Several heat treatment methods for bearing steel

Aug 08, 2024

Bearing steel, a vital material in machinery manufacturing, automobiles, aerospace, ships, and other fields, directly influences the service life and performance of the equipment. Heat treatment is a crucial method for altering the properties of steel, particularly for bearing steel. The following are several main heat treatment methods for bearing steel and their detailed analysis, each with a significant impact on the properties of the steel.

1. normalize

Normalizing is a heat treatment process in which bearing steel is heated to a temperature above the critical point AC3 (or ACM), kept warm for some time, and then cooled naturally. This process can eliminate bubbles, oxides, and residual stress in the steel and balance the composition and structure inside the steel. After normalizing, the toughness and workability of the steel are significantly improved, laying the foundation for subsequent processing and use. The key to normalizing is to control the heating temperature and holding time to avoid excessive grain growth that affects performance.

2. Annealing

Annealing is a kind of heat treatment process in which the hypereutectoid steel workpiece, a type of steel with a carbon content higher than the eutectoid composition, is heated to 20-40°C above AC3, kept warm for some time, and slowly cooled (such as furnace cooling, sand burial, or lime cooling) to below 500°C, and then naturally cooled to room temperature. The main purpose of annealing is to soften the steel, improve its processing properties and plasticity, and eliminate internal stress. For bearing steel, annealed steel is easier to machine and ready for subsequent quenching.

3. Quenching

Quenching is the most critical step in the heat treatment of bearing steel. This process heats the steel above the critical temperature to austenitize the steel completely and then rapidly cools it to low temperatures to form a martensite structure. Quenching can significantly improve the hardness, wear resistance, and fatigue resistance of steel and is an important means for bearing steel to obtain excellent performance. When quenching, the control of the cooling rate is crucial. Fast and slow will prevent the performance of the steel from decreasing or producing defects.

Heat treatment workshop for FV bearings

4. Tempering treatment

Tempering is a heat treatment process in which the steel is heated up to a certain temperature below the critical point AC1 after quenching, kept warm for some time, and then cooled. The primary goal of tempering is to remove internal stress and structural instability created during the quenching process while also adjusting the hardness and toughness of the steel to achieve optimum usability. The ultimate properties of the steel are determined by carefully selecting the tempering temperature and time to meet specific application requirements.

5. Solid solution treatment

Solid solution treatment is a heat treatment process in which alloy materials are heated to a high-temperature single-phase region to dissolve the excess phase in the solid solution fully and then rapidly cooled to obtain a supersaturated solid solution. For the alloying elements in bearing steel, solution treatment can strengthen the solid solution, improve toughness and corrosion resistance, eliminate stress and softening, and provide good conditions for subsequent processing and use.

6. Timeliness treatment

Aging treatment is a heat treatment process in which the alloy material is placed at room temperature or slightly above room temperature after solid solution treatment or cold plastic deformation to make its properties change over time. Aging treatment can promote the precipitation and hardening of the strengthening phase in the alloy, further improving the strength and hardness of the material. For bearing steel, aging treatment helps stabilize the structure and improve performance.

7. Carbonitriding

Carbonitriding (also known as cyanidation) is a heat treatment process that simultaneously infiltrates carbon and nitrogen into the surface of steel. Medium-temperature gas carbonitriding and low-temperature gas carbonitriding are two commonly used methods. Medium-temperature gas carbonitriding can improve the hardness, wear resistance, and fatigue strength of steel. In contrast, low-temperature gas carbonitriding mainly improves the wear resistance and anti-seize properties of steel. Bearing steel treated with carbonitriding has better surface properties and longer service life.

8. Conditioning and tempering treatment

Quenching and tempering is a heat treatment process involving rapid cooling and subsequent reheating and cooling of a metal. Quenching and tempering treatment can make bearing steel reach a balanced state in four aspects: hardness, strength, toughness and fatigue resistance. It is suitable for important structural parts such as connecting rods, bolts, gears, and shafts. The tempered sorbate structure obtained after quenching and tempering treatment has excellent mechanical properties and service life.

The picture above is a large bearing housing customized by FV for customers. The heat treatment method of the bearing steel is quenching and tempering, and the hardness is HB350-420.

9. Brazing

Brazing is a heat treatment process that bonds two workpieces together through brazing filler metal. In the bearing manufacturing process, brazing is often used to connect the inner and outer rings or other components of the bearing. Brazing has the advantages of high connection strength and good sealing, which can ensure the overall performance and reliability of the bearing.

To sum up, there are various heat treatment methods for bearing steel, and each method has its own unique role and applicable scenarios. Through reasonable selection and application of these heat treatment methods, the performance and service life of bearing steel can be significantly improved to meet the use requirements under various complex working conditions, such as high-speed rotation in machinery manufacturing, extreme temperatures in aerospace, heavy loads in shipbuilding, and high-stress environments in automobiles.