1. Broadband electroslag surfacing technology
(1) Background: The surface of the thick-walled pressure vessel, such as the hydrogenation reactor, the raw-stream synthesis tower, the coal liquefaction reactor and the nuclear power plant, in the petrochemical industry requires large-area surfacing to resist high temperature, oxidation and hydrogen sulfide. Stainless steel lining. In the 1970s, in this field, a large number of submerged arc surfacing (SAW) technologies were used at home and abroad. The width of the strip also develops from a narrow strip to a broadband direction of 60 mm, 90 mom, 120 mm, 150 mm. This technology has made great progress in the dilution rate and deposition rate compared with the wire submerged arc welding. However, with the increasing size and high parameterization of the pressure vessel, the surfacing technology has been developed to a higher quality and more efficient direction. In the early 1970s, Germany first invented the electroslag surfacing technology, which was further improved by Japan, the United States, the former Soviet Union and other countries. It has higher production efficiency, lower dilution rate and lower density than the submerged arc. The advantages of good weld formation and so on have been rapidly developed and widely used at home and abroad in recent years.
(2) Technical content and technical key The electrode electroslag welding is to use the resistance of the conductive slag to thermally melt the surfacing material and the base material. Except for the introduction stage, the whole surfacing process should be provided with arc generation. In order to obtain a stable electroslag surfacing process, there are several technical key points:
1) Welding power supply. In the electroslag surfacing process, the stability of the slag pool has a great influence on the quality of the surfacing, and the fluctuation of the voltage is the most critical factor affecting the stability of the slag pool. Therefore, it is desirable to minimize the voltage fluctuation during the surfacing process. DC power supply with pressure characteristics. In addition, the power supply should have low voltage, high current output, high control accuracy, strong ability to compensate for network voltage fluctuations and reliable protection. The rated current of the power supply varies depending on the bandwidth used. Generally, it is 60mm × 0.5mm, the rated current is 1500A, the 90mm × 0.5mm is 2000A, and the 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 electrical conductivity of the electroslag surfacing flux needs to be 2 to 3 Ω-1 cm-1, which is 4 to 5 times that of the ordinary submerged arc welding flux. At present, the electroslag flux used at home and abroad is mostly sintered. The conductivity of the flux depends on the amount of chloride (NaF, CaF2, Na3AIF6, etc.) in the flux composition. When the chloride (mass fraction) is less than 40%, the surfacing process is an arc process, at 40% to 50%. The range is roughly the arc and electroslag combination process; when the chloride is greater than 50%, a full electroslag process can be formed. CaF2 is both a good conductive material and a major slagging agent, so CaF2 is usually the main component of electroslag surfacing flux.
In addition to electrical conductivity, the flux also needs to have good surfacing process (slag removal, forming, wettability) and good metallurgical properties (small alloy elements burn less, less adverse element increments), suitable particle size (general ratio Submerged arc welding flux is fine). At present, the above requirements have been met, and there are many types of fluxes that have been used for production, such as foreign FJ-1 (Japan), EST122 (Germany), Sandvik 37S (USA), domestic SJ15, SHD202, and the like.
4) Control of process parameters. The use of reasonable surfacing process parameters is an effective means to ensure stable electroslag surfacing process and good weld quality. The most important process parameters affecting the quality of electroslag surfacing are welding voltage, current and welding speed, followed by dry elongation, thickness of flux layer, overlap between welds, and welding position.
1 Precise control of the welding voltage is of great significance for the electrode slag surfacing. When the voltage is too low, there is a tendency to adhere to the base metal. The voltage is too high, the arc phenomenon is significantly increased, the molten pool is unstable, and the splash is also increased. The recommended welding voltage can be preferably between 20 and 30V.
2 The welding current has a great influence on the quality of the electroslag surfacing. As the welding current increases, the penetration depth, melt width and stack height of the weld bead increase with this, while the dilution rate decreases slightly, but the current is too large and the splash increases. Different widths of the strip should be selected for different welding currents. For example, for a strip of φ75 mm × 0.4 mm, the current can be preferably between 1000 and 1300 A.
3 As the welding speed increases, the weld width and stack height decrease, the penetration depth and dilution rate increase, and the welding speed is too high, which will increase the arc rate, and control the certain dilution rate to ensure the performance of the weld overlay. The welding speed is generally controlled at 15 to 17 cm/min.
4 When the graded electroslag surfacing is used, the inclination of the base metal will affect the dilution rate and bead forming. It is generally recommended to use the horizontal position or the slope with a slight slope (1o~2o).
5 Other recommended values ​​are: the extension length of the strip is 25~35mm, the thickness of the flux is 25~35mm, and the weld overlap is 5~l0mm.
(3) Advantages, Disadvantages and Application Ranges The electroslag surfacing and the submerged arc surfacing have the following advantages:
1) The deposition efficiency is high, at medium current, 50% higher than submerged arc welding;
2) The melting depth is shallow and uniform, the base material dilution rate is low, generally can be controlled below 10%, which is twice as small as submerged arc welding, and single layer surfacing can meet the performance requirements.
3) The surfacing layer is well formed, it is not easy to have defects such as slag inclusion, the surface quality is excellent, the surface irregularity is less than 0.5mm (more than lmm during submerged arc surfacing), so the surface does not need mechanical processing, saving material and saving time.
4) The burning of the alloy elements in the strip and the increase of the unfavorable elements are rare, and the plasticity and toughness of the surfacing layer are higher than that of the submerged arc.
5) Because the carbon diffusion layer of the joint fusion zone is narrow and the martensite zone width is small, the joint fusion zone performance is better than that of the submerged arc surfacing.
Due to the above advantages, the electrode slag surfacing has been widely used in large-area surfacing of pressure vessels in hydrogenation control reactors, gas engineering hot wall exchange furnaces and nuclear power plant equipment in recent years.
Due to some characteristics of the electroslag belt surfacing itself, it also has a fixed range of applications:
1) Due to the high heat input of the electrode slag surfacing, it is generally used for surfacing thick-walled workpieces of 50-200mm. The recommended minimum diameter and wall thickness of the workpiece are shown in Table 1.
3) Due to the low welding speed of the electroslag slag welding, the heat input is large, resulting in coarse grain boundary layer between the base metal and the surfacing layer, which makes the oxygen-resistant peeling performance of the hard-to-weld layer poor. On the one hand, foreign countries have carried out research on the base materials, strips and fluxes used in order to obtain a more purely surfacing metal to improve the resistance to hydrogen stripping.
2. Broadband ultra-high-speed surfacing technology (1) Background As mentioned above, with submerged arc surfacing and with electrode electroslag surfacing, due to low welding speed and large heat input, the hydrogen-resistant peeling performance of the hard-to-weld layer is not Ideally, in the late 1980s, high-speed strip surfacing technology with high surfacing speed and low heat input was developed at home and abroad.
(2) Technical content and technology The key high-speed strip surfacing is developed on the basis of electroslag surfacing, which increases the welding speed. As the welding speed increases, the arc incidence increases, thus making the welding conductive. The process changes from an electroslag process to a combined process of electroslag and arc. However, the electroslag process is still the mainstay. Therefore, the requirements for power supply, flux, etc. are the same as those for electrode electroslag surfacing. Only some of the characteristics of high-speed strip stacking are now covered.
1) Magnetic control device. As the welding speed increases, the welding current also increases accordingly, the magnetic contraction force will increase squarely, and the undercut will be more serious. Therefore, the magnetron current must be increased, but the magnetron current is too large, and the excitation winding is heated. Serious, reducing the service life. In order to increase the magnetron effect at a lower magnetron current, corresponding measures should be taken. For example, it has been studied to use a magnetic pole head with a certain angle to rotate and an additional magnetic pole pole shoe, and has received good results.
2) Control of process parameters. Under the premise of improving the anti-hydrogen stripping performance of surfacing, according to the comprehensive performance requirements of the product, factors such as weld bead shape and dilution rate are considered. For the different strip widths, the best welding parameters should be tested, as shown in Table 2.
1) Since the boundary layer of high-speed surfacing has M+A dual-phase structure and fine grain, it has good resistance to hydrogen stripping; the surfacing layer also has good corrosion resistance due to fine grains.
2) Since the heat input to the base metal by the high-speed surfacing is small, the deformation of the base material is small, which is more meaningful for thin-plate surfacing.
This technology has been used in production. For example, the chemical machinery factory of Nanhua (Group) Co., Ltd. has produced a hydrogenation control reactor with high-speed strip welding, which achieves the purpose of high-speed high-quality surfacing.
Although the effective thickness of the surfacing welding of this technology is larger than that of the submerged arc surfacing, it is still necessary to surfacing two layers. It is better to carry out the electroslag surfacing and the single layer can meet the requirements.
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