Roller bearings are the bearings used on the rolls and drums of metallurgical, mining and other types of rolling machines. Here are the detailed explanations of 11 heat treatment processes for roller bearings.

1、Reduce the heating temperature

The normal quenching heating temperature for hypereutectoid carbon steel is above Ac3 by 30 to 50℃, while for eutectoid and hypereutectoid carbon steels, it is above Ac1 by 30 to 50℃. However, recent research has shown that hypereutectoid steel can be quenched and heated in the α+γ two-phase region slightly below Ac3 (i.e., subcritical quenching) to improve the strength and toughness of the steel, reduce the brittle transition temperature, and eliminate the temper brittleness. The quenching heating temperature can be reduced by 40℃. 

For high-carbon steel, applying low-temperature rapid short-time heating for quenching can reduce the carbon content of austenite, which is beneficial for obtaining plate martensite with good strength and toughness. This not only enhances its toughness but also shortens the heating time. 

For certain transmission gears, replacing carbon nitriding with carburizing can increase the wear resistance by 40% to 60%, and the fatigue strength by 50% to 80%. The total treatment time is the same, but the carburizing temperature (850℃) is 70℃ lower than the carburizing temperature (920℃), and it also reduces the thermal treatment deformation.

2、Shorten the heating time

Production practice has shown that the traditional heating time determined based on the effective thickness of the workpiece is overly conservative. Therefore, the heating coefficient α in the heating and holding time formula τ = α·K·D needs to be revised. According to the traditional processing parameters, when heating in an air furnace to 800-900℃, the recommended value of α is 1.0-1.8 minutes per millimeter. This is obviously conservative. If the α value can be reduced, the heating time can be significantly shortened. The heating time should be determined through experiments based on the steel type, workpiece size, and loading quantity. Once the optimized process parameters are determined, they must be strictly followed to achieve significant economic benefits.

3、Cancel the tempering process or reduce the number of tempering sessions

Canceling the tempering process for carburized steel, such as the double-sided carburized piston pins used in loader machines, can increase the fatigue limit by 16% compared to those that undergo tempering; Canceling the tempering process for low-carbon martensitic steel will simplify the pin sleeve of the loader to 20 steel in quenched state (low-carbon martensite), with hardness remaining stable at around 45HRC. The product's strength and wear resistance are significantly improved, and the quality is stable; Reducing the number of tempering processes for high-speed steel, such as the saw blades used in machine tools made of W18Cr4V steel, replacing the traditional three tempering processes at 560℃× 1h with a single tempering process at 560℃× 1h, the service life is increased by 40%.

4、Replace high-temperature tempering with medium-low temperature tempering

For medium-carbon or medium-carbon alloy structural steels, medium and low-temperature tempering can be used instead of high-temperature tempering to achieve higher multi-pass resistance. The W6Mo5Cr4V2 steel drill bit with a diameter of Φ8mm, after quenching, undergoes a secondary tempering at 350℃ for 1 hour followed by 560℃ for 1 hour. The cutting life of this drill bit is 40% higher than that of the drill bit subjected to three tempering at 560℃ for 1 hour.

5、Reasonably reduce the depth of the infiltration layer

The chemical heat treatment process is lengthy and consumes a large amount of electricity. Reducing the depth of the diffusion layer to shorten the time is an important means for energy conservation. By using stress measurement to determine the necessary depth of the hardened layer, it is indicated that the current hardened layer is too deep. Only 70% of the traditional hardened layer depth is sufficient. Research shows that carburizing and nitriding together can reduce the layer depth by 30% to 40% compared with carburizing alone. At the same time, if the depth of the diffusion layer is controlled at the lower limit of the technical requirements in actual production, it can also save 20% of energy, shorten the time, and reduce deformation.

6、Carry out high-temperature and vacuum chemical heat treatment

High-temperature chemical heat treatment involves increasing the chemical heat treatment temperature under the conditions where the equipment's operating temperature is within the allowable range and the austenite grains of the steel being treated do not grow. This approach significantly accelerates the carbon diffusion process. By raising the carbon diffusion temperature from 930°C to 1000°C, the carbon diffusion rate can be increased by more than twice. However, there are still many issues, and the future development is limited. 

Vacuum chemical heat treatment is carried out in a gas-phase medium under negative pressure. Due to the purification of the workpiece surface under vacuum conditions and the use of higher temperatures, the diffusion rate is significantly increased. For example, vacuum carburizing can increase productivity by 1 to 2 times; when aluminum or chromium is diffused under 133.3×(10^-1 to 10^-2) Pa, the diffusion rate can be increased by more than 10 times.

7、Ion chemical heat treatment

　　It is a chemical heat treatment process in which the element to be infiltrated is introduced into a gaseous medium at a pressure lower than one atmosphere. This process utilizes the glow discharge between the workpiece (cathode) and the anode to simultaneously infiltrate the desired element. Examples include ion nitriding, ion carburizing, and ion sulfurizing, etc. This process has the advantages of fast infiltration speed, good quality, and energy saving.

8、Use induction for self-hardening

　　Instead of using furnace annealing, induction self-tempering is adopted. Since the heat generated by induction heating is transmitted to the layer outside the quenched area, during the quenching cooling process, the residual heat is not completely removed, thus achieving short-term tempering. Therefore, it has the advantages of high efficiency and energy saving. In many cases (such as for high-carbon steel and high-carbon high-alloy steel), it can avoid quenching cracking. At the same time, once the process parameters are determined, it can be mass-produced. The economic benefits are significant.

9、Utilizing post-forging preheating and quenching

Post-forging preheating and quenching not only reduces the energy consumption of heat treatment and simplifies the production process, but also improves the product performance.

By using post-forging residual heat quenching + high-temperature tempering as the pre-treatment, the disadvantages of coarse grains and poor impact toughness caused by post-forging residual heat quenching as the final heat treatment can be eliminated. Compared with spheroidizing annealing or general annealing, it takes less time and has higher productivity. Moreover, the temperature of high-temperature tempering is lower than that of annealing and normalizing, so it can significantly reduce energy consumption, and the equipment is simple and the operation is easy. Post-welding residual heat normalizing, compared with ordinary normalizing, not only can enhance the strength of the steel, but also can improve its plasticity and toughness, reduce the cold brittleness transition temperature and notch sensitivity. For example, for 20CrMnTi steel after forging, when cooled at a rate of 20℃/h from 730 to 630℃, good results were achieved.

10、Replace carburizing quenching with surface quenching

Systematic studies on the properties (such as static strength, fatigue strength, multiple impact resistance, residual internal stress) of medium-high carbon steels with carbon content ranging from 0.6% to 0.8% after high-frequency quenching have shown that it is entirely possible to partially replace carburizing quenching with induction quenching. We successfully manufactured gearbox gears using 40Cr steel through high-frequency quenching instead of the original 20CrMnTi steel gears that were quenched by carburizing.

11、Replace overall heating with local heating

For some parts with specific local technical requirements (such as the wear-resistant shaft diameter, roller diameter of the rolling mill, etc.), local heating methods such as bath furnace heating, induction heating, pulse heating, and flame heating can be adopted instead of overall heating methods like box furnaces. This can achieve appropriate cooperation between the friction and bonding parts of each component, extend the service life of the parts, and because it is local heating, it can significantly reduce quenching deformation and lower energy consumption.