Abstract: The main form of failure in the rolling process of metallurgical rolls is roll surface wear. The repairing technologies of worn roller surfaces mainly include brush plating, thermal spraying, thermal spray welding, traditional surfacing, laser cladding, etc. The advantages and disadvantages of various repairing technologies are analyzed. In order to ensure the performance of the roll after repair, it is necessary to reduce the residual tensile stress of the roll repair layer. Reducing the residual tensile stress is mainly carried out from two aspects: the matching of the repair layer material and the linear expansion coefficient of the substrate, and the reasonable preheating and post-repair heat treatment. Measures to reduce the residual tensile stress during the repair of the worn roller surface are proposed.
Roller is a direct tool to deform metal during rolling. Its quality and service life are directly related to the production efficiency, product quality and production cost of rolling production. In the actual use process, the roll has to withstand great rolling cyclic stress, instantaneous high temperature and strong impact force, strong friction and squeezing force, which will inevitably cause the failure of the roll. If the roll fails, the entire roll will be scrapped, resulting in huge waste and huge economic losses to the enterprise. Roll surface wear is a major failure form of metallurgical rolls. How to repair the worn roll surface has always been a major concern in the roll industry. Through the introduction of brush plating technology, thermal spray technology, thermal spray welding technology, traditional surfacing technology, and laser cladding technology for the repair of worn roller surfaces, the advantages and disadvantages of various repair technologies are analyzed, and the repair of worn roller surfaces of metallurgical rollers is proposed. Measures to reduce residual stress during the process provide a theoretical reference for on-site repair of worn roll surfaces.
1. Failure form of worn roller surface
Metallurgical roll wear roll surface includes two forms of normal wear and abnormal wear.
Normal wear is that during the actual rolling process of the roll, with the extension of the rolling time, the surface of the roll and the rolled product grind and contact each other, resulting in the gradual separation of metal particles on the surface of the roll from the surface of the roll, resulting in a smaller roll diameter and wear resistance. reduce. When the roll diameter wear is small to a certain extent and the roll surface hardness drops to a certain value, it cannot be used even if there are no other defects. Because the roll diameter is small, after the hardness is reduced, the rolled product will not be able to meet the specified size and shape requirements.
The wear mechanisms of abnormal wear mainly include fatigue wear, adhesive wear, corrosive wear, abrasive wear, and fretting wear. In the actual rolling process, these several forms of wear exist mutually and alternately work together. The main form of action is the surface cracks of the roll, which can be divided into axial cracks, circumferential cracks, cracks, ring-shaped cracks and net-shaped cracks. According to statistics, the failure rate of surface cracks of cold work rolls can reach more than 60%, and that of hot work rolls can reach more than 20%. The cracks mostly occur in the contact parts of the roll and the rolled product. As the wear intensifies, cracks, ring-shaped and net-like cracks will spread from the outside to the inside along the grain boundary, and shallow peeling of the roll surface or large area deep peeling may occur. At this time, the roll surface can be repaired; but with As the rolling time is prolonged, roll breakage may occur further, and the roll cannot be repaired and can only be scrapped.
2. Repairing technology of worn roller surface
Before repairing metallurgical rolls with worn roll surfaces, mechanical processing must be used to remove crack defects and fatigue layers on the roll surface to prevent them from being the source of cracks and affecting the quality of the repaired roll. There are mainly the following techniques for repairing worn roller surfaces.
2.1 Brush plating technology
The brush plating technology was invented by the French "Dalic" Research Institute in 1938. It is a method of quickly electrochemically depositing a metal or alloy coating on the local surface of the roll at room temperature and without grooves. Wang Hongmei et al. brush-plated nano-composite coatings on the surface of the rolls. The coatings have excellent fatigue and wear resistance properties.
Brush plating technology has the advantages of simple equipment, flexible operation, fast plating speed, low surface roughness of the coating, and many types of coatings. However, this technology has serious environmental pollution in production, which restricts its application to a certain extent.
2.2 Thermal spray technology
Thermal spraying technology is to heat the sprayed material to melt or semi-melt, and then use high-speed gas to disperse and refine the sprayed material and hit the surface of the substrate at high speed to form a micro-metallurgical bond or mechanical bond coating preparation technology. Si Jian et al. used oxyacetylene flame spraying nickel-aluminum composite powder F 505 (transition layer) and nickel-based alloy G 112 powder (working layer) to repair cast iron Km TBMn 5 W 3 rolls. The porosity between the coating and the roll surface was 25%. The tensile bond strength of the coating and the substrate is 25 MP, and the shear bond strength is 130 MPa. The working coating has been subjected to 1500 h, and the wear after working is small, which meets the technical requirements. Japan Fujii M et al. conducted abrasion resistance friction test by thermal spraying AI 2 O 3-Ti O 2 composite coating on the surface of steel rolls. The results showed that the coating with a thickness of 0.2 mm has good wear resistance when resisting rolling contact fatigue. .
Thermal spraying technology repairs the surface of the roll with small matrix deformation and very shallow heat-affected zone. However, the disadvantage of this technology is that the bonding strength between the coating and the substrate is low, and there are pores and residual stress inside the coating, the toughness is poor, and the machinability is poor, which also limits its wide application to a certain extent.
2.3 Thermal spray welding technology
Thermal spray welding technology is a coating technology developed on the basis of thermal spray technology. It uses a heat source to re-melt or partially melt the coating material on the surface of the substrate to achieve the gap between the coating and the substrate and between the particles in the coating. Metallurgical combination eliminates pores. Jiang Fuhui and others used SPH-C spray welding remelting equipment to spray weld G 112 powder on the surface of the roll. The hardness of the spray welding layer is 58 ~ 62 HRC, and the bonding strength of the spray welding layer and the substrate has exceeded or approached the base material. Ni Zhenhang et al. used flame as the heat source to spray-weld nickel-based alloy Ni 60 A powder on the CSP conveyor roller. The spray-welded layer and the substrate have high bonding strength and low porosity. Thermal spray welding technology The spray welding layer has a dense structure and few metallurgical defects. The metallurgical bonding strength of the spray welding layer and the substrate is high, but the deformation and heat-affected zone of the substrate are larger than that of the thermal spray technology.
2.4 Traditional surfacing technology
Traditional surfacing technology is a technology of cladding an alloy layer with special properties such as heat resistance, wear resistance and corrosion resistance on the surface of the roll. Traditional surfacing welding technology is the most commonly used repairing technology for the worn surface of metallurgical rolls. The general process route of the surfacing repairing technology is shown in Figure 1.
There are three types of surfacing welding technology, including automatic submerged arc welding, manual arc welding and argon tungsten arc welding. See Table 1 for the repair characteristics of different surfacing welding technologies. Various wear-resistant welding materials have been developed at home and abroad to be used in the repair of worn surface of metallurgical rolls, such as Japan's MF-30 / US-H 600 N, HF-1000; Sweden's UTP-CDU R 600, SK-C 600 -O; Face weld 12 in the United States; D 667, D 687, GFH-423-S / GXH-82 in China, etc.
2.4.1 Automatic submerged arc welding
Submerged arc automatic welding is currently the most commonly used metallurgical roll surface surfacing repair technology. Usually, the roll defect area is first used for transition layer materials, and then used for working layer materials. This method is automated production and has the advantages of high production efficiency and good working conditions. At the same time, due to the protective effect of the molten slag on the molten pool, the intrusion of nitrogen, oxygen and hydrogen in the air into the molten pool is reduced, and the performance and quality of the surfacing metal is good. The submerged arc automatic surfacing technology has a large heat input and a high dilution rate to the roll. It usually requires multiple layers of surfacing to ensure the required surfacing metal properties. In order to prevent cracks from being excessively cooled during roll repair, measures such as preheating and slow cooling before welding are usually required. In order to reduce the residual stress of the surfacing welding and improve the performance of the surfacing material, heat treatment is required after welding.
Shi Qiuhong and others used H 2 Cr 13 submerged arc welding wire and flux SJ 260 to repair the 70 Mn low alloy high-strength steel roll by submerged arc welding. The surfacing layer can not only meet the quality requirements, but also the high hardness and high resistance of the surfacing layer. The grindability nearly doubles the service life of the roll. Niu Ben et al. used self-made cored welding wire and flux to perform surfacing repair tests on Cr 5 cold rolls. The surfacing process is stable and beautiful in shape. The surfacing layer metal is based on austenite structure, and the hard phase particles are dispersed in the matrix. Medium, the structure is uniform and small; the average hardness of the surfacing layer is 59 HRC, and the wear resistance is better than that of Cr 5 steel.
2.4.2 Manual arc welding
Manual arc welding is a method in which the manually operated electrode and the workpiece to be welded are treated as two electrodes, and the arc heat between the electrode and the workpiece is used to melt the metal for welding. This technology is mainly used for surfacing repair of local defects on the surface of rolls. Japan invented a continuous casting roll surfacing type 1 Cr 16 Ni 4 Cu 4 Nb electrode with high strength and corrosion resistance. Lu Cheng et al. developed a wear-resistant surfacing D 600 electrode. The surfacing metal has a higher hardness value (about 58.8 HRC) and better wear resistance. Cheng Zhonggeng used self-made electrodes to perform manual arc surfacing on MC 5 cold-rolled work rolls. The metal hardness of the surfacing layer is 59.3 HRC, which is better than the performance of domestic D 322 electrodes.
The characteristics of manual arc welding are simple surfacing equipment, flexible operation, not limited by welding position and roller surface shape, but its production efficiency is low, the dilution rate is high, it is not easy to obtain a thin and uniform surfacing layer, and the working conditions are poor. .
2.4.3 Tungsten arc welding
Tungsten argon arc welding is a welding method that uses tungsten rods as electrodes and argon gas as shielding gas. During welding, argon gas is continuously sprayed from the nozzle of the welding torch to form a gas protection layer around the arc to isolate the air and prevent oxygen in the air from affecting the air. Oxidation of the tungsten electrode, the molten pool and the adjacent heat-affected zone to obtain a beautiful weld metal. This technology is mainly used for local defect areas on the surface of metallurgical rolls. Compared with manual arc welding technology, the heat input is easier to control, the gas protection effect is good, and the surfacing layer metal performance is excellent, but the working conditions are poor and the production efficiency is low.
2.5 Laser cladding technology
Laser cladding technology is a new type of surface modification technology with the most development prospects in roll repair. It uses preset powder or synchronous powder feeding to place selected cladding materials on the surface of the roll substrate, and make it through laser irradiation. It melts at the same time with the thinner layer on the surface of the substrate, and quickly solidifies to form a surface cladding layer with extremely low dilution and metallurgical bonding with the substrate material. Different cladding layer materials have different surface properties. This technology has the characteristics of small heat-affected zone, small roll deformation, compact structure, high bonding strength between the cladding layer and the substrate, and it is not restricted by the welding position and the shape of the roll surface.
The cladding materials used in laser cladding technology are generally powders, mainly iron-based alloy powders, nickel-based alloy powders, and cobalt-based alloy powders, as well as ceramic materials, pure metal powders, and so on. Iron-based alloy powders are inexpensive and widely used, and are suitable for parts that require local wear resistance and easy to deform; nickel-based alloy powders are suitable for parts that require local wear resistance, heat corrosion resistance and thermal fatigue resistance; cobalt-based alloy powder prices Expensive, generally suitable for parts requiring high hardness, high temperature wear resistance, high temperature corrosion resistance and thermal fatigue resistance; ceramic material powder has high strength at high temperatures, good thermal stability, and high chemical stability, generally suitable for Parts requiring wear resistance, corrosion resistance, high temperature resistance and oxidation resistance. Yan Kai et al. used a cross-flow CO 2 laser to cladding 50% Cr 3 C 2 and 50% Ni-Cr alloy powder on the surface of high chromium cast iron rolls. The surface structure of the cladding layer is Cr 3 C 2 and M 7 C 3 type carbides. , The hardness is significantly increased to 1100 HV, which is about twice the hardness of the substrate, and the wear resistance of the cladding layer is also improved. Zhang Hui and others prepared Ti VC 2 reinforced iron-based cladding on 42 Cr Mo roll substrates by using a mixed powder of ferro-vanadium, ferro-titanium, reduced iron powder, and graphite using a synchronous powder feeding method. Floor. Studies have shown that the cladding layer is well formed, compact, free of porosity and cracks, and has high metallurgical bonding strength with the substrate. Fu YM et al. analyzed the temperature field distribution on the surface of the continuous caster roll through finite element software, calculated the limit value of the load during the working process of the continuous casting roll, and then used laser cladding technology to coat the surface of the continuous casting roll with a thickness of The 3 mm Ni 45 alloy powder doubles the surface hardness of the roll. A factory installs a rectangular nozzle on a semiconductor laser robot, and uses a synchronous powder feeding method to cladding Fe Ni Cr BSi alloy powder on the support roller 70 Cr 3 Mo, as shown in Figure 2.
3. Analysis of repairability of worn roll surface
The metallurgical roll on the worn roll surface has a large mass and volume. During the repair process, the molten pool cools quickly and is easy to form a large temperature gradient. At the same time, there is a difference in the expansion coefficient between the repair layer material and the substrate, and it is easy to produce a large amount of The residual tensile stress causes cracks in the repair area of the metallurgical roll and reduces the service life of the roll. Therefore, in order to ensure the service performance of the roll repaired, it is necessary to reduce the residual tensile stress of the roll repaired layer. Reducing the residual tensile stress is mainly carried out from two aspects: the matching of the repair layer material and the linear expansion coefficient of the substrate, and the reasonable preheating and post-repair heat treatment.
3.11 The matching of the repair layer material and the linear expansion coefficient of the substrate
One of the important reasons for the residual tensile stress in the repair layer is the difference between the expansion coefficient of the repair layer material and the base material. When the expansion coefficient of the repair layer is greater than the expansion coefficient of the substrate, tensile stress will be generated, otherwise, compressive stress will be generated. When the residual tensile stress is greater than the ultimate strength of the material, cracks are likely to occur, causing cracking and peeling of the repair layer. The expansion coefficient of the repair layer is not as small as possible for the matrix, and it needs to have a certain range limit. Therefore, selecting a repair layer material close to the expansion coefficient of the substrate is one of the effective methods to reduce the residual tensile stress of the repair layer and reduce the cracking sensitivity.
3.22 Reasonable preheating and post-repair heat treatment
Preheating the substrate before the roll repair can reduce the temperature gradient and reduce the residual tensile stress. The heat treatment after the repair can reduce the residual stress, improve the performance of the repair layer, and ensure the quality of the roller after the repair. However, in the actual on-site repair of worn metallurgical roll surfaces, it is difficult to ensure that the roll substrate is preheated before the repair and the heat treatment after the repair. Preheating before repairing worn rolls and heat treatment after repairing will prolong the repairing time, increase costs, and worsen the working environment. The laser cladding technology can use a low-power laser to preheat the roll before repair, reduce the temperature gradient between the repair layer and the substrate, reduce the residual tensile stress, and improve the quality of the roll after repair.
4. Conclusion
What kind of technology is used to repair the worn surface of metallurgical rolls and improve the service life of the rolls has always been a major concern in the roll industry. Conventional wear roller surface repair technologies such as brush plating technology, thermal spray technology, thermal spray welding technology, automatic submerged arc welding, manual arc welding, and argon tungsten arc welding can not effectively improve the ability of the roller surface to resist high temperature wear and cracking. Therefore, it is of great significance to carry out laser cladding technology to repair the worn surface of metallurgical rolls and will be an important direction for future development. With the rapid development of laser cladding technology, composite laser cladding technologies have emerged on the market, such as laser cladding with ultrasonic, electromagnetic stirring, alternating magnetic field, mechanical vibration and other technologies. These technologies are beneficial to the internal structure of the cladding layer. The rate decreases, the grain size decreases, and the hardness and wear resistance of the cladding layer are greatly improved. In recent years, with the rapid development of nanomaterials, laser cladding technology can be used to cladding nanomaterials on the surface of rolls, which can increase the overall strength of the material to a certain extent, forming a new technology for nanometering the surface of metallurgical rolls, which is helpful for scientific research Workers' in-depth research on the overall performance of nanomaterials.
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