Wear-Resistant Boronizing for 17-4PH Components of Fluid Pump

Abstract

The fluid pump was the key component of the formation tester; the pump cylinder, piston, and piston rod of the fluid pump often suffer from wear scratches and seal failure, which greatly reduces the service reliability of the instrument. To improve the wear resistance of the fluid pump, 17-4PH steel specimens were treated by boronizing at 750 °C for 20 h. Specimens with and without boronizing were studied by OM, SEM, XRD, microhardness test, and wear resistance test. Layers of about 60 μm thickness formed during boronization contain a mixture of FeB, CrB, and α(B)-Fe phases, which leads to a significant improvement in microhardness (from 336 to 980 HV) and wear rate (from 16.4 × 10⁻⁵ mm³/Nm to 3.3 × 10⁻⁵ mm³/Nm). The pump cylinder and the fluid-pump piston rod were boronized and assembled into the pumping module, which passed the indoor durability test for 90 h and did not show obvious surface wear after 60 h of field experience. For the first time, the boronization process extends the service time of the fluid pump, improving the wear resistance of the pump cylinder and piston rod.

1. Introduction

As the core component of the formation tester, the pumping module is mainly used to clean the polluted zone in the sampling process, to obtain high-purity formation fluid samples and provide direct parameters for reservoir evaluation [1,2]. The fluid pump is the main structure of the pumping module, which is often made of stainless steels. Due to the effects of fluid pressure and hard-cuttings wear over a long time, the pump cylinder, piston, and piston rod of the fluid pump often suffer from wear scratches and seal failure, which greatly reduces the service reliability of the instrument [3,4]. A combination of chemical attack and mechanical abrasion is especially damaging to alloys that form protective surface films [5]. In high-pressure systems, streaming-current-driven corrosion can cause wear on metal orifices, particularly where there is significant variation in wall shear stress. These wear mechanisms can significantly impact pump performance and lifespan, necessitating a careful material selection and preventive measures to mitigate their effects in various operating environments, including marine applications [6]. Therefore, improving the surface performance of the key components of the fluid pump has become an urgent problem that needs to be solved in the application of the instrument.

There are many surface engineering solutions available, such as thermal spraying, anodizing, electroplating, plasma-assisted technology, physical vapor deposition, laser surface treatment to resist abrasion, erosion, and corrosion [7,8]. However, some of these technologies have limitations to being applied on the inside surface of a long tube with a small diameter, such as the pump cylinder block. Additionally, materials obtained by these technologies can suffer from problems such as delamination, flaking, and debonding when exposed to severe conditions involving mechanical contact.

A range of methods have been explored to improve the surface hardness of 17-4PH stainless steel with strong joint strength. Plasma nitrocarburizing at 460 °C has been found to significantly increase surface hardness, with the 8 h treatment showing the best corrosion resistance [9]. Low-temperature plasma nitrocarburizing has also been shown to increase surface hardness, with the modified layer thickness increasing with treatment time [10]. Laser powder bed fusion (LPBF) has been found to enhance the corrosion resistance of 17-4PH stainless steel in sulfuric acid, with the LPBF alloy showing a significantly improved electrochemical performance [11]. Surface mechanical attrition treatment (SMAT) has been found to improve the corrosion resistance of 17-4PH stainless steel, with the formation of a nanostructured surface layer and a protective oxide layer [12].

The steel boronizing process, which is a thermal diffusion process based on chemical vapor deposition (CVD) principles, can be used successfully for the surface protection of long tubes, as well as complex shape components [13]. Additionally, because boronization can generate a reinforced layer on the workpiece surface in situ, this process has the advantages of a strong bonding force with the matrix and is not easily soiled [14,15]. The objective of this study is to improve the wear resistance of key components of fluid pumps by using surface boriding technology.

2. Materials and Methods

2.1. Engineering Background

The main structure of the pumping module is the fluid pump shown in Figure 1, which is a pipeline-type conveying equipment. The piston, piston rod, and pump cylinder block are the key components of the fluid pump. The two sides of the piston are fixed at both ends of the piston rod, and the middle piston is fixed on the pump cylinder block. In the process of downhole operation of the fluid pump, the piston rod moves reciprocally in the pump cylinder under the push of hydraulic oil, to realize the transfer of materials, including mud, formation fluids, etc.

The fluid pump was made of 17-4PH martensitic, precipitously hardened stainless steel. After 60 h of underground work, scratches appeared on the inner wall of the pump cylinder (Figure 2a) along with serious wear of the piston seal ring (Figure 2b), and more particulate wear appeared on the piston rod (Figure 2c), as shown in Figure 2. 17-4PH steel samples were used in order to study the strengthening effect of the surface boriding technology.

2.2. Boronizing Process

The 17-4PH steel produced by Chongqing Iron and Steel (Group) Co., Ltd. (Chongqing, China) was the source material for this essay.

Table 1. Chemical composition of the investigated 17-4PH steel (wt.%).

C	Mn	Si	Cr	Ni	Cu	S	P	Fe
0.038	0.39	0.15	15.45	4.33	3.16	0.016	0.026	Balance

displays its chemical makeup. Before testing, the as-forged 17-4PH steel was annealed for one hour at 1050 °C, water-quenched, and then tempered for two hours at 620 °C. Following heat treatment, 17-4PH steel’s Rockwell hardness was found to be 35 ± 0.5 HRC.

The heat-treated specimens were sliced. Among them, the average surface roughness was finely smoothed to reach a Ra value of 0.8 μm. The wear specimen is a disc with a diameter of 30 mm and a thickness of 8 mm.

All specimens were cleaned in ethanol using an ultrasonic cleaner for ten minutes prior to the boronization process. The powder that was created was a blend of SiC, BF4K, and amorphous boron. The samples were treated for 20 h at 750 °C in a specially designed powder furnace.

2.3. Structural Characterization

Using field-emission scanning electron microscopy (SEM; FEI Quanta 250; Shanghai, China) and Optec MIT15010006 optical microscopy (Shanghai, China), surface structural alterations were examined on specimen cross-sections. The main technical indicators of FEI Quanta 250 are as follows: the maximum magnification is 200,000×, and the resolutions of the secondary electron and backscattered electron images are 5 μm and 2.5 μm, respectively. Optec MIT15010006 is equipped with an adjustable lighting system that includes color filters and a 20 W/6 V halogen lamp. Before observations, cross-sections of the specimens were mechanically polished by a conventional metallographic procedure, and a ferric chloride solution [16] (5 g FeCl₃ + 500 mL HCl + 100 mL H₂O) was used to infiltrate the cross-section for 10 s for the chemical assault.

Cu Kα radiation was used in X-ray diffraction (XRD) (Ultima IV; Figure 3) to detect the phase components. In this experiment, the scanning speed was 10 degrees per minute, and the scanning angle varied from 10 to 90 degrees. JADE 6.0 software was used for the data analysis and phase calibration.

2.4. Microhardness Measurements of Surface and Microhardness Depth Profiling

The HXD-1000TMC microhardness tester in Figure 4 (Shanghai, China) was used to assess the surface microhardness, using a 300 g test load and a 15 s holding period. Prior to testing, the oxide coating was removed from the specimen surfaces by wet grinding them with SiC grinding papers. In addition, measurements of cross-sectional hardness were conducted using a 300 g test load and a 15 s holding period. A minimum of three measurements were averaged to obtain each hardness value. Traditionally, the depth at which the hardness rises to 50 HV above the core hardness has been considered to be the hardened case depth [17,18].

2.5. Wear Experiments

The BKD-BMHX 002 wear and friction testing system (University of Science and Technology Beijing) is shown in Figure 5. The specimen and the Si₃N₄ cemented carbide ball were used as friction pair in this wear test, with the specimen spinning in the axial direction and the Si₃N₄ ball (5 mm in diameter) rolling over the specimen surface. The specimens were worn at 205 °C (the maximum temperature of the inner wall of the fluid pump cylinder in service is 205 °C). The angular velocity of the specimens was 400 r/min, the test load was 100 N, and the wear time was 10 min. The specimens were cooled to room temperature before being removed from the tests.

The volume wear rate (W_s) had been accumulated as follows [19].

$$W_S={\displaystyle \frac{V}{N\times d}}$$

(1)

$$V={\displaystyle \frac{Lh\left(3h^2+4b^2\right)}{6b}}$$

(2)

where V is the wear volume (m³), N is the load (N), d is the sliding distance (m), L is the length (perimeter) of the wear tracks (mm), h is the depth of the wear tracks (mm), and b is the width of the wear tracks (mm). The morphology of the wear tracks was acquired using a profile-analyzing laser microscope VK-X250K (produced by KEYENCE Corporation in Shanghai, China) with shape-measure function, which yielded the ‘h’ and ‘b’.

3. Results and Discussion

3.1. Microstructure and Morphology Analysis

The results of the microstructure and surface phase analysis of boronized samples are shown in Figure 6. It can be seen that the prepared permeating layer has a uniform thickness and can completely cover the surface of the 17-4PH matrix. However, the morphology observed by optical microscope (OM) and scanning electron microscope (SEM) is different, mainly reflected in what follows: (1) the linear distribution of compounds between the white layer and the diffusion layer is different in color, the former being black, and the latter being bright white; (2) OM cannot observe the diffusion layer, while SEM can clearly see the contrast difference between the diffusion layer and other parts. To facilitate expression, the morphology observed by SEM was used to describe the infiltration layer. From the surface to the center, the infiltration layer can be divided into the compound layer, diffusion zone, and substrate. Combined with the morphology observation and phase analysis, it can be determined that the compound layer on the surface is composed of FeB and CrB, in which FeB is the main component of the compound layer, and CrB is distributed in a white and bright line near the diffusion layer [20]. The compound layer is the most critical layer in boriding, which greatly improves the hardness of stainless steel [21]. The diffusion zone is the transition region between the compound layer and the substrate. This region can be clearly observed in Figure 6b, and its corresponding phase is mainly α(B)-Fe [22].

According to the experimental results in Figure 6, Figure 7 shows that the compound layer is composed of FeB and CrB with a thickness of approximately 10 μm, the diffusion layer is composed of α(B)-Fe with a thickness of approximately 50 μm, and the total thickness of the infiltration layer is approximately 60 μm.

3.2. Cross-Sectional Hardness Profile

Figure 8 delineates a plot of microhardness as a function of depth for the cross-sectional analysis of a boronized specimen. A pronounced decrease in hardness is evident, with a rapid transition from a surface hardness of 980 HV to 620 HV at a depth of 25 μm, and further decrease to 387 HV at a depth of 60 μm. The hardness of the core region of the 17-4PH steel after boronization was found to be approximately 336 HV. Consequently, the depth of the hardened case, defined as 50 HV above the core hardness, was estimated to be roughly 60 μm. This finding aligns with the total thickness of the infiltration layer as inferred from the micromorphology analysis presented in Figure 7. Figure 6 illustrates that the thickness of the compound layer, denoted as the ‘white’ portion in Figure 6b, is approximately 10 μm. Beneath this ‘white’ layer, a diffusion zone is observed. The microstructural differences between the ‘white’ layer and the diffusion zone are significant, which accounts for the abrupt decrease in cross-sectional hardness.

3.3. Wear

After being worn at 205 °C, the 17-4PH stainless steel and its boronized surface-enforced specimen are shown in Figure 9. The microscopic morphology and size of the wear marks are shown in Figure 10. To ensure the accuracy of the data, the width and depth of the wear marks of the sample in this experiment were measured three times. In Figure 10, the top color lines corresponded to positions of the three measurements.

Firstly, the average values of the three maximum depths and widths of the wear marks of the samples before and after boriding were calculated from Figure 10. Then, the bulk worn loss was calculated using Equation (2) (see

Table 2. Bulk worn loss of 17-4PH steel with and without boriding after wear at 205 °C for 10 min.

	Average of Three Maximum Wear Depths/μm	Average of Three Maximum Wear Widths/μm	Bulk Worn Loss/mm3
17-4PH steel	69.5	1415.1	4.125
Boronized 17-4PH steel	26.7	744.7	0.833

). Finally, the wear rate was calculated using Equation (1) (see Figure 11). After wear at 205 °C for 10 min, the wear rate of boronized 17-4PH steel was only 3.3 × 10⁻⁵ mm³/Nm, while that of bare 17-4PH steel was 16.4 × 10⁻⁵ mm³/Nm. After boronizing, the wear rate was reduced by about 5 times, and the boronizing layer had a good wear resistance.

4. Indoor Durability Test and Field Experience

Using the boronizing process, the surface treatment of the pump cylinder and piston rod is carried out, and the pumping module is assembled. An indoor durability test was carried out to verify the reliability of the sealing system of the pumping module and the surface wear resistance of the boronized pump cylinder and piston rod.

The test scheme is shown in Figure 12. The measurement and control unit, hydraulic power unit, and pumping module (fluid pump module) are connected. To detect the status of the pumping module, the values of each pumping module sensor are transmitted to the data-processing system through the information exchanging system and the acquisition system. The resistance wire heats the pumping module to simulate the downhole service temperature, the flow valve controls the flow to simulate the suction pressure of the fluid pump, and the pumping module continuously pumps the mud and holds the pressure test halfway to simulate the downhole operation status. After 90 h of indoor testing, all functions and sensors of the pumping module work normally, which can meet the requirements of a long-term downhole pumping operation.

Until now, the pumping module has been operated more than 40 times in the Bohai Sea and the South China Sea. It worked well, and problems such as the low sampling efficiency and poor sample identification effect of complex reservoirs were successfully solved. The field application of the pumping module is shown in Figure 13.

After the fluid pump worked for 60 h, the instrument was disassembled and properly cleaned, and then the inner wall of the pump cylinder, the piston, the piston rod’s surface and the seal ring were checked. The results were as follows: The inner wall of the pump cylinder was inspected by endoscope, and the surface infiltration layer was intact without obvious scratches and wear (Figure 14a); The surface of the piston and piston rod were intact without obvious scratches and wear (Figure 14b,c); There was no obvious damage to the integrated sealing system (Figure 14b). According to the above field application, compared to the macromorphology of the untreated key components of the fluid pump after 60 h of underground operation (Figure 2), the boronized fluid pump can better solve the problem of surface wear, improve the reliability of the pump module, and meet the requirements of a long-term application in the downhole environment.

5. Conclusions

The boronized layer was prepared on the surface of 17-4PH steel samples, followed with structure characterizations and properties tests. After that, the pump cylinder and the fluid-pump piston rod were boronized, and the indoor durability test and field application was carried out. The conclusions that can be drawn from this investigation are:

(1)

The boronized layer consists of a compound layer and a diffusion layer, with a total thickness of 60 μm and a surface hardness of 980 HV. The compound layer is composed of FeB and CrB, while the diffusion layer is composed of α(B)-Fe. The wear rate of 17-4PH steel was reduced from 16.4 × 10⁻⁵ mm³/Nm to 3.3 × 10⁻⁵ mm³/Nm after boronizing.

(2)

The pump cylinder and the fluid-pump piston rod were boronized and assembled into the pumping module, and the pumping module worked well after 90 h of indoor testing and 60 h of field experience.

(3)

Boronization can better solve the problem of surface wear and extend the service time of the fluid pump due to improving the wear resistance of the pump cylinder and piston rod.