As one of the advanced surface modification technologies, laser cladding provides a new direction for the modification and repair of high-end valve products, and effectively solves the problems of easy detachment, pore crack defects, and low production yield existing in traditional surface modification technologies. This paper first introduces several typical failure forms of valves and explains the significant advantages of laser cladding technology in the modification and enhancement of valve sealing surfaces. Then, combined with the actual demands of surface modification and repair in the valve industry, the research status of laser cladding technology for key components such as nuclear valves, throttle valves, butterfly valves, ball valves and pump valves in valve products is reviewed from the aspects of cladding material design, cladding process optimization and statistical simulation. Finally, the current problems to be solved, future key research directions and development trends of laser cladding of high-end valves are analyzed and prospected.
1 Introduction
As the core components of various types of key equipment, high-end valves play a key role in the material transportation hub in the fields of petroleum, petrochemical, metallurgy, nuclear power and military industry, and play a vital role [1-3]. In recent years, the valve industry has developed rapidly and made significant contributions to the national economic construction. According to reports, the main business income of large-scale enterprises in the pump and valve industry reached 260 billion yuan in 2021 [4]. As my country vigorously promotes the localization of high-end equipment manufacturing, higher requirements are put forward for the quality and reliability of domestic high-end valve products, which also brings new challenges and opportunities to valve companies. The “Hualong One” nuclear primary regulator fast pressure relief valve, nuclear secondary main steam isolation valve, and CAP1400 series key valves developed by China Nuclear Su Valve have broken the foreign monopoly on key technologies in nuclear power engineering and successfully filled the domestic gap [5]; Chongqing Chuanyi Control Valve has developed and manufactured the NPS6 Class1500 high-pound ultra-low temperature deep-cold ball valve for LNG, which has reached the international advanced level in performance indicators and can be promoted and applied in low-temperature devices such as liquefied natural gas [6].
However, my country’s valve manufacturing industry developed relatively late, and there is still a gap in overall technical level with developed countries, especially in the reliability and service life of high-end valves. The valve sealing surface is one of the key factors affecting valve quality. When the product is in a harsh service environment and the working conditions are complex and changeable, it is very easy to suffer from surface damage in the form of corrosion, wear and other failures, which seriously endangers the safe service performance of the equipment. In particular, in the fields of coal chemical industry and nuclear engineering, more stringent requirements are put forward for valves. In order to improve the service life and reliability of valves, enterprises currently usually use surface modification technologies such as spraying [7-8], plasma arc cladding [9-10], and argon arc cladding [11-12] to improve the wear resistance and corrosion resistance of the product surface, and have achieved good economic benefits. Huang Yusheng et al. [13] prepared Cr3C2-NiCr coating on 30CrMo steel substrate by supersonic spraying process to improve the wear resistance and corrosion resistance of sealing ball valves. Studies have shown that the coating has excellent wear resistance and corrosion resistance, and the bonding strength with the substrate exceeds 70 MPa. Lin Zhong et al. [14] used nanosecond laser to ablate micro-pits on the substrate surface and combined it with supersonic flame spraying technology to prepare WC-10Co-4Cr coating. The corrosion resistance of the coating was improved by about 31.98% compared with surface polishing or sandblasting. Xie Yuanfeng et al. [15] applied plasma surfacing technology to the surface hardening of all-metal sealed butterfly valves to improve the product surfacing efficiency and quality. Compared with traditional arc surfacing, the overall production cost was reduced by 50%. With the rapid development of high-end manufacturing industry, the demand for high-performance valve products is becoming more and more urgent. When facing complex application scenarios, the existing surface modification technology is increasingly difficult to meet the production and manufacturing requirements. As a “universal processing tool”, laser processing technology can be used for material surface modification, providing a new option for the modification and repair of high-end valve products.
Laser cladding technology is an advanced surface modification technology with advantages such as green energy saving, enhancement and remanufacturing[16]. It can solve the problems of easy detachment, high porosity and large deformation of current surface modification technology, and meet the needs of performance improvement, life extension and repair of high-end valves. This paper introduces several common failure forms of valve surface, and combines the actual needs of surface modification in the valve industry. It summarizes the research progress of laser cladding technology in the valve industry from three aspects: cladding material design, process optimization and statistical simulation. Finally, it looks forward to its future.
2 Valve failure forms
Valves will have different types of failure during service, that is, the valve cannot complete the expected function due to structural damage during use[17]. According to the common failure phenomena and characteristics of valves, their failure forms can be divided into leakage failure, wear failure, corrosion failure, fracture failure, etc., as shown in Figure 1. Fracture failure refers to the fracture failure forms such as plasticity, brittleness, fatigue and stress corrosion in valve parts. This type of failure often occurs in the valve body, among which stem fracture is the most common. Failure forms such as leakage, wear, and corrosion often occur on the valve sealing surface. For example, too little contact between the valve sealing surface and the valve seat, grooves on the sealing surface caused by high-speed erosion of the fluid medium, repeated collision and friction between the valve seat and the valve core, and interference contact between the valve seat and the valve stem resulting in a reduction in the sealing pressure ratio can all lead to failure of the valve sealing surface. Among them, wear failure and corrosion failure are the two most typical ways of valve sealing surface damage.
2.1 Wear failure
Wear failure generally refers to the failure form in which the valve product repeatedly contacts with solid particles and fluid media during service, causing wear, the surface state of the valve components changes, and the original function is lost [18]. Wear failure forms can be divided into two categories: corrosive wear and abrasive wear. Corrosive wear refers to the failure form in which the flow channel structure is destroyed due to corrosive media during valve service. It often occurs in the flow-passing components (impeller, pump body, etc.), the inner wall of the pipeline, and the sealing pair. Abrasive wear of valves is caused by the presence of a large number of solid particles in the flowing medium, which repeatedly washes and impacts the inner surface of the valve during flow, eventually leading to valve failure. The main causes of wear failure are: service environment, structural design and material factors. When the service environment of the product cannot be changed, optimizing the structure and optimizing the material are the main measures to improve valve wear failure.
2.2 Corrosion failure
Corrosion failure refers to the failure form caused by the corrosive medium when the valve product is in service. It can generally be divided into two categories: galvanic corrosion and crevice corrosion [19]. Galvanic corrosion usually refers to the corrosion of dissimilar metals between valve components. Due to the difference in corrosion potential, the low-potential metal corrodes faster and the high-potential corrosion slows down. Galvanic corrosion often occurs in valve components that are welded or plated with materials different from the base material. Therefore, the galvanic failure form in the valve is first manifested as the failure of the welded layer or the plating layer, which then leads to direct contact between the product and the corrosive medium, resulting in galvanic corrosion. Crevice corrosion is a failure form caused by the existence of gaps in valve products. In essence, crevice corrosion is a special galvanic corrosion generated in extremely small gaps. In order to avoid corrosion failure, anti-corrosion substrates and surface coating treatments are often used. The use of anti-corrosion substrates can directly and effectively solve the problem of corrosion failure, but the price of anti-corrosion substrates is high and it is extremely difficult to achieve large-scale application; surface coating treatment is currently the most widely used anti-corrosion method by various valve companies. According to the service conditions of the workpiece, the required corrosion-resistant materials can be locally welded, which can greatly improve the corrosion resistance of the valve. Table 1 shows the common failure forms and causes of valve sealing surfaces.
In view of the wear, corrosion and other failure problems that are prone to occur in valve products, conventional surface modification technologies have the disadvantages of low coating bonding strength and large workpiece deformation, which are difficult to meet the stringent manufacturing requirements of high-performance valve products. Laser cladding technology has the characteristics of green, high efficiency, and easy automation. While meeting the requirements of valve surface performance enhancement, it also takes into account high reliability and durability. It has become a reliable and highly potential surface modification and repair technology in the valve industry.
3 Application of laser cladding in valve surface strengthening and repair
Laser cladding is a typical representative in the field of laser processing. It significantly improves the wear resistance, corrosion resistance and oxidation resistance of the workpiece surface by adding one or more layers of selected materials on the substrate surface. The processing process is as follows: formulate a laser cladding plan based on the appearance and size of valve parts, the surface modification/repair parts and technical requirements; carry out laser cladding processing; inspect and test the surface modified/repaired products, and complete the cladding final forming after machining[20]. Using laser cladding technology to modify/repair the valve surface can make high-end valve products have excellent comprehensive performance and even further enhance the wear resistance and corrosion resistance of the surface.
At present, the research hotspots of laser cladding in the valve industry are mainly concentrated in the directions of cladding material design, cladding process optimization, statistical simulation and path planning. The research work relationship is shown in Figure 2. The selection of cladding materials fundamentally determines the performance of the cladding layer; cladding process optimization controls the quality of valve surface modification; statistics and simulation provide theoretical guidance for cladding layer performance prediction and cladding process optimization; cladding path planning provides a feasible solution for the practice of laser cladding of complex valves. With the large-scale application of laser cladding technology in the valve industry, higher requirements will inevitably be placed on the performance of the cladding layer. Through the research and development of materials, process optimization and statistical simulation, a systematic laser cladding technology research and development system for the valve industry will gradually be formed.
3.1 Cladding material design
The selection and proportion of cladding materials are crucial to the performance of the cladding layer on the valve surface[24]. Generally, the performance requirements of the cladding layer depend on the service environment of the valve product. Due to the large number of valve types and complex working conditions, the hardness, wear resistance and corrosion resistance of the cladding layer under high temperature and high pressure conditions are mainly required. Among them, the enhancement of the hardness, wear resistance and corrosion resistance of the cladding material can be achieved through second phase strengthening, fine grain strengthening and other methods.
Adding ceramic particles and rare earth oxides is currently a feasible solution to improve the hardness and wear resistance of cladding materials. Li Mingzhe et al. [25] added Al2O3 ceramic particles on the basis of Ni/Al, and refined the structure to achieve a hardness of 650HV. This is mainly because there are more dispersed Al2O3 particles in the NiCrAl/Al2O3 cladding layer, which inhibits the plasticity of the matrix during friction, enhances the supporting capacity of the matrix, and thus improves the wear resistance of the cladding layer. Liu Jia et al. [26] prepared a nickel-based tungsten carbide coating containing rare earth Y2O3 on the surface of 40Cr10Si2Mo valve steel. The addition of rare earth Y2O3 can refine the grains, purify the molten pool and reduce the dilution rate, significantly improving the hardness and wear resistance of the coating. Among them, the average hardness of the nickel-based WC coating with 1% Y2O3 added is 1054 HV, and the wear mechanism is abrasive wear. The wear resistance is improved by 26.7% compared with the nickel-based WC coating without rare earth addition.
In addition, in the application of nuclear power valves, in order to avoid the radiation activation of elemental cobalt and meet the operating requirements of the nuclear valve sealing surface, relevant scholars have developed a variety of cobalt-free alloy powders for strengthening the nuclear valve sealing surface. Shi-hong Shi et al. [27] designed a cobalt-free iron-based alloy powder FeCr-1 for the nuclear valve sealing surface. The coating contains high-hardness fine carbides such as Fe3C, Fe5C2 and Fe0.4Mn3.6C, thereby achieving dispersion strengthening. At the same time, these carbides produce a large number of dislocations in the coating, forming solid solution strengthening and second phase strengthening. This type of iron-based powder has the characteristics of high temperature resistance, good wear resistance and good corrosion resistance. Geyan Fu et al. [28] developed a cobalt-free Ni-Cr based alloy powder (Ni-3) to meet the requirement of cobalt-free surface coating for nuclear power valve seals. The cladding layer mainly contains strengthening phases such as M23 (CB)6, M7C3, Ni3Al, etc., with an average hardness of 500 HV, and has good wear resistance, good corrosion resistance and high temperature resistance. Aiqin Xu et al. [29] developed two types of alloy powders, nickel-based and iron-based, according to the requirement of cobalt-free for nuclear power valves. The performance of nickel-based alloy is close to that of cobalt-based alloy, and it has excellent performance in high temperature corrosion environment; iron-based alloy coating has good weldability and wear resistance. Compared with cobalt-based alloy, iron-based alloy has a huge price advantage in the large-scale application of nuclear power plant valves. T. Shi et al. [30] developed a cobalt-free nickel-based alloy (Ni-SD) and applied hollow annular spot laser cladding technology to the surface modification of nuclear valve sealing surface. The results show that the main phase of the Ni-SD coating is γ-Ni, as well as a small amount of carbides and borides (Figure 3). The microhardness of the cladding layer can reach 700 HV0.3, which is much higher than the commonly used cobalt-based alloy Stel-lite6 (500 HV0.3) currently used in nuclear valves. It has high temperature resistance and excellent wear resistance.
3.2 Laser cladding process
The laser cladding process parameters mainly include laser power, powder feeding rate, scanning speed, cladding path, etc. The process parameters will directly change the interaction between laser, powder and substrate, resulting in changes in the surface morphology and microstructure of the cladding layer, thereby affecting the cladding quality, mechanical properties and cladding efficiency of the coating.
Liu Fuguang et al. [31] conducted a laser cladding Stellite 6 cobalt-based alloy repair test to solve the problem of on-site repair of valve sealing surfaces of supercritical thermal power units. The results showed that the hardness of the cladding layer reached 450~500 HV, and the impact energy at room temperature reached 40~60 J. The repair layer had a good metallurgical bond with the SA182 F91 heat-resistant steel substrate. However, the cladding layer was prone to defects such as poor fusion and pores, so it was necessary to select appropriate cladding process parameters. Cheng Hao et al. [32] used iron-based powder as the sealing surface cladding material to solve the problems of frequent failure of the combination valve sealing surface caused by the large number of water injection pumps in Changqing Oilfield and long-term operation. After cladding, the service life of the product was extended by more than 0.6 times, and the production cost was saved by more than 38% compared with the new product, achieving cost reduction and efficiency improvement of oilfield water injection pumps. Wu Qiang et al. [33] used semiconductor laser to laser clad Ni-based alloy coating on the surface of 2205 duplex stainless steel of butterfly valve. The minimum corrosion rate of the cladding layer was 3.47×10-5A/cm2, and the maximum hardness was 680 HV, which was about 2.5 times the hardness of the substrate. Cui Lujun et al. [34] used a high-power fiber-coupled semiconductor laser to prepare multiple cobalt-based alloy coatings on a ZG45 plate of pump valve material. The average microhardness of the cladding layer reached 586.5 HV0.3, which is more than 2.8 times that of the substrate. The salt spray corrosion resistance of the cladding layer was significantly improved. Cui Gang et al. [35] laser clad a cobalt-based tungsten carbide metal ceramic coating on a 410 stainless steel valve seat substrate. The results showed that under the conditions of a laser power of 3.5 kW and a scanning speed of 100-150 mm/min, a cladding layer with good surface formation and performance could be obtained. Shu-Shuo Chang et al. [36] used coaxial laser cladding process to clad the cobalt-based alloy Stellite 6 on the valve seat surface of the control valve. The valve seat cladding layer showed excellent impact wear resistance, that is, impact cracking resistance under high load. Moo-Keun Song et al. [37] used different parameters to conduct laser cladding tests on the surface of marine engine exhaust valves. The prepared gradient coating had no pores and crack defects, and the average hardness of the cladding layer was higher than 529 HV. Yinping Ding et al. [38] conducted laser cladding cobalt-based alloy tests on the sealing surface of the control valve seat and prepared a mixed cladding layer composed of 70% Stellite 3 + 30% Stellite 21. Compared with Stellite 6, the hardness and wear resistance of the mixed coating were enhanced, and the cracking tendency of the laser cladding layer was improved. Xingchen Lin et al. [39] proposed a three-beam laser source cladding method to solve the crack problem of stainless steel ball valve laser cladding layer. The three beams were used for dynamic local preheating, laser cladding and dynamic local tempering respectively, and the Ni60+WC powder cladding processing of stainless steel ball valve was realized. Compared with the traditional cladding method, the ball valve sample had no cracks and pore defects, as shown in Figure 4. Table 2 is a statistical table of the characteristics of laser cladding coatings of various valves.
For the laser cladding of complex valve structures, relevant scholars have carried out research by using path planning and R&D special machines. Jiang Xiaoxia [23] designed a trajectory planning algorithm for laser cladding of the sealing surface of the three-eccentric butterfly valve disc under the conformal inclined cone fixture and the flat universal fixture to strengthen the sealing surface of the three-eccentric butterfly valve disc. The results show that the laser cladding platform based on a six-axis robot + rotary table can realize the cladding process of the three-eccentric butterfly valve. Yu Mingwei [41] developed a set of semiconductor lasers combined with a five-axis four-linkage mechanism laser cladding processing system for ball valves in response to the needs of laser cladding processing. At the same time, according to the spatial motion trajectory of the ball valve during laser cladding, the algorithms and programs for part positioning, parameter setting, feed calculation, etc. were realized, thereby realizing the laser cladding conformal processing program control of the ball valve. In order to achieve high-quality repair of damaged valve cores, Shu Linsen et al. [42] proposed a valve core laser cladding process method based on path posture planning and geometric reconstruction, which can effectively follow the valve core remanufacturing boundary surface for cladding. The prepared coating has no defects such as pores and cracks. Figure 5 is a schematic diagram of the prefabrication design process of the laser cladding remanufacturing boundary surface of the valve core part.
3.3 Statistics and simulation
The laser cladding process is a dynamic process affected by multiple factors and involves complex physical and chemical changes. This characteristic leads to a large investment in the research of laser cladding, but the efficiency is very low. Therefore, relevant scholars use statistical and simulation methods to accelerate the research process of valve laser cladding technology.
The use of statistical algorithms can optimize iterative cladding test work, reduce trial and error costs, and accelerate the completion of product development cycles. Song Shaodong et al. [43] used the stepwise regression analysis method to establish a regression model for cladding process parameters, and optimized it based on a multi-objective genetic algorithm to obtain the optimal process parameters. After algorithm optimization, the microhardness of the cladding layer increased by 21.7%, the coating formation was better, and the quality of the cladding layer was significantly improved. Lei Che et al. [44] studied the effects of laser power, scanning speed, powder feeding speed and WC content on the cladding layer through orthogonal experiments, and proposed a fuzzy comprehensive evaluation method including macro surface quality, microstructure, microhardness, wear resistance, oxidation resistance, density and corrosion resistance. The results show that when the laser power is 1.1 kW, the scanning speed is 800 mm/min, the powder feeding rate is 20%, and the WC content is 20%, the hardness, oxidation resistance and corrosion resistance of the coating are optimal. Jianbin Wang et al. [45] calculated the optimal process parameters of laser cladding by genetic algorithm, and established a quantitative relationship model between process parameters and valve performance by neural network method. Through parameter encoding, initial group setting, fitness function design and algorithm control parameter setting, the optimal configuration scheme of valve laser cladding genetic algorithm control parameters was obtained.
Based on heat transfer, laser heat source model, thermoelastic-plastic theory, phase field theory, etc., a finite element model is established to simulate the temperature field and stress field of laser cladding, and predict the performance of the cladding layer, which can provide a theoretical reference for actual cladding experiments. Cai Chunbo et al. [46] used SYSWELD finite element analysis software to numerically simulate the laser cladding process of the gate surface, analyzed the laser cladding stress field and temperature field, and obtained the distribution law of laser cladding residual stress on the gate surface. The study shows that the residual stress in the gate cladding layer gradually increases with the negative direction of the z axis. The closer to the circular hole area in the gate, the greater the tensile stress, and the crack tendency increases significantly. Figure 6 shows the z direction and equivalent residual stress distribution of the gate. Zhang Weibo [47] used numerical simulation to analyze the laser cladding process of valve core parts, realized the temperature field simulation of valve core parts, and measured the residual stress value of the valve core part surface cladding layer based on the blind hole method. The results show that the simulation value is basically close to the measured value, which can meet the actual working conditions.
4 Summary and Outlook
At present, my country and Western countries are increasingly competing in the fields of economy, trade, high-end technology, etc., and the problem of import restrictions on high-end valve products is becoming increasingly severe. Domestic substitution has become imperative. In view of this, domestic high-end valve manufacturing companies and scientific research institutions have carried out a series of valve laser cladding related research, mainly focusing on valve cladding materials, cladding processes, statistics and simulation.
(1) The research and development of valve cladding materials has the characteristics of high hardness, high temperature resistance, excellent wear resistance and corrosion resistance. Cladding materials are the decisive factor in the comprehensive performance of high-end valve cladding layers. The development of materials with excellent performance and low price is a prerequisite for the large-scale application of laser cladding technology in high-end valves. Therefore, in order to meet the high reliability and high service life requirements of high-performance valve products and improve the competitiveness of domestic high-end valves, the development of a genome system for high-end valve special materials is one of the key research directions.
(2) The laser cladding process improves the quality of valve cladding layers, cladding efficiency, and realizes the cladding manufacturing of complex structure valves. Cladding technology plays a key role in the manufacturing process of high-end valves. Through parameter matching and process optimization to avoid the generation of pores and cracks, trajectory planning algorithm + special machine method is adopted to realize the cladding production of complex structure valves. Laser cladding of complex structure valves is a difficult point in current application.
(3) Statistics and simulation can reduce the R&D cost and trial-and-error cycle of valve laser cladding. The method based on regression analysis can quickly form an optimized cladding process plan. The application of path planning and geometric reconstruction lays a theoretical support for laser cladding remanufacturing. The simulation of stress field and temperature field provides a theoretical reference for the laser cladding of valve products.
(4) Embed laser cladding technology into the design process of valve products. From the perspective of product life cycle management, valve modification, enhancement and repair technology should be considered to avoid structural form and material selection restricting the application of laser cladding technology.
