Rollers are important parts of rolling mills. They are easy to wear out during the steel rolling process, which is related to the economic benefits and production efficiency of steel rolling enterprises. Rollers are usually bimetallic structures, consisting of a metallurgically bonded working layer and a roller core. The working layer is high-speed steel or high-chromium cast steel with good red hardness, wear resistance and hardenability, and the roller core is ductile iron. During the rolling process, the rolls will fail due to sticking, wear, etc., which will not only reduce the surface quality of the rolled parts, but also affect the normal use of the rolling mill in severe cases, and even cause large economic losses.

Most rolls fail due to local working layer failure, and scrapping will cause resource waste. The causes of roll failure should be explored, and the failed rolls should be remanufactured. Roller remanufacturing mainly repairs failed rolls and prepares new working layers with high bonding strength, wear resistance and corrosion resistance to extend the service life of the rolls. The use of advanced technology to remanufacture failed rolls is of great significance to improving the level of steel rolling equipment and the development of the metallurgical industry.

1 Failure forms and causes of rollers

Rollers are in high temperature, high humidity and corrosive atmosphere for a long time. During use, they are not only subjected to external loads, but also generate thermal stress due to the circulation of high temperature and cooling water, which leads to fatigue damage failure after a period of use. The main failure forms are surface wear, fatigue cracks, surface peeling and fracture.

Surface wear failure is the most important form of roller loss and failure. According to the wear mechanism, wear can be divided into fatigue wear, abrasive wear, adhesive wear, corrosive wear, etc. The main reasons for its occurrence are: periodic contact stress, thermal stress and alternating stress generated by the alternating action of the rolled piece and cooling water on the surface, resulting in fatigue wear on the roller surface; bonding points are formed on the contact surface between the roller and the rolled piece, and wear debris is formed after the bonding points are broken, resulting in adhesive wear on the roller surface; debris and oxide scale on the surface of the rolled piece are broken into hard particles under the action of rolling force, resulting in abrasive wear on the roller surface; oxidation reaction occurs on the roller surface in an environment containing water vapor and oxygen to generate corrosive reactants, resulting in corrosive wear on the roller surface.

The main reason for fatigue cracking failure is that during hot rolling, the surface or weak part of the roll surface generates thermal stress and tensile stress under the action of hot and cold cycles. The thermal stress causes plastic deformation on the roll surface, and the roll core prevents the surface from shrinking and generates tensile stress to crack the surface.

The main reason for roll peeling failure is that the residual stress and serious metallurgical defects generated during the production of the roll cause fatigue cracks to form at the weak part of the roll surface, and expand under the action of cyclic shear stress. When the shear stress is greater than the strength of the roll surface, peeling occurs.

Fracture failure is the most serious and irreparable failure form, which generally occurs at the joint ends on both sides of the roll neck, the transition zone between the roll neck and the roll body, and the middle of the roll body. It is divided into brittle fracture and plastic fracture. The main reasons for fracture failure are: organizational defects are generated during the roll casting process and expand under the action of mechanical stress, the working load of the roll exceeds the strength limit, and the roll surface temperature is too high.

2 Current status of roll remanufacturing technology

The roll remanufacturing technology mainly includes subtractive technology based on traditional grinding and additive technology based on surface modification. The traditional turning reduction technology mainly removes the wear layer and cracks on the surface of the annealed failed roll and repairs the roll. This technology will consume a lot of roll material, and the roll that has been repaired by multiple reductions will be scrapped due to its small diameter. In addition, the roll body often produces defects such as grinding burns and roll surface scratches during the reduction process, so the application of turning reduction technology is becoming less and less. In recent years, surface additive remanufacturing technology can not only significantly improve the wear resistance, corrosion resistance and thermal stability of mechanical parts, but also extend their service life, and has developed rapidly. At present, the remanufacturing technologies of rolls at home and abroad mainly include spraying, brush plating, surfacing, cladding, etc.

2.1 Spraying technology

Spraying is to heat the alloy material to a molten or semi-molten state, spray it onto the workpiece surface at high speed, and form a mechanically bonded metal coating after solidification. There are plasma spraying, arc spraying, flame spraying, etc. The principle of plasma spraying technology is shown in Figure 1.

Commonly used spraying materials are powders, wires and rods, all of which are high-hardness metals, ceramics or composite materials. This technology has high productivity, controllable coating thickness, low cost, low substrate heating, parts are not easy to deform, and can be prepared on a large area. Since the substrate and the spraying material are mainly mechanically bonded, the bonding strength is low. The pretreatment requirements for the roller spraying part are high. If the pretreatment is improper, the coating is easy to fall off; if the protection is poor, the coating is prone to pores and cracks. Spraying technology is not suitable for the remanufacturing of rollers used under high temperature, high pressure, and high impact conditions.

In order to prepare coatings with high bonding strength, low porosity, and good comprehensive performance, new technologies such as cold spraying, high-speed arc spraying, supersonic plasma spraying, and supersonic flame spraying have been developed, and the types of spraying materials and the temperature range of heat sources have been expanded. Improving the stability and controllability of the process is the development direction of spraying technology.

2.2 Brush plating technology

Brush plating is to provide electrolyte through a pad or brush in contact with the anode, and the plated surface of the workpiece is connected to the cathode. Under the action of the current, the pad or brush moves on the plated surface, so that the metal ions in the plating solution discharge and crystallize on the surface of the part to form a metal coating. The principle is shown in Figure 2.

Brush plating technology has the advantages of simple process, wide application range, fast deposition rate, small residual stress, small substrate deformation, and on-site repair. However, the coating and the substrate are mainly mechanically bonded. Under high contact stress, the coating is prone to cracking and peeling. It can only remanufacture pits or wear within 1 mm below the roller surface. The coating thickness is limited, and metal ions will pollute the environment. This technology can be used to repair large machine parts, but is not suitable for workpieces that are subjected to rapid cooling and heating, and workpieces with deep cracks or large wear areas.

Improving the surface performance of parts, increasing the bonding force between coating and substrate, developing electric contact strengthening technology (a high-energy density surface heat treatment process that uses the resistance heat and pressure generated by the contact between the workpiece and the electrode to strengthen the surface), and developing composite coating preparation technology are the development directions of brush plating technology research.

2.3 Overlay welding technology

Overlay welding is to deposit heat-resistant, wear-resistant and corrosion-resistant metal materials on the surface of the workpiece to give the workpiece surface different properties or repair failed workpieces. There are arc overlay welding, submerged arc overlay welding, tungsten inert gas arc overlay welding, plasma arc overlay welding, laser overlay welding, etc. Compared with other overlay welding technologies, plasma overlay welding has the characteristics of less mutual melting between the substrate and the overlay welding material and small changes in the properties of the overlay welding material. It has been widely used. Its working principle is shown in Figure 3.

In addition to the advantages of other surfacing technologies, plasma surfacing technology also has the characteristics of high temperature, high energy density, melting refractory materials, high material utilization rate, high productivity, etc., and the surfacing layer is metallurgically bonded to the substrate, with no obvious defects on the surface, not easy to produce pores, low dilution rate, stable process, low cost, easy to realize automation and intelligence. Surfacing is a process of rapid melting and solidification, which produces large thermal stress, and the workpiece is prone to deformation and cracking. The welding process has a great influence on the structure and performance of the surfacing layer, especially multi-layer surfacing is prone to impurities, weld scars, holes and other defects. Surfacing metal has been widely used in aviation, petrochemical, machinery manufacturing and other fields, mostly for workpieces with low precision and performance requirements, not suitable for alloy cast iron and high carbon rollers with poor welding performance, and the repair quality of large rollers is difficult to guarantee. Reducing the defects of the surfacing layer, optimizing the welding process, improving the high temperature wear resistance of the surfacing layer, improving the formability of the surfacing layer, developing new welding materials, and adopting a process method combining auxiliary processing with surfacing technology are the development direction of surfacing technology.

2.4 Laser Cladding Technology

Laser cladding technology uses a laser beam to melt metal materials on the surface of the workpiece to form a cladding layer that is metallurgically bonded to the workpiece surface. The principle is shown in Figure 4. The feeding methods of cladding materials include pre-setting powder, synchronous powder feeding and wire feeding cladding. Pre-setting powder is to pre-set the powder on the substrate so that the powder and the substrate melt and solidify at the same time to obtain a cladding layer, as shown in Figure 4(a). Synchronous powder feeding is to feed the cladding powder directly into the beam, and form a cladding layer as the beam moves on the workpiece surface. Synchronous powder feeding mainly includes coaxial powder feeding (Figure 4(b)) and off-axis powder feeding (Figure 4(c)). Wire feeding cladding is to feed the metal wire into the beam, melt and solidify at the same time as the substrate to form a cladding layer, as shown in Figure 4(d).

Laser cladding technology has the advantages of concentrated heat, extremely low dilution rate, small heat-affected zone, small substrate damage, good wettability of the cladding layer joint, high bonding strength with the substrate, not easy to produce defects, precise control of process parameters, and basically no pollution. However, there are also problems such as low efficiency, easy shrinkage and deformation of workpieces, high equipment cost, and high cost. Laser cladding technology can repair almost any parts with complex shapes.

Solving the problem of cracking of the cladding layer through heat treatment, developing new laser cladding processes, developing new laser cladding equipment, and developing ultra-high-speed laser cladding technology are the development directions of laser cladding technology.

2.5 Characteristics of remanufacturing technology

The main advantages, disadvantages and application scope of different remanufacturing technologies are shown in Table 1. The comparison shows that the laser cladding layer and the cladding layer are metallurgically bonded to the substrate, with higher bonding strength than the spray layer and the electroplating layer, and better wear resistance and corrosion resistance; the heat affected zone of laser cladding is smaller than that of cladding, and slightly larger than that of spraying and brush plating; the dilution rate of laser cladding technology is the lowest; compared with brush plating and spraying, laser cladding technology has less pollution and is more in line with green remanufacturing.

3 Application of laser cladding technology in roller remanufacturing

3.1 Effect of cladding materials on the organization and properties of remanufactured rollers

Cladding materials affect the wear resistance, corrosion resistance and oxidation resistance of laser cladding layers. The thermal expansion coefficient, melting point and elastic modulus of powder materials should be similar to those of the substrate material, otherwise the coating and the substrate are prone to cracking, and the coating will peel off in severe cases. Commonly used laser cladding materials include iron-based, cobalt-based, nickel-based alloys and composite materials. The hardness and wear resistance of the remanufactured cladding layer can be improved by second phase strengthening, fine grain strengthening, solid solution strengthening, microstructure optimization, and coating amorphization.

Liu Yingbo used spherical WC with different mass fractions (10%, 20%, 30%, 40%) of Stellite 6 powder as cladding material and laser cladding technology to prepare Stellite 6/WC gradient composite cladding layer on the surface of cold rolling roller. When 30% Stellite 6 powder was added, the forming quality, hardness and wear resistance of the cladding layer were the best. Yan Kai et al. [53] used a transverse CO2 laser to laser clad 50% Cr3C2 and 50% Ni-Cr alloy powder on the surface of a high chromium cast iron roller, and tested the hardness and wear resistance of the cladding layer. The results showed that the hardness of the cladding layer was as high as 1 100 HV, which was twice that of the substrate. Yin Yan et al. [54] used ultra-high-speed laser cladding technology to prepare a cobalt-based alloy cladding layer on the surface of a 32Cr3Mo1V steel cast roller sleeve. The results showed that the cladding layer had a uniform and fine structure, produced very small oxide chips, formed a large area of ​​enamel layer with friction reduction and wear resistance, and the roller sleeve had excellent high-temperature friction and wear resistance.

3.2 Effect of laser cladding process parameters on the structure and performance of remanufactured rollers

Laser cladding process parameters mainly include laser power, overlap rate, scanning rate, powder-carrying gas flow rate, powder feeding rate, etc., which all affect the geometry, microstructure, mechanical properties, surface quality, stress state and cladding efficiency of the cladding layer. Studies have shown that the quality of the cladding layer mainly depends on the scanning rate, laser power, powder feeding rate, etc. There are many laser cladding process parameters, and the combination of parameters is more complex. It is difficult to test each parameter or its combination separately. Optimizing laser cladding process parameters through numerical simulation can not only save test costs, but also shorten the test cycle.

Dong Hui et al. studied the effect of scanning rate on the structure and properties of the cladding layer, and prepared NiCr/Cr3C2 composite coating on the roller surface by laser cladding technology. The study found that when the scanning rate increased from 2 mm/s to 4 mm/s, the structure of the cladding layer changed from dendrites and cellular crystals to equiaxed crystals, and the defects changed from pores to large-sized gaps and cracks; with the increase of laser scanning rate, the hardness of the cladding layer increased significantly; the coating prepared at a scanning rate of 3 mm/s had the smallest wear and the best wear resistance. Zheng Hongbin et al. prepared M2 high-speed steel coating on the surface of 9Cr2Mo steel roller by ultra-high-speed laser cladding technology, and tested the structure and properties of the cladding layer. The results show that the hardness of the laser cladding layer is higher than that of the substrate and has good wear resistance. Lian et al. established a mathematical model of the relationship between process parameters and roller coating quality, which can predict the geometric characteristics, performance and optimization process parameters of the coating. The coating surface quality prepared by the optimized process parameters is excellent.

4 Conclusion

The development of roller remanufacturing technology has brought good social, economic and environmental benefits to the remanufacturing of failed rollers. In order to ensure the remanufacturing quality of failed rollers, the future development direction of roller remanufacturing technology is:
(1) Develop remanufacturing equipment for high-speed steel rollers. High-speed steel rollers have higher wear resistance and heat resistance and have been promoted and applied. Therefore, it is necessary to develop a processing technology combining laser cladding with artificial intelligence, as well as intelligent, small, portable and low-cost laser cladding equipment and special laser cladding materials.
(2) Develop new processes for remanufacturing high-speed steel rollers, such as laser cladding + surfacing, laser cladding + brush plating, etc.; develop simple and easy-to-use heat treatment processes with high production efficiency.
(3) The size of the remanufactured roller has been restored, and its hardness, wear resistance and corrosion resistance have met the requirements for reuse, but its performance is still difficult to accurately evaluate. A fatigue life assessment model for remanufactured rollers should be established to study the relationship between the fatigue life of remanufactured rollers and process parameters, cladding materials, manufacturing equipment, etc., and formulate performance assessment standards for remanufactured rollers.