PRODUCT CATEGORIES

Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

Vacuum Brazing Furnace

Vacuum Heat Treatment Process of Rolling Bearings

1.Process requirements

Chromium bearing steel is the main steel for manufacturing rolling bearing parts, of which GCr15 steel is used the most. Rolling bearings work in a state of variable stress, and each element is subjected to variable loads. Therefore, the main failure forms of rolling bearings are fatigue pitting and fatigue wear, which requires rolling bearings to have high strength, high hardness and wear resistance. GCr15 steel has a hypereutectoid composition, and a small amount of chromium is added to improve hardenability and recycling resistance. It can obtain high hardness, high strength and good wear resistance through proper vacuum heat treatment, and the vacuum quenching deformation is small.

2.Rolling bearing process analysis

2.1.1 Working conditions

When the rolling bearing is working, the inner ring and the balls rotate and roll relative to each other, and are subjected to periodic loads. Relative sliding occurs between the rolling elements, the ferrule and the cage, resulting in mutual friction, which causes the bearing to wear, and the temperature of the friction surface will increase during operation. The atmosphere and lubricants also have a certain corrosive effect on the bearing.

2.1.2 Failure form

The main failure modes are fatigue failure and permanent deformation.

Fatigue damage: During the working process of the rolling bearing, the rolling element rotates continuously relative to the inner ring (or outer ring), so the contact surface of the rolling element and the raceway is stressed, and fatigue cracks are first generated at a certain depth below the surface of the rolling element and the raceway. Then it spreads to the contact surface, forming fatigue pitting corrosion, causing the bearing to not work properly. Generally, fatigue pitting is the main form of rolling bearing failure.

Permanent deformation: When the bearing speed is very low or intermittently oscillates, there is generally no fatigue damage, but under the action of a large load or impact load, permanent deformation pits will be generated at the contact between the bearing raceway and the rolling body, so that the The bearing produces severe vibration and noise during operation, so that the bearing cannot work normally.

2.1.3 Performance requirements

As a precise mechanical element, the working performance of rolling bearing directly affects the working performance of the main engine, and even the working performance of some bearings installed in the key parts of the main engine almost determines the working performance of the machine. For example, the P2-grade precision tapered roller bearings 30306 and 30307 used on the spindle of the precision optical coordinate boring machine have a radial runout of 0.001 to 0.0025mm and a raceway runout of 0.002 to 0.003mm. This largely determines the rotation accuracy of the machine tool spindle, that is, the performance index of the host. In addition to this high-precision bearing, the quality indicators of bearings with special performance requirements such as high temperature resistance, low temperature resistance, rust resistance, vibration resistance, high speed, high vacuum and corrosion resistance are also very strict.

Generally speaking, rolling bearings should have the basic properties of long life, low noise, small rotational torque and high reliability. To achieve these properties, it is necessary to first ensure the following indicators of the bearing parts in the machining process:

Rotational accuracy: The geometrical accuracy and positional accuracy of each part of the bearing are required to be no more than a few microns.

Dimensional accuracy: The dimensional accuracy of each part is required to be within a few microns.

Roughness: The Ra value of the installation surface roughness is not more than 0.63μm, the raceway requirements are higher, and the Ra value is less than 0.16μm.

Dimensional Stability: No significant size and shape change during long-term storage and work.

Anti-rust ability: The parts are not allowed to rust.

Vibration and noise: Bearing vibration and noise should be limited within a certain range, which requires that the various qualities of bearing parts should be as high as possible.

Residual magnetism: The residual magnetism of the bearing should be controlled below 0.4 to 1.0 mT.

3.Steel material

GCr15 bearing steel is a kind of high carbon chromium bearing steel with less alloy content, high carbon content, good performance and the most widely used. After quenching and tempering, it has high and uniform hardness, good wear resistance and high contact fatigue performance. The steel has medium cold working plasticity, general cutting performance, poor welding performance, high sensitivity to white spot formation, and temper brittleness. The chemical composition of GCr15 bearing steel is chromium and carbon (C: 0.95-1.05; Mn: 0.20-0.40; Si: 0.15-0.35; S: ≤ 0.020; P: ≤ 0.027; Cr: 1.30-1.65.).

4.Processing process

The thermal processing process of the vacuum heat treatment process design of rolling bearings has been improved many times to form the following process flow: blanking→forging→preparatory heat treatment (spheroidizing annealing, normalizing)→cutting process→quenching→low temperature tempering→grinding process→ assembled.

GCr15 steel belongs to high carbon steel, and its composition is as follows

4.1 Specific vacuum heat treatment

The vacuum heat treatment of GCr15 bearing steel includes initial forging and pre-heat treatment, in order to reduce the hardness of the billet to facilitate subsequent machining, and also to provide excellent original structure for subsequent cutting, quenching and tempering.

Analysis of vacuum heat treatment process of GCr15 bearing steel:

①Because of the high carbon content of GCr15 bearing steel, preliminary heat treatment should be carried out in the early stage of forging to reduce the hardness, and at the same time, the ribbon or network carbides will be converted into fine carbides for subsequent vacuum quenching and cutting. Provide good original tissue.

The purpose of spheroidizing annealing is to change the structure into fine-grained pearlite with uniform distribution, obtain the best machinability and provide a good original structure for vacuum quenching, and obtain the best mechanical properties after vacuum quenching and tempering. The annealing temperature of GCr15 bearing steel is generally 780 ~ 810 ℃, but after preliminary heat treatment, the annealing temperature should be

Decrease by 10-20℃.

③ The preliminary heat treatment process includes carbide refinement and grain refinement of bearing steel, that is, double refinement process. After the double treatment process, the forgings can be refined by 1.5 to 2.0 grades than the original grains, thereby improving the impact hardness, flexural strength and fatigue life of the steel. This process can obtain a uniform martensite structure after quenching and tempering, and improve the hardness uniformity.

4.2 Preparatory heat treatment process

Annealing is a heat treatment process in which the steel is heated to a temperature above or below the critical point Ac1, and then slowly cooled with the furnace to obtain a nearly equilibrium state. GCr15 bearing steel mainly adopts the spheroidizing annealing heat treatment process of spheroidizing the carbides in the steel to obtain granular pearlite.

The purpose of annealing is to:

1.Eliminate the composition segregation of the steel ingot and make the composition uniform.

2.Reduce hardness and improve plasticity to facilitate cutting and cold deformation processing.

3.Refine the grains, make the structure uniform, and prepare the structure for the subsequent heat treatment.

4.Eliminate the internal stress in the steel to prevent deformation and cracking.

4.3 Vacuum quenching + low temperature tempering heat treatment process

①Vacuum quenching

Vacuum quenching is a vacuum heat treatment process in which the steel is heated to a certain degree above the critical temperature point Ac3 or Ac1 in a vacuum environment, and then cooled at a rate greater than the critical cooling rate to obtain martensite (or lower bainite) after heat preservation. .

The purpose is to make the austenitized workpiece obtain as much martensite as possible, and then tempering at different temperatures to obtain various required properties, so that the parts can obtain high hardness and wear resistance, and high contact fatigue life. and reliability, high dimensional stability.

②Tempering at low temperature

Tempering is a heat treatment operation in which vacuum quenched steel is heated to a temperature below A1, after heat preservation, and then cooled to room temperature by a certain cooling method.

Its purpose is to:

1.Reduce brittleness and eliminate internal stress. After the workpiece is quenched, there is a lot of internal stress and brittleness. If it is not tempered in time, the parts will be deformed or cracked.

2.Obtain the required mechanical properties of the workpiece. After the workpiece is quenched, the hardness is high and the brittleness is high. In order to obtain the required properties of the workpiece, the tempering temperature can be used to adjust the hardness, reduce the brittleness, and obtain the required plasticity, strength and hardness.

3.Stable workpiece size. GCr15 steel has two metastable microstructures—martensite and retained austenite in the quenched microstructure. Both microstructures are unstable and tend to be spontaneously transformed or induced to be transformed into stable microstructures, thus causing workpieces. Changes in size and shape. By tempering, these tissue transformations can be induced. Reach a more stable state so that it does not deform during subsequent use.

After the above vacuum heat treatment, the toughness, hardness and wear resistance of GCr15 bearing steel have even been greatly improved.