The main failure mode in the rolling process of metallurgical rolls is rolled surface wear. Roll surface repair technologies mainly include brush plating, thermal spraying, thermal spray welding, traditional surfacing, laser cladding, etc. In order to ensure the serviceability of the roll after repair, it is necessary to reduce the residual tensile stress of the roll repair layer. The reduction of residual tensile stress is mainly carried out from two aspects: the matching of the linear expansion coefficient of the repair layer material and the substrate, and reasonable preheating and post-repair heat treatment. Measures to reduce the residual tensile stress in the process of repairing the worn roller surface are proposed.
A roll is a direct tool for deforming metal during the rolling process, and its quality and service life are directly related to the production efficiency, product quality, and production cost of rolling production. During the actual use of the roll, it must bear a large rolling cycle stress, instantaneous high temperature, strong impact force, strong friction, and extrusion force, which will inevitably cause the roll to fail. 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 mode of metallurgical rolls. How to repair the worn roll surface has always been a major concern of the roll industry. Through the introduction of brush plating technology, thermal spraying technology, thermal spray welding technology, traditional surfacing technology, and laser cladding technology for the repair of worn roll surfaces, the advantages and disadvantages of various repair technologies are analyzed, and a repair method for metallurgical roll worn roll surface is proposed. The measures to reduce the residual stress in the process provide a certain theoretical reference basis for on-site repair of the worn roller surface.
1. Failure form of worn roller surface
Metallurgical roll wear roll surface includes two forms normal wear and abnormal wear.
1.1 Normal wears
Normal wear is that during the actual rolling process of the roll, with the prolongation of the rolling time, the surface of the roll and the rolled product are in ground contact with each other, causing the metal particles on the surface of the roll to gradually separate from the surface of the roll, making the diameter of the roll smaller and improving the wear resistance. reduce. When the wear of the roll diameter is small to a certain extent and the hardness of the roll surface drops to a specified value, it cannot be used even if there are no other defects. Because the diameter of the roll is small and the hardness is reduced, the rolled product will not meet the specified size and shape requirements.
1.2 Abnormal wear
The wear mechanism of abnormal wear mainly includes fatigue wear, adhesive wear, corrosion wear, abrasive wear, and fretting wear.
In the actual rolling process, these types of wear coexist and interact with each other. The main form of action is cracking on the surface of the roll, which can be divided into axial cracks, circumferential cracks, cracks, annular and reticular cracks. According to statistics, the failure rate of cracks on the surface of cold work rolls can reach more than 60%, and the surface of hot work rolls can reach more than 20%. With the intensification of wear, cracks, annular and network cracks will expand from outside to inside along the grain boundary, and shallow peeling or large-scale deep peeling of the roll surface may occur. At this time, the surface of the roll can still be repaired; but as The extension of the rolling time may further cause roll breakage, and the roll cannot be repaired and can only be scrapped.
2. Worn roller surface repair technology
Before the metallurgical roll with the worn surface is repaired, it is necessary to remove the crack defect and fatigue layer on the roll surface by mechanical processing, so as to prevent it from affecting the quality of the repaired roll as a crack source. There are mainly the following types of worn roller surface repair technologies.
2.1 Brush plating technology
The brush plating technology was invented by the French “Dalic” Institute in 1938. It is a method of rapidly electrochemically depositing a metal or alloy coating on the local surface of the roll under normal temperature and no groove conditions. The nanocomposite coating is brush-plated on the surface of the roll, and the coating has excellent anti-fatigue and anti-wear properties.
The brush plating technology has the advantages of simple equipment, flexible operation, fast plating speed, low surface roughness of the coating, and various types of coatings. However, the environmental pollution of this technology is serious in production, which restricts its application to a certain extent.
2.2 Thermal spraying 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 it on the surface of the substrate at high speed to form a micro-metallurgical bond or a mechanically bonded coating preparation technology. Si Jian et al. used oxyacetylene flame spraying nickel-aluminum composite powder F505 (transition layer) and nickel-based alloy G112 powder (working layer) to repair the cast iron KmTBMn5W3 roll. The surface porosity of the coating and the roll was 25%, and the transition coating and the substrate were tensile The bonding strength is 25MP, and the shearing bonding strength is 130MPa. After 1500 hours of working coating, the wear is small after work, which meets the technical requirements. Japanese Fujii M et al. thermally sprayed AI2O3-TiO2 composite coating on the surface of steel roller for wear resistance friction test. The results show that the coating with a thickness of 0.2mm has good wear resistance when resisting rolling contact fatigue.
Thermal spraying technology to repair the surface of the roll has a small matrix deformation and a 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, which leads to poor toughness and poor machinability, 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 melts the coating material on the surface of the substrate to realize the bonding between the coating and the substrate and between the particles in the coating. Metallurgical bonding to eliminate porosity. Jiang Fuhui and others used SPH-C spray welding and remelting equipment to spray G112 powder on the surface of the roll. The hardness of the spray welding layer is 58~62HRC, and the bonding strength of the spray welding layer and the substrate has exceeded or approached the base metal. Ni Zhenhang et al. used flame as a heat source to spray-weld nickel-based alloy Ni60A powder on the CSP conveyor roller table. The spray-welded layer and the substrate have high bonding strength and low porosity. The thermal spray welding technology has a dense spray layer structure, fewer metallurgical defects, and high metallurgical bonding strength between the spray weld layer and the substrate, but the substrate deformation and heat-affected zone are larger than those of the thermal spray technology.
2.4 Traditional surfacing technology
The traditional surfacing technology is a technology of cladding a layer of alloy layer with special properties such as heat resistance, wear resistance, and corrosion resistance on the surface of the roll. Traditional surfacing technology is the most commonly used repair technology for the worn surface of metallurgical rolls.
Surfacing welding techniques include submerged arc automatic welding, manual arc welding, and argon tungsten arc welding. Different surfacing welding techniques have different repair characteristics. Various wear-resistant welding materials have been developed at home and abroad for repairing the worn surface of metallurgical rolls, such as MF-30 /US-H600N, HF-1000 in Japan; UTP-CDUR 600, SK-C600-O in Sweden; Face weld12; Chinese D667, D687, GFH-423-S /GXH-82, etc.
2.4.1 Submerged arc automatic welding
Submerged arc automatic welding is currently the most commonly used metallurgical roll surface surfacing repair technology. Usually, the transition layer material is used first in the defective area of the roll, and then the working layer material is used. The method is automatic production and has the advantages of high production efficiency, good working conditions, and the like. At the same time, due to the protective effect of 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 welding metal are good. The submerged arc automatic surfacing technology has a large heat input and a high dilution rate to the roll. Usually, multiple layers of surfacing welding are required to ensure the required surfacing metal properties. In order to prevent cracks from occurring due to excessive cooling during roll repair, measures such as pre-welding preheating and slow cooling are usually required. In order to reduce the residual stress of surfacing welding and improve the properties of surfacing welding materials, heat treatment is required after welding.
Use H2Cr13 submerged arc welding wire and flux SJ260 to carry out submerged arc automatic welding surfacing repair on the low-alloy high-strength steel 70Mn roll. almost doubled. The surfacing welding repair test was carried out on the Cr5 cold roll with self-made pharmaceutical cored wire and flux. The surfacing process is stable and the shape is beautiful. The metal of the surfacing layer is based on an austenite structure, and the hard phase particles are dispersed in the matrix. The structure is uniformly Small; the average hardness of the surfacing layer is 59HRC, and its wear resistance is better than that of Cr5 steel.
2.4.2 Manual arc welding
Manual arc welding is a method in which a manually operated electrode and the workpiece to be welded are used as two electrodes, and the metal is melted by the heat of the arc between the electrode and the workpiece for welding. This technology is mainly used for surfacing repair of local defects on the roll surface. Japan invented a continuous casting roller surfacing type 1Cr16Ni4Cu4Nb electrode, which has high strength and corrosion resistance.
Manual arc welding is characterized by simple surfacing equipment, and flexible operation, and is not limited by the welding position and the surface shape of the roll, but its production efficiency is low, the dilution rate is high, it is difficult to obtain a thin and uniform surfacing layer, and the working conditions are poor.
2.4.3 Argon tungsten arc welding
Argon tungsten arc welding is a welding method in which a tungsten rod is used as an electrode and argon gas is used as shielding gas. During welding, argon gas is continuously ejected from the nozzle of the welding torch, forming a gas protective layer around the arc to isolate the air and prevent oxygen in the air from Oxidation of the tungsten electrode, molten pool, and adjacent heat-affected zone to obtain a well-formed 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 easy to control, the gas protection effect is good, and the metal performance of the surfacing layer 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 repairing. The selected cladding materials are placed on the surface of the roll substrate by means of preset powder or synchronous powder feeding. The thinner layer on the surface of the substrate is melted at the same time, and after rapid solidification, a surface cladding layer with a very low dilution and a metallurgical bond with the substrate material is formed. Different cladding layer materials have different surface properties. This technology has the characteristics of a small heat-affected zone, small roll deformation, compact structure, and high bonding strength between the cladding layer and the substrate, and is not limited by the welding position and the surface shape of the roll.
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 and pure metal powders. Iron-based alloy powder is cheap and widely used, suitable for parts that require local wear resistance and easy deformation; nickel-based alloy powder is suitable for parts that require local wear resistance, heat corrosion resistance, and thermal fatigue resistance; cobalt-based alloy powder price 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 temperature, good thermal stability, high chemical stability, generally suitable for Parts that require wear resistance, corrosion resistance, high-temperature resistance, and oxidation resistance. A cross-flow CO2 laser is used to clad 50% Cr3C2 and 50% Ni-Cr alloy powder on the surface of high-chromium cast iron rolls. The surface structure of the cladding layer is Cr3C2 and M7C3 carbides, and the hardness is significantly increased to 1100HV, which is about 2 times the hardness of the matrix. At the same time, the wear resistance of the cladding layer is also improved. Using ferrovanadium, ferrotitanium, reduced iron powder, and graphite mixed powder, the TiVC2 reinforced iron-based cladding layer was prepared on the 42CrMo roll substrate by laser cladding in situ autogenous reactions by synchronous powder feeding. The research shows that the cladding layer has a good shape, compact structure, no pores and cracks, and high metallurgical bonding strength with the substrate.
3. Analysis of repairability of worn roller 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 a large temperature gradient is easily formed. At the same time, there is a difference in expansion coefficient between the repair layer material and the matrix, which is easy to produce a large temperature gradient in the repair layer. Residual tensile stress causes cracking in the repair area of metallurgical rolls and reduces the service life of the rolls. Therefore, in order to ensure the service performance of the roll after repair, it is necessary to reduce the residual tensile stress of the roll repair layer. The reduction of residual tensile stress is mainly carried out from two aspects: the matching of the linear expansion coefficient of the repair layer material and the matrix, and reasonable preheating and post-repair heat treatment.
3.1 The matching of the repair layer material and the linear expansion coefficient of the matrix
One of the important reasons for the residual tensile stress in the repair layer is the difference in expansion coefficient between the repair layer material and the base material. When the expansion coefficient of the repair layer is greater than the expansion coefficient of the matrix, 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, which will cause cracking and peeling of the repair layer. The expansion coefficient of the repair layer is not as small as possible for the substrate, and it needs to be limited to a certain range. Therefore, choosing a repair layer material close to the expansion coefficient of the base material is one of the effective methods to reduce the residual tensile stress of the repair layer and reduce the crack sensitivity.
3.2 Reasonable preheating and post-repair heat treatment
Preheating the substrate before 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 repaired roll. However, in the process of repairing the worn metallurgical roll surface on the actual site, it is difficult to ensure the preheating of the roll substrate before repair and the heat treatment after repair. Preheating before repairing worn rolls and heat treatment after repairing will prolong the repair time, increase costs and deteriorate the working environment. The laser cladding technology can use a low-power laser to preheat the roll before repairing, reduce the temperature gradient between the repaired layer and the substrate, reduce the residual tensile stress, and improve the quality of the repaired roll.
What kind of technology to use to repair the worn surface of metallurgical rolls and improve the service life of the rolls has always been a major issue concerned by the roll industry. Conventional worn roller surface repair technologies such as brush plating technology, thermal spraying technology, thermal spray welding technology, automatic submerged arc welding, manual arc welding, and argon tungsten arc welding cannot effectively improve the ability of the roll surface to resist high-temperature wear and cracking. Therefore, it is of great significance to carry out laser cladding technology to repair the worn roll surface of metallurgical rolls, and it will be an important direction for future development. With the rapid development of laser cladding technology, composite laser cladding technology has appeared on the market, such as laser cladding supplemented by ultrasonic, electromagnetic stirring, alternating magnetic field, mechanical vibration, and other technologies, these technologies are conducive to the organization of pores inside the cladding layer The rate decreases, the grain size decreases, and the hardness and wear resistance of the cladding layer are greatly improved. With the rapid development of nano-materials in recent years, laser cladding technology can be used to clad nano-materials on the surface of the roll, which can improve the overall strength of the material to a certain extent, and form a new technology for nano-scale metallurgical roll surface, which is helpful for scientific research Workers have conducted in-depth research on the overall performance of nanomaterials.