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 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 for 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 area 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 pump stations and large-scale rural small and medium-sized pump station popularization and 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 maintenance period can be carried out; (4) the coating does not require special insulation curing, after coating can be quickly cured 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 generally should meet the following principles [4]: ​​(1) for small and medium-sized pump blades and impeller chamber to deal with the less corrosive 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 require low-cost Fe60 and Fe-WC25 alloy powders, which can be used directly without spray processing. (4) Spray welding of Fe-WC35, Ni-WC25, Ni-WC35 alloy powder with 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 cause a large phase transition and interlayer stress in the matrix metal during cooling, 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 powder feeding speed, 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