In the field of modern mechanical manufacturing, the forging process of bearing steel is a key step in producing high-precision, high-performance rolling bearings. Bearing steel, as the core material of rolling bearings, not only determines the shape and dimensions of the bearings but also directly affects their service life and reliability. This article will explore each stage of bearing steel forging, from material properties and forging processes to heat treatment, unveiling the secrets of this precision manufacturing process.
Bearing steel, fully known as rolling bearing steel, is a high-quality steel primarily used for manufacturing rolling bearing components such as balls, rollers, and bearing rings. These components bear extremely high concentrated alternating loads during operation. Due to the small contact area and the presence of sliding friction, bearing steel is required to have the following characteristics:
High hardness is one of the important characteristics of bearing steel. High hardness ensures that components maintain shape stability under high loads, reducing deformation and wear. During operation, balls and rollers must withstand enormous pressure, and high hardness effectively resists these pressures, extending the service life of the bearings.
High wear resistance is another key property of bearing steel. During the operation of rolling bearings, balls and rollers generate significant friction with the bearing rings. High wear resistance can reduce the wear caused by this friction, thereby extending the bearing's service life. For example, in high-speed, high-load mechanical applications such as automobile engines and industrial machine tools, wear-resistant bearing steel can significantly improve operational efficiency and reliability.
High fatigue strength of bearing steel is essential for its performance in high-load, high-speed environments. During operation, bearing components are subjected to repeated alternating stresses. High fatigue strength allows bearing steel to maintain performance under repeated stress, preventing premature component failure. For example, in the aerospace field, the high fatigue strength of bearing steel is a key factor in ensuring flight safety.
The metallurgical structure and chemical composition of bearing steel are crucial to its performance. The carbon content of bearing steel is usually between 0.95% and 1.1%, with added alloying elements such as chromium and manganese to improve its performance. The addition of these alloy elements improves the steel's hardenability, hardness, and wear resistance. At the same time, strict metallurgical control ensures material uniformity and reliability. For example, uneven distribution of carbides can cause cracks during quenching, reducing the bearing's service life. Therefore, modern bearing steel refining processes are continuously improved, using techniques such as electric furnace smelting, electroslag remelting, and vacuum smelting, strictly controlling sulfur, phosphorus, and non-metallic inclusions in content and distribution.
Pre-forging preparation is a key factor in ensuring successful bearing steel forging. Before forging, the steel must undergo strict inspection and treatment to remove surface defects such as cracks, slag inclusions, and burrs. If these surface defects are not removed, they may propagate during forging and ultimately affect bearing performance.
Removing surface defects is an important pre-forging step. Common surface defects include cracks, slag inclusions, burrs, scabs, and oxide scale. These defects not only affect the smooth progress of forging but may also reduce the performance of the final product. For example, cracks may expand during forging, forming larger defects and ultimately leading to part scrap. Therefore, mechanical processing, grinding, and other methods are used to thoroughly remove these surface defects before forging.
Preheating and heating before forging are also crucial. Steel needs to be preheated and then quickly heated to forging temperature to reduce thermal stress and prevent surface oxidation. The forging temperature is generally around 1150°C, but the following dangerous temperature zones must be avoided:
Blue brittleness zone (200°C–400°C): strength decreases, toughness worsens.
High-temperature brittleness zone (>1250°C): steel may become brittle due to grain growth.
Thermal brittleness zone (800°C–950°C): steel is prone to cracking.
Precise control of heating temperature ensures that steel maintains good plasticity and toughness during forging. For example, in forging large bearing rings, preheating and heating must be strictly controlled to ensure uniform deformation at high temperatures and avoid cracks.
Forging is the core step of bearing steel processing. Through hammering, extrusion, and rolling at high temperatures, steel is shaped into the required form and size. The forging process requires strict control of forging temperature, force, and speed to ensure uniform deformation at high temperature and the formation of an ideal internal structure.
Different forging equipment is suitable for different forging needs. Common forging equipment includes:
Air Hammer: Suitable for small-batch production of large rings, with free forging and die forging modes. The air hammer drives the hammer shaft with compressed air, providing large forging force suitable for large components. For example, in producing large bearing rings, the air hammer can quickly forge steel into the required shape.
Crank Press: Suitable for stamping sheet metal and die-forging rings, especially for assembly line production. The crank press drives the slide via a motor-driven crankshaft, achieving high-precision die forging. In small bearing ring production, the crank press improves efficiency and ensures product quality.
Ring Rolling Mill: Used for ring rolling, forming rings through the coordination of rollers. The rolling mill precisely controls ring wall thickness and diameter, ensuring dimensional accuracy. For example, in high-precision bearing ring production, ring rolling achieves high-precision shaping and improves product quality.
During forging, temperature and forging force must be strictly controlled. Initial forging temperature is usually 1150°C, and final forging temperature is 850°C. Forging operators must adjust parameters according to steel characteristics and forging requirements to ensure smooth progress. For example, when forging large bearing rings, forging force and speed must be adjusted according to steel thickness and shape to ensure uniform deformation and ideal internal structure.
After forging, steel requires heat treatment to further enhance its performance. Heat treatment includes pre-treatment (spheroidizing anneal) and final treatment (quenching + low-temperature tempering).
The purpose of spheroidizing anneal is to reduce steel hardness, facilitate subsequent processing, and prepare for quenching. The process is as follows:
Heat steel to 750–770°C.
Hold at this temperature to homogenize internal structure.
Slowly cool below 600°C, then air-cool.
Spheroidizing anneal reduces steel hardness and makes the internal structure more uniform, laying a good foundation for quenching. For example, in high-carbon bearing steel production, spheroidizing anneal improves machinability and processing efficiency.
Quenching involves heating steel to a specific temperature and then rapidly cooling it to obtain a high-hardness, wear-resistant martensitic structure. Quenching temperature and medium significantly affect steel performance. Common quenching parameters:
GCr6: Quench 800–820°C, medium water or oil; temper 150–170°C; hardness HRC62–64.
GCr9: Quench 800–830°C, medium water or oil; temper 150–170°C; hardness HRC62–64.
GCr15: Quench 820–846°C, oil; temper 150–160°C; hardness HRC62–64.
After quenching, low-temperature tempering reduces internal stress and brittleness while maintaining high hardness and wear resistance. The tempering temperature is usually 150–250°C, with post-tempering hardness of HRC58–64. For high-precision bearings, low-temperature tempering improves toughness and fatigue resistance, extending bearing life.
Forging equipment and heating technology are important supports for the forging process. Equipment selection and heating quality directly affect efficiency and quality.
Different forging equipment suits different requirements:
Air Hammer: Small-batch large rings, free forging or die forging; large forging force suitable for large components.
Crank Press: Sheet metal stamping and die-forging rings; high precision for assembly line production.
Ring Rolling Mill: Forms rings; precisely controls wall thickness and diameter, ensuring dimensional accuracy.
Heating quality affects internal quality, material utilization, production efficiency, energy consumption, working conditions, labor intensity, and environmental impact. Furnace structure and fuel type are key factors. Common domestic heating furnaces include coal, oil, gas, and electric furnaces. Electric furnaces provide uniform heating and precise control, suitable for high-precision bearing steel, while coal furnaces are cost-effective but less uniform.
Quality control spans the entire production process, from raw material inspection to final product testing, with every stage critical.
The quality of raw materials directly affects the final product. Chemical composition, metallography, and surface quality must meet standards. For example, a spectrometer can verify carbon and alloy element content within specified ranges.
During forging and heat treatment, process parameters must be strictly controlled. Parameters such as forging temperature, forging force, quenching temperature, and tempering temperature require precise control. Automated control systems enable real-time monitoring and adjustment, ensuring process stability and consistency.
Final products undergo strict inspections including dimensional accuracy, hardness, and metallography. These checks ensure bearing steel performance meets design requirements, guaranteeing bearing life and reliability. For example, hardness testers verify HRC values, and metallographic microscopes confirm uniform internal structure.
Bearing steel forging is the core process for manufacturing high-precision rolling bearings. By strictly controlling pre-forging preparation, forging, heat treatment, and quality control, bearing steel achieves excellent performance, meeting modern mechanical manufacturing requirements for high-precision, high-performance bearings. With continuous technological advancement, forging processes are increasingly optimized, providing strong support for the mechanical industry. In the future, with new materials and processes emerging, bearing steel forging will become more efficient, environmentally friendly, and intelligent, driving high-quality development in mechanical manufacturing.
