Spare Parts

If the materials processed by large vertical mills are of low hardness and not very abrasive, such as limestone, gypsum, etc. (Mohs hardness 3-4), and are more sensitive to cost, cast steel roller sleeves may be a suitable choice; but if large vertical mills need to process medium-to-high hardness and highly abrasive materials, such as iron ore, quartz, etc., and have high requirements for wear resistance and operating efficiency, high-chromium cast iron roller sleeves are more suitable and can play a greater role in improving production efficiency and reducing overall costs.

From the perspective of mechanical properties, cast steel roller sleeves have obvious advantages. The tensile strength of cast steel is generally between 400MPa and 600MPa, and the yield strength is around 200MPa to 350MPa. It has good strength and toughness and can withstand the large pressure and impact generated by large vertical mills during operation. For example, in some large vertical mills of cement production enterprises, when using cast steel roller sleeves to process relatively soft materials such as limestone, they can run stably for a long time and are not prone to breakage, ensuring the stable operation of large vertical mills. Although high-chromium cast iron roller sleeves have high hardness, their toughness is relatively poor. The impact toughness of high-chromium cast iron is generally between 3J/cm² - 10J/cm², which is much lower than that of cast steel. Under the high-load and high-pressure operating conditions of large vertical mills, high-chromium cast iron roller sleeves may have the risk of cracking or even breaking due to brittleness. According to relevant statistics, in some large vertical mills that use high-chromium cast iron roller sleeves to process high-hardness ores, the probability of cracks is 20% - 30% higher than that of cast steel roller sleeves.

Wear resistance is an important consideration in selecting roller sleeves. The wear resistance of cast steel roller sleeves is relatively general. When processing materials with high hardness and strong abrasiveness, the wear rate is relatively fast. For example, when processing quartz with a Mohs hardness of 6-7, the service life of cast steel roller sleeves may only be 3000-5000 hours. However, high-chromium cast iron roller sleeves contain a large amount of chromium, forming high-hardness carbides, and their wear resistance is much better than that of ordinary cast steel roller sleeves. The hardness of high-chromium cast iron can usually reach HRC50-60, while the hardness of ordinary cast steel is generally around HRC20-30. When processing medium-to-high hardness and abrasive materials, high-chromium cast iron roller sleeves can effectively reduce wear and extend service life. Data show that when processing iron ore of the same hardness, the service life of high-chromium cast iron roller sleeves can be extended by 50%-100% compared with cast steel roller sleeves, reaching 6000-10000 hours, greatly reducing the replacement frequency and improving the overall operating efficiency of large vertical mills.

In terms of processing and manufacturing, cast steel roller sleeves have obvious advantages. The processing performance of cast steel roller sleeves is good. Through various casting processes and subsequent processing methods, the size and shape that meet the requirements of large vertical mills can be accurately manufactured. Generally speaking, the processing accuracy of cast steel roller sleeves can be controlled within ±0.05mm, and the surface roughness can reach Ra3.2-Ra6.3μm, which can meet the requirements of large vertical mills for roller sleeve accuracy and surface roughness. However, high chromium cast iron roller sleeves are difficult to process due to their high hardness and high brittleness. During the processing, the tool wear is serious, the processing efficiency is low, and the processing cost is high. For example, the processing time of high chromium cast iron roller sleeves is usually 30%-50% longer than that of cast steel roller sleeves, and the processing accuracy is relatively difficult to control. The general processing accuracy is about ±0.1mm, and the surface roughness is Ra6.3-Ra12.5μm.

From a cost-effectiveness perspective, the raw material cost of cast steel roller sleeves is relatively low, and the manufacturing cost is also not very high, requiring a smaller initial investment. Generally speaking, the price of cast steel roller sleeves with the same specifications is 20%-30% lower than high-chromium cast iron roller sleeves. However, due to their limited wear resistance, the replacement and maintenance costs may be higher during the long-term operation of large vertical mills.
For example, in the case of a large cement vertical mill with an annual processing capacity of 1 million tons, using cast steel roller sleeves may require replacement 3-4 times a year. Each replacement costs approximately 20,000 - 30,000 USD, including spare parts and labor costs, meaning the total annual replacement cost would be between 60,000 - 120,000 USD.
On the other hand, high-chromium cast iron roller sleeves, although their purchase cost is typically higher than that of cast steel sleeves, offer superior wear resistance, reducing the frequency of replacements and downtime. For the same cement vertical mill, using high-chromium cast iron roller sleeves may only require 1-2 replacements annually, with total replacement costs between 30,000 - 60,000 USD. In the long run, the overall cost may be more advantageous.

The service life of the vertical mill roller sleeve tire is affected by many factors, including material characteristics, roller sleeve material, equipment operating parameters, etc. The following is an introduction to the service life of the vertical mill roller sleeve from the aspects of wear resistance of different materials and roller sleeves of different materials.

The wear resistance of different materials has different effects on the service life of the roller sleeve. For soft materials, such as gypsum, limestone, etc., the Mohs hardness is generally around 3-4, and the abrasiveness is relatively weak. When processing such materials, if ordinary cast steel roller sleeves are used, the service life may be around 8000-10000 hours;

if cladding layer roller sleeves or ceramic roller sleeves are used, the service life can be extended to 12000-15000 hours or even longer, because the wear-resistant layer or ceramic material on its surface can better resist the friction of soft materials. For medium-hard materials, such as coal, shale, etc., the Mohs hardness is between 4-6, and there is a certain degree of abrasiveness.

When using high chromium cast iron roller sleeves to process such materials, the service life is usually 6000-8000 hours; while using ceramic composite roller sleeves with better wear resistance, the service life can reach 10000-12000 hours, thanks to the high hardness and low friction coefficient of ceramics, which can effectively reduce wear. For hard materials, such as quartz stone, iron ore, etc., the Mohs hardness is greater than 6, and the abrasiveness is very strong. For such materials, roller sleeves made of ordinary materials wear quickly and the service life may be only 3000-5000 hours. If tungsten carbide welded layer roller sleeves or high-performance ceramic roller sleeves are used, the service life can be increased to 8000-10000 hours through their ultra-high hardness and wear resistance.

The advantages of roller sleeves made of different materials and their impact on service life also have their own characteristics. The surfacing material of the surfacing layer roller sleeve can be customized according to the material characteristics, and can be repaired after wear. By forming a high-hardness, high-toughness wear-resistant layer on the roller sleeve surface, it can effectively resist the grinding and erosion of the material.

For various materials, the adaptability can be improved by adjusting the surfacing material. When processing medium and low hardness materials, the service life can be greatly extended. When processing high hardness materials, repair can also guarantee a certain length of use. Overall, it can effectively improve the comprehensive service life of the roller sleeve. The cast steel roller sleeve has high strength and rigidity, can withstand greater pressure and impact, has good processing performance, can be precisely processed to meet the needs of different equipment, has good surface quality, can reduce material adhesion and accumulation, and can guarantee a long service life when processing soft and medium hardness materials with its good comprehensive performance.

When processing materials with high hardness but not too abrasive, it can also maintain a certain use time. However, for highly abrasive hard materials, its service life is relatively short. High chromium cast iron roller sleeves contain a large amount of chromium, forming hard carbides with high wear resistance. They also have good toughness, strength, high temperature resistance and corrosion resistance. They are particularly suitable for processing materials with medium to high hardness and certain abrasiveness. Under such working conditions, their wear resistance effectively slows down the wear rate, and their service life can be increased by about 20%-50% compared with ordinary cast steel roller sleeves.

Ceramic roller sleeves have extremely high hardness and wear resistance far exceeding that of metal materials. They have good chemical stability and are not easy to react with materials. Their low friction coefficient can reduce energy consumption and material adhesion. They show excellent wear resistance when processing materials of various hardness. Especially when processing high hardness and high abrasive materials, their service life can be extended several times compared with ordinary metal roller sleeves, which can greatly reduce the replacement frequency of roller sleeves and improve production efficiency.

Different roller sleeve materials have different hardness, and hardness is one of the key factors affecting grinding efficiency. Generally speaking, the harder the roller sleeve, the more effectively it can crush and grind the material during the grinding process. For example, the hardness of the roller sleeve made of Tongli high chromium cast iron can usually reach HRC50-60. When processing medium-hardness materials such as quartz with a Mohs hardness of 6-7, the high-hardness high-chromium cast iron roller sleeve can quickly cut into the material, causing the material to be strongly squeezed and ground between the roller sleeve and the grinding disc. In contrast, the hardness of the roller sleeve made of Tongli ordinary cast steel is generally HRC20-30. When grinding the same material, its cutting depth and crushing effect are relatively poor, and the grinding efficiency is low. Tongli Design Institute has concluded based on customer feedback over the years that under the same grinding conditions, the vertical mill using high chromium cast iron roller sleeves has a grinding efficiency of quartz that is 30%-50% higher than that of the vertical mill using ordinary cast steel roller sleeves.

Wear resistance is also an important indicator to measure the impact of roller sleeve material on grinding efficiency. Roller sleeves with good wear resistance can maintain good surface shape and dimensional accuracy during long-term grinding, thereby ensuring stable grinding effects. Take tungsten carbide alloy roller sleeves as an example. They have extremely high wear resistance and perform well in processing high-hardness and high-abrasive ore materials. In large-scale vertical mill applications of some mining companies, when tungsten carbide alloy roller sleeves process iron ore with a Mohs hardness of 7-8, after 5000 hours of continuous operation, the wear of the roller sleeves is only 5-8mm, and they can continue to maintain efficient grinding capabilities. If ordinary alloy steel roller sleeves with poor wear resistance are used, under the same working conditions, after about 3000 hours of operation, the wear may reach 10-15mm. At this time, the gap between the roller sleeve and the grinding disc increases, the grinding force decreases, and the grinding efficiency is significantly reduced, usually 20%-40% lower than that of tungsten carbide alloy roller sleeves.

Toughness is also crucial to the stability and service life of the roller sleeve during the grinding process, which in turn affects the grinding efficiency. Roller sleeves with good toughness can withstand large impact forces without being easily broken or damaged. For example, the impact toughness of roller sleeves made of bainitic steel is generally 30-50J/cm². When processing some materials with certain brittleness and hardness, such as coal gangue, they can maintain good integrity even in high-speed rotation and high-pressure grinding environments. When the vertical mill encounters uneven material particles or sudden impact during operation, the bainitic steel roller sleeve can effectively absorb and buffer these impact forces to avoid problems such as cracks or breaks, thereby ensuring the continuity and stability of the grinding process. If the roller sleeve is not tough enough, such as some ceramic roller sleeves with high hardness but low toughness, it is easy to break when subjected to large impact forces. Once the roller sleeve is damaged, it needs to be stopped and replaced, which will seriously affect the grinding efficiency and cause production interruption. According to statistics, when processing materials such as coal gangue, the vertical mill using ceramic roller sleeves has 40%-60% more downtime due to roller sleeve damage than the vertical mill using bainitic steel roller sleeves, and the overall grinding efficiency is greatly reduced.

Some special materials of roller sleeves also have unique properties, which can further improve the grinding efficiency. For example, roller sleeves made of composite materials combine the advantages of multiple materials, and have high hardness and wear resistance, as well as certain toughness. One common composite roller sleeve is a metal matrix with ceramic particles added. This composite structure allows the ceramic particles to enhance the grinding effect on the material during the grinding process, improving the fineness and efficiency of the grinding. When ultra-fine grinding materials such as calcium carbonate, this composite roller sleeve can increase the fineness of the product by 10% - 20%, and the grinding efficiency is increased by 20% - 30% compared to roller sleeves made of a single material.

The service life of high-chromium cast iron roller sleeves varies greatly depending on the application scenario, material characteristics and equipment operating conditions.

1 Coal mill in thermal power plant: In the coal mill of a thermal power plant, if ordinary raw coal is processed, its hardness is moderate and the abrasiveness is relatively mild. Under normal circumstances, the service life of high-chromium cast iron roller sleeves can reach 8-12 months. However, in actual operation, the quality of raw coal varies. If a large amount of impurities with higher hardness such as coal gangue are mixed in the coal, these impurities will continue to scratch and impact the surface of the roller sleeve during the grinding process like sandpaper, greatly increasing the degree of wear. At this time, the roller sleeve may need to be replaced in about 6 months. Taking some small thermal power plants as an example, their units are small in scale and the coal mill processing capacity is limited. Every year, due to the wear and tear caused by poor coal quality, at least 6 roller sleeves need to be replaced. However, with the development of material technology, the performance of the grinding roller sleeve made of the improved high-chromium cast iron material FCW6 with a chromium content of not less than 20% has been greatly improved, and the service life can reach 10-12 years, which significantly reduces the replacement frequency and maintenance costs.

2 Cement vertical mill: In the vertical mill equipment for cement production, the service environment of the high-chromium cast iron roller sleeve is relatively complex. Under normal working conditions, that is, when the material particle size distribution is uniform and the grinding pressure and temperature are stable, the service life of the roller sleeve can usually reach 2-3 years. However, cement production raw materials are diverse. If raw materials such as limestone contain more hard minerals such as flint, or in the production process, the grinding pressure of the vertical mill is too high due to improper process adjustment, the wear of the roller sleeve will be accelerated. Under such harsh working conditions, the roller sleeve may need to be replaced every 1.5-2 years. Some large cement companies have multiple cement production lines with a daily output of thousands of tons, and each production line is equipped with multiple vertical mills. The annual investment in the replacement of high-chromium cast iron roller sleeves is considerable, and the demand for improving the service life of roller sleeves is extremely urgent.

3 Bituminous coal mill: For the mill that processes bituminous coal, the hardness of bituminous coal is relatively low and the grinding property is average. If the mill is operating under stable conditions, the service life of the high chromium cast iron roller sleeve can usually reach 8-10 months. However, if there is a certain amount of impurities in the bituminous coal, or the operating parameters of the mill are improperly controlled, such as excessive grinding pressure, too fast speed, etc., the wear of the roller sleeve will be accelerated, and the service life may be shortened to 6-8 months.

4 Lignite mill: Lignite has a high moisture content and low hardness, and the wear on the roller sleeve is relatively small. Under normal operating conditions, the service life of the high chromium cast iron roller sleeve in the lignite mill can reach 10-12 months. However, if there are some minerals with higher hardness in the lignite, or there is a problem with the drying system of the mill, resulting in uneven humidity of the material during the grinding process, the service life of the roller sleeve may also be reduced to 8-10 months.

5 Lean coal mill: Lean coal has a higher hardness and is more difficult to grind. High chromium cast iron roller sleeves face great wear challenges in lean coal mills, and generally have a service life of 6-8 months. If the lean coal has a high ash content and contains a lot of minerals such as quartz, the wear will be further aggravated, and the service life of the roller sleeve may be only 4-6 months.

6 Cement raw material mill: When the raw materials of cement raw materials are mainly limestone, clay and other materials with good grindability, the wear of high chromium cast iron roller sleeves is relatively slow. When the mill operates normally and the parameters such as material particle size and moisture are reasonably controlled, the service life of the roller sleeve can usually reach 1.5-2 years. Complex raw materials: If the cement raw material contains a lot of materials with high hardness such as iron ore and sandstone, or the raw material has a large particle size and unstable moisture content, it will increase the difficulty of grinding and aggravate the wear of the roller sleeve. In this case, the service life of the high chromium cast iron roller sleeve may be only 1-1.5 years.

7 Cement clinker mill Ordinary cement clinker: For clinker mills that produce ordinary Portland cement, under normal grinding process conditions, the service life of high chromium cast iron roller sleeves is generally 1-1.5 years. However, if the cooling effect of the clinker is not good, resulting in more active substances such as free calcium oxide in the clinker, it will react chemically with the roller sleeve during the grinding process, accelerating the corrosion and wear of the roller sleeve, and the service life may be shortened to 10-12 months. Special cement clinker: The production of special cement, such as high-aluminum cement, sulphoaluminate cement, etc., has a clinker composition and performance that is quite different from ordinary cement clinker, and may have higher hardness and stronger abrasiveness. The service life of high chromium cast iron roller sleeves in this type of cement clinker mill is relatively short, generally 8-10 months. If the production process is not properly controlled, such as too high grinding temperature, too fast material flow rate, etc., the service life of the roller sleeve will be further reduced, which may be only 6-8 months.

The wear-resistant material consumption of high chromium cast iron roller sleeves per ton of material in different mills varies depending on material characteristics, equipment operating conditions and other conditions. In coal mills, when a bituminous coal mill processes bituminous coal with a grindability index of 60-80, it normally consumes 100-150 grams of wear-resistant materials per ton of bituminous coal.

When impurities are mixed in and the grindability index drops to 40-60, the consumption reaches 150-200 grams; lignite mills normally consume 80-120 grams per ton of lignite, and when the moisture content is too high and the fluidity is poor, it rises to 120-150 grams; lean coal mills process lean coal with a grindability index of 40-50, generally consuming 180-250 grams per ton, and when it contains more hard minerals such as quartz, it exceeds 250 grams or even reaches 300 grams. In the cement raw material mill, when the raw materials are mainly easy-to-grind limestone, clay, etc., with an average particle size of 5-10 mm and uniform particle size distribution, the grinding consumes 120-180 grams per ton of raw material. When the moisture content exceeds 5%, the material agglomerates and the weight rises to 180-220 grams.

When the material contains more iron ore, sandstone and other materials with high hardness, the average particle size exceeds 10 mm and there are large pieces of material, the consumption per ton is 200-300 grams or even more than 350 grams. In the cement clinker mill, when grinding ordinary Portland cement clinker with uniform particle size and specific surface area of ​​300-350 square meters/kg, 150-200 grams are consumed per ton of clinker. In order to increase production, the grinding pressure or speed is increased, and the consumption rises to 200-250 grams. When grinding special cement clinkers such as high-alumina cement and sulphoaluminate cement, the consumption is generally 220-300 grams per ton. When the production process is unstable and the clinker contains unreacted raw materials or impurities, it reaches 300-350 grams.

1 When welding the vertical mill roller sleeve, the first thing to do is to prepare for welding, including checking and debugging welding machines, welding crosses, water-cooled welding heads, workpiece rotating devices and other welding equipment to ensure that the electrical connection, water cooling system, motion mechanism, etc. are stable and reliable and the parameters are accurately adjustable; select suitable welding wire according to the material, wear condition and use requirements of the vertical mill roller sleeve, and prepare protective gases such as carbon dioxide and argon with stable purity and supply; use wire brushes, grinding wheels, sandblasting and other methods to clean the surface of the roller sleeve. Oil, rust, dust and other impurities to expose the original color of the metal, after cleaning, perform flaw detection and repair cracks, pores and other defects in advance; use measuring tools to accurately measure the diameter, length, depth and width of the wear part of the roller sleeve and other dimensions and record the original data to control the welding amount and dimensional accuracy.

2 Then clamp and position, install the cleaned roller sleeve on the workbench of the surfacing equipment, fix it with a chuck fixture or a special fixture to ensure that it does not move or rotate during surfacing, adjust the position and angle of the roller sleeve to make the surfacing part in the best welding position, and ensure that the rotation axis of the roller sleeve is parallel or perpendicular to the movement axis of the welding equipment to ensure uniform surfacing layer.

3 Then preheat treatment, select the appropriate preheating temperature according to the material and size of the roller sleeve, generally at 200℃-300℃, and use flame heating, induction heating or heating furnace heating to evenly heat the roller sleeve so that the overall temperature reaches the requirement and is maintained for a certain period of time to ensure uniform internal temperature.
Then carry out the surfacing operation. Common surfacing processes include open arc surfacing and submerged arc surfacing. Open arc surfacing is simple to operate and fast, but the penetration is shallow. Submerged arc surfacing has a large penetration and high quality, but the equipment is complex and difficult to operate. According to the surfacing process and welding wire specifications, set appropriate welding current, voltage, welding speed, welding wire extension length and other parameters; proceed in the order of from outside to inside and surfacing in layers, and control the thickness of each layer to 2mm-5mm. After each layer is completed, clean up the slag and spatter, check and repair defects; monitor the roller sleeve temperature in real time to avoid too high or too low. Spray cooling or air cooling can be used to keep the temperature in the appropriate range.

4Finally, post-weld treatment is performed. After surfacing, the roller sleeve is wrapped with insulation material to cool it slowly, reduce welding stress and prevent cracks; after cooling to room temperature, the surfacing layer is ground to make its surface smooth and reach the designed size and precision, and pay attention to controlling the amount of grinding; quality inspections such as appearance inspection, size measurement, hardness inspection, and flaw detection are carried out to check whether there are defects such as cracks, pores, slag inclusions, and whether the size and hardness meet the requirements.

5There are certain differences between ceramic roller surfacing and ordinary wear-resistant surfacing processes. In terms of welding materials, ordinary wear-resistant surfacing generally uses wear-resistant welding wires, etc., and ceramic roller surfacing usually uses special welding materials containing ceramic particles to ensure that the surfacing layer has the high hardness, high temperature resistance, wear resistance and other properties of ceramics. In terms of process parameters, due to the high melting point of ceramic particles and the large difference in physical properties with the metal matrix, ceramic roller surfacing requires higher welding energy input, such as larger welding current and voltage, and the welding speed may be relatively slow to ensure that the ceramic particles and the metal matrix can be well fused.

In terms of preheating and post-heating requirements, in order to reduce the thermal stress between ceramics and metals and prevent defects such as cracks, the preheating temperature of ceramic roller surfacing may be higher, and more stringent post-heating and slow cooling treatment may be required after welding. Sometimes special heat treatment processes such as tempering are required to improve the performance of the surfacing layer. In terms of surfacing equipment, conventional welding equipment can be used for ordinary wear-resistant surfacing. Ceramic roller surfacing may require more advanced and precise welding equipment due to its high requirements for welding energy control, such as pulse welding power supplies with higher stability and control accuracy. In some cases, special powder feeding devices may be required to evenly transport ceramic powder.

1 Preparation before repair: Tongli personnel need to arrive at the site in advance, inspect and evaluate the wear of the vertical mill roller sleeve in detail, record the wear area, depth, shape and other information, and measure the key dimensions of the roller sleeve, such as diameter, length, roller surface roundness, etc., to determine the specific location and amount of surfacing; transport the surfacing equipment to the site and install and debug it to ensure that the equipment can operate normally; after the manufacturer cleans the mill, enter the mill and use tools such as wire wheels or grinding wheels to clean the oil, rust, impurities, etc. on the surface of the roller sleeve. The non-surfacing area should be well protected to prevent mis-welding. In addition, the roller sleeve is inspected for cracks and other defects. If there are defects that seriously affect the surfacing, it is necessary to communicate with the manufacturer in advance.

2 Hydraulic roll turning: The hydraulic roll turning device converts electrical energy into hydraulic energy by driving the hydraulic oil pump through the motor. Under the action of the oil pump, the hydraulic oil is transported to the hydraulic cylinder through the hydraulic pipeline. When the roller needs to be flipped, the control element (such as the reversing valve) is activated to change the flow direction of the hydraulic oil, so that the pressure oil enters the rodless chamber of the hydraulic cylinder, pushing the piston and the piston rod to extend, thereby driving the roller to flip around its support shaft and flip the roller out of the mill for easy inspection, maintenance or replacement of the roller parts. When the operation is completed and the roller needs to be reset, the reversing valve is activated again to allow the hydraulic oil to enter the rod chamber of the hydraulic cylinder, the piston rod retracts, and the roller returns to its original working position. During the whole process, the accumulator can play a role in maintaining pressure and buffering, making the flipping action of the roller smoother. Convenient maintenance: The roller can be flipped out of the mill without disassembling the whole body, which greatly reduces the maintenance workload and maintenance time, improves the maintenance efficiency of the equipment, and reduces the maintenance cost. Improve equipment utilization: Shortening the maintenance time means that the equipment can resume production faster, improves the operating efficiency and equipment utilization of the vertical mill, and increases the production benefits of the enterprise.
Ensure operational safety: During the roller flipping process, the control components of the hydraulic system can accurately control the flipping speed and force to avoid collision, falling and other safety accidents during the flipping process, ensuring the safety of operators and safe operation of equipment.

3 Overlay welding process: Tongli selects appropriate overlay welding wire according to the material, wear condition and use requirements of the vertical mill roller sleeve. The chromium content of the wear-resistant welding wire used in the transition layer can reach 22-28, and the overlay hardness can reach 57-62. The wear-resistant welding wire used in the hard surface layer has a higher chromium content and the maximum hardness can reach 63; according to the specific conditions of the welding material and the vertical mill roller sleeve, set the appropriate welding current, voltage, welding speed, weld overlap and other parameters. For example, the automatic overlay standard current can be set to 320-360A, voltage 28-32V, welding speed 900-1500mm/min, weld overlap 20-30%, and wire extension length 25-35mm, polarity is DC reverse polarity; automatic surfacing or semi-automatic surfacing is used for construction. For local deep pits and other severely worn parts, manual surfacing can be used for repair and leveling first, and then automatic surfacing can be performed. During the surfacing process, the construction technical plan and standard welding process must be strictly followed. The dimensions of the workpiece should be checked while surfacing to ensure the quality and progress of surfacing.

4 Post-repair processing: After surfacing, the appearance of the grinding roller sleeve is checked to see if there are any welding defects, such as pores, slag inclusions, cracks, etc. At the same time, the dimensions of the grinding roller sleeve are checked using a fixture model or a measuring tool to ensure that the dimensions after repair meet the design requirements; the surface of the grinding roller sleeve after surfacing is polished to remove sharp objects, burrs, etc., to make the surface smooth and flat, and at the same time, the spatter and slag on the surface of the grinding roller sleeve are cleaned; after the repair is completed, the equipment is tested and the operation of the grinding roller sleeve is checked, including whether the rotation is stable, whether there is abnormal vibration and noise, etc., and it can be delivered for use only after acceptance.

1Vertical mill is a large-scale grinding equipment that integrates multiple functions such as crushing, drying, grinding, and graded transportation. It is widely used in cement, electricity, metallurgy, chemical industry, non-metallic minerals and other industries. Taking the cement industry as an example, my country's cement production reached 1.86 billion tons in 2010. According to this production scale, thousands of vertical mill cement production lines are required to operate day and night. In the vertical mill equipment, the roller sleeve is an important component for realizing the rolling and grinding process, and it is also the main vulnerable part. Extending the service life of the roller sleeve of the vertical mill and reducing the consumption of product tons are of great significance to enterprises. However, under the actual complex working conditions, the surface of the roller sleeve will continue to suffer wear, which will not only directly affect the production efficiency and product quality, but in severe cases, it will even produce cracks, which will lead to shutdown and production.

2Facing the wear and crack problems of roller sleeves, there are currently two main solutions. The first is to replace spare parts, but imported spare parts are expensive, which will greatly increase the cost of enterprises; the second is repair. Although repair can save costs, it is difficult to repair. At present, my country mainly uses surfacing technology to repair roller sleeve wear, but the repair of large roller sleeve cracks is still in the exploratory stage. Since repairing cracks and performing wear-resistant surfacing can effectively increase the service life of roller sleeves and reduce enterprise costs, this repair technology has become the first choice for enterprises. This article will take the DFRM2650 vertical mill roller sleeve as an example to conduct an in-depth exploration of the repair of its cracks and surface wear.

3 Roller sleeve crack analysis and chemical composition detection After measurement, the specific structural parameters of the DFRM2650 vertical mill roller sleeve are as follows: the outer diameter of the large end is 1969.96mm, the outer diameter of the small end is 1710mm, the inner diameter of the small end is 1480mm, the height is 500mm, the maximum wall thickness reaches 240mm, and there are rounded corners on the inner walls of the large end and the small end, and a ring groove is provided in the middle, and its weight is greater than 3t. Through observation, it was found that the roller sleeve had cracks along the axial direction, and the crack width was between 1 and 3mm. Irregular shrinkage holes can be seen from the crack fracture, and iron beans are embedded in the fracture metal. The metal grains on the fracture surface show the characteristics of fine surface and coarse inside. In terms of chemical composition, it was found through testing that the chemical composition (mass fraction) of the roller sleeve is C3.8%, Cr23%, Mo3.5%, Nb4.0%, W2.8%, Si0.8%, Mn0.7%, Cu1.0%, P, S≤0.03%, and its surface hardness is 57HRC, which belongs to high-chromium cast iron material (chromium content exceeds 11%, Cr/C exceeds 3.5).

4 Analysis of the formation mechanism of roller sleeve wear and cracks: When grinding high-hardness materials such as cement raw materials and slag in the vertical mill, the roller sleeve and the grinding disc liner will be subjected to the extrusion force, and relative slip will occur between their contact surface and the ground material. In this case, the wear of the parts surface is mainly micro-cutting. Specifically, the large end of the roller sleeve mainly plays the role of fine grinding and grinding, and the wear is more obvious; while the small end mainly plays the role of coarse breaking and cloth, and the wear mode is mainly collision friction wear. In order to ensure the normal operation of the vertical mill, a certain amount of ventilation must be ensured inside the mill. Therefore, different parts inside the mill will be eroded by high-concentration dust-containing gas for a long time, resulting in jet wear. In addition, the vertical mill roller sleeve will also have physical wear forms such as adhesive wear, material fatigue erosion and chemical reaction corrosion. Under complex working conditions, the roller sleeve will gradually reduce the wall thickness due to continuous wear, and then produce stress concentration. Under the combined effect of large grinding force and alternating stress, the roller sleeve is easy to crack. Moreover, the greater the wear, the thinner the wall thickness, and the higher the risk of cracking.

At the same time, chemical wear will also have a certain degree of wear on the roller sleeve of the vertical mill. The root cause of the cracks in the roller sleeve is the failure of the casting process and the heat treatment process. Irregular shrinkage holes inside the roller sleeve, iron beans embedded in the metal, and slag remaining during the processing process can all cause cracks. The rapid heating rate of the vertical mill at startup and the rapid cooling rate when the mill is stopped will cause thermal stress on the roller sleeve. In addition, during the grinding process, the dynamic load caused by the vibration of materials and metal waste will also cause cracks. For roller sleeves with cracks, the cracks need to be repaired before surface surfacing. However, for large roller sleeves, welding cracks are very likely to occur during crack repair or after welding. There are two main reasons for this: one is the characteristics of high-chromium cast iron materials themselves, which have poor welding performance; the other is that the process measures are difficult to be effectively guaranteed. For example, the roller sleeve has a large diameter and the tensile force generated during welding is large. If the preheating temperature does not meet the requirements or the stress relief annealing after welding is incomplete, it is easy to cause cracks. If the surfacing material is not selected properly, cracks will also appear during the operation. When the selected material has a relatively ideal hardness, that is, it shows good wear resistance, but poor impact toughness, the roller sleeve may crack after working for a period of time.

5 Repair of roller sleeve cracks and surface wear
High chromium alloy cast iron is very easy to crack during welding. In order to solve this problem, the roller sleeve needs to be preheated as a whole. The preheating temperature is controlled within the range of 600-700℃. This temperature will not change the organizational structure of the roller sleeve, thereby affecting its mechanical properties. At the same time, during the slow cooling process after welding, the graphitization process can be fully carried out, which can effectively prevent the generation of white spots and hardened structures, thereby avoiding the appearance of cracks. The selection of groove form is a key link in the repair process. In order to better solve the welding tension problem and reduce the dilution of the alloy elements of the parent material, a trapezoidal groove without through holes is opened on the inner and outer surfaces near the crack of the roller sleeve. After practical testing, this groove form can achieve good repair results.

6 In the selection of welding materials, A302 welding rods can be used for base welding. The filling welding needs to be customized according to the chemical composition of the roller sleeve to ensure that it can be well integrated with the parent material. The welding equipment uses ZX4-500S welding machine, which adopts the DC positive connection method. During the welding process, parameters such as electrode diameter, welding current, arc voltage and welding speed need to be reasonably selected according to the actual situation, and both the base welding and the filling welding adopt multi-layer and multi-pass welding methods. In order to eliminate the residual stress of the welded joint and improve its mechanical properties, heat treatment is required after welding. The specific heat treatment process is to heat the welded joint to 670℃ and keep it for 2 hours, and then furnace cool it.

7 According to the material of the parent material and its specific working conditions, welding wires with different compositions need to be selected to repair the roller sleeve. First, the original roller sleeve material should be sampled and tested to determine the chemical composition of the required welding wire. The cladding layer is generally divided into a transition (base) layer and a hardening layer. The transition layer welding wire must not only ensure good fusion with the parent material, but also cannot produce brittle phases, and also prevent cracks from extending to the matrix; the hardening layer is mainly to ensure that the roller sleeve has good wear resistance.

8 In the selection of surfacing method, two methods can be used: one is to use a self-shielded flux-cored wire with a diameter of 1.2-3.2mm for open arc surfacing. Before surfacing, the roller sleeve needs to be preheated to about 200℃, and then the above method is used to surfacing in sections and symmetrically; the other is to reasonably design and select automatic submerged arc welding with a suitable flux layer according to the wear condition and actual working conditions. By reasonably designing and selecting wear-resistant surfacing technology, the best surfacing repair effect can be achieved.

Quality inspection and use
9 The appearance inspection of the repaired roller sleeve crack repair weld showed that there was no surface defect. Subsequently, a 100% coloring inspection was carried out, and no weld cracks were found, which showed that the repair achieved the expected purpose. In terms of hardness surface surfacing quality inspection, although the surface hardness of the repaired roller sleeve has been reduced, it has been verified by actual use that its service life exceeds 2800h, which meets the design requirements and has good wear performance. With the continuous improvement and perfection of the roller sleeve repair process, the repair effect of the roller sleeve will be more ideal in the future.

There are two methods for manufacturing the wear-resistant parts of the vertical mill roller: casting and surfacing. The roller sleeve of the slag vertical mill adopts a structure of a casting body plus a wear-resistant surfacing layer. Because the slag has poor wearability and the mill has large working vibration, the roller sleeve of this structure can be repeatedly surfacing to extend the service life.

1 Problems with surfacing of slag vertical mill roller sleeve materials
At present, the main problems of surfacing of slag vertical mill roller sleeve materials in China are the structure of the surfacing wear-resistant layer and the weldability of the surfacing base material. The weldability of the metal of the vertical mill roller sleeve refers to the characteristics of the metal adapting to welding processing and forming a complete welding joint or working surface with certain performance, which mainly depends on the chemical composition of the surfacing metal. Flux-cored welding wire is commonly used in the surfacing process, in which the content and ratio of elements such as Cr, Si, C, Ni, and Mn directly affect the strength, toughness, and hardness of the deposited metal, i.e., the surfacing layer. Therefore, when surfacing, it is necessary to select the appropriate welding wire according to the surfacing base material, process conditions and use environment to ensure the wear resistance of the surfacing deposited metal.

2 Reasons for the shedding of the wear-resistant layer
The wear-resistant surfacing technology of domestic professional manufacturers is mature, and the shedding of the wear-resistant layer is mainly due to structural problems. The wear-resistant layer of the roller sleeve of the slag vertical mill and the cement vertical mill has fallen off or been worn out of the groove. This is due to the different structure of the casting matrix and the different thickness of the wear-resistant layer. The vibration of the mill during use will cause the wear-resistant layer to fall off, or the wear-resistant layer will be quickly worn out of the groove. The size of the junction between the body and the wear-resistant layer is too low, and the wear-resistant layer is too thick and easy to fall off; the size of the junction is too high, and the wear-resistant layer is too thin and easy to be worn out of the groove, which will damage the body, affect the grinding effect and output of the mill, and is difficult to repair.

Structural design factors: The size of the junction between the body and the wear-resistant layer is unreasonable. If the size of this part is too low, the wear-resistant layer will be too thick. During use, due to the action of various forces, the wear-resistant layer will quickly fall off; if the size of the joint point is too high, the wear-resistant layer will be too thin. During use, the wear-resistant layer will quickly be worn out of grooves, thereby damaging the body. This damage may also indirectly cause the wear-resistant layer to fall off.

Differences in the characteristics of the casting matrix and the wear-resistant layer: There are differences in the structure of the matrix of different castings, and the thickness of the wear-resistant layer is also different. When the mill is running, the vibration is large. Due to the different characteristics of the matrix and the wear-resistant layer, under the influence of vibration, the wear-resistant layer is prone to fall off or be worn out of grooves.
Slag's own characteristics and working environment: Slag has poor wearability, which makes the mill vibrate more when working. This large vibration acts on the roller sleeve for a long time, causing impact and fatigue on the wear-resistant layer, thereby accelerating the shedding of the wear-resistant layer.

3 Optimization of roller sleeves
After continuous summarization of experience and exploration, our company has designed a roller sleeve structure size suitable for the use of slag vertical mills. The optimized grinding roller sleeves have been widely used in our company's slag vertical mill. Under the premise that the slag is easy to grind and the feed moisture, feed particle size and iron ore content meet the requirements, the average life of the roller sleeve is more than one year, and the repair welding cycle is about 2-3 months (when the mill is running stably). In theory, each roller sleeve body can be repaired more than 4 times without affecting the use effect.

The grinding roller of the vertical mill is a key consumable part of the cement vertical mill, and its use status is directly related to the production efficiency and quality of the vertical mill. The technology of surfacing wear-resistant carbide on the roller sleeve of the grinding roller is widely used in the cement vertical mill industry and has good effects. It is of great significance to improve the production efficiency of the vertical mill, extend the service life and reduce costs. The wear of the vertical mill roller of Sinoma Zibo Heavy Machinery Co., Ltd. is mainly particle wear and low-stress wear. Recently, with the help of open arc automatic surfacing, by setting the base layer, transition layer, cover layer, strictly controlling the interlayer temperature and other measures, the offline surfacing of the wear-resistant carbide of the vertical mill roller sleeve has been successfully completed.

Determination of process plan
(I) Surfacing substrate
The base material of the surfacing roller sleeve is ZG20SiMn low-carbon cast steel, which has good welding performance. Its chemical composition has specific technical requirements, and the actual chemical composition also has corresponding test data.
(II) Surfacing process formulation
Because the parent material and the required carbide have great differences in hardness, linear expansion coefficient and other characteristics, in order to ensure the quality of surfacing, three types of welding wires are used for surfacing in sequence. The deposited metal of 1# welding wire has good crack resistance and toughness. It is used for base layer cladding to ensure good bonding with 2# and 3# welding wires and base materials with higher hardness, reduce the dilution rate of base materials, and prevent the cladding layer from peeling off. The cladding alloy structure of 2# and 3# welding wires is mainly carbide, martensite, and residual austenite, which are used for transition layer and cover layer cladding respectively. 3# welding wire appropriately adds Cr, Mo and other elements on the basis of 2# welding wire to obtain better hardness and roughness.
(III) Welding parameter formulation
The diameter of the welding wire used for cladding is 2.8mm. According to the empirical formula and welding equipment, the welding current is designed to be 280-320A, the voltage is 28-32V, and the cladding speed is 1300-1400mm/min. In order to ensure the toughness and bonding strength of the cladding layer, thin weld bead multilayer welding is adopted, with a weld bead thickness of 1.5-2.0mm and a width of 8-10mm. The surfacing equipment is ARC-NMG7-1 coal grinding roller automatic surfacing machine. During the welding process, the heat input is well controlled, multi-layer swing welding is performed, and the interlayer temperature is strictly controlled. The base layer is 150-180℃, and air cooling is used; the transition layer and the cover layer are 75-85℃, and water mist is used for cooling when the temperature is too high. Excessive interlayer temperature will make the grains in the heat-affected zone coarse, reducing the weld strength and low-temperature impact toughness.

II. Implementation of cladding process
(I) Preparation before welding
Inspect the chemical composition of the incoming roller sleeve base material to ensure that it meets the requirements; perform preliminary machining on the base material to remove shallow defects on the casting surface, process the outer diameter of the roller sleeve according to the design, and ensure the number of cladding layers; perform coloring or ultrasonic flaw detection on the surface of the cladding base material, and use carbon arc air gouging to treat deep defects, pay attention to the gouging depth, clean the iron carbide after treatment and prevent slag inclusion, preheat the workpiece to 100-120℃, use J422 welding rod to repair defects, and inspect again to pass; according to the carbon content of ZG20SiMn, use acetylene gas to preheat the roller sleeve base material to 150-180℃, and treat the oil and dirt on the cladding surface; check whether the circuit, gas circuit and cooling system of the welding equipment are normal.
(II) Surfacing operation
The operator adjusts the current and voltage in time according to the thickness and width of the weld bead, and the weld bead thickness does not exceed 2mm, and the over-thick part is polished. After welding one layer, the next layer of weld bead is arranged in sequence between the previous layer, and pay attention to the slag and dust removal of the previous layer. If there are local defects in the surfacing, they should be repaired in time. Evenly distributed and dense cracks are allowed, but there should be no through cracks and dense pores. Control the interlayer temperature, generally without interruption. If interrupted, heat it in time to maintain it at 100-120℃. Due to the special shape of the roller sleeve, the number of surfacing layers is different. The surfacing thickness should be adjusted according to the shape to ensure the flatness and size of the plane.
(III) Post-weld treatment
After welding, visually inspect the surface of the surfacing layer, deal with the welding pits and local small defects; after surfacing, perform hardness test on the parts, and measure the hardness of each part at least 3 points, and take the average value; heat the welded workpiece to 250℃, keep it warm for 5-6h, take it out of the furnace at 80℃ and air cool it to room temperature, and perform post-weld heat treatment to ensure wear resistance and toughness and eliminate residual stress.
III. Conclusion
The key to using open arc automatic surfacing to surfacing cemented carbide on the vertical mill roller sleeve is to prevent the surfacing wear-resistant layer from peeling off and ensure wear resistance. The company improved the surfacing process in actual production and successfully completed the surfacing of grinding roller carbide through measures such as layering and temperature control, so that the surfacing roller sleeve meets the use requirements and the effect is good.

1 Maintenance basis and current equipment status
This maintenance is because the 2# grinding roller bearing is damaged. The grinding roller of the current vertical mill cannot be used normally and can only rely on the other two rollers to barely maintain work.

2 Personnel preparation
The project leader is 11 mechanics from the Tongli installation team, and the team leader Richard is responsible for on-site operation command; the company is also equipped with 1 technician to handle power outages and on-site organization and coordination.

3 Tool preparation
The tools include 2 2T hand chain hoists, 2 5T hand chain hoists, 1 10T hand chain hoist, 1 50T hydraulic synchronous top, 1 300T hydraulic jack, 2 5P sledgehammers, 1 set of open-end dual-purpose wrenches, 1 percussion wrench of specifications 85 and 100, 1 set of socket wrenches, 2 2P hammers, 1 set of flip hydraulic cylinder and accessories, 1 roller skin tooling, 6 steel wire ropes, and 1 angle grinder.

4 Material and spare parts preparation
The material and spare parts include 1 set of grinding rollers and roller sleeves, 6 sealants, 10 gauze pieces, 10 sheets of 40-grit sandpaper, 2KG of molybdenum disulfide, 25KG of diesel, and 25KG of cotton cloth.

5 Maintenance plan
First prepare the tools and check their integrity, including the hydraulic cylinder, hydraulic jack, hand chain hoist, wire rope, socket wrench, percussion wrench, etc.; move the tools to the construction site in advance; in terms of personnel arrangement, 1 person will handle the power outage procedures and contact the process personnel to confirm that there is no lubricating oil inside, 2 people will prepare the lifting tools, and 2 people will prepare to remove the maintenance door bolts. The specific steps are: turn the grinding roller into place within 6 hours; remove the roller skin block within 4 hours; install the roller skin tooling within 4 hours; lift out the old roller core within 6 hours; install the new roller core within 4 hours; install the new roller skin within 4 hours; complete the grinding roller turning into place and roller skin block installation within 4 hours; complete the grinding door installation and oil pipe assembly within 4 hours; remove the hydraulic cylinder within 3 hours; check all parts and prepare for the test within 1 hour.

VI. Coordination Project
When the roller is turned over, a 25T crane is required for assistance. The old roller core is hoisted out, the new roller core is installed, and the new roller skin is installed with the assistance of a 130T crane. The roller is turned over in place and the hydraulic cylinder is removed with the assistance of a 25T crane.

VII. Maintenance Precautions
Ensure that the power outage procedures are in place; when hoisting objects, it is strictly forbidden to stand under the boom; during the installation process, careful operation should be performed to avoid strong pulling and knocking.