As an economical and fast process method of material surface modification, overlay welding is increasingly used in the manufacture of parts in various industrial sectors for repair. In order to make the most effective use of the cladding layer, the desired cladding method has a small dilution of the base material, a high melting speed, and excellent cladding performance, i.e., high quality, efficient, low dilution rate cladding technology.
A welding method in which metal is melted by electric or gas welding and piled on top of a tool or machine part. They are usually used to repair worn and crumbling parts.
The dilution rate and melt rate of several cladding methods are shown, for example.
Cold welding and overlay welding
Cold welding overlay technology is the use of high-frequency electric discharge principle, the workpiece non-thermal overlay welding, to repair the surface defects and wear of the metal workpiece, to ensure the integrity of the workpiece; also can use its strengthening function to strengthen the workpiece to achieve the wear resistance, heat resistance, corrosion resistance, etc. Cold welding and cladding equipment for metal products after repair workpiece is not deformed, not annealed, high bonding strength, wear resistance. The metallurgical combination of the weld material and the substrate ensures the robustness of the weld. Commonly used for pinholes, holes, burrs, flying edges, knocks, scratches, chipping, corners, holes, cracks, wear, sagging, manufacturing errors, manufacturing defects, welding defects, repair, and mechanical surface strengthening in precision castings.
The surface of plastic mold is wooled to increase the aesthetics and service life; helmet plastic mold fractional surface welding repair; aluminum alloy die-casting mold shunt cone surface reinforcement; mold cavity super weak, wear, scratch repair and reinforcement.
Plastic and rubber
Rubber and plastic machinery parts repair, rubber, and plastic parts for the mold are poor, wear, and repair.
Aircraft engine parts, turbine, turbine shaft repair or repair, rocket nozzle surface strengthening repair, aircraft outer plate parts repair, artificial satellite shell strengthening or repair, titanium alloy parts local carburizing strengthening, iron-based high-temperature alloy parts local carburizing strengthening, magnesium alloy surface carburizing A1 anti-corrosion coating, magnesium alloy parts local defect welding repair, nickel-based / cobalt-based high-temperature alloy blade parts local welding repairs, such as blade crown damping surface and leaf tip wear and guide leaf ablation, etc.
Manufacturing and maintenance
Used in the automobile manufacturing and repair industry for cam, crankshaft, piston, cylinder, brake disc, impeller, hub, clutch, friction plate, exhaust valve, etc. to patch and repair, and surface weld defect correction of an automobile body.
Repair of electric crankshafts, bushings, shaft tiles, electrical components, resistors, etc., welding of electric railroad wheels to bottom line rail connecting plates, welding of electroplating plant conductor rollers, metal oxidation treated copper and aluminum electrodes.
Correction of super-differential workpieces and repair of machine guides, various shafts, cams, hydraulic presses, hydraulic press pistons, cylinder walls, journals, rollers, gears, pulleys, arbor for spring forming, plug gauges, ring gauges, various rolls, rods, columns, locks, bearings, etc.
Repair of iron, copper, aluminum casting trach holes, and other defects, aluminum model, wear repair.
In the 1970s, a large number of domestic and international use of submerged arc welding (SAW) technology in this field. The width of the band poles has also evolved from a narrow band to broadband of 60mm, 90mm, 120mm, 150mm. The technology in the dilution rate and melting speed than the filament submerged arc welding has made great progress, but with the pressure vessel increasingly large, high parameterization, to promote the development of welding technology to a more high-quality and efficient direction. The early 70s, Germany first invented, after being Japan, the United States, the former Soviet Union, and other countries to further improve the electrode slag welding technology as it has higher production efficiency than submerged arc welding, lower dilution rate and good weld forming and other advantages, at home and abroad has been rapid development and more common application.
Electrodeposited slag overlay welding is the use of conductive slag resistance hot melt overlay material and base material, except for the lead stage, the entire overlay process should have an electric arc generation. In order to achieve a stable electron slag overlay process, there are several technical keys
In the electric slag welding process, slag pool stability on the slag welding quality has a great impact, and the voltage fluctuation is the most critical factor affecting the stability of the slag pool, it is hoped that the slag welding process voltage fluctuations are minimal, so the requirement to use the constant voltage characteristics of the DC power supply. 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 performance. The rated current of the power supply varies depending on the bandwidth used, generally, 60mm x 0.5mm with a pole, the rated current is 1500A, 90mm x 0.5mm for 2000A, 120mm x 0.5mm for 2500A.
Another necessary condition for obtaining a stable electro slag process is that the flux must have good electrical conductivity. General electric slag cladding flux conductivity needs to reach 2 ~ 3Ω-1cm-1, 4 ~ 5 times the ordinarily submerged arc solder flux. Most of the electro slag fluxes used at home and abroad are of the sintered type. The size of the flux conductivity, depending on the flux component in the fluoride (NaF, CaF2, Na3AIF6, etc.), when the fluoride (mass fraction) is less than 40%, the arc welding process for the arc process, in the range of 40% to 50% is roughly the arc, joint slag process; when the fluoride is greater than 50%, can form a full electric slag process. CaF2 is both good conductive material and the main slag agent, so CaF2 is usually the main component of the electric slag flux overlay. In addition to electrical conductivity, the flux also needs to have a good cladding process (slag removal, forming, wettability) and good metallurgical properties (small alloy element burnout, less adverse element increment), suitable particle size (generally finer than submerged arc flux particle size). To meet the above requirements has been used in the production of many types of flux, such as foreign FJ-1 (Japan), EST122 (Germany), Sandvik37S (United States), domestic SJ15, SHD202 and so on.
For broadband pole (with pole width greater than 60mm) electric slag overlay welding, due to the magnetic contraction effect, will make the overlay layer produce biting edge, with the pole width increases, the current overlay increases, the heavier the biting phenomenon, so must use the external magnetic field method to prevent the generation of biting edge (magnetron method). As shown in the figure. At the same time, the magnetic pole position must be reasonably arranged, choose a reasonable size of the excitation current, the external magnetic field is too strong or too weak will affect the formation of the overlay weld path (Figure 2). The magnetron current of the two poles should be adjustable separately. For example, for non-preheated workpieces in the flat welding position, when the band is extremely 60mm × 0.5mm, the south and north pole control currents of the magnetron device are 1.5A and 3.5A respectively; for 90mm × 0.5mm band pole is 3A and 3.5A respectively.
The use of reasonable welding process parameters is an effective means to ensure the stability of the electric slag welding process and good weld quality. The process parameters that affect the quality of slag overlay welding are mainly welding voltage, current and welding speed, followed by dry elongation, flux layer thickness, amount of overlap between channels, welding position, etc. When the voltage is too low, there is a tendency to stick to the base material with the electrode. The voltage is too high; the arcing phenomenon increases significantly, the melt pool is unstable, the splash also increases, the recommended welding voltage can be between 20 to 30V preferred. ② The welding current also has a large effect on the quality of the electro slag cladding with the pole. The weld current increases and the depth, width, and stack height of the weld path increases with this, while the dilution rate decreases slightly, but the current is too high, and splash increases. Different welding currents should be selected for different widths; for example, for φ75mm x 0.4mm band, the current can be between 1000 and 1300A. ③ With the increase of welding speed, the melting width of the welded pipe and stack height is reduced, the melting depth and dilution rate are increased, welding speed is too high, will make the arc incidence increase, in order to control a certain dilution rate, to ensure that the performance of the layer, welding speed is generally controlled at 15 ~ 17cm / min. ④ with the level of electric slag overlay welding, the base material inclination will affect the dilution rate and weld pipe forming; it is generally recommended that the horizontal position or a slight slope of the uphill welding is appropriate. ⑤ The recommended values for other parameters are 25-35mm band length, 25-35mm flux thickness, 5-l0mm lap volume.
Scope of application
With electro slag overlay welding and submerged arc overlay welding with the poles have the following advantages: 1) high welding efficiency, in the medium current, 50% higher than submerged arc welding; 3) the overlay layer is well-formed, not easy to have slag and other defects, surface quality is excellent, surface unevenness is less than 0.5mm (submerged arc overlay welding is greater than 1 mm), so the surface without mechanical processing, saving material and time. 4) with the poles in the alloy element burnout and adverse element increment is very small, the plasticity and toughness of the overlay layer are higher than submerged arc difficult to weld. 5) due to the fusion zone of the joint carbon diffusion layer narrow, the martensite bandwidth is small, so the fusion zone of the joint performance is better than submerged arc overlay welding with poles. Because of the above advantages of electro-slag overlay welding, it is widely used in hydrogenation control reactors, gas engineering hot wall exchange furnaces, nuclear power plant equipment in large areas of the inner surface of pressure vessels at home and abroad. Due to the characteristics of electro slag welding, it also has a certain scope of application: electro slag welding with high heat input, so it is generally used for welding thick-walled workpieces of 50-200mm. Table 1 Recommended minimum diameter and wall thickness electrode sizes for electro slag overlay welding with electrodes Minimum substrate thickness Minimum surface diameter Outer surface inner surface 60×0.5 40 250 45090×0.5 80 500 900
As a basic industry of the national economy, the power industry has been a key target of national development. The last two decades have been the fastest growing and most successful period in the history of China’s power development. As of 1998, China’s installed electricity capacity reached 277 289 MW, the annual generation capacity reached 1,157.6 billion kWh, and the number of large thermal power plants (with an installed capacity of more than 1,000 MW) in the country reached 68 . As the number of power plants grows and individual capacities and parameters increase, unit maintenance and rehabilitation become increasingly complex and important. As the heart of the turbine generator set – the generator rotor, its high operating accuracy, fast running speed, and high manufacturing costs, once damaged, will directly lead to the entire unit output power decline or even paralysis. It has been repaired by thermal spraying, argon arc welding, patching machine, electric brush plating, and other processes , but the actual practical results after repair are not satisfactory. This paper adopts the DZ-1400 EDM overlay welding equipment (ESD) developed and produced by the Institute of Surface Engineering Technology, China Academy of Agricultural Mechanization Science and Technology, to repair the worn rotor seal section shaft diameter of the generator on-site, which has obtained satisfactory results and successful experience. The EDM process has been proven to play an important role in the repair of power plant components, yielding significant economic and social benefits.
Shaft diameter wear
Once the shaft diameter is worn or strained, the oil pressure in the sealing layer is difficult to maintain equalization, hydrogen will leak, the sealing layer between the shaft diameter and the tile is completely destroyed, and the high-speed operation of the rotor is impeded, and in serious cases, the unit cannot work. Figure 1 is the rotor shaft diameter after the wear of the schematic diagram 3. EDM overlay welding and related process comparison ~ in order to ensure high-speed rotor operation and cooling effect, the rotor shaft and the tile between the three levels of hydrogen, oil, water sealing layer composed of. The process of maintaining a 0.075 EDM gap between the shaft diameter and the shaft tile during operation is different from processes such as welding, spraying, or elemental penetration. In short, it is a process in between, with some characteristics of welding and other processes, and a process that has unique advantages such as small heat input and metallurgical combination of the weld layer and the base material. In some applications with special requirements, the EDM cladding process makes up for the shortcomings of other processes (working principles published separately). Table 1 shows a comparison of the EDM cladding process with other processes.
In the past two years, the EDM deposition overlay welding process has been used to successfully solve the repair and surface strengthening problems of key components of power plants, such as the repair of cylinder sealing surface erosion of the turbine cylinders, repair of spindle wear surface of heat grid circulation pump. Figures 9 and 10 show photographs of the work restored using the EDM cladding process, respectively.
⒈ EDM cladding layer and the base material metallurgical combination, the heat-affected zone of very narrow cladding, the residual stress is negligible. Then, we can repair the damage of the key parts of the power plant by electric spark stacking process, which can be operated online, and the process is simple. Smaller post-replenishment throughput and reduced downtime. 3.3 EDM overlay process has a wide range of applications in power plants, with significant economic and social benefits.
Manual arc welding, overlay welding of the double roll crusher roll surface, achieved a service life of 10 months, crushing 150,000 tons of clinker effect. The main points of cladding are as follows: (1) Selection of welding rods: To select D-65, D-667 and 506 welding rods, before cladding, according to the instructions of welding rods, dry the rods and put them in the holding tank for backup. (The use of Class 506 welding material can cause severe flaking. (2) (2) Roll surface treatment: Roll surface repair can be divided into two methods: local direct welding and overall removal of the overall welding after an overall repair, it can also be said that the two processes. Uneven wear and pattern along the wide direction of the roller, uneven wear of the hardpoint and the overall wear of the roller surface, you can take local repair method direct welding; after 5-6 times direct welding, because the mother body repeatedly undergoes high extrusion stress, welding micro-cracks continue to expand, the wear roller surface will produce a certain thickness of fatigue layer, at this time, if the wear repair welding rod direct welding, easy to produce layer shedding, so the wear layer on the surface of the wear roller fatigue layer needs to be thoroughly cleaned before overlay welding. The repair of the roll face at Xixing Cement Plant was a post-cleaning filler weld. Whether it is direct welding or overall cleaning after welding, the roundness error of the grinding roll and the diameter error of the two roles should not be too large. Otherwise, it will cause horizontal vibration of the roll press and increase the uneven load of the two grinding rolls. To clean the fatigue layer of the roll surface, it can be cleaned by a carbon arc air planer, the fatigue layer of the roll surface should be planed to expose the base material layer. Before cladding, the welding rod should be dried according to the instructions for the use of the welding rod, preheat the welding parts, and slow cooling after welding. (3) To choose the power of 10 kVA or more DC or 20 kVA or more AC welding machine. Use a DC welder to reverse the connection (positive electrode to electrode). When cladding, AC welding machine requires no-load voltage ≥ 70V, the current should be mastered in about 200A. If the no-load voltage is lower than 70V, the current should be increased, and the welding rod and the master material should be fully dissolved. The weld width to height ratio of 3:1 is appropriate. This is what really fuses firmly with the base material and forms the required wear-resistant tissue. (4) The order and thickness of overlay welding: after the roll surface is preheated, it is necessary to first use 506 welding rods overlay welding 1-3 layers; the roll will find round. Then evenly overlay several layers of D-667 to achieve the desired thickness, D-667 layer overlay, then overlay a layer of D-65, overlay thickness of 3-5mm; D-65 layer overlay, then D-65 overlay a layer of lingzhi pattern. (The wear and tear of the roll surface must be generated by both the pressure required to crush the material and the relative sliding. The pressure is determined by the nature of the material and is often difficult to change. It is easier to reduce the relative sliding of the material against the roll surface during the extrusion process by means of the roll pattern. The herringbone pattern used in the early years of the country, although it can stop the circular sliding of the material, but does not limit the axial sliding of the material in the extrusion process, especially in the extrusion of small particles of material, wear is more serious. (In contrast, roll surfaces with a lingering pattern and hardened dots in the middle have the best wear resistance. The edge length of the Ling pattern is 4-5cm, the width of the weld path is about 1cm, and the height is about 4mm. The thickness of each wear layer should be uniform so that the squeeze rolls remain rounded forever during use. (5) When overlay welding, it is necessary to work three shifts, stopping people and not stopping horses, so that the welded parts maintain a high temperature for a long time.
Arc overlay welding
The plasma arc overlay technology for wear-resistant materials is a plasma arc overlay method that takes advantage of the high temperature and high current density of the plasma arc. The high hardness particles are uniformly brazed into the cladding metal with no or little melting of the hard particles. A composite cladding layer is formed. This composite layer is made up of more than two different materials with macroscopically different properties. One is the hard carbide particles that play a major role in the wear resistance of the layer, usually cast tungsten carbide, chromium carbide, boron carbide, sintered tungsten carbide, etc. In principle, all kinds of carbides, borides, and even diamonds with higher hardness can be used as components of the composite overlay layer. Domestic and foreign industries on the application of composite plasma arc welding applications welding more hard particles are cast tungsten carbide; it is composed of eutectic, the hardness of 250 ~ 300. It is generally believed that the combination of hard particles and placental metal is brazed, welding layer, and the combination of the parent material for the metallurgical combination.
The quality of the composite cladding layer obtained by plasma arc overlay technology is stable and reliable. The latest advances in composite plasma arc overlay technology allow the layer to be free of porosity, cracks, carbide burnout, melting, and other defects. Carbide particles are evenly distributed in the overlay layer. The wear resistance of the overlay layer is high. The wear resistance of the composite overlay layer is particularly high under severe wear conditions, which can increase the wear life by several times or more than ten times compared to the usual surface protection layer of iron, cobalt, and nickel-based alloys. It has a high bond strength. Due to the metallurgical bonding of the overlay layer to the protected surface of the workpiece, high strength requirements can be met. The bonding strength of the overlay layer is 3 to 8 times higher than that of the thermal spraying layer. Composite overlays can meet certain requirements for impact resistance. For example, the hammerhead of limestone crusher of cement production equipment, due to the impact force in the wear process, generally used high manganese steel material, low service life, after welding high carbon and high chrome alloy on the surface,