Laser cladding remanufacturing technology can effectively repair the surface wear of parts and components of cement production equipment, but the cracking of the cladding layer has become a key bottleneck restricting its wide application. The causes of cladding layer cracking were analyzed from the perspectives of thermodynamics and kinetics, the factors affecting the cracking of the cladding layer and the methods of suppressing it were discussed, and the laser cladding remanufacturing optimization technology for solving the cracking of the cladding layer by using the substrate preheating and slow cooling method was proposed. Practice has shown that the use of this technology to repair shaft, inner hole, and bushing cement equipment can increase the service life of the equipment by more than double. Taking the maintenance of roller presses or vertical mills as an example, the annual maintenance cost can be saved by 100,000 to 300,000 yuan, with significant economic benefits.

Cement production equipment mostly operates under high temperature, high dust, and heavy load conditions. The operating conditions of some key components are complex and harsh. When problems such as wear occur, if they are not maintained in time, they will easily fail, thereby affecting the safe and stable operation and service life of the host equipment. Traditional surface repair processes for failed parts mainly include thermal spraying, arc welding, etc., which often have problems such as complex repair processes, high costs, and difficulty or even inability to repair, resulting in a great waste of resources.

In recent years, laser cladding remanufacturing technology (abbreviated as: laser cladding technology) has been widely used in the surface repair of parts and components of cement production equipment. Compared with traditional repair processes such as thermal spraying and arc welding, laser cladding coatings are relatively dense, the cladding powder dilution rate is low, and the surface hardness of the repaired workpiece is evenly distributed, which not only improves the surface corrosion resistance, wear resistance and fatigue resistance of parts, but also extends the service life of parts. Using laser cladding technology, the thickness of the cladding layer is increased compared with the thermal spraying process, and the workpiece bonding strength and repair efficiency are improved. At the same time, the laser cladding technology has a low degree of environmental pollution and is an advanced manufacturing technology with low carbon, environmental protection, energy saving and emission reduction.

Our company has successfully applied laser cladding technology to parts of cement equipment that are prone to wear, such as the roller shaft and bearing seat inner hole of the roller press, the main shaft and hub inner hole of the vertical mill roller, the sealing bushing, and the shaft hole of the reducer housing. However, in the early stage of the application of this technology, the cracking problem of the cladding layer became a key bottleneck restricting its widespread application. This article conducts an in-depth analysis of the causes, influencing factors and suppression methods of cracking of the cladding layer, provides a basis for solving the problem of cracking of the laser cladding layer, and briefly introduces the company’s typical application cases in repairing cement equipment using laser cladding technology.

Analysis of the causes of cracking of the laser cladding layer:

① The causes of cracking of the laser cladding layer on the surface of cement production equipment can be analyzed from two aspects: thermodynamics and kinetics. In terms of thermodynamics, affected by the fast melting and cooling characteristics of laser cladding technology, when laser cladding is used to repair workpieces, the melt in the molten pool will increase the cracking sensitivity between dendrites during the cooling and solidification process due to the uneven composition of the cladding material grains, and then crack the cladding layer under the action of residual stress. This type of crack mainly occurs at the junction of the cladding layer and the substrate. This is because the bottom of the cladding layer is usually coarse dendrites, which are prone to intergranular weakening, and the bottom of the cladding layer is usually in a state of residual tensile stress, which makes it easier to crack. In addition, analysis shows that the fundamental cause of cracking of the cladding layer is due to the mismatch of the thermal physical properties of the substrate and the cladding layer materials. Such cracks mainly manifest in two forms: transverse cracks and longitudinal cracks.

② In terms of dynamics, during the laser cladding process, the temperature gradient perpendicular to the laser scanning direction is the largest, and the residual tensile stress is the highest, so cracks are easy to occur. Such cracks mostly occur on the surface of the cladding layer, which is easy to be found and detected, and is one of the most common crack forms.

③ In addition, the parts with internal defects such as pores and inclusions in the cladding layer are also prone to become crack sources. During the service of the cladding layer, these parts are prone to stress concentration, resulting in the generation and expansion of microcracks until the parts fail.

At present, the company mainly uses iron-based alloys as cladding materials for surface repair of cement production equipment. The main reasons for the cracking of the cladding layer of such materials include the stress state of the cladding layer, microscopic defects and hardening tendency.

Factors affecting cracking of laser cladding layer:

① Thermophysical properties of cladding layer materials
During the laser cladding process, the difference in thermal expansion coefficient of cladding layer materials is the key factor that causes the different thermal expansion and cooling contraction rates of the cladding layer and the substrate, and leads to tensile stress and interface cracks at the interface between the two. Reasonable matching of the thermal expansion coefficients of the cladding layer material and the substrate material can significantly reduce the cracking sensitivity of the cladding layer.

② Laser cladding process parameters
Laser cladding process parameters determine the dynamic characteristics of the molten pool and the change of temperature gradient, which affects the generation of cracks in the cladding layer. Among them, with the increase of process parameters such as laser scanning rate, powder feeding rate and number of cladding layers, the cracking tendency of the cladding layer increases; with the increase of laser power, the cracking tendency of the cladding layer decreases.

③ Equipment surface heat treatment
The surface heat treatment methods mainly include preheating, heat preservation and slow cooling. Through equipment surface heat treatment, the temperature gradient distribution of the cladding layer can be optimized, the residual tensile stress inside the cladding layer can be reduced, and the cracking phenomenon of the cladding layer can be reduced. Practice has found that equipment surface heat treatment is the simplest and most effective means to reduce the cracking tendency of the cladding layer.

Methods to inhibit cracking of laser cladding layer:

The root cause of cracks in laser cladding layer is that the strain induced by thermal stress of laser cladding layer exceeds the plastic limit of cladding layer. Therefore, releasing or eliminating thermal stress is the core method to inhibit cracking of cladding layer. The commonly used methods to inhibit cracking of laser cladding layer are shown in Figure 1.

① Reasonable control of the thermophysical properties of cladding layer materials
As mentioned above, among the thermophysical performance parameters, the thermal expansion coefficient has the greatest influence on the cracking of cladding layer. Reasonable matching of the thermal expansion coefficient between cladding layer material and substrate material can significantly reduce the generation of interface stress, thereby inhibiting the cracking tendency of cladding layer.

② Optimizing laser cladding process parameters
A scientific and reasonable combination of laser cladding process parameters can inhibit the cracking of cladding layer to a certain extent. Appropriately reducing process parameters such as laser scanning rate, powder feeding rate and number of cladding layers, and appropriately increasing laser power can inhibit the cracking tendency of cladding layer.

③ Heat treatment of the equipment surface (substrate preheating)
Through surface heat treatment, the residual tensile stress of the cladding layer and the temperature gradient between the cladding layer and the substrate can be significantly reduced, thereby reducing the generation of thermal stress. In addition, slow cooling measures after laser cladding can further reduce or eliminate the residual thermal stress of the cladding layer, thereby effectively reducing the tendency of the cladding layer to crack.

④ Reasonable design of cladding layer material
Adding toughening and plasticizing elements, rare earth elements or their oxides (such as MgO, TiO2, V2O5, Y2O3, etc.) to the cladding layer material can reduce the cracks in the cladding layer to a certain extent and improve the quality of the cladding layer. Studies have shown that adding only 1% Y2O3 to the cladding layer material can make the cladding layer structure more dense, without defects such as cracks and pores [6].

⑤ External auxiliary energy field (force)
Ultrasonic vibration technology acts on the solidification process of laser cladding metal, which can significantly refine the cladding coating structure and effectively improve the mechanical properties such as hardness of the cladding layer. When the ultrasonic vibration action angle is 5°, there are no pores and cracks inside the cladding layer, and the wear resistance effect is most significant. The electromagnetic stirring technology is applied to the laser cladding molten pool intervention, which can improve the solidification process of liquid metal, promote the grain refinement of metal materials, enhance the hardness of the cladding layer, and effectively inhibit the formation of cracks in the cladding layer. Studies have found that the rotating magnetic field can make the temperature gradient distribution of the molten pool tend to be uniform, effectively inhibiting the generation of cracks in the cladding layer.
The composite process technology of auxiliary application of ultrasonic vibration and electromagnetic stirring and other external energy fields can help laser cladding break through the bottleneck of single process optimization and regulation, and provide a new way to improve the quality of the cladding layer. However, the different effects brought about by the applied multiple external energy fields may have a “coupled competition” relationship, and the control effects of different energy field parameters and application methods on the defects of the cladding layer are also different. Therefore, there is still a large uncertainty in the use of multiple external energy fields to control the quality of the cladding layer.

⑥Other crack suppression methods
Laser remelting technology can reheat the cladding layer and keep it within the melting temperature range, so that the cladding material undergoes secondary crystallization and secondary bonding between crystals, reducing residual stress, thereby reducing the risk of cracking of the cladding layer.
Ultra-high-speed laser cladding technology has a low dilution rate, and the heat-affected zone and temperature gradient of the substrate are small, which is conducive to reducing the stress generation in the cladding layer, thereby reducing the risk of cracking of the cladding layer.
Through a systematic analysis of the causes of cracking of the laser cladding layer and a comprehensive attempt at a variety of suppression methods, it was finally determined to adopt the method of preheating the substrate and slow cooling. After application, the generation of cracks during the laser cladding process was successfully suppressed.

Application of laser cladding remanufacturing technology in cement equipment repair:

① Shaft surface repair
The failure forms of shaft parts mainly include shaft deformation, shaft fracture, shaft surface wear, etc. Among them, shaft deformation and shaft fracture cannot be repaired, and shaft surface wear failure can be repaired by using advanced technology. In cement production equipment, the bearings and seals of roller press rollers, vertical mill roller main shafts, fan main shafts, cylinder piston rods and other shaft parts are prone to wear.
In recent years, our company has accumulated rich experience in the surface wear repair of roller press rollers, vertical mill roller main shafts and other shaft parts of cement production equipment. According to statistics, the degree of surface wear of roller press rollers, vertical mill roller main shafts and other equipment used by different cement companies is also different. The wear parts of these equipment are mainly concentrated in the bearing positions, transmission connection assembly positions, sealing bushing assembly positions, etc., and the wear thickness is about 1~3mm. For this type of wear, our company’s 6kW laser cladding machine can be used for precise repair. During the repair process, the selection of cladding powder materials must match the material of the workpiece parent material. First, semi-finish the surface of the worn part to remove the fatigue layer of the worn part; then reasonably adjust the operating parameters of the cladding machine and reasonably allocate the cladding powder material; the thickness of the equipment surface wear is different, and the thickness of the cladding layer used is also different; during winter construction, it is necessary to take relevant insulation measures around the workpiece according to the cladding environment temperature to ensure the bonding quality of the cladding layer and the workpiece. The repair and remanufacturing process of the worn part of the roller bearing of a certain model of roller press is shown in Figure 2.
During the entire repair process, the surface temperature of the workpiece cladding area is <70℃, the residual stress of the workpiece production is small, and it will not affect the strength and performance of the workpiece. Compared with the traditional cladding repair process, the repair cost of laser cladding technology can save 1/3, and the repair period can be shortened by about 50%.

② Repair of inner holes of seat and housing
The inner hole parts in cement production equipment, such as the inner hole of roller press bearing room, the inner hole of vertical mill roller hub, the inner hole of reducer housing and planetary gear, the inner hole of oil cylinder barrel, etc., are also prone to wear in the bearing position and sealing position. These wears are mainly caused by factors such as bearing impact overload, loose fasteners, poor lubrication or unreasonable fit tolerance design. The inner hole repair process is basically the same as the shaft surface repair process, and its repair and remanufacturing process is shown in Table 1.

③ Repair of the outer surface of thin-walled bushings
Common thin-walled bushings, such as vertical mill roller sealing bushings, roller press roller shaft sealing bushings, crusher bearing sealing rings and other sealing surfaces, will not come into contact with dust under normal circumstances. However, when the external dust ring, sealing air or other protective facilities fail, dust will intrude, and small grooves will gradually form on the outer surface of the bushing. If not handled in time, the wear will be aggravated, and the grooves will be widened and deepened. At the same time, a large amount of dust enters the bearing chamber, which will cause the bearing to heat up or even damage, thereby causing equipment failure and shutdown. According to statistics, the wear depth of the outer surface of the thin-walled bushing is about 2mm, the wear width is basically the same as the width of the seal ring, and the proportion of the entire bushing width is small. If the bushing is directly replaced, it will cause material waste, and the production cycle of the new bushing is long and the maintenance cost is high. If the traditional surfacing method is used for repair, it is very easy to deform, affecting installation and use. Therefore, the laser cladding repair process is more suitable. This process does not require the removal of the bushing, the thermal deformation and stress of the cladding layer are small, and it does not affect the matching accuracy with the roller shaft. At the same time, by optimizing and adjusting the ratio of the cladding powder material, the hardness of the cladding layer surface can be increased, thereby further improving the surface wear resistance and service life of the bushing. After the thin-walled bushing is repaired by cladding, its surface hardness can be increased from about HRC40 to more than HRC55, and the service life of the bushing can be extended by more than double, which can save about 50% of the cost compared with replacing a new bushing. The outer surface wear repair of the roller shaft and bushing of the roller press is shown in Figure 3.

Conclusion

This paper deeply explores the application of laser cladding remanufacturing technology in the repair process of cement production equipment parts, and conducts a detailed analysis of the causes, influencing factors and suppression methods of cracking of the cladding layer. By adopting the substrate preheating and cladding layer slow cooling technology, the generation of cracks in the laser cladding layer was successfully suppressed.

Practice shows that the surface of shaft and inner hole workpieces repaired by laser cladding remanufacturing technology has no quality problems such as cracking and peeling, and the service life is greatly improved compared with the traditional repair method. At the same time, the frequency of equipment disassembly and seal replacement is reduced, the maintenance cost and labor intensity are greatly reduced, the amount of alloy steel and the cost of heat treatment process are saved, and the energy-saving and environmental protection benefits are significant. Taking the maintenance of cement production line roller press or vertical mill roller main equipment as an example, the annual maintenance cost can be saved by 100,000 to 300,000 yuan, with significant economic benefits.