Overlay welding

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As an economical and fast process for surface modification of materials, surfacing welding is increasingly used in the manufacturing and repair of parts in various industrial sectors. In order to play the role of the cladding layer most effectively, it is hoped that the cladding method used has smaller base metal dilution, higher deposition speed and excellent cladding layer performance, that is, high-quality, efficient, low dilution rate cladding technology. At present, there are many surfacing methods used in production. The dilution rate and deposition rate of several surfacing methods are compared as shown below. Comparison of the characteristics of several surfacing methods. Dilution rate of surfacing method (%) Deposition rate (kg/h) Submerged arc surfacing single wire 30～60 4.5～11.3 Multi-wire 15～25 11.3～27.2 Series arc 10～25 11.3～15.9 Single strip pole 10～20 12～36 Multi strip pole 8～15 22～68 Plasma arc cladding automatic powder feeding 5～15 0.5～6.8 Manual wire feeding 5～15 0.5～3.6 Automatic wire feeding 5～15 0.5～3.6 Double hot wire 5～15 13～27 Melting electrode gas shielded arc cladding among them: Self-shielded arc cladding 10～40 0.9～5.4 15～40 2.3～11.3 Electroslag cladding with electrode 10～14 15～75 As can be seen from Table 3, strip cladding has a higher deposition speed, and plasma arc cladding has a lower dilution rate. In recent years, on this basis, researchers have further developed advanced strip cladding technology and plasma arc cladding technology that are both efficient and low dilution rate. Strip welding technology 1. Broadband electrode electroslag surfacing technology (1) Production background The internal surfaces of hydrogenation reactors, raw flow synthesis towers, coal liquefaction reactors and nuclear power plants' thick-walled pressure vessels in the petrochemical industry require large-area surfacing stainless steel linings that are resistant to high temperature, oxygen and hydrogen sulfide corrosion. In the 1970s, submerged arc cladding (SAW) technology was widely used in this field at home and abroad. The width of the band pole has also developed from narrow band to wide band of 60mm, 90mm, 120mm and 150mm. This technology has made great progress compared to wire submerged arc welding in terms of dilution rate and deposition speed. However, as pressure vessels become increasingly large-scale and highly parameterized, surfacing welding technology is driven to develop in a higher quality and more efficient direction. In the early 1970s, the electroslag surfacing technology with electrodes, first invented in Germany and later further improved by Japan, the United States, the former Soviet Union and other countries, has been rapidly developed and widely used at home and abroad in recent years because it has the advantages of higher production efficiency, lower dilution rate and good weld formation than difficult submerged arc welding with electrodes. (2) Technical content and technical key points Electroslag welding uses the resistance heat of conductive slag to melt the surfacing material and base metal. Except for the induction stage, the entire surfacing process should be equipped with arc generation. In order to obtain a stable electroslag cladding process, there are the following technical keys:: 1) Welding power source. During the electroslag cladding welding process, the stability of the slag pool has a great influence on the quality of the cladding welding, and the voltage fluctuation is the most critical factor affecting the stability of the slag pool. Therefore, it is hoped that the voltage fluctuation during the cladding welding process will be minimal, so a DC power supply with constant voltage characteristics is required. In addition, the power supply should have low voltage, large current output, high control accuracy, strong ability to compensate for network voltage fluctuations and reliable protection performance. The rated current of the power supply varies depending on the bandwidth used. Generally, for 60mm×0.5mm pole, the rated current is 1500A, 90mm×0.5mm is 2000A, and 120mm×0.5mm is 25O0A. 2) Flux. Another necessary condition for obtaining a stable electroslag process is that the flux must have good electrical conductivity. Generally, the conductivity of electroslag cladding flux needs to reach 2 to 3Ω-1cm-1, which is 4 to 5 times that of ordinary submerged arc welding flux. At present, most of the electroslag fluxes used at home and abroad are sintered. The conductivity of the flux depends on the amount of chloride (NaF, CaF2, Na3AIF6, etc.) in the flux component. When the chloride (mass fraction) is less than 40%, the surfacing process is an arc process. In the range of 40% to 50%, it is roughly a combined arc and electroslag process. ； When the chloride is greater than 50%, a full electroslag process can be formed. CaF2 is both a good conductive material and the main slag-forming agent, so CaF2 is usually the main component of electroslag cladding flux. In addition to electrical conductivity, the flux also needs to have good surfacing processability (slag removal, forming, wettability) and good metallurgical properties (small burning loss of alloy elements, small increment of unfavorable elements), and suitable particle size (generally finer than submerged arc welding flux). Currently, there are many types of fluxes that meet the above requirements and have been used in production, such as foreign FJ-1 (Japan), EST122 (Germany), Sandvik37S (USA) ； Domestic SJ15, SHD202, etc. 3) Magnetic control device. For electroslag cladding with wide-band poles (strip pole width greater than 60mm), due to the magnetic shrinkage effect, the cladding layer will produce undercuts. As the strip pole width increases, the cladding current increases, and the undercut phenomenon becomes more severe. Therefore, an external magnetic field must be used to prevent the occurrence of undercuts (magnetic control method). As shown in the picture. At the same time, the magnetic pole position must be reasonably arranged and a reasonable excitation current size must be selected. If the external magnetic field is too strong or too weak, it will affect the formation of the cladding bead (Figure 2). The magnetic control current of the two magnetic poles should be adjustable separately. For example, for the workpiece in the non-preheated flat welding position, when the strip pole is 60mm×0.5mm, the south and north pole control currents of the magnetic control device are 1.5A and 3.5A respectively. ； For the 90mm×0.5mm strip pole, they are 3A and 3.5A respectively. 4) Control of process parameters. Adopting reasonable surfacing process parameters is an effective means to ensure a stable electroslag surfacing process and good weld quality. The most important process parameters that affect the quality of strip electroslag surfacing are welding voltage, current and welding speed, followed by dry elongation, flux layer thickness, overlap amount between weld beads, welding position, etc. ① Precisely controlling the welding voltage is of great significance for electroslag cladding with electrodes. When the voltage is too low, there is a tendency for the electrodes to stick to the base metal. If the voltage is too high, the arc phenomenon will be significantly increased, the molten pool will be unstable, and spatter will also increase. The recommended welding voltage can be between 20 and 30V. ② The welding current also has a great influence on the quality of strip electroslag surfacing. As the welding current increases, the penetration depth, width, and pile height of the weld bead increase, while the dilution rate decreases slightly. However, if the current is too large, spatter will increase. Different welding currents should be selected for strips with different widths. For example, for strips with φ75mm×0.4mm, the current can be preferably between 1000 and 1300A. ③ As the welding speed increases, the welding width and pile height decrease, the penetration depth and dilution rate increase. If the welding speed is too high, the arc incidence rate will increase. In order to control a certain dilution rate and ensure the performance of the cladding layer, the welding speed is generally controlled at 15-17cm/min. ④ During step electroslag surfacing welding, the inclination angle of the base metal will affect the dilution rate and weld bead formation. It is generally recommended to use horizontal position or uphill welding with a slight slope (1º to 2º). ⑤ Recommended values ​​for some other parameters are: The protruding length of the strip pole is 25~35mm, the thickness of the flux is 25~35mm, and the overlap amount of the weld bead is 5~10mm. (3) Advantages, Disadvantages and Application Scope Electroslag surfacing with electrodes has the following advantages over submerged arc surfacing with electrodes: 1) High deposition efficiency, 50% higher than submerged arc welding at medium current ； 2) The penetration depth is shallow and uniform, and the dilution rate of the base metal is low. It can generally be controlled below 10%, which is twice as small as submerged arc welding. A single layer of surfacing can meet the performance requirements. 3) The surfacing layer is well formed and is not prone to defects such as slag inclusions. The surface quality is excellent and the surface unevenness is less than 0.5mm (larger than 1mm during submerged arc surfacing), so the surface does not require mechanical processing, saving materials and time. 4) There is very little burning loss of alloy elements and increment of unfavorable elements in the strip pole, and the plasticity and toughness of the surfacing layer are higher than those of submerged arc refractory welding. 5) Since the carbon diffusion layer in the joint fusion zone is narrow and the martensite band width is small, the performance of the joint fusion zone is better than that of strip submerged arc surfacing. Due to the above-mentioned advantages of electroslag cladding with electrodes, it has been widely used at home and abroad in recent years in large-area cladding of the inner surfaces of hydrogenation control reactors, gas engineering hot wall exchange furnaces, and pressure vessels in nuclear power plant equipment. Due to some characteristics of electroslag surfacing welding, it also has a certain range of applications.: The heat input of electrode electroslag cladding is relatively high, so it is generally used for cladding thick-walled workpieces of 50 to 200mm. The recommended minimum diameter and wall thickness of the workpiece are shown in Table 1. Table 1 Recommended minimum diameter and wall thickness suitable for strip electroslag cladding Electrode size Minimum substrate thickness Minimum curved surface diameter Outer surface Inner surface 60×0.5 40 250 450 90×0.5 80 500 900 Application of spark cladding process in the repair of key components in power plants Wang Ruijun Huang Xiaoou, Institute of Surface Technology, Chinese Academy of Agricultural Mechanization Sciences Abstract: As the basic industry of the national economy, the electric power industry has always been * * focus of development. This article uses the electric spark cladding welding process to successfully repair the shaft diameter wear defect of the turbine generator rotor seal section, and successfully passed the two-and-a-half-year operation test. In addition, the repair of the sealing surface of the steam turbine cylinder and the wear surface of the main shaft of the heating network circulation were completed. So far, this process has been used to successfully repair nearly twenty rotors of related types and dozens of related parts, achieving huge economic and social benefits. keywords: Spark surfacing, power plant, wear, repair 1. Introduction As the basic industry of the national economy, the electric power industry has always been * * focus of development. The past two decades have been the period of fastest development and greatest achievements in the history of my country's electric power development. As of 1998, my country's installed electric power capacity reached 277,289MW, the annual power generation reached 1,157.6 billion kilowatt hours, and there were 68 large thermal power plants (with an installed capacity of more than 1,000MW) nationwide. As the number of power plants increases and unit capacity and parameters continue to improve, unit maintenance and repair are becoming increasingly complex and important. As the heart part of the steam turbine generator unit, the generator rotor has high operating precision, fast operation speed and high manufacturing cost. Once damaged, it will directly lead to a decrease in the output power of the entire unit or even paralysis. Various processes such as thermal spraying, argon arc welding, chip placement machines, and brush plating have been used for repairs, but the actual practical results after repair were not satisfactory. This article uses the DZ-1400 electric spark surfacing equipment (ESD) developed and produced by the Surface Engineering Technology Institute of the Chinese Academy of Agricultural Mechanization Sciences to repair the shaft diameter of the worn generator rotor seal section on site, and has obtained satisfactory results and successful experience. Nearly twenty worn rotor shaft diameters have been repaired, and the longest running time after repair has been more than two and a half years. Practice has proven that the spark cladding process plays an important role in the repair of power plant components and produces huge economic and social benefits. 2. The wear of the generator rotor shaft diameter is 0.125mm. Once the shaft diameter is worn or strained, the oil pressure in the sealing layer is difficult to maintain balance, hydrogen will leak, the sealing layer between the shaft diameter and the tiles is completely destroyed, and the high-speed operation of the rotor is hindered. In severe cases, the unit may not work. Figure 1 is a schematic diagram of the rotor shaft diameter after wear. 3. Comparison of spark surfacing and related processes ~ In order to ensure the high-speed operation and cooling effect of the rotor, a three-level sealing layer composed of hydrogen, oil, and water is used between the rotor shaft and the tiles. During operation, the gap between the shaft diameter and the bearing bush is maintained at 0.075. The spark surfacing process is different from welding, spraying or element penetration and other processes. Simply put, it is a process in between, which has some characteristics of welding and other processes, but also has unique advantages such as small heat input and metallurgical combination of the welding layer and the base metal. In some applications with special requirements, the spark cladding process makes up for the shortcomings of other processes (the working principle is published in another article). Table 1 compares the EDM cladding process with other processes. Figures 2 and 3 are respectively a schematic diagram of the EDM surfacing work and a cross-sectional metallographic photo of the surfacing layer. 3. Tests and results 3.1 Preparation of the sample 50mm10mm According to the common material of the rotor shaft diameter, 35CrMoA was selected as the sample base material, and Inconel high alloy rod with a size of 25mm6.0mm was used as the cladding electrode. The chemical composition is as shown in Table 2 Composition of the cladding electrode. 3.2 The selection of surfacing electrode materials is based on factory requirements: The hardness of the surfacing layer is consistent with that of the substrate, and it has self-lubricating, self-polishing, anti-cavitation properties and high alloy content. Table 1 Process comparison of turning method, brush plating, patch method, spark surfacing method Advantages 1. High processing accuracy 1. Suitable for on-site processing 2. Low cost 3. Guaranteed original design size 1. Suitable for on-site processing 2. High efficiency 1. On-site construction 2. Short cycle 3. Guaranteed original size 4. High bonding strength 5. Minimal residual stress Disadvantages 1. Reduced shaft diameter size 2. Need to redistribute tiles, poor interchangeability of parts. 3. Long cycle, high cost, and difficult on-site processing 1. Low bonding strength 2. Limited coating thickness 3. Copper coating has poor wear resistance and is prone to galvanic cell corrosion. 1. High porosity between patch layers 2. Low bonding strength - low efficiency Table 2 Composition of surfacing electrode Ni Cr Fe Mo Mn Si Other 70 14 4.5 --- 7.8 0.5 Bar 3.2 Test analysis Use sandpaper and copper wire brush to remove oxides on the surface of the sample, and then use acrylic solution to clean the oil stains on the surface of the sample. Tightly connect the sample base to the ground wire of the EDM cladding machine, and clad the sample surface with a thickness of 1 mm. 3000), it can be seen that: ′After spark surfacing, wire cutting is used to obtain the cross-section of the surfacing layer and prepare metallographic samples. Figure 5 is a cross-sectional metallographic photo of the surfacing layer (1) The surfacing layer has no pores, oxide slag inclusions, cracks and other welding defects. (2) The grains of the surfacing layer and base metal transition layer are small and have no tendency to grow. (3) The structure of the surfacing layer is an extremely fine columnar crystal structure, which proves that the surfacing layer has good corrosion resistance and wear resistance. ︳Surface welding layer︱︱Substrate︱3000)' Figure 4 Metallographic image of the surfacing layer (Figure 5 Determination of the heat affected zone Figure 6 Microhardness measurement Microhardness measurement of the substrate, it can be seen that the average hardness of the surfacing layer and heat affected zone is HV220, which is very close to the hardness of the substrate. T heat affected zone Tm. This not only proves that spark cladding can obtain a cladding layer with good metallurgical bonding with the base metal, but also shows that the heat-affected zone of cladding is extremely narrow and the welding residual stress is negligible. Figure 6 is the surfacing layer m. Figure 5 is the energy spectrum analysis of Ni and Cr elements in the surfacing layer. After measurement, the thickness of the heat affected zone of the spark surfacing is only 10 3.3 m), and the hardness of the surfacing layer is consistent with the hardness of the substrate. The above analysis results are in line with the requirements of the power station for repairing the motor rotor shaft diameter. Figures 7 and 8 show the generator rotor shaft diameter after repair. m test proves that when the Inconel alloy is cladded using the EDM cladding process, the cladding layer is metallurgically bonded to the base metal and there are no welding defects. The heat-affected zone after welding is extremely narrow (when the surfacing layer reaches 1mm, the heat-affected zone is only 0.01) Figure 7 Rotor shaft diameter after surfacing Figure 8 Rotor shaft diameter after grinding and polishing 4. In the past two years, other components of the power plant have been used. The EDM deposition cladding process has been used to successfully solve the repair and surface strengthening problems of key components of the power plant, such as the erosion repair of the sealing surface of the steam turbine cylinder, the repair of the worn surface of the main shaft of the heating network circulation pump, etc. Figures 9 and 10 are photos of repair work using the spark surfacing process. 5. Conclusion 1. The EDM cladding layer is metallurgically bonded to the base metal, the heat-affected zone of the cladding is extremely narrow, and the residual stress is negligible. 2. The electric spark cladding process is used to repair damage to key components of the power plant. It can be operated online and the process is simple. The amount of processing after repair welding is small, reducing downtime. 3. The spark cladding welding process has broad application prospects in power plants and contains huge economic and social benefits. References Power Reliability Management Center, National Large Unit Handbook, 1998, 4 Prospects of Power Station Welding Technology, Special Collection of Academic Reports on the 60th Anniversary of the Chinese Society of Electrical Engineering, Williams A, D., Humphries JL The 15th International Thermal Spray Conference (ITSC'98) Paper Selected Extrusion Roller Overlay Welding Method Qian'an Xiexing Cement Plant adopts manual arc welding and overlay welding of the roller surface of the double-roller crusher, achieving a service life of 10 months and crushing 150,000 tons of clinker. The key points of surfacing welding are as follows: (1) Selection of welding rods: D-65, D-667 and 506 welding rods should be used. Before surfacing, follow the instructions for using the welding rod, dry the welding rod and put it in an insulating box for later use. (2) Roller surface treatment: Roller surface repair can be divided into two methods: local direct repair welding and overall repair welding after overall removal, which can also be said to be two processes. For uneven wear and patterns along the width of the roller, uneven wear of hard points, and overall wear of the roller surface, local repair methods can be used to directly repair welding. ； After 5-6 times of direct repair welding, as the parent body is repeatedly subjected to high extrusion stress, the welding micro-cracks continue to expand, and a fatigue layer of a certain thickness will be produced on the surface of the grinding roller. At this time, if the wear-resistant repair electrode is used for direct repair welding, interlayer detachment is likely to occur, so the fatigue layer on the surface of the grinding roller needs to be thoroughly cleaned before the wear-resistant layer is surfacing. The repair of the roller surface in Xiexing Cement Plant is overall cleaning and repair welding. Whether it is direct repair welding or repair welding after overall cleaning, the roundness error of the grinding roller and the diameter error of the two rollers cannot be too large, otherwise it will cause horizontal vibration of the roller press and increase the uneven load of the two grinding rollers. To clean the fatigue layer on the roller surface, carbon arc gouging can be used to clean the fatigue layer on the roller surface so that the base material layer of the roller is exposed. Before surfacing welding, follow the instructions for using the welding rod, dry the welding rod, preheat the weldment, and slowly cool down after welding. (3) Choose a DC welding machine with a power of more than 10 kVA or an AC welding machine with a power of more than 20 kVA. When using a DC welder, the connection must be reversed (the electrode should be connected to the positive pole). When surfacing welding, the no-load voltage of an AC welder is required to be ≥70V, and the current should be controlled at about 200A. If the no-load voltage is lower than 70V, the current should be increased until the welding rod and base metal are fully integrated. The appropriate ratio of weld bead width and height is 3:1. Only in this way can it be truly sintered firmly with the base material and form the required wear-resistant structure. (4) Overlay welding sequence and thickness: After the roller surface is preheated, 1-3 layers of 506 welding rod should be used to surfacing the roller to round it. Then evenly build up several layers of D-667 to reach the required thickness. After the D-667 welding layer is overlayed, another layer of D-65 is overlayed, with a thickness of 3-5mm. ； After the D-65 welding layer is overlayed, use D-65 to overlay a layer of ridge-shaped patterns. (The occurrence of roller surface wear must have both the pressure and relative sliding required to crush the material. The pressure is determined by the properties of the material and is usually difficult to change. It is easier to reduce the relative sliding between the object and the roller surface during the extrusion process through the pattern of the roller surface. Although the herringbone pattern used early in China can prevent the circumferential sliding of the material, it does not restrict the axial sliding of the material during the extrusion process. Especially when extruding materials with smaller particles, the wear is more serious. In comparison, roller surfaces with ridge-shaped patterns and hard points in the middle have the best wear resistance. ) The side length of the ridge-shaped pattern is 4-5cm, the width of the weld bead is about 1cm, and the height is about 4mm. The thickness of each wear-resistant layer should be uniform so that the squeeze roller always maintains a round shape during use. (5) During surfacing welding, it is necessary to work in three shifts without stopping, so that the welding parts can maintain a high temperature for a long time.