cemented carbide can be significantly improved after adding inhibitors. Nino A studied the effect of Cr3C2 on WC-SiC hard ceramics and found that with the addition of Cr3C2, the coarse plate-like particles were transformed into fine grains, and the hardness of the ceramics was significantly improved. Luyckx S et al. [20] studied the effect of Cr3C2 on the grain size of WC-Co. The results showed that Cr3C2 has a significant effect on grain refinement. In addition, Cr3C2 itself has a high hardness and is often used as a reinforcing phase in laser cladding. Lin Chenghu et al. used laser cladding technology to in-situ synthesize chromium carbide-nickel-based composite coatings on the surface of 45# steel. The study found that the microhardness of the coating was significantly improved, reaching 3.5 times that of the substrate; the coating had a dense structure and excellent performance. Zhong Wenhua et al. [22] studied the microstructure and properties of laser cladding nickel-based chromium carbide coatings. The results showed that the hardness and wear resistance of the chromium carbide coating were significantly improved.
The above studies mostly used Cr3C2 as a grain growth inhibitor in cemented carbide sintering, and it was only used as a reinforcing phase in laser cladding. There is no report on the use of Cr3C2 as a particle inhibitor in ceramic metal-based cladding layers. This paper introduces Cr3C2 as a grain growth inhibitor into laser cladding to explore the influence of Cr3C2 content on the particle size of in-situ generated WC reinforcing particles and its influence on the structure, phase and mechanical properties of nickel coatings.
1 Experimental materials, equipment and methods
1.1 Experimental materials
The experimental substrate is H13 steel with a size of 200 mm×40 mm×8 mm. In order to achieve the in-situ generation of WC reinforced particles, the coating powder is a mixed powder of 65% Ni60 and 35% (W+C), on which 1%, 1.5%, 2%, and 2.5% Cr3C2 particles are added respectively. The powders are weighed and put into the ball milling jar, and the experiment is carried out after two hours of ball milling on the planetary ball mill. The specific powder ratio and experimental process parameters are shown in Table 1.
1.2 Experimental equipment and methods
Before cladding, the substrate surface was milled to ensure the surface flatness, and then the surface was cleaned with anhydrous ethanol and dried. The cladding coating was prepared using the ILAM-3000 fiber laser processing system of Chengdu Qingshi Laser Technology Co., Ltd. (LAMLH-SV laser cladding head, HR-PFH-DT5NO6 synchronous powder feeder, and both the powder carrier gas and the protective gas were nitrogen). The cladding specimens were cut into 20 mm × 20 mm specimens by warp cutting, and the coating sections were polished, cleaned and inlaid with 200 mesh, 600 mesh and 800 mesh sandpaper respectively. The coating surface was polished to a mirror surface without scratches using a metallographic grinder and polisher. The coating was corroded with a corrosive solution (hydrofluoric acid, nitric acid and water volume ratio of 2:4:7) for 15 s, then wiped with anhydrous ethanol and dried. The physical composition of the coating was detected by EDAX Genesis 2000 X-ray diffractometer from the United States. The metallographic structure of the coating was observed by Hitachi S-3400N scanning electron microscope from Japan. The microhardness of the coating was tested by HVS-1000 microhardness tester produced by Shanghai Juhui. The friction and wear properties of the coating were tested by MW-W1B vertical universal friction and wear testing machine produced by Jinan Shijin.
2 Results and discussion
2.1 Phase analysis
Figure 1 shows the XRD spectrum of the in-situ generated WC reinforced coating under different Cr3C2 contents. When no Cr3C2 inhibitor is added to the coating, the main reinforcement phase of the coating is WC particles and the generated diffraction peak is higher, and there is a small amount of Cr7C3. This is because there is chromium in the Ni60 powder. During the cladding process, Cr will react with C element to generate Cr7C3 through in-situ reaction. When the Cr3C2 content is 1%, the WC diffraction peak is significantly reduced, and Fe3W3C interstitial compounds appear in the coating, indicating that the addition of Cr3C2 inhibits the formation of WC, and some W elements combine with Fe elements to form other phases. At the same time, the FeNi3 iron-nickel solid solution and FexC cementite in the coating increase significantly.
As the Cr3C2 content increases, although the content of WC reinforcement phase in the coating does not change much, the peak value of WC diffraction peak increases, and W2C is generated in the coating, which shows that Cr3C2 has an important influence on the synthesis and growth of WC. When the Cr3C2 content continues to increase to 2.5%, the synthesis and growth of WC are greatly inhibited, the WC diffraction peak becomes significantly smaller, and the content of Cr3C2 and Cr7C3 increases further.
2.2 Microstructure Analysis
Figure 2 shows the interface diagram of the in-situ generated WC reinforced nickel-based coating and the substrate at different Cr3C2 contents. It can be seen that a dense and reliable bond is formed between each coating and the substrate, and there are no defects such as pores and cracks.
Figure 3 shows the microstructure diagram of the in-situ generated WC reinforced nickel-based coating at different Cr3C2 contents. In order to determine the corresponding enhanced area in the organization, the energy dispersive spectrometer (EDS) was first used to analyze the composition of the WC reinforced nickel-based coating area at different Cr3C2 contents. Table 2 shows the element mass percentage and atomic percentage of each point (P1, P2, P3, P4, P5) of the Cr3C2 coating. At point P1, the atomic ratio of C and W is about 1.5:1, indicating that the main phase in this area is WC, with a small amount of chromium carbide and cementite. At points P2 and P3, after removing the C atoms with the same molar amount as W, the remaining C and Cr atomic ratio is about 2:3, indicating that the phase in the dotted area is mainly WC, and the overall phase is mainly a eutectic of WC and Cr3C2. At points P4 and P5, the atomic percentage of W is significantly reduced, and the atomic percentage of C and Cr is increased, indicating that the dotted area is still a eutectic of WC and Cr3C2, but the amount of WC is significantly reduced, which is consistent with the results of phase analysis in Section 2.1.
From the microstructure: the microstructure morphology, particle size and distribution of the coating are significantly different with different Cr3C2 contents in the coating. In the coating without Cr3C2 in Figure 3a, the size of the reinforcing particles is mostly concentrated between 6 and 10 μm, and is in the shape of coarse petals; in Figure 3b, when the Cr3C2 content is 1%, the size of the petal-shaped area in the coating does not change much, and the size is mostly concentrated in 6 to 8 μm, but the local petal-shaped material decomposes, forming aggregates with small gaps. This is because a small amount of Cr3C2 can inhibit the growth of reinforcing particles to a certain extent, but the inhibitory effect is limited, and the grains still tend to aggregate. In Figure 3c, when the Cr3C2 content is 1.5%, the original coarse petal-shaped material and aggregates in the coating are significantly reduced, the gaps between the tissues are significantly enlarged, and small petal-shaped and quadrilateral flake materials appear. This is because the inhibition effect on the size of the reinforcing particles will be more obvious after the Cr3C2 content increases, and the overall size of the grains is concentrated in 2 to 4 μm. As shown in Figure 3d, when the Cr3C2 content is 2%, the particle aggregation phenomenon in the coating disappears significantly, and the particles become small flake particles with consistent shape and uniform distribution, with a size of 1-2 μm. This shows that the Cr3C2 content at this time fully inhibits the growth of the reinforcing particles in the coating, and the particles have no chance to grow and aggregate. The obvious refinement effect makes the reinforcing particles appear as small flake particles. The refinement of the reinforcing particles increases the fluidity of the molten pool and reduces the collision, making the distribution of the reinforcing phase more uniform. When the Cr3C2 content is 2.5%, the grain refinement effect does not change much compared with 2%, but the particle morphology changes, and obvious edges and corners appear. This shows that the more Cr3C2 grain inhibitors, the better, but there is an optimal value. When this optimal value is exceeded, the grain size will no longer change significantly, and Cr3C2 is a hard phase with high hardness. When flowing in the molten pool, it is easy to collide with other phases, causing collision defects and causing edges and corners in the particles.
The reason why Cr3C2 has the effect of inhibiting grain growth is that it reduces the driving force of grain growth and slows down the process of dissolution and precipitation. Small particles such as W and C are first dissolved in the bonding phase. When the dissolved W and C atoms reach saturated solubility in the bonding phase, WC particles begin to precipitate. The added Cr3C2 inhibitor can be preferentially dissolved in the bonding phase, reducing the solubility of WC crystals in the bonding phase. Therefore, the growth trend of WC in the bonding phase will be inhibited, showing that the WC grain size is small and the grain growth morphology is uniform.
2.3 Microhardness analysis
The cladding layer is wire cut along the vertical section, and the section is ground, polished, and corroded before hardness measurement. Figure 4 shows the microhardness change curve of WC reinforced nickel-based coating with different Cr3C2 contents. The hardness test points are distributed as 7 cladding layers, 3 heat-affected zones, and 2 substrates. As can be seen from the figure, when the Cr3C2 content is 2%, the microhardness value of the coating is the highest, with an average of 1064HV0.5; when the Cr3C2 content is 0, 1%, 1.5% and 2.5%, the average microhardness of the coating is 755HV0.5, 852HV0.5, 900HV0.5, 995HV0.5, respectively, which are more than 3 times the microhardness of the substrate (232 HV0.5).
The microhardness is related to the particle size and distribution of the reinforcing phase in the coating. First, after adding Cr3C2, the WC reinforcing particles are finely and evenly distributed in the coating, which plays a role in fine grain strengthening, and there is no significant fluctuation in the hardness change of each test point. When the Cr3C2 content is 2%, the WC reinforcing phase particles in the coating are round and the inter-organization gaps are small, and the microhardness of the coating is the highest. When the Cr3C2 content is 2.5%, the fine grain strengthening effect is no longer significant; on the contrary, too much Cr3C2 damages the reinforcement particles during the mass transfer process in the molten pool, increasing the particle defects. In addition, the gaps between the grains in the coating are large, and the microstructure distribution is not uniform, resulting in a decrease in the hardness of the coating.
Comparing the hardness of coatings with different Cr3C2 contents, it can be seen that the lowest hardness point of each coating appears at the bottom of the coating. Figure 5 shows the bottom microstructure of the coating when the Cr3C2 content is 2%. It can be seen from the figure that the reinforcement particles at the bottom are neatly arranged and distributed more; the bottommost area close to the substrate is gray nickel-based structure, and the microhardness is significantly reduced. This is because this is mainly the bonding area. During the cladding process, the substrate is melted, causing part of the iron element to enter the bottom of the cladding layer, and there are more iron-nickel solid solutions in the phase, which causes the hardness of the bottom of the coating to decrease.
2.4 Friction and wear performance analysis
In order to analyze the effect of Cr3C2 on the wear performance of the coating, the friction and wear performance of the in-situ generated WC-reinforced Ni-based coating without Cr3C2 and the coating containing 2% Cr3C2 were tested on a MW-W1B friction and wear tester. Before the test, the coating surface was processed using a surface grinder to make its roughness reach Ra 3.2 μm. The test pressure during the test was 30 N, the speed was 60r/min, and the friction torque was 2 000 N·mm. The grinding pair material was 99% Al2O3, and the hardness value was 1 400～1 600 HV.
Figure 5 shows the wear changes of the WC-reinforced coating without Cr3C2 and the WC-reinforced coating with 2% Cr3C2 at 30 min, 60 min and 90 min respectively.
As shown in Figure 5, the wear loss of the coating containing Cr3C2 is less than that of the coating without Cr3C2 inhibitor in three different time periods. This is because after adding Cr3C2 inhibitor, Cr3C2 can significantly inhibit the growth of WC crystals. The WC in the coating is not only small in particles but also evenly distributed in the coating, which increases the hardness of the coating. At the same time, the fine grains can effectively hinder the occurrence of wear movement. Therefore, the wear loss of the coating with Cr3C2 inhibitor is smaller.
Figure 6 shows the wear morphology of WC reinforced nickel-based coating after 90 minutes of wear. As can be seen from Figure 7, there are a lot of adhesion aggregation in the wear area of the WC reinforced coating without Cr3C2 inhibitor, and the surface roughness after friction is poor. From the enlarged figure, it can be seen that most areas are mainly adhesive wear, the reinforcement particles are relatively aggregated, and the particle size is large. There is no obvious adhesion aggregation in this area, indicating that the reinforcement particles play a supporting role, which significantly reduces the wear amount, while there may be no or little support from the reinforcement particles in the adhesion aggregation area, causing the nickel-based coating to be worn and aggregated in large quantities. The wear mechanism of the coating is mainly adhesive wear.
Figure 8 shows the wear morphology of the WC reinforced nickel-based coating with 2% Cr3C2 inhibitor. The boundary between the wear area and the coating morphology is not obvious, and it can be seen from the enlarged figure that the reinforcement phase in the wear area is fine and evenly distributed, and there is only a small amount of adhesion aggregation and furrowing after wear. This is because Cr3C2 has a significant refinement and uniform distribution effect on the WC particles, which greatly improves the wear resistance of the coating and improves its consistency. The wear mechanism of the coating is still manifested as adhesive wear and abrasive wear.
Conclusion
Cr3C2 as an inhibitor of WC reinforcing particles will mainly have the following effects on the structure and properties of the coating:
(1) When the Cr3C2 content is 0, 1%, 1.5%, and 2%, the reinforcing particles in the coating are all refined, gradually changing from large petal-shaped substances to small flake particles. When the Cr3C2 content is 2%, the reinforcing particles are rounded and evenly distributed in the entire coating; but when the Cr3C2 content continues to increase to 2.5%, the grain size in the coating no longer decreases, and even tends to increase again and aggregate. This shows that there is an optimal value for the addition of Cr3C2 as a grain inhibitor, and 2% is the best in this group of experiments.
(2) The microhardness of the coating with the addition of Cr3C2 inhibitor is significantly increased, and the microhardness of the coating is the highest when the Cr3C2 content is 2%. The average microhardness is 1 064 HV0.5, which is more than 4 times that of the substrate. This is mainly because Cr3C2 makes the WC reinforcing particles fine and evenly distributed in the coating, which plays a role in fine grain strengthening. The friction and wear properties of the coating with the addition of Cr3C2 inhibitor are also significantly better than those of the coating without Cr3C2, and the wear mechanism is mainly manifested as a small amount of adhesive wear.
