Failure Analysis of a Chromium Plating Layer on a Piston Rod Surface and the Study of Ni-Based Composite Coating with Nb Addition by Laser Cladding

Abstract

The failure of a chromium plating layer on the surface of a piston rod was analyzed, and an Ni-based alloy mixed with a niobium (Nb) composite coating was investigated by means of stereomicroscopy, metallographic microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Vickers hardness testing, and wear testing. The results show that penetrating cracks were present in the chromium layer. Subsequently, the corrosion intensified, resulting in the bubbling, cracking, and peeling of the chromium layer. The cladding layer presented a structure morphology of planar crystalline at the bottom, dendritic at the middle, and equiaxial crystalline at the top. The solidification parameters, which were derived from simulation results, confirmed that the ratios between temperature gradient (G) and solidification speed (S) decreased, and the values of the cooling speeds increased from the bottom to the top of the cladding layer. With an increase in Nb content, the structure became gradually refined and uniformly dense. The cladding layer of the Ni-based alloy mixed with Nb was mainly composed of γ-Ni, Cr₂₃C₆, Cr₇C₃, NbC, and Ni₃Nb. NbC could be formed in situ and presented itself in four forms: particles, dendrites, polyhedrals, and networks. The microhardness of the coating with 15% added Nb was enhanced to 400 HV0.2, and the wear resistance thereof was 11.14 times higher than the substrate. After the 15% Nb coating had aged for 16 h, the diffraction peak intensities of Cr₂₃C₆ and Cr₇C₃ significantly increased, and the volume fraction of granular NbC increased from 1.67% to 2.54%. The microhardness was enhanced to 580 HV0.2, and the wear resistance thereof was better than that of the chromium plating layer.

1. Introduction

As an essential component in hydraulic transmission systems, piston rods have a chromium plating layer on the surface. However, piston rods are exposed to corrosive environments such as the atmosphere, seawater, and mines for long periods of time [1]. After long periods of operation, the chromium plating layer will peel, causing damage to the piston rod [2]. For repairing damaged piston rods, the traditional process includes dechromium treatment, welding repairs of defects, and chromium plating. Piston rod damage is a mode of failure that requires the highest maintenance cost and the longest maintenance time [3]. Utilizing a dense and uniform coating, a small thermal input, and environmentally friendly production, laser cladding for piston rod repair has seen an increasing amount of research [4]. When piston rods are repaired by means of laser cladding, the damage is repaired locally, and the plating process is omitted. This is done by using a laser cladding layer in the corrosion cavities, with the rest of the surface retained. The corrosion cavity is cleaned, expanded, and then local laser cladding is applied, and finally the repaired surface is fine machined. For piston rod repair, laser cladding has obvious advantages in terms of performance, economy, and environmental protection.

An Ni-based alloy mixed with Nb powder was used as a cladding material, which possesses a better wetting ability, corrosion resistance, wear resistance, and toughness [5]. Sun et al. [6] investigated the in situ NbC-enhanced composite coating of Ni45 and found that the coating was uniform and, with the increase in Nb content, the microhardness of the coating gradually increased. Dong et al. [7] prepared an Ni-based alloy cladding layer with different Nb contents on the AISI1045 steel substrate and concluded that NbC was present in the coating, with the composite coating having high wear resistance. Due to the fast heating and cooling features of laser cladding, the microstructure presents the characteristics of stratification [8]. The temperature field distribution of laser cladding can be mastered by means of numerical simulations, which can predict the structure change of the cladding layer [9]. Quan et al. [10] analyzed and verified the temperature field and solidification of an Ni base alloy and concluded that the spacing of the secondary dendrite coincided with the solidification theory. Aging treatment is an effective heat treatment method that can further adjust a structure after laser cladding and improve the performance of materials [11]. J. Ke et al. [12] explored the wear mechanism of a composite coating after 700 °C aging treatment and concluded that the composite coating had good micro-wear properties.

In the present study, the failure of the chromium plating layer of a piston rod was analyzed. For piston rod repair, an Ni-based alloy mixed with Nb powder by means of laser cladding was investigated. By using ABAQUS software to simulate the temperature field distribution of the laser cladding, the microstructure of the cladding layer was verified. The microhardness and wear resistance of the cladding layer were tested. The Ni-base + 15% Nb cladding layer was aged to measure the change in hardness and wear resistance and compared with that of the chromium plating layer. The present study can provide a reference for improving the chromium plating layer of piston rods and research on repairs carried out via laser cladding.

2. Materials and Methods

2.1. Material

The piston rod is shown in Figure 1. The length of the piston rod was 400 mm, and the diameter of the main part was 45 mm. The surface of the piston rod was a milky white chromium plating layer with a thickness of 0.04 mm. The substrate material of the piston rod was 40 Cr, and the chemical composition thereof is shown in

Table 1. Chemical composition of 40 Cr (wt%).

Element	C	Cr	Si	Fe	Ni	Cu	Mn	P	S
Wt%	0.41	0.97	0.29	Bal.	0.047	0.049	0.64	0.011	0.01

. For the piston rod, liquid was used as the working medium and power was transmitted through the change of the sealing volume. The liquid used was No. 4 resident solution, which was developed and manufactured by the Armory factory and was composed of ethylene glycol, distilled water, corrosion inhibitor, and other additives. The piston rod was used to make reciprocating linear motions in a hydraulic cylinder to lift heavy objects. When the piston rod was in a protruding state, the protruding part of the piston rod was inevitably affected by corrosive media and dust pollutants from within the unclean surrounding environment. The piston rod was in service for two to three years, with chromium layer damage in the middle of the piston rod, as shown in Figure 1.

The cladding materials were Ni-based alloy powders with different Nb contents, and the compositions are shown in

Table 2. Chemical composition of Ni-based alloy powders (wt%).

Number	C	Si	Fe	Cr	Ni	Nb
1#	0.3	3	5	15	Bal.	0
2#	0.3	3	5	15	Bal.	2
3#	0.3	3	5	15	Bal.	5
4#	0.3	3	5	15	Bal.	10
5#	0.3	3	5	15	Bal.	15
6#	0.3	3	5	15	Bal.	20

. The powder morphology is shown in Figure 2, and the diameters of the powders were about 40–100 μm. The cross-section of the piston rod was obtained as the cladding object, with a size of 40 mm × 20 mm × 8 mm.

2.2. Laser Cladding Process

Before laser cladding, the substrate surface needed to be polished to be free of oxides and rust. The samples were then ultrasonically cleaned using absolute ethyl alcohol. A model BXQM-4L planetary ball mill was used to mix and shake the powders. The powders were then placed in a drying oven to remove any moisture. The experimental equipment for laser cladding was a solid laser, with a maximum power of 500 W and a spot diameter of 2 mm, and the cladding mode was via coaxial powder delivery. Argon was used as a protective gas. After cladding, the samples were cut by means of wire-cut electrical discharge machining (WEDM) in the vertical scanning direction. The optimal process parameters of laser cladding were determined by means of orthogonal experiments, as shown in

Table 3. Optimized process parameters.

Laser Power (W)	Scanning Speed (mm/s)	Feeding Speed (g/min)	Spot Diameter (mm)
250	2	10	2

. Figure 3a shows the cross-section of the cladding layer using the optimal process parameters. The width, height and melting depth of the cladding layer were 1910 μm, 800 μm and 80 μm, respectively, and the cladding layer had no cracks and pores. Multiple overlaps help to prepare a large area of the cladding layer. The surface flatness of the cladding layer was good when the overlap rate was 35%, as shown in Figure 3b.

2.3. Aging Treatment Process

After cladding, the sample was placed in a heat-treatment furnace. The 5# cladding layer was treated when the aging temperature was 650 °C and the aging times were 2 h, 8 h, 16 h, and 24 h, respectively. Air-cooling treatment was used after the aging process was completed.

2.4. Microstructure Observation and Phase Analysis

The microstructure of the cladding layer was observed using a ZEISS inverted optical metallographic microscope (OM) and a TESCAN-VEGA3 scanning electron microscope (SEM). Composition analysis was performed by means of EDS, and the phases in the cladding layer were characterized by means of XRD of D/MAX-2500 with Cu–Kα radiation (λ = 0.154178 nm).

2.5. Microhardness and Wear Resistance

The cross-section hardness of the cladding layer was measured with a TMVS-1 Vickers hardness meter at a distance of/from the cladding layer surface to the substrate. Three points were taken in the same horizontal direction with a load of 0.2 kg, a loading time of 10 s, and a step length of 0.15 mm. The wear resistance was measured using the MMW-1A abrasion machine. The grinding ring was quenched 45 steel and GCr15. The test material was the cladding layer and the chromium plating layer, and the sample size was 12.7 mm × Φ 4.8 mm. As shown in Figure 4, the grinding ring was fixed, and the sample was embedded in the grinding plate and slightly contacted with the grinding ring. The grinding plate would rotate after the wear time, load, and speed were set. The wear sample was first worn on quenched 45 steel with a wear time of 40 min, a load of 30 N, and a rotation speed of 120 r/min. A cladding layer with optimal wear resistance was chosen for comparison with the chromium plating layer, and the grinding ring was GCr15, with a wear time of 60 min, a load of 100 N, and a rotation speed of 120 r/min. An electronic balance (precision of 0.1 mg) was used for weighing. Each result was measured three times and averaged. SEM was used to observe the wear morphology of the specimen after wearing.

3. Finite Element Simulation

3.1. Assumptions

The following assumptions were included in the simulation: (1) the entire surface of the subject was considered as a plane; (2) classical heat transfer theory was applied to the interaction of the laser and materials; (3) the influence of temperature on the material density was excluded, and the material was isotropic; (4) the influence of the liquid flow in the molten pool was not considered; and (5) the room temperature was 20 °C [13].

3.2. Laser Heat Source Selection

As shown in Figure 5, the heat flow density distribution of the laser heat source of a solid laser was Gaussian. The formula is as follows:

$$q\left(r\right)=\frac{\alpha P}{\pi R^2} \exp (-\frac{r^2}{R^2}),$$

(1)

where q(r) is the heat flow density; r is the distance of A’ from the center of the laser spot; R is the radius of the laser spot; α is the laser absorption rate of 0.9; and P is the laser power. When r = 0, the heat flow density of the laser center is indicated; when r = R, the heat flow density of the laser edge is indicated [14].

3.3. Establishment of a Finite Element Model

The substrate size of the model was 40 mm × 20 mm × 8 mm, and the cladding layer size was 40 mm × 2 mm × 0.8 mm. Stratification and chunking modeling were used, and the minimum mesh size was 0.4 mm × 0.4 mm × 0.4 mm. The ABAQUS software was version 2021 and is a product of Dassault Systemes Simulia Corp., Johnston, RI, USA. The 3D model was built using ABAQUS, as shown in Figure 6. The laser scanning speed was v (m/s), and the laser spot scanning track was L (m), thus the time required for the laser to pass through the surface was t = L/v (s). The DFLUX subroutine in ABAQUS was written to simulate mobile heat sources. The “life-and-death unit” technology in ABAQUS was used to achieve real-time filling of the cladding layers.

Initial conditions: when t = 0, the workpiece had a uniform initial temperature T0. The formula is as follows:

T0 = 20 °C, (T = T0).

(2)

Boundary conditions: during the laser cladding process, the heat was mainly lost in the form of free convection and radiation.

The heat loss caused by convective heat exchange adopted the Newtonian cooling formula. The formula is as follows:

qc = −hc (TS − T0).

(3)

where qc is convective heat; hc is the convection heat transfer coefficient of air and coating, which was 10 W/(m² × °C); TS is the temperature of the coating surface; and T0 is the temperature of the ambient environment.

Heat loss due to radiative heat transfer was calculated by the Stefan–Boltzmann law. The formula is as follows:

qr = −εσ[(TS + 273.15)⁴ − (T0 + 273.15)⁴].

(4)

where qr is the radiation heat; ε is the surface radiation emittance of 0.8; and σ is the Stefan–Boltzmann constant of 5. 67 × 10⁻⁸ W/(m² × K⁴).

3.4. Finite Element Simulation Parameters

Simulating the thermophysical property parameters of Ni-based alloy powder was performed using JMatPro. The JMatPro software was version 7.0.0 and is a product of Sente Software Ltd., Surrey Technology Center, 40 Occam Road, Guildford GU2 7YG, United Kingdom. The relevant element content was input into the software, and the temperature step method was selected to simulate the thermophysical properties, as shown in Figure 7. At 1280 °C, the thermophysical properties parameters changed due to the phase change of the material. The density of the substrate 40 Cr was 7800 (kg/m³). The thermophysical properties parameters are shown in

Table 4. The thermophysical property parameters of the 40 Cr steel.

T/°C	0	400	800	1000	1500
Conductivity/W × m−1 × °C−1	0	28.35	31.08	32.76	31.50
Specific Heat/J × Kg−1 × °C−1	580	756	840	882	756

.

4. Results

4.1. Failure of Chromium Plating Layer of the Piston Rod

As shown in Figure 8a, after the piston rod was in operation for a long period of time, the chromium layer in the middle area of the piston rod bubbled, cracked, and peeled. As shown in Figure 8b, the corrosion pit was black, and the chromium plating layer completely fell off, exposing the substrate. A further observation could be made that the chromium plating layer on the edge of the corrosion pit was rough and uneven.

According to Figure 9a, the thickness of the chromium plating layer was 43.15 μm, which was physically combined with the substrate material. In Figure 9b, cracks penetrating the chromium plating layer were found, which provided a channel for the corrosive medium. In Figure 9c, the substrate had been corroded, and corrosion sources existed at the interface between the chromium plating layer and the substrate. Here, the chromium plating layer had not peeled off. The corrosion in Figure 9d extended further, and the chromium plating layer cracked and peeled off, making the corrosion medium easier to penetrate, with the corrosion being more pronounced. In Figure 9e, part of the chromium plating layer had separated from the substrate, resulting in the steel substrate being completely exposed. The chromium plating layer belonged to the cathodic coating. The combination of a large cathode and a small anode aggravated the corrosion, resulting in the corrosion area of the substrate being larger. In Figure 9f, the steel substrate was further corroded, resulting in the fracture and peeling of the chromium plating layer at the edge of the corrosion pit. Corrosion of the steel substrate proceeded along the width and depth direction of the corrosion pit. An observation can be made that the pit depth was 137 μm in Figure 9f.

4.2. Microstructure Observations of the Cladding Layer

The typical microstructure of a laser cladding coating is shown in Figure 10. The interface between the cladding layer and the substrate had a planar crystalline form. The bottom and the middle transition of the cladding layer had the form of cellular crystal. The middle part mainly had the form of columnar crystal, with a small number of equiaxial crystals. The top of the cladding layer presented an equiaxial crystal morphology.

Figure 11 shows the OM microstructure of different cladding layers in the middle. The columnar grains in the cladding layer without Nb were relatively thick. As the content of Nb increased, the columnar grains became gradually refined. When 15% Nb and 20% Nb contents were added, the columnar grains were the smallest. The cellular grains in the cladding layer without Nb were larger and had different sizes and uneven distributions. When 2% Nb content was added, the cellular grains were refined and even. When 5% Nb content was added, the cellular grains disappeared, and fine dendrites appeared in the cladding layer. With continuous increase in the Nb content, the dendrites became gradually refined. An observation can be made that when 15% Nb and 20% Nb contents were added, the microstructures were uniform, and the grains were finer.

Figure 12 shows the SEM microstructures of different cladding layers in the middle. The microstructure of the cladding layer without Nb was presented as a large dendritic solid solution. With the increase in the content of Nb, the dendritic solid solution became gradually refined. When 2% Nb and 5% Nb contents were added, substantial white matter was formed in the inter dendrite. When 10% Nb, 15% Nb, and 20% Nb contents were added, white granular matter was also formed in the dendrites and interdendrites. Moreover, when 15% Nb was added, the microstructure changes were most obvious. In comparison with the cladding layer without Nb, the dendritic solid solution exhibited refinement, and there was a uniform distribution of white granular matter and a eutectic structure in the cladding layer with 15% Nb added.

4.3. Temperature-Field Simulation and Verification

The stratification of the cladding layer microstructure was related to temperature gradient (G), solidification speed (S), and cooling speed [15]. To determine the reason for the stratification of the cladding layer, the Ni-based alloy temperature field was simulated by using ABAQUS software. The melting point of the Ni-based alloy designed in the experiment was 1085 °C [16].

Table 5. Comparison of the measured and simulated values of the melting height.

Type		Process Parameters
		P = 250 W V = 2 mm/s	P = 250 W V = 4 mm/s	P = 350 W V = 4 mm/s
Melting Height	Measured value	805 μm	438 μm	923 μm
	Simulation value	715 μm	537 μm	879 μm

shows the measurements of multiple melting heights, and the obtained experimental results were close to the numerical simulation results, verifying the applicability of the built model. Appropriate assumptions were made when building the model, since the value of the absorption rate and the determination of the thermal property parameters of the cladding layer could cause errors [17].

Three points, A, B, and C, at different depths in the cladding layer were selected, respectively, at the top, middle, and bottom of the cross-section, for cooling speed and G/S calculations, as shown in Figure 13.

The temperature gradient (G) is calculated as follow:

$$G=\frac{\Delta T}{\Delta d}$$

(5)

where $\Delta$T is the temperature change per unit distance, and $\Delta$d is the unit distance. Here $\Delta$d was 0.4 mm.

The cooling speed calculation formula was the differential calculation of the temperature curve of the cladding layer. The formula is shown below.

$$G \times S=\frac{\mathrm{d}T}{\mathrm{d}t}$$

(6)

Figure 14 shows the A, B, and C three-point temperature curves inside the cladding layer, which showed that the temperature increased rapidly to the maximum within 2 s when the laser scanned the point. After the laser left the point, the temperature dropped sharply. By calculation of the cooling speed and temperature gradient, the highest cooling speeds of points A, B, and C were 1381 °C/s, 1083 °C/s, and 621 °C/s, respectively, and the temperature gradients increased from 3.86 × 10⁵ °C/m of point B to 1.51 × 10⁶ °C/m of point C. When the melting pool began to cool and solidify, the temperature gradient tended to increase from the surface to the bottom because the surface of the melting pool was closer to the laser heat source. The solidification rate of the cladding layer was gradually reduced from 2.81 mm/s on the top to 0.411 mm/s at the bottom. The G/S values decreased, and the values of the cooling speeds increased from the bottom to the top of the cladding layer. The simulation results showed that part A was equiaxial crystalline and the grain was fine, part B was dendritic, and part C was planar crystalline. The solidification parameters derived from the simulation confirmed the transition trend of the cladding layer.

4.4. Phase and Composition Analysis

From Figure 15, γ-Ni, Cr₂₃C₆, and Cr₇C₃ were detected in the cladding layer. In addition to Cladding layer 1#, Cladding layers 2#, 3#, 4#, 5#, and 6# exhibited the generation of NbC diffraction peaks. With the increase in Nb content, the NbC diffraction peaks became stronger. At the same time, with the increase in Nb content, the compound Ni₃Nb of Nb and Ni was formed because of a lack of carbon content.

As can be seen from Figure 16, a straight white band appeared at the fusion point of the cladding layer and the substrate. The line scan energy spectrum was analyzed here. Combined with the chemical composition of 40 Cr and EDS line scanning analysis, diffusion of the element occurred between the coating and the substrate, indicating that a good metallurgical binding had formed between the coating and the substrate [18].

As can be seen from Figure 17, the microstructure of Cladding layer 1# consisted of a solid solution matrix and black bulk matter. Point-atomic analysis was performed, as shown in

Table 6. Chemical composition of A–G points (atom%).

Point	C	Si	Fe	Cr	Ni	Nb
A	33.02	0.85	1.63	60.57	3.93	0
B	5.65	2.30	19.63	13.29	59.13.	0
C	13.62	2.49	5.23	4.42	55.69	18.5
D	32.78	2.26	9.80	9.53	39.52	6.11
E	49.89	1.86	1.34	1.76	2.92	42.23
F	10.33	5.93	8.72	7.59	51.4	16.04
G	33.46	2.09	8.06	7.62	8.09	40.68

. Point A was rich in Cr and C elements, and point B contained a large percentage of Ni elements and Fe and Cr elements. Combined with the XRD results, sections A should be a carbide of Cr, and sections B should be γ-Ni solid solutions. Cladding layer 2# was composed of a γ-Ni solid solution, carbide of Cr, white matter, and granular white matter. The sizes and volume fractions of each phase were assessed using Image J software. Because of the addition of Nb, the size of Cr₂₃C₆/C_r7C₃ was greatly reduced from 23.9 μm to only 2.7 μm in Cladding layer 2#. Point C was measured and contained a large number of Ni elements and Nb elements. Combined with the XRD results, the white matter should be a eutectic structure composed of Ni₃Nb and NbC, the size of which was between 2 and 5 μm. Composition analysis of point D showed high Ni content, but the granular white matter size ranged between 0.5 and 1.5 μm. The NbC phase was smaller, and the composition of the matrix material was included. The size of the carbide of Cr in Cladding layer 3# was again reduced to 0.4–1.5 μm. Cladding layer 4# exhibited diamond and triangular matter generations, with sizes of between 1.82 and 3.27 μm. Point E contained a large percentage of Nb and C elements, and the atomic ratio was close to 1:1. Combined with the XRD results, NbC could be found. Furthermore, no carbide of Cr was found in Cladding layer 4#. There were a large number of network eutectic structures generated in Cladding layer 5#. Point F contained large amounts of Nb and Ni elements and, combined with the XRD results, should be a eutectic structure of NbC. Here, the maximum size of the diamond NbC increased to 5.6 μm in Cladding layer 5#. The size of the diamond NbC in Cladding layer 6# further increased to a maximum of 11.9 μm.

Figure 18 shows the distribution diagram of NbC in Cladding layer 5#, which exhibited a diamond and triangular form along dendritic and interdendritic distributions. Face-scanning analysis was performed on the NbC region. The Nb and C elements were mainly enriched in diamond and triangular matter and were highly consistent, demonstrating the presence of NbC.

4.5. Aging Treatment Analysis

Figure 19 shows the OM observation after the aging treatment of Cladding layer 5#. After aging for 2 h, the grain size did not change from the bottom to the top, and no precipitate was produced, but the cladding structure became more uniform and denser. After aging for 16 h, the cladding layer produced substantial white matter from the bottom to the top. An observation can be made that the white matter presented network and particle morphology, and there was mainly an intercrystalline distribution.

Figure 20 shows the XRD spectrum of Cladding layer 5# after aging treatment. After different time-aging treatments, the phase structure did not change, but the shape of the X-ray diffraction peak did change. After 2 h, the main peaks corresponding to γ-Ni and Cr₂₃C₆ were slightly higher, with the other peaks unchanged. The peak intensities of γ-Ni and Cr₂₃C₆ increased significantly after 16 h. The peak intensities corresponding to Ni₃Nb and Cr₇C₃ also significantly improved after 16 h.

Figure 21 shows the SEM observation of Cladding layer 5# after aging treatment, and an observation can be made that the eutectic structure and granular NbC in the cladding layer changed significantly compared with the unaged cladding layer. With the extension of aging time, the network eutectic structure grew, and the granular NbC gradually increased. At an aging time of 24 h, the eutectic structure became considerably large, and the volume fraction of granular NbC increased from 1.67% to 2.54%. Composition analysis of the solid solution matrix using EDS showed that the Cr element content decreased with the extension of aging time, as shown in

Table 7. Chemical composition of P1–P4 points (wt%).

Point	C	Si	Fe	Cr	Ni	Nb
P1	1.07	1.19	12.36	14.77	59.69	10.91
P2	2.31	1.04	15.00	13.23	59.91	8.50
P3	2.43	1.06	16.83	11.92	59.16	8.60
P4	2.38	1.13	16.85	9.40	61.59	8.66

. Combined with the XRD spectrum, the Cr element was precipitated in the form of Cr₂₃C₆/Cr₇C₃, as shown in Figure 22. The size of the carbide was about 1.2 μm.

4.6. Micro-Hardness and Wear Resistance

As can be seen from Figure 23, the micro-hardness distribution curves of different cladding layers could be divided into three sections, corresponding to the hardness of the cladding layer, the heat-affected zone, and the substrate. The hardness increased in the heat-affected zone due to the presence of martensite [19]. The hardness of the cladding layer without Nb was 180 HV0.2. With the increase in Nb content, the hardness of the cladding layer gradually increased. The hardness levels of cladding layers 2#, 3#, 4#, 5#, and 6# were 205 HV0.2, 240 HV0.2, 300 HV0.2, 400 HV0.2, and 400 HV0.2, respectively. The hardness of Cladding layer 6# fluctuated greatly.

After aging treatment, the hardness of Cladding layer 5# was higher than that without aging treatment. The hardness after aging for 2 h reached about 490 HV0.2. With the extension of aging time, the hardness of Cladding layer 5# gradually increased. The hardness of Cladding layer 5# after aging for 16 h reached a maximum value of about 580 HV0.2, and the hardness after aging for 24 h slightly decreased.

As shown in Figure 24, the mass loss of the substrate was 54.6 mg, which was the largest. With the increase in Nb content, the mass loss decreased gradually. The coating with 15% Nb added had a minimum mass loss of 4.9 mg, and the wear resistance thereof was 11.14 times higher than that of the substrate. When Nb content was 20%, the mass loss was 9.1 mg, and the wear resistance slightly decreased. Moreover, the wear resistance of 15% Nb coating was 8.12 times higher than the coating without Nb.

The wear resistance of Cladding layer 5# after aging for 16 h was compared with the chromium plating layer. The wear amount of Cladding layer 5# after aging for 16 h was 0.818 mg/mm², and that of the chromium plating layer was 1.03 mg/mm². From the wear amount, an observation can be made that the wear resistance of the cladding layer was better than the chromium plating layer.

Figure 25 shows the wear morphology. A large area of shedding and migration occurred on the substrate surface, and the wear mechanism was adhesive wear. There were deep scratches, grooves, and large abrasive dust on Cladding layer 2#, and the wear mechanism was abrasive wear. The surface of Cladding layer 5# was relatively smooth with only slight scratches and fatigue spalling. The surface of Cladding layer 6# had clear scratches, a large amount of abrasive dust, and fatigue spalling. The wear surface of Cladding layer 5# after an aging treatment of 16 h had scratches and fatigue spalling, and the wear surface was relatively smooth. The chromium plating layer was damaged and did not protect the substrate, resulting in the substrate surface being exposed.

5. Discussion

5.1. Peeling Mechanism of the Chromium Plating Layer on the Surface of the Piston Rod

The chromium plating layer was physically combined with the substrate material, in which the binding force was weak [20]. Additionally, the surface of the chromium plating layer had many microcracks due to the electroplating process [21]. The piston rod was an accessory piece to the artillery, and the working medium was No. 4 resident solution, of which the main ingredient was water. However, the water easily evaporated, resulting in the rupture of the water film. Due to the artillery gas, the oxygen and sulfur atmosphere mixed into the water film and became a corrosive medium. The penetrating cracks in the chromium plating layer provided channels for the corrosive media, forming corrosion sources, as shown in Figure 9c [22,23]. As corrosion developed, the corrosion products gradually increased. With the action of the O and S elements, FeS and Fe(OH)₂ were generated. The specific volume of the corrosion products was much larger than that of iron, thereby resulting in expansion [24]. Corrosion pits formed as the corrosion intensified, with iron becoming the anode and the chromium plating layer becoming the cathode. The combination of the large cathode and the small anode made the substrate corrode rapidly [25]. The substrate was further corroded towards the depth and width of the pit, as shown in Figure 9f.

5.2. Microstructure Evolution of the Cladding Layer

The stratification of the cladding layer structure had a significant relationship with the solidification parameters. The grain size of the structure decreased as the cooling rate increased during the cladding process. As G/S decreased, the morphology of the cladding layer became successively planar crystalline, cellular and columnar dendritic, and equiaxial crystalline [26]. The solidification parameters, which were derived from simulation results, confirmed the transition trend of the structure morphology of the cladding layer.

The solid-solution matrix in the Ni-based alloy without Nb was thick, and the size of the carbide was large. With the increase in Nb content, the cladding layer structure became gradually refined. Nb is a strong carbide-forming element, and in the initial solidification, Nb formed an NbC phase with C in the molten pool [27]. NbC has a high melting point and could act as a preferential nucleation matter to promote grain refinement. At the same time, the size of Nb atoms is larger than that of Ni atoms, and so the Nb atoms preferentially occupied the grain-boundary position, reducing the interface energy of the γ-Ni austenite grain-boundary and hindering grain growth [28]. The binding of Nb to C led to a sharp decrease in C content in the molten pool, and the size of Cr₂₃C₆/Cr₇C₃ gradually decreased before disappearing.

The in situ generated NbC phase existed in the form of particles, dendrites, polyhedrals, and networks [29]. Upon solidification, a large number of NbC particles were engulfed by the liquid–solid interface, and the diffusion rate of Nb and C in the solid phase was lower than in the liquid phase, resulting in the formation of smaller particles. The NbC particles not engulfed by the interface were retained in the liquid phase, and a binary phase diagram of NbC-Ni presented a larger phase field with both Ni and NbC present in a semi-solid state, indicating that the NbC particles had enough time to grow into a polyhedral morphology [30]. According to the binary phase diagram of Nb and Ni, Ni₃Nb was generated during the cladding process [31]. After eutectic transformation, the eutectic NbC exhibited dendritic and network distributions along the crystal boundary.

The rapid solidification of laser cladding suppressed the precipitation of carbide and promoted the alloy elements in the solid solution [32]. The microstructure changed greatly after the aging treatment of Cladding layer 5#. Under the combined action of aging temperature and aging time, the alloy elements in the oversaturated solid solution had enough time to diffuse and recombine, resulting in precipitation [33]. The C element first became bound to the Nb element and emerged in the dendrites and interdendrites in the form of NbC particles. Nb elements had limited solubility in the Ni-based solid solution, and as the aging time increased, C elements would bind with Cr elements to form Cr₂₃C₆/Cr₇C₃. In parallel, the diffusion ability of the atoms was enhanced so that the eutectic structure grew after prolonged heat preservation at 650 °C [34].

5.3. Hardness Analysis and Wear Mechanism

Laser cladding made C, Cr, Fe, Si, and Nb dissolve into the dendrite regions, that is, to produce solid solution reinforcement. A portion of the Nb added to the Ni-based alloy existed in the form of an NbC hard phase, and the NbC had a higher microhardness, which increased the hardness of the coating [35]. Moreover, the addition of Nb elements refined the grain and enhanced the binding force of the grain boundary, which made grain-boundary motion difficult [28]. As such, the addition of Nb produced a hard phase, strengthened the grain boundary, and refined the grain, thereby increasing the hardness. After aging treatment, the precipitation of Cr₂₃C₆/Cr₇C₃ and NbC gave the coating a high hardness. However, when the aging time reached 24 h, the hardness slightly decreased. Such findings could be attributed to the weakening of solid-solution strengthening, the growth of the eutectic structure, and the coarsening of the precipitate [32].

The substrate had a low hardness and binding properties similar to that of the grinding ring, being prone to adhesive wear [36]. When the Nb element was added to the coating, the grains refined gradually, and the NbC phase was generated. NbC had a mixture of covalent bonds and metal bonds, rendering it difficult to perform solid-state welding during wear and greatly reducing adhesive wear [37]. NbC had a high hardness and was evenly distributed in the cladding layer, forming a skeleton resistant to wear and greatly improving the wear resistance of the cladding layer [6]. When 20% Nb was added, the NbC increased in size and volume fraction, which might have resulted in an increase in structural stress and fragility in the coating, resulting in a decrease in the wear resistance [38]. After aging treatment, the volume fraction of granular NbC increased, and Cr₂₃C6/Cr₇C₃ was precipitated, with a smaller size, producing the effect of diffusion enhancement.

6. Conclusions

In the present study, the failure of the chromium plating layer of a piston rod was analyzed, and an Ni-based composite coating with the addition of Nb, fabricated by laser cladding, was investigated. The following conclusions were drawn:

As the corrosion intensified, the chromium layer would bubble, crack, peel off, and eventually form a corrosion pit, leaving the piston rod unusable. The peeling of the chromium plating layer on the surface of the piston rod was caused by penetrating microcracks, working media, and a harsh environment.

The cladding layer presented a structure morphology of planar crystalline at the bottom, dendritic at the middle, and equiaxial crystalline at the top. The transition trend of the structure of the cladding layer was verified by the solidification parameters obtained from the simulation. By adding Nb powder, a uniformly dense and grain-refining cladding layer could be obtained. The Ni-based cladding layer with the addition of Nb was mainly composed of γ-Ni, Cr₂₃C₆, Cr₇C₃, NbC, and Ni₃Nb. With the increase in Nb content, the carbide size of Cr gradually reduced before disappearing, and NbC presented itself in four forms: particles, dendrites, polyhedrals, and networks. After aging treatment, the volume fraction of granular NbC increased, the carbide of Cr precipitated, and the network eutectic structure grew.

The Ni-based coating with 15% Nb had a hardness of 400 HV0.2 and an optimal wear resistance before receiving any aging treatment. After an aging treatment of 16 h, the Ni-based coating with 15% Nb had a hardness of 580 HV0.2 and had a better wear resistance than the chromium plating layer.