The surface treatment technologies and processes commonly used in the valve remanufacturing process are introduced, while the current status of the valve remanufacturing industry is analyzed. The development status of plasma surface treatment technology and laser surface treatment technology is summarized, the application of plasma surface treatment method and laser surface treatment method in the valve remanufacturing industry is highlighted, and finally, the future development direction of valve remanufacturing surface treatment technology prospects.
With the increasing industrialization of the manufacturing industry, the demand for industrial valves is increasing, and various valves are used in various industries. In contrast, more and more valves are scrapped or eliminated because of failure or damage. Due to the weak technical capacity of China’s valve manufacturing industry, many major installations’ key valves need to be imported from abroad, especially in extreme conditions of the use of valves. Valve remanufacturing can better solve these problems; valve remanufacturing is also an effective measure to deal with trade protectionism. Valve remanufacturing refers to the bulk production process of repairing used valves and their components. At the same time, remanufactured products should be required to have the same quality and performance as the original product. Therefore, improving the level of remanufactured valves and the ability becomes an urgent problem. Among them, valve surface treatment technology is the key to the valve remanufacturing process. Through the remanufacturing of used valves, effective surface treatment can improve the valve surface hardness, strength, wear resistance and corrosion resistance, etc., to extend the valve’s service life and reduce the probability of valve failure.
1. Traditional surface treatment process
Electroplating refers to the process of electrolytic principle in the workpiece surface plating a layer of metal alloy, alloy metal film can improve the workpiece’s wear resistance, corrosion resistance, oxidation resistance, and fatigue resistance. One of the most widely used alloys in the industry is those formed by zinc and iron group metals. There are 2 main types of zinc-iron alloys for industrial applications: one is an alloy with high iron content, and the other is a zinc-iron alloy with trace amounts of iron. Liu Jian et al. studied the relationship between the gold, copper, and nickel content in the plating solution and the composition of the plating layer after the plating of gold, copper, and nickel alloys on the surface of copper substrates when the gold, copper, and nickel content reaches a certain level, the alloy plating layer has better wear resistance and corrosion resistance.
Phosphating is the process of chemically and electrochemically reacting to form a phosphate chemical conversion film on the surface of the workpiece. The purpose of phosphating is to protect the base metal of the workpiece to prevent corrosion, wear, or oxidation of the base metal of the workpiece. Phosphating can also improve the adhesion of the paint film layer while playing a lubricating role in the metal cold working process. Yi Xiaoyong et al. studied the valve worm gear by phosphating pretreatment of the workpiece solid lubricating film’s surface. Combining the substrate with a potassium titanate whisker can improve the workpiece’s wear resistance and surface strength. The workpiece surface’s friction coefficient is less than 50% of the surface’s friction coefficient without phosphating treatment.
Passivation is oxidizing a metal surface using strong oxidizing agents or electrochemical methods to make the metal surface inactive. Passivation enables the formation of a thick oxide film on the metal surface that is stable and can repair itself in the air. Cheng Yawei et al. studied the passivation treatment method of liquid rocket engine valves, analyzed the valve spool with passivation and bright surface treatment methods through a corrosion resistance screening test, and finally concluded that the valves with passivation treatment method can meet the requirements of use. Sun Guocai and other analyses of the valve in the process of pickling passivation, due to incomplete cleaning and corrosion of the valve bolt causes, the valve bolt connection gap in the residual passivation paste and acid is the main influence of corrosion factors.
1.4 Heat treatment strengthening
Heat treatment is the process of heating, holding, and cooling solid metals and alloys according to predetermined requirements to change their internal organization and obtain the relevant properties. Among them, the purpose of quenching is to make subcooled austenite for martensite or bainite transformation to martensite or bainite organization, and then with different temperatures of the tempering process, to significantly improve the rigidity, hardness, wear resistance, fatigue strength, and toughness of steel to meet the requirements of various mechanical parts and tools. The surface quenching process is widely used in medium carbon tempered steel and ductile iron workpiece because the medium carbon tempered steel, after pretreatment, and then surface quenching, not only can make the workpiece has better overall mechanical properties but also the surface of the workpiece has a better hardness and wear resistance, etc. For example, the spool in the valve is often heat-treated to strengthen its surface hardness and wear resistance; heat treatment strengthening with simple operation, low cost, and high efficiency; however, after strengthening by heat treatment, the workpiece itself will be deformation and cracking and other problems, the need for non-destructive testing of the surface of the workpiece after heat treatment strengthening to ensure that the workpiece has good integrity.
1.5 Surface surfacing
Surface overlay welding is a highly efficient surface treatment process that has been widely used in the manufacturing and repair process of parts in various industries. Fu Huiming et al. introduced the overlay process and selection principles of valve sealing surface overlay materials, compared the relevant domestic and foreign valve sealing surface selection standards, and the performance of some valve overlay materials launched an analysis of the study. Liu Ying compared the different valve overlay process evaluation standards and found the different points of valve overlay process evaluation in terms of test requirements, acceptance criteria, and coverage. Hu Gaolin et al. used automatic overlay welding of Co-based alloys by melting electrode gas-shielded welding on valves. They analyzed the hard valve seal automatic overlay samples’ chemical composition, microstructure, and surface quality. He Fen studied the automatic overlay welding process of nuclear turbine valve sealing surface. In the specified automatic overlay welding equipment, select 16Mn instead of G20Mo5 and select the appropriate welding process parameters so the final welding quality can meet the requirements of the relevant standards.
Carburizing is the process of using some method to make carbon atoms infiltrate into the surface layer of the metal; after carburizing the surface layer of the metal with high carbon content, and then quenched and low-temperature tempering so that the surface layer of the workpiece has good wear resistance and high hardness. In contrast, the inner part of the workpiece still maintains the toughness and plasticity of low-carbon steel. Yanyan Zhang et al. studied the stainless-steel spool treated with an active screen ion carburizing surface hardening process and analyzed the hardness, corrosion resistance, and carburizing layer thickness of the spool, and found that the treatment process did not affect the surface corrosion resistance of the spool and effectively improved the surface strength of the spool. Liu Wei studied the application of a low-temperature ion carburizing process, the use of a low-temperature ion carburizing process for austenitic stainless steel valve parts for surface hardening treatment, and found to obtain a good carburizing hardening effect.
Nitriding is a chemical heat treatment process at a temperature of about 400-600 ℃in the medium so that nitrogen atoms penetrate the workpiece’s surface layer. Xia Shengjian et al. used glow plasma low-temperature nitriding process of austenitic stainless steel valve workpiece was studied, compared the surface roughness, position tolerance, and hardness of the workpiece before and after nitriding, but also after nitriding salt spray test, the results show that: the surface finish and position tolerance of the workpiece afterglow plasma low-temperature nitriding slightly decreased, hardness significantly increased, the surface of the workpiece after salt spray test no corrosion phenomenon The results showed that the surface finish and position tolerance of the valve workpiece was slightly reduced, the hardness was significantly improved, and the surface of the workpiece was corrosion-free after salt spray test. Zhang Min et al. used Nd: YAG pulsed laser to nitride TC4 alloy valves in different ratios of argon and nitrogen gas mixture and studied the effect of nitrogen concentration on the corrosion resistance and hardness of the nitride layer tissue.
2. New surface treatment techniques
2.1 Plasma spraying
Plasma spraying refers to using plasma or plasma jet to heat some metal alloy materials or ceramic materials to melt or partially melt and then shoot at high speed to the surface of the workpiece to form a coating attached to the surface of the workpiece. Plasma spraying technology has the advantages of low cost, high production efficiency, high coating quality, and a wide range of coating materials. Xu Weipu et al. introduced the main thermal spraying technologies such as plasma spraying, flame spraying, and arc spraying, explained the characteristics of the three thermal spraying coatings and their applications in the valve industry, and compared the principles and processes of the three thermal spraying technologies. Gao Mingyu, etc., introduced the progress and research status of plasma spraying technology in copper and copper alloy materials and compared and analyzed the advantages and disadvantages of several surface treatment technologies. The plasma spraying equipment, process, and parameters are studied to make the coating have high bonding strength with the substrate.
2.2 Plasma coating
Plasma cladding is a process that uses an argon-transfer plasma arc as the heat source to melt the alloy powder and the workpiece surface together to form a molten pool, thus forming a layer of molten alloy on the workpiece surface. Plasma cladding is also commonly used to remanufacture parts with complex structures and high surface requirements. Current research developments in plasma cladding technology are introduced by Shi Yun et al. in terms of the influence of the plasma cladding process on the quality of the clad layer, plasma clad alloy coating, plasma clad particle enhanced metal-based composite coating, plasma clad layer quality control methods, and plasma clad technology applications.
2.3 Laser surface hardening
Laser surface treatment technology is to apply a high energy density laser beam to the surface of the workpiece, causing the surface of the workpiece to dissolve and change the microstructure of the material, forming a reinforced layer on the surface of the workpiece, thus improving the corrosion resistance, heat resistance, wear resistance, thermal fatigue resistance, and oxidation resistance of the workpiece. The laser surface hardening process can reduce the workpiece scrap rate, improve the workpiece’s service life, and the workpiece’s surface hardness is higher than that of the workpiece after conventional flame quenching. Liangliang Huang et al. introduced the research status of laser surface hardening and numerical simulation and the development and application at home and abroad, and also analyzed the strengthening mechanism and characteristics of laser surface hardening technology and pointed out the problems of laser surface hardening numerical simulation at the present stage. Wang Chao et al. hardened the surface of Cr12MoV steel with a high-power semiconductor laser. They studied the effects of process parameters such as heating temperature, scanning speed, and lap rate on the surface hardness of Cr12MoV steel and the depth of the hardened layer.
2.4 Laser surface fusion
Laser surface cladding is a process in which external material is added to the molten pool formed by laser irradiation on the substrate surface. The two are rapidly solidified together to form a cladding layer. The laser cladding process has the following characteristics:
- (1) High bonding between the molten metal and the workpiece base;
- (2) High quality of dense and fine cladding layer;
- (3) Controllable thickness of the cladding layer;
- (4) Small heating area and low impact on the workpiece itself. Lin Jixing et al. used CO2 laser to laser clad Co-based alloy coating on the surface of 316 stainless steel valve material. They focused on the effect of laser power on microstructure, corrosion resistance, and coating dilution rate. Shi Shihong, etc., in the petrochemical valve sealing surface austenitic matrix melting Ni-based self-fusing alloy, laser melting of the petrochemical valve sealing surface can obtain a 3.0mm alloy layer thickness.
3. The prospect of valve remanufacturing surface treatment technology
At present, the rapid development of the low-carbon manufacturing industry, research, and development of valve remanufacturing surface treatment technology for the realization of long-term safe use of valves and valve remanufacturing production are of great significance due to the traditional surface treatment technology, low efficiency, high energy consumption, pollution of the environment, etc., will gradually be replaced by new surface treatment technology. The continuous development of green manufacturing, plasma surface treatment technology, laser surface treatment technology, mechanical product remanufacturing and 3D printing technology, and other green manufacturing methods will usher in a broad development prospect.
The use of plasma surface treatment technology is low cost, simple to use, and can obtain excellent performance surface coating. Plasma surface treatment technology in the valve remanufacturing industry has a broad space for development, research, and exploration of more applicability. Plasma surface treatment technology is bound to become a popular direction.
Laser surface treatment in the field of valve remanufacturing technology research and development focused on laser reinforced processing and surface repair technology, laser rapid forming process technology, functional laser coating and material properties, and other directions; these studies can provide the necessary material and process support for the valve key components surface laser reinforcement, remanufacturing, rapid laser forming, etc. As the basic theory of laser surface treatment research continues to deepen, future research on the mechanism of laser surface treatment technology, laser heat treatment technology, nano-surface engineering technology, and other research continues to innovate, laser surface treatment technology will play a greater role in the valve remanufacturing industry.
Author: Han Shoupeng