Analysis of Microstructure and Performance of Cr₃C₂/Ni60A Coating on 45 Steel for Laser Cladding Piston Rod

Abstract

This study investigates the preparation of a high-performance Cr₃C₂/Ni60A coating on 45# steel through laser cladding technology. The microstructure, residual stress, phase composition, hardness, and wear resistance of the coating are analyzed. The results reveal that the solidification structure of the molten pool exhibits a progression from coarse columnar crystals and dendrites near the interface with the substrate to equiaxial crystals at the coating surface. The coating primarily consists of Fe-Ni solid solution, Cr₇C₃, and Cr₂₃C₆ phases. As the Cr₃C₂ mass percentage in the Cr₃C₂/Ni60A composite powder increases, the formation of the Cr₇C₃ and Cr₂₃C₆ phases is suppressed. A pronounced stress concentration occurs at the interface between the coating and the substrate, leading to an increased dislocation density and localized grain deformation. When the Cr₃C₂ mass percentage reaches 45% and 55%, the coating surface exhibits a higher density of induced cracks due to the combined effects of microstructural changes and thermal influences. The maximum microhardness of the coating ranges from 520 HV₁ to 556 HV₁, approximately three times that of the substrate. The wear resistance of the coating initially increases and then decreases with increasing Cr₃C₂ content. The wear resistance is optimal at a 35% Cr₃C₂ mass ratio, with a wear loss of 0.15 mg—five times lower than that of the substrate. The primary wear mechanism is abrasive wear, although localized fatigue and adhesive wear are also observed.

1. Introduction

Natural gas is abundant in the world and, as a clean and low-carbon energy source, it can help mitigate the shortage of renewable energy. The transportation and utilization of natural gas primarily involve pressurization, with reciprocating piston compressors playing a crucial role in the exploitation of oil and gas fields. These compressors are widely used in various industrial applications [1,2,3]. A reciprocating piston compressor is a type of positive displacement compressor, with the piston assembly being one of its key components. In long-term use, the relative movement between the piston rod and the sealing assembly causes wear, fatigue, and other failures in the contact area [4,5]. Typically, piston rods are made from materials such as 45# steel, 40Cr steel, and stainless steel. Among these, 45# steel is favored for its advantages in heat treatment, ease of processing, cost-effectiveness, and abundant availability, making it a widely used material. Regarding the surface strengthening of piston rods, chromium plating is the traditional method. To repair a worn piston rod, the chromium coating must first be removed, followed by surface repair and re-chroming of the entire rod. Although the whole process is simple, it is cumbersome, its cost is high, the cycle is long, and, in addition, it causes heavy pollution in the environment [6]. To streamline this process and reduce both maintenance costs and repair time, there is a need for an alternative surface reinforcement technology to replace the traditional repair methods [7,8,9].

In order to solve the above problems, researchers currently apply laser cladding remanufacturing technology to strengthen and repair metal parts [10,11,12]. Zhai Jianhua et al. [13] used laser cladding to repair the surface wear of piston rods and compared the thickness and performance of the cladding layer with that of conventional chromium plating. Their analysis revealed that the laser clad coating provided superior performance, especially with the use of diverse cladding powders, which significantly enhanced specific properties of the piston rod surface. Wu Linhu et al. [14] compared traditional electroplating methods with laser cladding for repairing two piston rods of the same size, conducting salt spray corrosion resistance tests on the repaired areas. The results showed that the piston rod repaired by laser cladding exhibited significantly better corrosion resistance than the electroplated chromium rod. Additionally, laser cladding eliminates the longest and most costly step in the traditional electroplating process, further demonstrating its feasibility as a repair method. While the hardness of the electroplated chrome layer was higher than that of the laser cladding layer, the corrosion resistance of the laser cladding layer was over four times greater. These findings underscore the high potential of laser cladding for repairing hydraulic cylinder piston rods. Nan Zhang et al. [15] utilized ultra-high-speed laser cladding to prepare M2 alloy coatings on 42CrMo steel. The results indicated that a strong metallurgical bond was formed between the M2 coating and the steel matrix consisting of residual austenite, flaky twin martensite, and carbide. The coating exhibited high hardness and improved wear resistance, although it also developed significant residual stress due to rapid cooling, extending to a depth of about 100 µm in the heat-affected zone of the matrix.

At present, researchers mainly develop composite alloy powders for laser cladding to prepare high-performance coatings, while there are few studies on the forming mechanism and wear failure mechanism of coatings prepared by the laser cladding of non-metallic enhanced composite powders on 45# steel metal surfaces [16,17,18]. This paper aims to investigate the forming mechanisms and primary failure modes of coatings produced by the laser cladding of nickel-based ceramic mixed powders. Specifically, it explores the effects of varying Cr₃C₂/Ni60A powder mass ratios on the surface wear resistance, microhardness, and microstructure of the coating.

2. Experimental Materials and Methods

2.1. Materials

For the test plates, 45# steel was selected as the base material, with dimensions of 100 mm × 90 mm × 20 mm. The surface oxide layer was removed by grinding with a belt sander, followed by cleaning with alcohol and acetone. The plates were then dried in a vacuum-drying furnace and packed in vacuum bags for later use. NNi60A alloy powder, which exhibits good self-fusibility, was chosen as the base powder. The powder has a particle size range of 48–106 µm, and its chemical composition is provided in

Table 1. Chemical composition of Ni60A powder (wt%).

Chemical Composition	C	Cr	Si	B	Fe	Ni
wt%	0.8	16.5	4.0	4.0	3.0~5.0	Balance

. Cr₃C₂ powder is used as a reinforcing base powder with a fineness of 500 mesh. Cr₃C₂/Ni60A mixed powders containing different mass ratios of 15%, 25%, 35%, 45%, and 55% are placed in a ball mill and mixed at a rotation speed of 230 r/min for 2 h. After mixing, the powders were dried in a drying oven for later use.

Figure 1 shows the laser cladding powders. Figure 1a shows Ni60A powder, which is spherical in shape. Approximately 80% of the powder particles have a size of about 55 µm, while 6% have a size of around 100 µm and the remaining particles are smaller than 50 µm. Figure 1b shows Cr₃C₂ powder, which has an irregular, flattened morphology and exhibits a relatively uneven particle size distribution. Larger powder particles are evident, and many of the particles have a fineness greater than 500 mesh. Figure 1c shows the distribution characteristics of Cr₃C₂/Ni60A mixed powder, and the distribution of mixed powder with different components is more uniform. During the ball milling process, the powder is reduced in size, resulting in a more uniform distribution. Notably, the amount of Cr₃C₂ ceramic powder is relatively low, but it is evenly distributed throughout the mixture. Figure 1d shows the XRD analysis results of Cr₃C₂/Ni60A mixed powder. The main components of the mixed powder are Cr₃C₂, γ-Ni, Cr₇C₃, Cr₂₃C₆, and CrB, which contribute to enhancing the hardness and wear resistance of the alloy.

2.2. Coating Preparation Process

A Laserline 3 kW semiconductor fiber laser with coaxial powder feeding was used to perform laser cladding on the surface of 45# steel sheets. Based on extensive preliminary laser cladding tests, the effects of process parameters on coating surface quality, hardness, residual stress, strain, and wear resistance were analyzed. These results were used to optimize the process parameters, ultimately yielding a coating with excellent comprehensive properties.

The laser power was set to 1400 W, with a scanning speed of 10 mm/s, a powder feeding rate of 6.0 g/min, a bonding rate of 50%, and a spot diameter of 3.6 mm. Argon gas was used for protection during the preparation of both single-channel and multi-channel cladding coatings. The detailed technological parameters for coating preparation are summarized in

Table 2. Coating preparation process parameters.

Number	Power (W)	Scanning Speed (mm/s)	Powder Delivery Rate (g/min)	Overlap Rate (%)	Spot Diameter (mm)	Mass Proportion of Mixed Powder (%)
						Cr3C2	Ni60A
1#	1400	10	6.0	50	3.6	15	85
2#	1400	10	6.0	50	3.6	25	75
3#	1400	10	6.0	50	3.6	35	65
4#	1400	10	6.0	50	3.6	45	55
5#	1400	10	6.0	50	3.6	55	45

. In order to avoid or reduce the crack derived from the laser cladding coating, the propane spray gun was used to preheat the sheet to 200~300 °C before the experiment, and the coated sheet after laser cladding was placed in the furnace to hold heat at 200 °C for 2 h while the room was cooled naturally.

2.3. Coating Forming Quality and Residual Stress

The surface of the laser cladding coating was cleaned with alcohol and dried for further use. The presence of surface cracks was detected using flaw detection reagents, and the macro morphology of the coating was examined using an ultra-depth microscope. After re-cleaning and drying the coating surface, a residual stress analysis sample was prepared by cutting the coating into a 25 mm × 10 mm × 20 mm sample using wire-cutting equipment. The coating surface was polished progressively with sandpaper of varying coarseness, followed by polishing with a W0.25 µm diamond spray polishing agent to achieve a scratch-free surface. The residual stress of the coating was detected with the help of the Canadian Proto stress meter, and three points on the surface of each sample were randomly detected to obtain the average stress. A schematic diagram of the coating sampling for the test stress is shown in sample 1 in Figure 2.

2.4. Microstructure and Phase Composition

All the samples are identical, as described above. After grinding and polishing, all samples were cleaned and dried using an ultrasonic cleaning machine. The cross-section of the coating was then electrolytically polished using an etching solution consisting of perchloric acid, n-butanol, and methanol, mixed in a volume ratio of 1:3:6 [19]. The electrolytic polishing was carried out at a current density of 0.5–0.8 A/cm² for 15 s. The metallographic structure in various regions of the coating cross-section was observed using an MDS400 optical microscope, and phase composition analysis was performed with a Smartlab 9 kW X-ray diffractometer. A schematic diagram of the samples used for microstructure and phase analysis is shown in sample 2 of Figure 2.

2.5. Microhardness and Wear Resistance Testing Method

All samples used for microhardness and wear resistance tests were the same as those described in Section 2.3. Microhardness measurements were taken on both the surface and cross-section of the coating. Four locations on the coating surface were randomly selected for hardness testing, and additional hardness measurements were taken at successively spaced intervals of 0.5 mm from the top of the coating. A schematic diagram of the microhardness test points is shown in Figure 3. The wear resistance of the coating surface was tested using an HT-1000 friction and wear-testing machine under normal temperature conditions. The friction pair consisted of a 6 mm diameter SiN ceramic ball with a rotational speed of 560 rpm, a load of 1000 g, and an experiment duration of 30 min. The friction radius was 3 mm. The weight difference before and after the wear test was recorded, and the wear morphology of the coating surface was analyzed using SEM to reveal the wear failure mechanisms. The sampling diagram of microhardness and wear resistance is shown in sample 3 and sample 4 in Figure 2.

3. Results and Discussion

3.1. Surface Topography and Residual Stress of the Coating

The macroscopic morphology of the laser cladding coating is shown in Figure 4. Panel (a) illustrates the overall macro morphology of the coating on the plate surface. The surface is divided into two regions: area “1” represents the single coating, while area “2” contains Cr₃C₂/Ni60A coatings with a 50% overlap ratio. The dimensions of the coated surface are 90 × 80 mm. The coating exhibits a good overall forming quality with no obvious defects, such as collapse, holes, or cracks. During the solidification of the molten pool, the liquid metal tends to settle, while the gas generated during solidification rises. This results in the oxidation of the molten pool’s lap boundary and the formation of local scum and pitting [20]. As shown in Figure 4b, the local topography of the coating reveals that the surface formed by overlapping layers of multiple coatings exhibits a sinusoidal waveform. The surface has distinct peaks and troughs, with the distance between adjacent peaks measuring 3.47 mm and the peak height reaching 0.238 mm.

The residual stress values of the coatings for different samples are presented in Figure 5. During the rapid cooling process, the molten metal solidifies and is constrained by the cooler substrate, generating tensile stress. As the mass ratio of Cr₃C₂ increases, the surface residual stress of the coating gradually increases. When the mass ratio of Cr₃C₂ is 55%, the maximum coating stress value is about 658.4 MPa.

The coating surface crack detection method employed in this study utilizes Hongda H-ST series color penetration flaw detection agents, which include a remover, colorant, and developer. The crack detection process for the laser cladding coating consists of four steps: the first step involves using the remover to clean the surface of the coating, eliminating contaminants such as oil and dust. This ensures that the surface remains clean and dry. The second step is to completely cover the surface of the coating with the red colorant and keep it for 5 to 10 min. The third step involves using the remover again to clean off the excess colorant from the surface of the coating. The final step is to evenly spray the developer over the coating surface from a height of 200–300 mm and allow it to rest for 5 min. If cracks or defects are present, red lines will appear on the developer’s surface, indicating the location of the crack or defect.

As shown in Figure 6, coating-derived cracks were observed on different samples. No cracks were detected on the surface of coating 1# (Figure 6a). However, for coatings 2# and 3# (Figure 6b,c), multiple parallel cracks are visible, originating from the coating overlap area and propagating along the fusion direction. For coatings 4# and 5# (Figure 6d,e), cracks not only appear parallel to the cladding direction but also perpendicular to the fusion zone. Notably, the surface of coating 5# exhibits a higher crack density and larger crack size compared to the other samples. As the mass ratio of Cr₃C₂ in the composite powder increases, the cracks on the coating surface become more pronounced. This suggests that the residual stress on the coating surface also increases, which is consistent with the residual stress data presented in Figure 5. The main causes of surface cracks in the coatings can be attributed to three major factors: structural stress, thermal stress, and restraint stress [21,22]. Among these, thermal stress plays a dominant role in the formation of surface cracks. Laser cladding is a powder metallurgy process characterized by “rapid heating and rapid cooling”. During this process, differences in the melting point, thermal expansion coefficients, and cooling shrinkage rates between the cladding material and the substrate cause thermal stress. As the coating material expands and contracts during heating and cooling, “volume differences” arise, leading to traction and extrusion at the solidification interface of the molten pool, thereby generating thermal stress [23]. In multi-channel laser cladding, the subsequent coating layer fuses with the previous one at a 50% overlap rate. The latter coating serves as the base for the previous layer during secondary remelting. The solidification boundary of the molten pool in the second coating is uneven: one half is in contact with the 45 steel substrate, while the other half is in contact with the previous coating. This uneven solidification boundary is prone to producing restraint stress. Additionally, any pitting or scum on the surface of the prior coating transforms into micro-voids, which concentrate stress around these defects. The continuous movement of the solid–liquid interface causes uneven microstructure transformation, contributing to microstructural stress, particularly in the lap zone. The complexity of temperature and flow field leads to the diversity and instability of the microstructure, which is also the main reason for the parallel cracks in the lap zone.

3.2. Coating Microstructure

The laser cladding process involves melting the coating powder with a laser beam and transforming it into liquid metal that rapidly solidifies on the substrate surface, in accordance with the theory of rapid solidification [24]. The molten pool created during cladding exhibits a complex temperature field, which leads to dynamic changes in the microstructure of different regions. The temperature gradient G and the grain growth rate R during the solidification process of the molten pool will affect the crystal structure and size together. The larger the G·R value, the more obvious the grain refinement, and, when the G/R value is smaller, the crystals evolve from planar to cellular, columnar, dendritic, and finally equiaxial crystals [25,26].

The cross-sectional microstructure of the laser cladded Cr₃C₂/Ni60 coating is shown in the “single-layer coating” region marked in Figure 7. The shape of this region is “trumpet-shaped”, with the diameter of the horn increasing as the distance from the coating surface grows. At the interface where the cladding alloy meets the substrate, the angle between the laser scanning velocity direction and the solid–liquid solidification is $\theta =90°$, and the G/R ratio is high at this time. Due to the preheating treatment applied to the substrate and the sequential cladding with multiple laser beams, heat accumulates on the surface, preventing the formation of large-scale flat crystals [27]. As a result, the solidification process at this junction produces coarse cladding crystals, columnar crystals, and some planar crystals along the direction of the temperature gradient, as shown in Figure 8a. As the solid–liquid interface continues to move forward to the top, the value of $\theta$ is gradually decreasing, the value of R is gradually increasing, and the value of G is decreasing, so the value of G/R is constantly decreasing. The crystal growth direction of the solidified microstructure in the molten pool aligns with the direction of the maximum temperature gradient, allowing the crystals to fully grow into coarse columnar structures, as shown in Figure 8b.

As the solid–liquid interface continues to advance, the G/R value decreases, causing the solidified microstructure in the molten pool to transition into dendritic crystals, as shown in Figure 8c. These dendrites grow along the center of the molten pool in the G direction, forming a “fan-shaped” structure. As the solid–liquid interface reaches the top, the G/R value reaches its minimum, and the solidification structure changes from dendrites to equiaxed crystals, forming a thin, dense layer of equiaxed microstructure on the coating surface, as shown in Figure 8d.

The “Combining interface” area, indicated in Figure 7, reveals a strip-shaped envelope where no defects, such as pores or cracks, are present. The bottom envelope, marked as 2 in Figure 8e, represents the initial bonding interface between the cladding layer and the 45 steel substrate, where metallurgical bonding occurs. Mark 1 in Figure 8e shows the second envelope, which is the interface between the subsequent cladding layer and the previous coating. This demonstrates a high degree of metallurgical bonding between the coating and the substrate. Figure 8f illustrates the metallographic structure of the heat-affected zone of the 45 steel substrate, which primarily consists of ferrite and pearlite. The 45# steel substrate undergoes thermal radiation from the laser beam, leading to dynamic recrystallization [28]. Compared to the untreated 45# steel, the grain size in the heat-affected zone is significantly refined after laser cladding.

The XRD patterns of the coatings on different samples are shown in Figure 9. The coatings mainly consist of the Fe-Ni solid solution phase, Cr₇C₃, Cr₂₃C₆, CrB, and other compound phases. The enrichment of carbon (C) and chromium (Cr) in the molten pool promotes the precipitation of Cr₇C₃ and Cr₂₃C₆, which inhibits the growth of dendrites and refines the microstructure. When the mass ratio of Cr₃C₂ in the composite powder is 25%, the amount of Cr₇C₃ and Cr₂₃C₆ precipitation increases significantly. When the mass ratio of Cr₃C₂ is 35%, the saturation of C and Cr elements in the molten pool inhibits the further precipitation of the Cr₇C₃, Cr₂₃C₆, and CrB phases.

The solid fusion boundary of the coating formed a high-density zone of grain dislocations, where stress was concentrated. The grains in this region underwent dynamic recrystallization, resulting in significant grain refinement, with the formation of equiaxed crystals. Figure 10a shows the recrystallization structure in the heat-affected region of the coating, with large-angle grains (>10°) accounting for more than 90%, indicating sufficient recrystallization. Region 2, which is closer to the solid fusion bonding area, was significantly impacted by the high-energy laser beam, leading to extensive recrystallization. In this region, the large-angle grains (>10°) make up about 93%, and the grain refinement is notably clear. Figure 10b illustrates the distribution of the weighted fraction of grain size in region 2, where the grain size is relatively uniform with a narrow variation range, and the average size is 3.81 um. On the other hand, Figure 10a shows that region 1, which is farther from the heat-affected area, was less influenced by the laser cladding heat. As a result, the recrystallization of the grains in this region is insufficient, and the large-angle grains (>10°) are about 87%. Figure 10c depicts the distribution of the weighted fraction of grain size in region 1. The grain size varies more widely here, and the consistency of the grain size is lower, with an average size of 5.01 µm.

As shown in Figure 11a, the recrystallized structures in different regions of the solid–liquid bonding boundary of the coating reveal distinct grain types: red grains represent deformed grains, yellow grains correspond to substructured grains, and blue grains denote recrystallized grains. The region marked “1” indicates the heat-affected zone of the 45 steel substrate, the region marked “2” corresponds to the solid–liquid bonding zone between the laser cladding coating and the substrate, and the region marked “3” refers to the internal coating zone. Figure 11b presents the percentage content of the recrystallization structure in various coating regions. The grains in the “1” region are primarily recrystallized grains and subcrystalline structure grains, accounting for 39% and 33%, respectively. In the vicinity of the solid–liquid bonding area between the coating and the substrate, the grains in the “2” region are predominantly deformed grains, constituting 53%. Meanwhile, in the “3” region, the grains are predominantly subcrystalline structure grains, which account for 85% of the total grain population in this area.

As shown in Figure 12a, dislocation densities in different regions of the coating exhibit a pronounced boundary zone with a high-grain dislocation density at the solid–liquid bonding interface between the coating and the substrate. In Figure 12a, the region marked “1” represents the heat-affected zone of the laser cladding coating, where the crystal dislocation density is 0.56. The region marked “2” corresponds to the coating boundary area, exhibiting a crystal dislocation density of 0.57. The region marked “3” is located at the base of the coating’s melting pool, where the crystal dislocation density is 0.46.

In summary, EBSD was employed to analyze the grains in various regions of the coating, including the recrystallized fraction and Kernel Average Misorientation (KAM). Laser cladding on the solid–liquid junction between the coating and the substrate results in a significant stress concentration, leading to a high dislocation density and a concentration of deformed grains. Recrystallization is particularly pronounced in the heat-affected zone of the 45 steel substrate, where fine equiaxed crystals are observed, consistent with the results from metallographic microstructure analysis.

3.3. Microhardness and Wear Resistance

As shown in Figure 13, the microhardness values across different regions of the coating cross-section indicate distinct hardness variations. The laser cladding coating area on the 45 steel is divided into several regions: the cladding coating, the combining interface, the heat-affected zone (HAZ), and the 45 steel substrate, from top to bottom. The average hardness of the coating in region 1# is notably high (520 HV₁–556 HV₁), whereas the hardness of the 45 steel substrate is approximately 174 HV₁, which is roughly three times lower than that of the coating. The microhardness values of both the coating melt pool and the combining interface (CI) area of the 45 steel substrate show a significant increase. This can be attributed to the high density of crystal dislocations and residual stresses in the combining interface, which enhances resistance to deformation. As a result, the microhardness in this region is relatively high. Furthermore, the microstructure of the heat-affected region of the coating is clearly refined, leading to a higher hardness compared to the 45 steel substrate.

Figure 14 shows the friction coefficient curves obtained from the friction and wear experiments of the coating surfaces with different serial numbers. Under the action of Si₃N₄ spherical friction pairs, the friction coefficient of the 45 steel substrate surface is approximately 0.92. In contrast, the friction coefficient of the Cr₃C₂/Ni60A composite coating surface is generally lower than that of the 45 steel substrate, ranging from 0.8 to 0.88. As the mass proportion of Cr₃C₂ powder in the composite cladding powder increased to 35%, the friction coefficient of the prepared coating surface significantly decreased to about 0.82. However, when the Cr₃C₂ powder mass proportion was further increased to 55%, the friction coefficient did not decrease further and actually increased slightly. Figure 15 presents the friction and wear amounts of the coating surface, which correspond to the friction coefficient. It can be observed that a higher friction coefficient correlates with greater friction and wear. The surface wear of the 45 steel substrate is 0.75 mg. As the mass proportion of Cr₃C₂ powder in the composite cladding powder increases, the surface wear of the Cr₃C₂-based coatings initially decreases and then increases. When the Cr₃C₂ powder mass proportion reaches 35%, the surface wear of the coating significantly decreases to 0.15 mg. At a 45% Cr₃C₂ powder proportion, the surface wear is not significantly reduced, with a wear amount of 0.13 mg. However, when the Cr₃C₂ powder mass proportion is further increased to 55%, the surface wear of the coating slightly increases to 0.14 mg.

Figure 16 shows the surface morphology of friction and wear. Figure 16a presents the macroscopic morphology of the friction and wear surface of the 3# coating. Distinct friction tracks of varying depths are visible on the coating surface, with these tracks alternating and superimposing on each other. According to the local topography of the friction and wear surface of the coating, as shown in Figure 16b, fine and dense plow grooves are evident on the coating surface, and a pronounced abrasive wear phenomenon is observed [29,30]. Additionally, fatigue spalling and material adhesion on the local surface are noticeable. As Si₃N₄ is a ceramic ball used as a friction pair, there is limited contact surface during friction. The experimental conditions, which involve high-speed and dry friction, generate substantial frictional heat, but Si₃N₄’s low thermal conductivity makes it difficult to dissipate this heat. This leads to the formation of an oxide layer on the friction and wear surface, which provides a protective and lubricating effect [31]. However, as the friction time increases, the surface oxide layer gradually detaches. Furthermore, edge material overflow accumulates in various layers, and the periodic action of the friction pairs induces fatigue wear [32], resulting in fatigue spalling. The spalled material migrates or accumulates locally, following the movement of the friction pairs.

Figure 16c presents the macroscopic morphology of the friction and wear surface of the 45 steel substrate. Wide and deep plow grooves are clearly visible on the substrate surface, with material extrusion bumps appearing on both sides of the plow grooves. The overall friction surface shape resembles a “ridge”, primarily due to the substrate’s relative softness compared to the Si₃N₄ friction pair. During the wear process, the friction pair embeds into the surface of the substrate. Upon closer inspection in Figure 16d, noticeable flake material accumulation and fatigue cracks are visible on the substrate surface. As the fatigue cracks expand over time, the flake material is peeled off by the friction pair. Some of this flake material migrates with the friction pair and adheres to the sides of the grooves while the remaining material detaches from the surface, resulting in the largest wear amount on the substrate surface.

4. Conclusions

A high-performance Cr₃C₂/Ni60A coating was successfully prepared on the surface of 45# steel using laser cladding technology. The coating’s microstructure, residual stress, phase composition, microhardness, and wear resistance were analyzed to evaluate the influence of the Cr₃C₂ powder mass percentage on the coating properties.

(1)

The residual stress and crack area on the coating surface increase as the Cr₃C₂ mass ratio in the coating rises. When the Cr₃C₂ powder mass percentage is less than 35%, the coating exhibits fewer surface cracks, presenting primarily short and fine parallel cracks at overlapping boundaries. When the Cr₃C₂ content exceeds 45%, the surface crack area expands and the cracks form a vertical and parallel cross pattern.

(2)

The cross-sectional microstructure of the laser cladded Cr₃C₂/Ni60A multi-channel coating was examined. As the molten alloy interacts with the substrate, the process aligns with the theory of rapid solidification of liquid metal. Moving from the solid–liquid interface toward the top of the coating, coarse columnar crystals, dendrites, and equiaxed crystals appear in succession. Stress concentration is evident in the solid–liquid bonding area between the coating and the substrate, leading to a higher dislocation density and a concentration of deformed grains. The heat-affected zone (HAZ) of the 45 steel matrix undergoes noticeable recrystallization. The coating is primarily composed of Fe-Ni solid phases, Cr₇C₃ and Cr₂₃C₆. As the mass percentage of Cr₃C₂ in the Cr₃C₂/Ni60A composite powder increases, the precipitation of the reinforcing phases, Cr₇C₃ and Cr₂₃C₆, is suppressed.

(3)

The obtained maximum microhardness for the coat was approximately 520 HV₁–556 HV₁ three times that of the No. 45 steel. As the Cr₃C₂ powder mass ratio increases in the composite cladding powder, the surface wear of the coating initially decreases and then increases. At a 35% mass proportion of Cr₃C₂ powder, the surface wear of the coating is significantly reduced to 0.15 mg, which is approximately one-fifth of the wear of the 45# steel substrate. When the mass ratio of Cr₃C₂ powder is 45% and 55%, the crack area of the coating surface increases and the wear resistance decreases. The friction surface of the coating is relatively smooth, with no significant material adhesion. The wear surface features small, dense plow grooves, which are indicative of abrasive wear, local fatigue wear, and adhesive wear.