Vacuum Heat Treatment Process of Bearing Steel

The full name of bearing steel is rolling bearing steel, which has high compressive strength and fatigue limit, high hardness, high wear resistance and certain toughness, good hardenability, and strict control of sulfur and phosphorus. It is a high-quality steel. Most of the bearing steel used by our company is imported from Japan, and some also use bearing steel developed in China with the same composition and similar processing technology as Japanese steel. The steel grade is SUJ2.
The production method of SUJ2 bearing steel: The steel factory adopts a vacuum degassing smelting method, continuously casting into steel bars (or die casting), with a forging and forming ratio of 6 or above. The steel is hot rolled, spheroidized, annealed, and then cold drawn; Then, supply according to various technical conditions specified (such as non-metallic inclusions, decarburization layer depth, dimensional tolerance, shape, appearance, hardness, organization, and other indicators).
After the bearing steel rod is formed through rotary machining in one step, it enters the previous heat treatment process of our company.
Before entering the vacuum heat treatment process, let's first understand what annealing is? What is quenching? What is tempering? Why do we need to carry out various different processes?
1. Annealing: Annealing is a commonly used preparatory heat treatment process in production, which involves heating steel to an appropriate temperature, holding it for a certain period of time, and then slowly cooling to obtain a heat treatment process close to equilibrium structure. Its purpose is to eliminate or reduce the internal stress and structural heterogeneity of chemical composition in castings, forgings, and weldments; It can improve and adjust the mechanical and technological properties of steel, and prepare the organization for our rotary machining process (in simple terms: reduce hardness, facilitate machining, and prepare for quenching). The steel we use actually undergoes "spheroidizing annealing" treatment at the steel factory (heating to 750-770 degrees, holding for a certain period of time, and slowly cooling to air cooling below 600 degrees). There are many types of annealing processes for steel, including complete annealing, incomplete annealing, isothermal annealing, spheroidizing annealing, and diffusion annealing; There are also recrystallization annealing, stress relieving, and hydrogen relieving annealing; I won't introduce it here.
2. Quenching and tempering: Quenching and tempering of bearing steel are important and widely used processes in heat treatment processes. Quenching can significantly improve the strength and hardness of steel. If tempered at different temperatures, the quenching internal stress can be eliminated (or reduced), and a combination of strength, hardness, and toughness can be obtained to meet different requirements. Therefore, quenching and tempering are two inseparable heat treatment processes.
(1) Quenching is a heat treatment process in which steel is heated above the critical point, insulated, and cooled at a rate greater than the critical cooling rate (Vc) to obtain martensite or lower bainite structure.
(2) Tempering is a heat treatment process in which quenched steel is heated to a temperature below A1 point and held for a certain period of time, and then cooled to room temperature in an appropriate manner. It is the next heat treatment process followed by quenching, which determines the microstructure and properties of the steel in its service state, and is related to the service life of the workpiece, so it is a key process.
After understanding some basic knowledge of heat treatment process, let's take a look at the heat treatment process of our rolling ball bearings:
Vacuum quenching: Its main characteristics on the production line are no oxidation, no deformation, and no cracking. For our company's precision bearings, vacuum heat treatment process was adopted from the beginning. Both Japanese and domestic vacuum furnaces have two vacuum chambers, cold and hot (the vacuum degree of the hot chamber is controlled below 2.0Pa and the cold chamber is below 9.3Pa). The heating process of bearing steel is gradually heated according to the curve (temperature and time) specified by the process, and the online temperature rises to 840 ± 10 ℃; However, oil or gas cooling is carried out through cooling media, with the oil temperature controlled at 60-80 ℃ and the upper limit not exceeding 100-120 ℃.
Oil tempering: Based on the characteristics of the bearing ring, we have adopted low-temperature oil tempering, with a temperature controlled at 180 ± 10 ℃. Its main purpose is to reduce internal stress and brittleness after quenching, stabilize size, and avoid cracking or premature damage during use while maintaining high hardness and wear resistance. Hardness HRC: 60-64.
Similarly, for stainless steel bearing products, such a heat treatment process is also necessary. Of course, the quenching curve (temperature and time) varies, with higher temperatures (1040 ± 10 ℃) and relatively longer time. Moreover, deep cooling treatment is required within half an hour after graded quenching (as discussed earlier), which mainly reduces residual austenite in the structure, stabilizes the structure, and improves the stability of the ring accuracy. However, the content of austenite should also be controlled during deep cooling treatment, as a certain amount of residual austenite is beneficial for improving the bearing life. Therefore, in the future, all precision bearings produced by our company should undergo cryogenic treatment.