Research Progress on Anti-cavitation and Wear Protection of Pumps

   Cavitation, abrasion and the joint damage of the pump have always been an important issue in the operation, maintenance and management of the pump. The traditional surface protection materials and processes are far from meeting the requirements of the pump for cavitation and erosion resistance. In order to enhance the ability of anti-cavitation and erosion on the surface of pump over-current components, besides the use of stainless steel or other cemented carbide materials to manufacture blades and impellers, the surface protection technology is continuously tested. This article describes its progress as follows.
1 surface protection technology research status quo
1 1 Introduction to surface protection technology 1 1 1 non-metallic coating research China in the 20th century, 60, 70 began to epoxy resin and its compounds applied to the pump anti-erosion protection. In the 1980s have also developed a composite nylon coating, polyurethane coating, imitation ceramic coating and rubber coating and other non-metallic coatings. In addition, there are some non-metallic coatings formed using materials such as quick-acting titanium rubber, rubber, enamel, ceramic, glass and the like, which are less used due to complicated processing techniques and the like. The 20th century, 90's, also introduced in the industrial areas of the United States Devcon repair agent, ARC composite coatings, synthetic rubber and other polymer materials. These non-metallic coating materials in pumping station harsh environment, often due to the coating and the metal matrix bonding ability and the material itself is not enough hardness, it is difficult to achieve the desired anti-cavitation, anti-erosion effect.
1 1 2 Metal Coatings [2] Metal surface coatings are also widely used in pump anti-abrasion surface protection technologies. The most widely used is electrode surfacing and wire coating. The use of stainless steel electrode surfacing method can guarantee the welding layer and the substrate has a high bonding strength, but the surfacing fade rate, welding thick and uneven processing margin, demanding solderability of the substrate material . The surface of the pump blade treated by the surfacing welding method is generally subjected to new cavitation destruction immediately around the surfacing spot until no cavitation damage occurs at the surfacing zone until the bottom of the surfacing layer. The stainless steel spray particle coating formed by wire spraying is mechanically combined, which is not suitable for the pump impact load and anti-cavitation repair. For some large pump parts, such as large diameter (3m diameter and above) axial pump impeller chamber, the surface can be mounted with a stainless steel plate to increase the anti-erosion ability. However, this method needs to be sent to large-scale pump plant specialized processing, turning, setting, welding, expensive, long cycle, non-ordinary pumping station can be implemented. Alloy powder coating is developed on the basis of wire spraying. Compared with the surfacing method, the molding is smooth, the thickness is easy to control, the fading rate is small, the method is simple, the heat source is easily obtained, and the processing is not limited by the climate and the site. However, since the sprayed layer is formed by regularly stacking alloy powder particles in a semi-molten state sprayed onto the surface of a substrate at a high speed and in a layered structure, the physical properties of the sprayed layer are directional and, in each spraying process, Powder particles appear condensation, shrinkage, deformation and other phenomena in the coating developed into an internal stress, so the alloy powder coating is generally only used for cavitation and erosion less serious surface protection of small and medium-sized pumps.
1 2 surface protection materials and process requirements 1 2 1 surface protection materials technical requirements Anti-wear coatings must have [1] :( 1) high strength and hardness to resist cavitation, abrasion damage; (2) with A certain toughness to absorb the impact energy; (3) has a high bond strength to ensure that the coating in the pump 30 ~ 35m / s under the impact of high-speed flow will not spall; (4) the coating material must be affordable , In order to ensure the large and medium-sized pumping station and a large amount of rural small and medium-sized pump stations in rural areas to promote the use; (5) coating materials should be non-toxic, non-flammable, explosive materials, easy to custody transport, do not pollute the surrounding environment.
1 2 2 Processing Requirements In order to ensure the promotion and application of surface protection technology, the processing technology must be: (1) the process is simple and can be mastered by different degree operators; (2) the tools (tools) Should be readily available in the market or necessary for general pumping station maintenance work, and at affordable prices, without special and expensive equipment; (3) The process should not be affected by the seasons and the surrounding environment to ensure that the pump station in the winter , The spring season can be carried out during the maintenance period; (4) The coating does not require special thermal curing, coating quickly solidified or put into use to shorten the maintenance cycle.
2 alloy powder spray welding technology Spray welding protection technology is the development of low melting point powder materials developed on the basis of spraying and welding of a metal surface protection technology. As the spray layer undergoes the process of remelting, the coating is dense and non-porous, the surface is smooth and flat, with the advantages of material saving, good quality and high efficiency. The surface hardness of the spray coating can be up to HRC60-70, which can be several times or even ten times longer Pump over-current components of the service life.
2 1 Spray welding alloy powder material optimization 2 1 1 optimization points In order to ensure the quality of spray coating, to prevent the deformation of the workpiece and the generation of coating cracks, coating material research and optimization of the technical route: (1) through the ratio optimization, Change the size of the hardened phase particles, the number and size of crystal grain size, in order to obtain a reasonable organizational structure and distribution status. (2) According to the cavitation and erosion characteristics of the pump, the performance indexes of the materials are adjusted in a targeted manner, which not only ensures the excellent abrasion resistance of the material, but also can suppress or reduce the crack generation to the maximum extent and improve the solderability. (3) Determine reasonable technical parameters and process parameters to improve the binding conditions under the conditions of use of the coating.
2 1 2 The basic composition of alloy powder and proportioning According to the basic requirements of the pump anti-abrasion, we spray a large number of alloy powder for screening and optimization. Mainly optimize the Fe-based, Ni-Cr-based and WC series of materials, the main components and proportions shown in Table 1.
2 1 3 alloy powder optimization choice Which powder material to use, is based on the pump working conditions, spray welding process and economy to decide. It is noteworthy that the abrasion resistance of the sprayed coating is not linearly proportional to its macroscopic hardness (HRC or HB), but rather to its microhardness (HV), so that the choice of solder layer should not be overemphasized Macroscopic hardness level, which has been confirmed in a large number of anti-erosion performance comparison test. The pump anti-cavitation, anti-wear alloy powder should generally meet the following principles [4]: ​​(1) for small and medium-sized pump blades and impeller chamber to deal with minor cavitation damage or sediment wear available Fe30, Fe280, Fe250 and other spraying treatment. Repair pump castings casting defects or damage, the choice of spray performance and toughness and better processing performance of Ni25, Ni20, with this welding instead of welding process, can be formed in the heat affected zone to prevent the formation of Fe3C brittle phase, to prevent Welding layer cracking. (2) for moderate wear and require better anti-cavitation performance of the conditions of use, consider the choice of Ni35 spray. It is adaptable to the base metal, will not cause cracking, spray coating is full of toughness, turning machining. For cavitation erosion or muddy sand pump blades, impeller room can choose Ni55 or Ni60, they have high hardness, toughness and impact resistance and good anti-cavitation, anti-wear properties. (3) Pumps that require less wear and erosion on the work surface at room temperature can be inexpensive Fe60, Fe-WC25 alloy powder, no need to process after spray welding can be used directly. (4) Spray welding of Fe-WC35, Ni-WC25, Ni-WC35 alloy powder containing WC can be selected for high-stress abrasive wear or erosion, serious cavitation erosion and demanding operating conditions.
2 2 key technologies of spray welding High hardness anti-cavitation, abrasive materials in the processing, we must focus on solving the spray-induced workpiece deformation and spray crack [5]. The main causes of deformation and cracking are as follows: (1) The high temperature in the process causes the generation of internal stress, which is superimposed with the increase of the thickness of the sprayed layer. (2) When the coating metal is solidified and crystallized, metal segregation occurs due to the existence of a large amount of low melting point eutectic, which creates the conditions for the crack generation. Coupled with the hard phase alloy crystal caused by coarse grains and grain boundary embrittlement greatly reduce the toughness of the material, the crack once produced extremely easy to expand. (3) Differences between the thermal physical properties of the coating and the substrate, especially when the coefficient of thermal expansion between the selected powder and the metal substrate is large, will lead to large phase transition and interlayer stress during the cooling process, resulting in deformation and Coating cracked. (4) The impact of environmental constraints on material processing, such as the ambient air temperature and humidity, operator proficiency, spraying speed, spraying and remelting temperature, and preheating, warming and Cooling rate and other factors have a direct impact on the spray quality and deformation cracks. For spray layer cracks and workpiece deformation problems, developed a rigorous process and technical parameters, including the speed of the powder, the pressure of the gas source, the nozzle aperture and the distance from the workpiece, the thickness of each spray layer, covering the way , Temperature control, cooling rate, holding time and measures.
2 3 Experimental Results In the laboratory, six kinds of test blocks with different proportions and different compositions were tested in a laboratory using cavitation disk machines and compared with other test blocks of protective materials. Ordinary carbon steel was used as a standard material, as shown in the table 1. It shows that coatings using Ni-Cr-based 60 Series spray coatings have 195 times the cavitation resistance of conventional carbon steel. Ni Cr-based alloy is added to the Ni, B, Si 15% to 18% of the Cr, microstructure of the coating can be seen in a large number of hard phase addition of fine grain refinement of the microstructure so fine and evenly distributed. In the fusion zone eutectic precipitation, the boundary can be seen diffusion layer. Figure 1 is a spray-welded microstructure photo.
Of the microstructure can be seen a lot of hard phase added to the grain refinement Table 1 cavitation test results comparison material name of the basic composition (%) hardness (HRC) connection corrosion resistance of carbon steel epoxy silicon carbide EDI enhanced Ni222Fe30AFe30ANi55Ni55Ni60Fe , C Epoxy liquid, corundum Fe, C, NiC0 5%, Cr15%, A14 5%, Fe8%, Si0 8%, Ni balance C0 6%, Ni34 5%, Cr13%, B1 5%, Si3% , Mo4 5%, Fe balance C0 6%, Ni34 6%, Cr13%, B1 5%, Si3%, Mo4 5%, Fe balance C0 6%, Cr15%, B4%, Si4%, Mo3%, Cu2 5%, Fe <12%, Ni balance C0 6%, Cr15%, B4%, Si4%, Mo3%, Cu2 5%, Fe <12%, Ni balance C0 9%, Cr15%, B4%, Si4 %, Co1 5%, Fe <14%, Ni balance 2030HB2503030555560 Sheet coating Spray coating Spray coating Spray coating Spray welding 1 0 Peeling 5 29 618 419 5
Table 2 1990 ~ 2000 process test research content and stage results Jiangsu Fangshan pumping station Jiangsu Lingcheng pumping station soap River pumping station Jiangdu three stations Hubei Fankou pumping station Laboratory Pump Model 28ZLB-7040ZLB-506HL-70ZL13 5-840CJ-95 Head H (m) 8 5121 589 5 Flow Q (m3 / s) 1 Impeller diameter (mm) Test date April 1990 November 1991 1992 April 1993 July 1997 December 1997 July key test content and purpose of the eight kinds of spray (coating) layer program comparison, the same cavitation operating conditions to study the spray material and the blade, impeller room protection technology to solve the welding process cracking and deformation problems Research large pump blade spray welding process, to solve the pretreatment, preheating, temperature control, deformation and other technology research large pump ha1f impeller room spray limit deformation, in order to take control measures ① a comprehensive inspection of the protection process, deformation control measures; ② The real machine application of experimental research; ③ research spray large parts of the preheating method and the impact of heat conduction ① study laser cladding material addition methods; ② study laser cladding processing parameters include: shock Optical power P, spot size d, optical cavity output configuration and focusing mode, laser scanning speed v, laser lap lapping coefficient a, etc .; ③ research laser processing material anti-cavitation test site Blade positive and negative all spray (Coated) Machining of 10 pump vanes Cavitation on both sides and impeller Cavity and impeller chamber 200mm width Cavitation zone Water inlet side of blade Vane width direction Strip width 250mm Impeller chamber 300mm Width Cavitation Venting Blade front Water inlet side 750 × 1500mm2 cavitation area and the impeller chamber 330mm width cavitation with cast iron and cast steel surface test results to May 1993 dismantling the pump inspection, each accumulated run time 3288h. Uneven blade cavitation area reached 2800mm2, depth 5mm. Spraying leaves in the cavitation zone off. Spray-welded leaves have not changed to verify the test results, spray processing, the blade angle from -10 ° to 0 ~ 2 °. To December 1996 inspection, the average operation of each 2860h, blade, impeller chamber surface smooth, no cavitation damage traces Ni-based tungsten carbide two-step spray welding, the effect is good, no cavitation damage traces without taking any measures , The ultimate deformation of the harf impeller chamber can reach 0.3% D, and the deformation can be controlled within ± 0 1% D (D is the impeller chamber diameter) by taking measures to control the deformation of the blade within ± 0 1% D , Impeller room deformation control within ± 0 05% D. Spray welding process, the measures taken, the heat has no effect on the pump oil bearing using the same spray with the Ni-based and WC coating material, the laser cladding, the average anti-cavitation ability of cast iron and steel were increased 15 91 times and 21 43 times. Compared with the spray welding process, the average anti-cavitation capacity increased by 13 times and 15 times 2 3 Field Test and Application Beginning in 1990, our school took the lead in the pump over-current components surface spray protection materials and key technologies Experimental Research. Through the continuous improvement of the material formula and the completion of the pump station test implementation of the pump, the basic solution to the large-scale axial flow pump, mixed flow pump impeller vane cavities serious damage to the area and the implementation of large parts of the coating shelling, the workpiece Deformation, cracks and other key technologies. The contents of the technological experiment from 1990 to 2000 and the stage achievements obtained are shown in Table 2. As a large number of experimental research work in the pumping station on the pump, the results have been verified over the years, already have the conditions for the promotion and application. Experimental research and practical application of spray welding technology is proven to solve the large and medium-sized pump cavitation, erosion and destruction of one of the best.
3 laser cladding surface modification technology
3 1 Characteristics of Laser Surface Cladding Laser Cladding technology refers to the surface of the cladding substrate after the coating material of choice, after laser irradiation and the substrate surface melting at the same time, and rapid solidification to form a very low dilution , With the substrate material metallurgical bonding surface coating, thereby significantly improving the substrate surface wear resistance, corrosion resistance of the process [6]. Compared with spray coating, it has the following characteristics: (1) fast condensation, resulting in rapid solidification of tissue characteristics, access to dense, uniform, high hardness coating; (2) thermal deformation is extremely small, especially suitable for Deformation of the impeller, impeller room; (3) the beam aimed at the constituency cladding and repair of other methods difficult to access the processing area; (4) the process is easy to automate and facilitate quality control.
3 2 Experimental Results [7] The laser surface modification of cast iron and cast steel was studied using 2kW CO2 continuous-flow laser with Ni-based and Ni-based WC alloy powders. The anti-cavitation performance of the sample was tested with J93025 magnetostrictive meter. The experimental results show that the anti-cavitation properties of the samples after laser cladding have been greatly improved. (1) The laser cladding is more dense and uniform than the spray coating, and the heat affected zone is narrower. Due to the high laser power, short scanning time, high melting speed after coating and substrate surface heating, large subcooling degree during rapid cooling, alloying elements in the molten pool can rapidly form a variety of compounds to increase the number of non-spontaneous nuclei, The nucleation rate greatly improved, the formation of small and uniform microstructure. Tissue refinement can improve the grain boundary bond strength and enhance the strength and toughness of the material, not only reduces the impurity content per unit grain boundary, but also reduces the degree of segregation of components in the rapid cooling process, thus ensuring the overall processing quality. (2) laser cladding surface modification is the use of a laser beam to stir the pool, making the pool of gas inclusions floating up to form a more dense coating to ensure the quality of the melt layer. The spray process is the use of oxyacetylene flame spray to the surface of the substrate powder melting, accompanied by the process of mixing air into the welding layer, the spray layer and the interface often have more pores and inclusions distributed in the thick branches Between the crystal. The chemical composition and unevenness of the metal structure reduce the corrosion resistance of the spray coating. (3) The surface of cast steel and cast iron after laser cladding treatment with the same coating material as spray coating increased the cavitation resistance by 15 91 times and 21 43 times respectively. Compared with spray welding, cavitation resistance increased by 13 and 15 times, respectively. (4) laser cladding itself, small thermal deformation, and the coating can produce a good combination of metallurgy, coupled with fast, beam targeting flexible processing, high degree of automation makes the laser cladding with other technological advantages can not be compared. With the development of laser technology, price reduction, material research and the development of broadband scanning laser equipment, laser cladding technology will provide a better solution for water pump anti-cavitation, wear prevention and repair.
4 Conclusion Surface protection technology can save high-quality materials, to maximize and greatly improve the general performance of the base metal surface properties, significantly improve the pump blades, impeller cavitation cavitation, erosion, and extend its service life.
references
1 Liu Chaofu hydraulic machinery over-current component surface coating technology application research pumping station technology, 1997; (3): 29 ~ 31
2 training training medium and large pumping station pump cavitation prevent damage and damage repair pump technology, 1994; (6): 37 ~ 40
3 storage training pump blades and impeller cavitation cavitation spray protection materials and process research report on the subject of pump technology, 1999; (5): 18 ~ 22
4 training training, Chen Lumped large pump station construction and renovation measures Nanjing: Hohai University Press, 2000: 90 ~ 92
5 CHEN Xiao-ping Turbine anti-wear protection technology progress Water Resources and Hydropower Engineering, 1996; (12): 33 ~ 34
6 XIE Pei-lin, WU Xin-yue Study on Cavitation Cavitation of Steam Turbine Blade Surface by Laser Cladding Journal of Naval Architecture Institute, 1999
7 MA Yu-dong Pump Anti-cavitation Laser Cladding Preventive Study Yangzhou University Water Conservancy Construction Engineering Master's Degree thesis, 2000

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