Zhang Jianyun, Chen Xin, Chi Bin, Liu He, Gao Jianbin

Shenyang Blower Group Reciprocating Machine Co., Ltd., Shenyang, Liaoning １１０８６９）

[Abstract]：In ultra-high pressure reciprocating compressors, the piston serves as the core component for achieving gas compression and works in conjunction with the packing ring to achieve dynamic sealing at extremely high pressures. This is crucial for the safe and stable operation of the entire compressor. Currently, some domestic application enterprises have attempted to carry out the domestic production of some compressor spare parts and have achieved certain results. However, the domestic substitution of the piston has not yet achieved a breakthrough. This paper analyzes the structure, material, and working characteristics of the piston, explores the entire process of piston domestication, including raw material preparation and subsequent precision processing, and verifies the method of piston domestication through actual manufacturing.

[Key Words]：Ultra-high pressure reciprocating compressor; piston; tungsten carbide; domestic production; Manufacturing

Chinese Library Classification Number：ＴＨ４５７ Document Code：Ｂ

Article Number：１００６－２９７１（２０２５）０２－００５８－０５

1引言

The ultra-high pressure compressor, as the core power equipment of the high-pressure polyethylene plant, is responsible for increasing the pressure of the medium to ３００ＭＰａThe level should be set to meet the requirements of subsequent production. Currently, only two foreign enterprises in the world are capable of manufacturing this type of large-scale ultra-high-pressure compressors, and they belong to highly technologically monopolized equipment. Some domestic application enterprises have attempted to carry out the domestic production of spare parts for this type of compressor, and have achieved some partial breakthroughs. Among them, the core key components such as the piston for compression and sealing, due to their significant importance in operation, large loss from failures, and high manufacturing difficulty, have not yet achieved a breakthrough in domestic substitution.

This article conducts a discussion on the domestication of the processes of mixing materials, sintering formation, and subsequent precision processing of the tungsten carbide plungers in ultra-high pressure compressors.

2. The structure and characteristics of the parts

２.１ Part structure

The gas compression in reciprocating compressors is achieved by forming a compression chamber through the cooperation of the piston and the cylinder. After the piston moves, the volume decreases, thereby increasing the gas pressure. As the pressure continues to rise, the cylinder diameter gradually decreases. In traditional piston structures, the design and manufacturing of piston rings and support rings have become more challenging. In this situation, eliminating the piston rings and support rings, simplifying the piston to a columnar structure, and utilizing the extremely small gap between the piston and the cylinder to achieve compression sealing have shown certain advantages.

During the development process of ultra-high pressure compressors, in the early stage, there were also pistons with a piston ring structure. The piston was of a combined type, and the entire component was assembled together by tightening the nuts onto the piston rod. The structure is shown in the figure.１As shown.

With the development of practical applications and chemical process requirements, it was discovered that ２５０ＭＰａIn the above-mentioned high-pressure working conditions, the lifespan of the piston rings in this structure significantly decreases, making it impossible to achieve stable operation over a long period. Therefore, the plunger packing type has gradually become the mainstream for ultra-high pressure compressors. The plunger packing type structure simplifies the traditional piston into a smooth and simple cylindrical structure, and combines it with ultra-high pressure packing to achieve dynamic sealing.

２.２ Piston working characteristics and features

It has been mentioned earlier that the pressurization of gases is achieved by the movement of the piston to change the volume of the compression chamber. Therefore, the piston is subjected to a compressive load, and as the intake and exhaust processes continue to cycle, the compressive load undergoes certain fluctuations. Therefore, the piston requires sufficient compressive strength and greater stiffness to ensure the stability of its operation.

Furthermore, under extremely high pressure sealing conditions, the metal sealing ring exerts a strong compressive stress on the piston. This also requires the piston to have sufficient surface hardness to prevent rapid wear or the formation of surface defects, which could lead to major accidents.

Based on these characteristics, sintered tungsten carbide has extremely high compressive strength, a large elastic modulus, and high surface hardness, making it the optimal choice for the material of the plunger.

3. Domestication of raw material preparation process

The raw material preparation process for tungsten carbide products mainly includes mixing, pressing and sintering. ３This process, each stage is of vital importance to the performance of the final product.

３.１ Mixing process

Mixing materials requires determining the proportion of components and the grain size grade. By sampling and analyzing the carbonized tungsten pistons that have been successfully in service on site, we can know that the pistons are mainly a mixture of carbonized tungsten and cobalt. Cobalt exists in hard alloy products as a binder, and the cobalt content is approximately 11%.Left and right, the average grain size is approximately １.０μｍ，The distribution uniformity of cobalt in hard alloys is generally not good.

At the same time, by comparing the finished product performance and sampling analysis results of the hard alloy enterprises with similar ratios and grain grades, their main properties are basically consistent. For example, the compressive strength≥２８００ＭＰａ，Hardness≥ＨＶ１３００，The elastic modulus is approximately ５４０ＧＰａ。Taking into account the working conditions of the plunger, the above indicators also meet the requirements for the strength, stiffness and wear resistance of the plunger.

By comparing with the mainstream hard alloys in the market, the current common alloy composition ratios, grain size grades and hardness of some brands are as shown in the table１As shown.

From this, it can be seen that the component ratio of the piston of the ultra-high pressure compressor belongs to the common ratio in hard alloys, with the grain size level at the medium level, and the mixing degree is generally. The conventional drum-type mixing can meet the requirements. Thus, it can be seen that there are no difficulties in the domestication of the mixing process of the piston, and it is easy to achieve.

３.２ Pressing process

The most common methods in the hard alloy pressing process are extrusion and cold pressing. Among them, the extrusion process is relatively simple. The mixed powder and forming agent are extruded out by the extrusion equipment, and can be directly extruded into rod-shaped billets. However, due to the fact that the raw materials in the single barrel of the production equipment are limited to the maximum ２００ｋｇ，It is impossible to extrude the product mold material of the piston at one time. Besides, the powder produced by the extrusion method contains moisture-containing materials, and the hardness is too soft, which is not conducive to the transfer of such large-scale parts as the piston. Therefore, the extrusion method is not suitable for piston production.

The cold pressing principle is that under the action of external pressure, the gaps between the powder particles are first reduced; as the pressure continues to increase, the powder particles will undergo severe deformation and rupture, and at the same time, under the action of friction force, the powder particles will undergo a certain degree of atomic diffusion, which promotes the further strengthening of the bonding between the powder particles. After the pressing process is completed, a preform with certain strength and density can be obtained. Common cold pressing methods include unidirectional pressing, bidirectional pressing, and floating pressing, etc. To ensure better density of the piston preform, the cold isostatic pressing method is selected for molding. Due to the simple overall structure of the piston and its ordinary cylindrical shape during the pressing stage, using an isostatic press to achieve powder pressing of the piston is the most suitable method.

Cold isostatic pressing can be further divided into dry bag isostatic pressing and wet bag isostatic pressing. The wet bag method is mainly used for large-sized plates or those with a height not exceeding 200mm.The cylindrical shape, after being pressure-molded, is usually placed in a bag, then evacuated, and finally put into another container. ＣＩＰImprove product density. Dry bag type isostatic pressing products can compress large-sized bars. They contain only wax and no water, making it convenient to transfer to carbon plate semi-sintering or direct sintering. Thus, it can be seen that using dry bag type isostatic pressing technology to compress piston powder is more suitable.

After the pressing process is completed, the piston preform needs to be subjected to simple processing to reduce the processing volume after sintering. This process is relatively simple and the main thing to pay attention to is not to damage the compactness of the preform during the processing.

Through the above analysis of the pressing process, the overall procedure is conventional. However, during the actual implementation of domestic production, some local bottlenecks were also identified. The main reason was that the diameter of the plunger was close to １５０ｍｍ，Lengths are close to each other １５００ｍｍ，

This specification size falls into the category of extremely large-sized products in hard alloy. This places higher demands on the production equipment. Among them, the capacity of the dry bag static isobaric equipment is the bottleneck of this process. Fortunately, the existing mature models of this equipment can meet the requirements of plunger pressing, and the domesticization difficulty has been solved by purchasing additional equipment.

３.３ Sintering process

After compression, the piston preform has achieved certain mechanical properties and density. However, this is merely a simple mechanical bonding state of powder particles. The performance is far from the target value. Therefore, the preform needs to be sintered. By applying certain temperatures and pressures, the powder particles interlock with each other and further shrink and densify. The bonding state of the powder particles in the preform is transformed into a polymer structure between crystal grains, achieving the migration of substances within the preform. Eventually, the pores in the piston are largely eliminated, the microstructure becomes uniform and dense, and the strength and density are significantly increased.

The common sintering methods currently in use include atmospheric sintering, hot-press sintering, microwave sintering, discharge plasma sintering, etc. Atmospheric sintering is not suitable for the sintering of tungsten carbide hard alloys due to its abnormal grain growth and unstable quality; microwave sintering technology is not yet fully industrialized; discharge plasma sintering is difficult to apply due to the size limitations of the products. Therefore, in the current hard alloy industry, the most common method is to combine vacuum with hot-press sintering, which is called vacuum hot-press sintering. This technology can reduce the pollution of harmful substances in the atmosphere (such as water, oxygen, nitrogen and other impurities), thereby avoiding undesirable reactions such as decarburization, carbon diffusion, reduction, oxidation and nitridation; at the same time, high-pressure gas is used as the pressure medium to act on the piston preform, allowing it to withstand uniform pressure in the pressurization process, and achieving densification of the material through the combined effect of high temperature and high pressure.

At present, the capacity of vacuum hot-pressing sintering equipment in China has been able to reach that of a single furnace. １７００ｋｇ，The largest size ２.４ｍ，It fully meets the requirements of piston sintering.

３.４ Hybrid firing process

As mentioned earlier, after the carbonized tungsten is sintered, its hardness is ≥ＨＶ１３００，It is extremely difficult to carry out processing on this material under this hardness condition. There are also inclined surfaces for positioning and center holes required for the processing technology on the piston. If all these processing steps are carried out after the complete round cylinder piston blank has been sintered, the overall processing volume will be large, the processing time will be long, and the economic efficiency and efficiency will be poor.

Therefore, before the complete sintering process, the piston pre-form material is subjected to pre-sintering to achieve a certain degree of density and mechanical strength, but the hardness is controlled at ＨＲＣ４０Here, the size variation after the final sintering is controlled within 20% to 25%. At this point, the semi-sintered piston is machined to process the dimensions of the inclined surface, center hole, reduction section, etc. to a certain size. Then, the vacuum hot-press sintering is continued to achieve the final performance requirements. Due to the presence of the semi-sintering process, during the semi-sintering process, the control of the carbon content of the piston preform, as well as the control of the carbon content and cobalt magnet, are also the core control points of this process.。

4Precision manufacturing localization

4.1 Processing Position and Requirements for Tungsten Carbide Pistons

As previously mentioned, the structure of the plunger is relatively simple. However, since it is made by sintering a single piece of tungsten carbide, it has extremely high hardness, making its processing very difficult. At the same time, when operating under extremely high pressure and the resulting load, the plunger must always maintain good alignment performance; otherwise, it will have a significant impact on the stability of the unit. Additionally, to ensure the service life of the sealing ring and avoid excessive frictional heat during operation, the surface roughness of the plunger needs to reach a high precision level. Therefore, the processing and manufacturing of the plunger must overcome its high hardness while also ensuring the ultra-high precision manufacturing quality.

The processing of the plunger must first ensure the accuracy of the 15° internal and external mating inclined surfaces, which is crucial for the centering performance of the plunger; in addition, the R20 transition rounded corner and the R2 transition rounded corner have complex processing shapes. However, they can effectively alleviate the stress concentration problem of the plunger; while the round runout, roundness, and coaxiality of the outer circle need to reach the 0.01mm level, the end face and inclined surface runout should be at the 0.01mm level, and the roughness of the friction surface requires Ra0.01 level. These are all the core concerns in the processing and manufacturing of the plunger.

４.２ Discussion on the processing procedure

First, a sample made of the entire tungsten carbide material was purchased for a process test. Based on the test results of the sample on the lathe, the cutting tool could not be processed, and during the machining process, the tool kept vibrating and the machining was unstable. This determined that the processing equipment could only be a grinding machine, and a diamond grinding wheel was used for grinding.

In view of the cylindrical structure of the plunger and the processing requirements of its end face, end face external cylindrical grinding machines are usually used for processing. The end face and outer circle of the plunger are processed separately using the external cylindrical grinding wheels. The extremely high roughness requirement for the outer circle requires that the grinding machine be equipped with a polishing wheel to achieve this. For the mating inclined surfaces, the following three grinding methods are commonly used:

（１）The worktable of the grinding machine is positioned at an appropriate angle, and the grinding is accomplished by the longitudinal feed of a common cylindrical grinding wheel. However, due to the inclined surface of the piston's mating part, there is １５°，The angle is too large. The maximum swing angle of the worktable of a general grinding machine is only ５°Left and right, and the longer the workbench, the smaller the swing angle. Moreover, the swing angle of the workbench cannot be made extremely precise, which is not conducive to ensuring the processing quality of the inclined surface. Therefore, this method is not applicable to the processing of plunger inclined surfaces.

（２）Using a formed grinding wheel, directly shape the wheel to the desired form and perform the grinding operation with longitudinal feed. However, the formed diamond grinding wheel requires a matching formed silicon carbide dressing wheel for dressing. If the angle needs to be changed subsequently, it is necessary to order a corresponding angle formed dressing wheel again, which can easily lead to structural rigidity.

（３）Using a common cylindrical grinding wheel, the wheel spindle rotates by a certain angle, and the interpolation program is used for inclined feed to achieve grinding. This method only requires that the wheel spindle can rotate, and there are no special requirements for the grinding wheel.

For the R20 transitional rounded corner, only a forming grinding wheel with longitudinal feed can be used for grinding. For the R2 transitional convex rounded corner, if a forming grinding wheel is used, it is necessary to shape it into an R2 concave rounded corner for grinding. Due to the small R2 angle, it is difficult to machine with a forming grinding wheel. Therefore, the R2 can be divided into 3 to 5 transitional inclined planes by rotating the grinding wheel, and then the edges can be ground manually to ensure no sharp corners and avoid stress concentration.

In conclusion, to meet the requirements of the plunger processing, a minimum of 3 grinding wheels (one end face and outer circle grinding wheel, one forming grinding wheel with R20 on one side, and one ordinary outer circle grinding wheel) and 1 polishing wheel are required. Considering the processing cost and efficiency, to avoid frequent wheel replacements during the processing, the grinding wheel holder of the end face and outer circle grinder needs to be rotatable and be equipped with multiple grinding wheels simultaneously.

Through previous technical exchanges with domestic and foreign grinding machine manufacturers, a certain domestic manufacturer's end face and outer circle combined grinding machine caught our attention due to its high precision and significant price advantage. And after more than one year of technical preparation, equipment adjustment and grinding wheel procurement, the piston specimens were subjected to trial processing. The processing procedures are as follows:

４.２.１ Preface One

As shown in Figures 2 and 3, the first step involves grinding the small head end. The equipment needs to be equipped with T1 forming grinding wheel, T2 end face outer circle grinding wheel, and T3 outer circle grinding wheel. One side of the T1 forming grinding wheel is shaped into an R20 round corner, and the other side is shaped into a 15° inclined surface, which is used to grind the outer circle of the intermediate transition part, the R20 round corner, and the 15° conical surface; the T2 end face outer circle grinding wheel is used to grind the end face and outer circle of the small head end; the T3 outer circle grinding wheel is used to grind the 15° outer conical surface; the transition round corner R2 of each surface is ensured to be without sharp corners by rotating each grinding wheel at a certain angle for 3-5 times, and then it is polished evenly by hand. Each surface is ground toＲａ０.２。

４.２.２ Part Two

As shown in Figures 4 and 5, the workpiece is clamped in reverse position, and the equipment remains equipped with the same grinding wheel. The T2 end face outer circle grinding wheel is used to grind the end face and outer circle of the large head end; the T3 outer circle grinding wheel is used to grind the 15° external conical surface; similarly, the transition circular arc R2 between each surface is ensured to be without sharp corners by rotating each grinding wheel at a certain angle for 3 to 5 times, and then it is polished evenly by hand. Each surface is ground to Ｒａ０.２。

４.２.３ Epilogue Three

This sequence requires replacing the grinding wheel. As shown in Figure 6, replace the T2′ fine diamond grinding wheel and the T3′ polishing wheel. Use the T2′ fine diamond grinding wheel to grind the outer circle of the large end to Ra0.05 and then to the final size; then use the T3′ polishing wheel to polish the outer circle to Ｒａ０.０１。

４.３ Development of specialized equipment

Through the above experiments, a customized domestic specialized development compound grinding machine was produced, and the technical details were continuously improved, which can precisely ensure the processing quality of the piston. The special grinding machine uses the Siemens numerical control system to separately control the feed of the grinding wheel frame, the rotation of the grinding wheel frame, and the movement of the worktable. The maximum grinding diameter is 500mm, the maximum grinding length is 1500mm, and the maximum weight of the workpiece is 500kg. The X-axis positioning accuracy of the machine is 0.004mm, the X-axis repeatability positioning accuracy is 0.002mm, the Z-axis positioning accuracy is 0.008mm, and the Z-axis repeatability positioning accuracy is 0.005mm. Moreover, according to the above processing process, grinding the piston requires 1 forming grinding wheel, 1 end face and outer circle grinding wheel, 1 rough diamond outer circle grinding wheel, 1 fine diamond outer circle grinding wheel, and 1 polishing wheel. The forming diamond uses diamond roller interpolation correction, and the outer circle diamond grinding wheel uses silicon carbide grinding wheel for correction. 

The grinding wheel frame of the compound grinding machine can be equipped with 3 grinding wheels. By rotating the grinding wheel frame, the 3 grinding wheels can be used respectively. During the processing, only one grinding wheel needs to be replaced to meet the requirements of piston processing. Thus, it avoids the need to replace the grinding wheels multiple times during the processing, which would affect the processing efficiency. The rotation angle of the grinding wheel frame of the compound grinding machine is 225°, and the rotation resolution can reach 0.0001mm. It can precisely control the rotation angle of the grinding wheel frame, thereby ensuring the accuracy of the inclined surface.

5Conclusion

The breakthrough in the domestic production of the piston of the ultra-high pressure compressor is an important part of the domestic production of the entire compressor. It will also provide more economical and convenient spare parts resources for in-service units, and offer new options for the economic, efficient and controllable operation of the owners.