About the selection of tube sheet surfacing equipment

Electroslag surfacing technology 2009-02-10 15:301． Broadband electrode electroslag surfacing technology? (1) Production background The internal surfaces of hydrogenation reactors, raw flow synthesis towers, coal liquefaction reactors and nuclear power plant 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 with 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 welding seam 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 voltage fluctuations are 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 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) The burning loss of alloy elements and the increment of unfavorable elements in the strip pole are very small, 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 electrode electroslag cladding, in recent years it has been widely used at home and abroad 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.: 1) Due to the high heat input of electroslag cladding welding, it is generally used for cladding thick-walled workpieces of 50 to 200 mm. The recommended minimum diameter and wall thickness of the workpiece are shown in Table 1. 2) Electroslag surfacing technology with electrodes is mostly used for corrosion-resistant surfacing. However, in the field of high-hardness wear-resistant surfacing, it cannot be applied due to the difficulty of rolling steel strips. Therefore, the development and research of flux cored electrode electroslag surfacing technology is being stepped up at home and abroad. 3) Due to the low welding speed and large heat input of electrode electroslag push welding, the boundary layer grains between the base metal and the surfacing layer are coarse, making the refractory layer poor in resistance to oxygen peeling. On the one hand, foreign countries have carried out research on the base metal, strip electrode, flux, etc., in order to obtain higher purity cladding metal to improve the resistance to hydrogen stripping, and gratifying results have been achieved. 2. Background of the development of broadband extremely high-speed cladding technology (l). As mentioned above, both strip submerged arc cladding and strip electroslag cladding have low welding speed and large heat input, which makes the resistance to hydrogen peeling of the refractory layer unsatisfactory. Therefore, in the late 1980s, high-speed strip cladding technology with high cladding speed and low heat input was developed at home and abroad. (2) Technical content and technical key points High-speed strip surfacing is developed on the basis of strip electroslag surfacing by increasing the welding speed. As the welding speed increases, the arc occurrence rate increases, so the conductive process of welding changes from the electroslag process to the combined process of electroslag and arc. But the electroslag process is still the main one. Therefore, the requirements for power supply, flux, etc. are the same as those for electrode electroslag surfacing. Now we only introduce some characteristics of high-speed belt pole stackers. 1) Magnetic control device. As the welding speed increases, the welding current will also increase accordingly, and the magnetic shrinkage force will increase squarely, and the undercut will become more serious. Therefore, the magnetron current must be increased. However, if the magnetron current is too large, the excitation winding will heat up seriously and reduce the service life. In order to increase the magnet control effect at lower magnet control current, corresponding measures should be taken. For example, measures such as the use of magnetic terminal heads with a certain angle that can be rotated and additional magnetic pole shoes have been studied, and good results have been achieved. 2) Control of process parameters. On the premise of improving the hydrogen stripping resistance of surfacing welding, factors such as weld bead shape and dilution rate should be considered based on the overall performance requirements of the product. For different strip widths, the best welding parameters should be obtained through experiments, as shown in Table 2. 3) Advantages, Disadvantages and Application Scope The ratio of strip high-speed cladding and strip electroslag cladding has the following advantages:: 1) Since the boundary layer of high-speed cladding has an M+A dual-phase structure and has fine grains, it has good resistance to hydrogen peeling. ； The surfacing layer also has good corrosion resistance due to its fine grain size. 2) Since the heat input of high-speed cladding to the base metal is small, the deformation of the base metal is small, which is more meaningful for thin plate cladding. This technology has been used in production. For example, the Chemical Machinery Factory of Nanhua (Group) Company produced a hydrogenation control reactor using high-speed strip surfacing, achieving the purpose of high-speed and high-quality surfacing. Although the effective thickness of cladding of this technology is larger than that of submerged arc cladding with electrode, it still requires two layers of cladding. It is not as good as electroslag surfacing with electrode, and a single layer can meet the requirements.