This article talks about the wear and processing technology of rolling mill bearings and its applications.
Keywords: rolling mill bearings; loss; processing technology; application
Common wear parts of rolling mill bearings and causes of wear
An analysis and introduction of common losses of rolling mill bearing parts and causes of rolling mill bearing failure can be roughly divided into the following aspects:
Fatigue spalling on the surface of rolling elements and ring raceways is referred to as fatigue spalling on the working surface of rolling mill bearings
Under the repeated action of contact stress on the surface of the rolling element and the ring raceway, the phenomenon that the surface metal peels off in flakes is called fatigue spalling, also known as pitting corrosion. This kind of peeling starts very small, then pits appear and gradually expand, and finally it peels off in a large area. This phenomenon will cause the rolling elements and ring raceways of the rolling mill bearing to gradually wear out, thereby shortening the life of the rolling mill bearing and causing the rolling mill bearing to fail.
The ring of the rolling mill bearing is broken
The cracking of rolling mill bearing rings can be divided into four types: fatigue cracking, overload cracking, defective cracking and assembly cracking.
The rolling elements of rolling mill bearings are broken.
Rolling element fracture can also be divided into four types: fatigue fracture, overload fracture, defect fracture and assembly fracture. The cause of cracking of rolling mill bearing rings and rolling elements is generally caused by prolonged use.
Cage chipping and wear
The reasons for cage fragmentation are mostly due to improper use of raw materials, unreasonable design, irregular processing and other reasons.
Wear and tear
Both the rolling and sliding surfaces of the bearing are likely to wear. The wear of rolling mill bearings generally includes abrasive wear, adhesive wear, creep wear, corrosion wear and strength wear. These wear may occur individually or in a mixed manner.
Indentation
(1) When the bearing is assembled, the inner and outer rings are tilted. After impact, the raceways and rollers are prone to indentations (scratches) in the axial direction.
(2) The bearing is subjected to excessive load or impact load under static conditions or low speed, causing plastic deformation of the rolling surface. For example, when steel is jammed in a rolling mill.
(3) External foreign matter is immersed in, causing indentation. Oxide scale, sand, etc.
(4) The hardness of bearing parts is low.
Bearing overheating or burning
Rolling mill bearings should not feel hot during normal operation. If the operating temperature of the bearing is close to or exceeds 100 degrees, it means the bearing is overheated. Excessive temperature will cause the bearing to change color (blue, purple, black). Burned or burned. There are many reasons for overheating or sticking of rolling mill bearings, such as the bearing itself, lubrication and sealing, rolling mill speed, bearing load, bearing installation, rolling mill adjustment, rolling mill transmission, etc.
Applications of rolling mill bearings
1. Hot rolling mill
Support rollers: four-row cylindrical roller bearings and double-row tapered roller bearings
Work roll: four-row tapered roller bearings or four-row cylindrical roller bearings plus four-point contact balls and other axial positioning bearings
2. Casting and rolling mill
Four-row tapered roller bearings are mainly used, but double-row tapered roller bearings are also used. There is also a trend of changing tapered rollers to cylindrical roller bearings and positioning bearings.
3. Rough rolling, intermediate rolling, finishing cold rolling, they are the same type of rolling mill, but the rolling force and rolling passes are different. The bearings used are four-row cylindrical roller bearings plus axial positioning bearings.
Support rollers: four-row cylindrical roller bearings and double-row tapered roller bearings
Work roll: four-row tapered roller bearings or four-row cylindrical roller bearings plus positioning bearings
4. Aluminum foil rolling mill
Support roller: four rows of tapered bearings plus axial positioning bearings
Work roll: four-row tapered roller bearings or four-row cylindrical roller bearings plus axial positioning bearings
5. Tension bending straightening machine back support roller bearing
6. Shearing machine ordinary bearings
The above mainly describes some main bearing products. There are also some reducer tensioners and coiler raceway support bearings in the equipment, which are standard series.
Rolling mill bearing heat treatment processing technology
Rolling mill bearings are bearings used on the roll necks and drums of metallurgy, mining and other rolling mill stands. The following is a detailed explanation of 11 heat treatment processes for rolling mill bearing processing.
Reduce heating temperature
Generally, the quenching heating temperature of hypoeutectoid carbon steel is 30~50℃ above Ac3, and the quenching heating temperature of eutectoid and hypereutectoid carbon steel is 30~50℃ above Ac1. However, research in recent years has confirmed that heating and quenching hypoeutectoid steel in the α + γ two-phase region slightly lower than Ac3 (i.e., sub-temperature quenching) can improve the strength and toughness of the steel, reduce the brittle transition temperature, and eliminate temper brittleness. The heating temperature for quenching can be reduced by 40°C.
Using low-temperature rapid short-time heating and quenching of high-carbon steel can reduce the carbon content of austenite and help obtain lath martensite with good strength and toughness. It not only improves its toughness, but also shortens the heating time.
For some transmission gears, carbonitriding is used instead of carburizing. The wear resistance is increased by 40% to 60% and the fatigue strength is increased by 50% to 80%. The co-carburizing time is equivalent, but the co-carburizing temperature (850°C) is higher than that of carburizing. Temperature (920℃) is 70℃ lower, and it can also reduce heat treatment deformation.
Shorten the heating time
Production practice shows that the traditional heating time determined based on the effective thickness of the workpiece is conservative, so the heating coefficient α in the heating holding time formula τ = α·K·D needs to be corrected. According to traditional treatment process parameters, when heated to 800-900°C in an air furnace, the α value is recommended to be 1.0-1.8 min/mm, which is obviously conservative. If the α value can be reduced, the heating time can be greatly shortened. The heating time should be determined through experiments based on the size of the steel workpiece, the amount of furnace charging, etc. Once the optimized process parameters are determined, they must be implemented carefully to achieve significant economic benefits.
Cancel tempering or reduce the number of tempering
Cancel the tempering of carburized steel. For example, the fatigue limit of double-sided carburized piston pins for 20Cr steel loaders can be increased by 16% compared with tempered ones. Cancel the tempering of low-carbon martensitic steel and replace the bulldozer pin with The set is simplified to 20. Use the quenched steel (low carbon martensite), the hardness is stable at around 45HRC, the product strength and wear resistance are significantly improved, and the quality is stable; high-speed steel reduces the number of temperings, such as W18Cr4V steel machine saw blades that use one tempering Fire (560℃×1h) replaces the traditional 560℃×1h three times of tempering, and the service life is increased by 40%.
Use low and medium temperature tempering instead of high temperature tempering
Medium carbon or medium carbon alloy structural steel uses medium and low temperature tempering instead of high temperature tempering to obtain higher multi-impact resistance. W6Mo5Cr4V2 steel Φ8mm drill bit is subjected to 350℃×1h + 560℃×1h secondary tempering after quenching. Compared with the drill bit with 560℃×1h triple tempering, the cutting life is increased by 40%.
Reasonably reduce the depth of seepage layer
The chemical heat treatment cycle is long and consumes a lot of power. If the depth of the penetration layer can be reduced to shorten the time, it is an important means of energy saving. The necessary hardened layer depth was determined by stress measurement, which showed that the current hardened layer was too deep and only 70% of the traditional hardened layer depth was sufficient. Research shows that carbonitriding can reduce the layer depth by 30% to 40% compared to carburizing. At the same time, if the depth of the permeable layer is controlled to the lower limit of the technical requirements in actual production, 20% of energy can be saved, and the time and deformation can also be reduced.
Use high temperature and vacuum chemical heat treatment
High-temperature chemical heat treatment is to increase the temperature of chemical heat treatment under the condition that the operating temperature of the equipment allows and the austenite grains of the steel to be infiltrated do not grow, thereby greatly accelerating the carburization speed. Increasing the carburizing temperature from 930℃ to 1000℃ can increase the carburizing speed by more than 2 times. However, because there are still many problems, future development is limited.
Vacuum chemical heat treatment is carried out in a negative pressure gas phase medium. Due to the purification of the workpiece surface under vacuum and the use of higher temperatures, the penetration rate is greatly increased. For example, vacuum carburizing can increase productivity by 1 to 2 times; when aluminum and chromium are infiltrated at 133.3× (10-1 to 10-2) Pa, the penetration rate can be increased by more than 10 times.
Ion chemical heat treatment
It is a chemical heat treatment process that uses glow discharge between the workpiece (cathode) and anode to simultaneously infiltrate the elements to be infiltrated in a gas-phase medium containing elements to be infiltrated at a pressure below one atmosphere. Such as ion nitriding, ion carburizing, ion sulfurizing, etc., which have the advantages of fast penetration speed, good quality, and energy saving.
Self-tempering using induction
Induction self-tempering is used instead of tempering in the furnace. Induction heating is used to transfer heat to the outside of the quenching layer. During quenching and cooling, all the remaining heat is not taken away and tempering is achieved in a short time. Therefore, it is highly energy-saving and has been used in many applications. Under certain circumstances (such as high carbon steel and high carbon high alloy steel), quenching cracking can be avoided. At the same time, once each process parameter is determined, mass production can be achieved, and the economic benefits are significant.
Use post-forging preheating and quenching
Preheating and quenching after forging can not only reduce heat treatment energy consumption and simplify the production process, but also improve product performance.
Using post-forging waste heat quenching + high-temperature tempering as pretreatment can eliminate the shortcomings of post-forging waste heat quenching as the final heat treatment of coarse grains and poor impact toughness. It takes shorter time and has higher productivity than spheroidizing annealing or general annealing. In addition, The temperature of high-temperature tempering is lower than that of annealing and annealing, so it can greatly reduce energy consumption, and the equipment is simple and easy to operate.
Compared with general normalizing, residual heat normalizing after forging can not only improve the strength of the steel, but also improve the plastic toughness, reduce the cold-brittle transition temperature and notch sensitivity. For example, 20CrMnTi steel can be heated at 730~630℃ at 20℃/h after forging. Rapid cooling has achieved good results.
Preheating and quenching after forging can not only reduce heat treatment energy consumption and simplify the production process, but also improve product performance.
Using post-forging waste heat quenching + high-temperature tempering as pretreatment can eliminate the shortcomings of post-forging waste heat quenching as the final heat treatment of coarse grains and poor impact toughness. It takes shorter time and has higher productivity than spheroidizing annealing or general annealing. In addition, The temperature of high-temperature tempering is lower than that of annealing and annealing, so it can greatly reduce energy consumption, and the equipment is simple and easy to operate.
Compared with general normalizing, residual heat normalizing after forging can not only improve the strength of the steel, but also improve the plastic toughness, reduce the cold-brittle transition temperature and notch sensitivity. For example, 20CrMnTi steel can be heated at 730~630℃ at 20℃/h after forging. Rapid cooling has achieved good results.
Use surface quenching instead of carburizing and quenching
A systematic study on the properties (such as static strength, fatigue strength, multiple impact resistance, residual internal stress) of medium and high carbon steel with a carbon content of 0.6% to 0.8% after high-frequency quenching shows that induction quenching can be used to partially replace carburizing. Quenching is entirely possible. We have achieved success by using 40Cr steel high-frequency quenching to manufacture gearbox gears, replacing the original 20CrMnTi steel carburizing and quenching gears.
Use local heating instead of overall heating
For some parts with local technical requirements (such as wear-resistant gear shaft diameter, roller diameter, etc.), local heating methods such as bath furnace heating, induction heating, pulse heating, and flame heating can be used instead of overall heating such as box furnaces. , can achieve appropriate coordination between the friction and engagement parts of each part, improve the service life of the parts, and because it is localized heating, it can significantly reduce quenching deformation and reduce energy consumption.
