Introduction to bearing heat treatment methods in the foreign machinery industry

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The quality of heat treatment has a direct impact on the quality of subsequent processing, which in turn affects the performance and lifespan of the parts. At the same time, heat treatment is a major consumer of energy and a significant source of pollution in the machinery industry. In recent years, with the advancement of science and technology and their application in heat treatment, the development of heat treatment techniques has mainly manifested in the following areas: (1) Clean heat treatment – The wastewater, waste gases, waste salts, dust, noise, and electromagnetic radiation generated during heat treatment processes can all cause environmental pollution. Solving the environmental pollution problems associated with heat treatment and adopting clean heat treatment (also known as environmentally friendly heat treatment) is one of the directions for the development of advanced heat treatment technologies. To reduce the emissions of SO2, CO, CO2, dust, and coal slag, the use of coal as fuel has been virtually eliminated; the amount of heavy oil used is also decreasing, with light oil being preferred in most cases. Natural gas remains the most ideal fuel. The utilization of waste heat from combustion furnaces has reached a high level; the optimization of burner design and strict control of the air-fuel ratio ensure reasonable combustion while minimizing NOX and CO emissions ； Use gas carburizing, carbonitriding, and vacuum heat treatment techniques to replace salt bath treatment in order to reduce pollution of water sources by waste salts and CN*-containing toxic substances ； Water-soluble synthetic quenching oil is used to replace part of the conventional quenching oil, and biodegradable vegetable oils are used to replace part of the mineral oils in order to reduce oil pollution. (2) Precision heat treatment has two meanings: on one hand, it involves using physical and metallurgical knowledge sowie advanced computer simulation and testing techniques, along with consideration of the part’s usage requirements, material properties, and structural dimensions, to optimize the process parameters in order to achieve the desired performance levels or to maximize the potential of the material ； On the other hand, it is necessary to fully ensure the stability of the optimization process, achieving a very low (or zero) variation in product quality as well as zero thermal treatment distortion. (3) Scientific production of energy-saving heat treatment and energy management are the most promising factors for the efficient use of energy; establishing specialized heat treatment plants to ensure full-capacity operation and make optimal use of equipment capabilities is the choice of scientific management. In terms of the energy structure for heat treatment, primary energy sources should be given priority ； Make full use of waste heat and residual heat ； Replace processes with long cycles and high energy consumption with those that have low energy consumption and short cycles. (4) Low-oxidation heat treatment involves replacing heating in an oxidizing atmosphere with heating in a protective atmosphere, as well as using a controlled atmosphere to precisely regulate carbon and nitrogen levels. This approach improves the properties of the parts after heat treatment, reduces heat treatment defects such as decarburization and cracks, decreases the amount of material that needs to be refined after heat treatment, and thus enhances the utilization rate of the material and the efficiency of machining. Vacuum heating gas quenching, vacuum or low-pressure carburizing, nitriding, nitrocarburizing, and boriding can significantly improve quality, reduce distortion, and increase lifespan. The quality control of heat treatment for bearing components is the strictest in the entire machinery industry. Great progress has been made in bearing heat treatment over the past 20 years, mainly in the following areas: research on the fundamental theories of heat treatment ； Research on Heat Treatment Processes and Application Technologies ； Development of new heat treatment equipment and related technologies. 1 ； The spheroidizing annealing of high-carbon chromium bearing steel is intended to achieve a microstructure in which fine, small, uniform, and round carbide particles are distributed evenly throughout a ferritic matrix, thereby preparing the material for subsequent cold working and the final quenching and tempering processes. The traditional spheroidizing annealing process involves holding the material at a temperature slightly above Ac1 (e.g., 780–810°C for GCr15), followed by slow cooling in the furnace at a rate of 25°C/h until the temperature drops below 650°C, after which the material is taken out of the furnace and cooled in air. This process requires a long heat treatment time (over 20 hours), and the carbide particles remain uneven after annealing, which affects subsequent cold working as well as the final quenching and tempering structure and properties. Thereafter, based on the transformation characteristics of supercooled austenite, an isothermal spheroidizing annealing process was developed: after heating, the material is rapidly cooled to a temperature range below Ar1 (690–720°C) where isothermal treatment takes place; during this isothermal phase, the transformation of austenite into ferrite and carbides occurs, and once this transformation is complete, the material can be taken out of the furnace for air cooling. The advantage of this process is that it saves heat treatment time (the entire process takes about 12–18 hours) ； In the treated tissue, the carbides are fine and uniform. Another time-saving process is repeated spheroidizing annealing: first heat to 810°C and cool to 650°C, then heat to 790°C and cool to 650°C before removing from the furnace for air cooling. Although this process can save some time, its operation is relatively complex. 2. Martensitic quenching and tempering of high-carbon chromium bearing steel. The microstructure and properties resulting from conventional martensitic quenching and tempering. Over the past 20 years, the development of the martensitic quenching and tempering process for conventional high-carbon chromium bearing steel has focused on two aspects: one is to study the influence of quenching and tempering parameters on the microstructure and properties, such as the microstructural changes during quenching and tempering, the decomposition of residual austenite, and the toughness and fatigue properties after quenching and tempering ； On the other hand are the process properties of quenching and tempering, such as the influence of quenching conditions on dimensions and deformation, as well as dimensional stability. The microstructure after conventional martensitic quenching consists of martensite, residual austenite, and undissolved (residual) carbides. Among them, the microstructural form of martensite can be divided into two categories: under a metallographic microscope (with a magnification generally below 1000 times), martensite can take two typical forms – lath-like martensite and plate-like martensite. After quenching, it usually presents as a mixture of these two forms, or as an intermediate structure known as date-pit martensite (what is referred to in the bearing industry as cryptocrystalline martensite or crystalline martensite) ； Under high-power electron microscopy, its substructures can be divided into dislocation entanglements and twins. Its specific microstructural form mainly depends on the carbon content of the matrix. The higher the austenite temperature, the less stable the initial structure is; consequently, a higher carbon content in the austenite matrix leads to more residual austenite in the quenched structure, as well as more lamellar martensite and larger sizes for it. Additionally, the proportion of twins in the substructure increases, and quenching-induced microcracks are more likely to form. Generally, when the carbon content in the matrix is below 0.3%, martensite is mainly lath martensite with a dislocation substructure ； When the carbon content in the matrix is higher than 0.6%, martensite is lamellar martensite with a mixed substructure of dislocations and twins ； When the matrix carbon content is 0.75%, large plate-like martensites with distinct mid-ridge surfaces appear, and microscopic cracks are present at the points where these plate-like martensites collide as they grow. Meanwhile, as the austenitization temperature increases, the hardness after quenching rises while toughness decreases; however, if the austenitization temperature is too high, excessive residual austenite after quenching leads to a decrease in hardness. In the microstructure resulting from conventional martensitic quenching, the content of residual austenite is generally 6–15%. Residual austenite is a soft, metastable phase; under certain conditions (such as tempering, natural aging, or during the use of the part), it becomes unstable and decomposes into martensite or bainite. The consequence of decomposition is an increase in the hardness of the parts, a decrease in their toughness, and changes in their dimensions, which can affect the dimensional accuracy of the parts and even their proper functioning. For bearing parts that require high dimensional accuracy, it is generally desirable to have as little residual austenite as possible; this can be achieved by performing supplementary water cooling or cryogenic treatment after quenching, as well as using higher-temperature tempering. However, residual austenite can improve toughness and resistance to crack propagation. Under certain conditions, the residual austenite in the surface layer of the workpiece can also reduce contact stress concentration, thereby increasing the contact fatigue life of bearings. In such cases, certain measures must be taken in terms of processing techniques and material composition to retain a certain amount of residual austenite and enhance its stability, such as by adding austenite-stabilizing elements like Si and Mn ； Perform stabilization treatments, etc.