As a key component of rolling bearing, steel ball plays the role of carrying and transferring load and movement in bearing, and has an important impact on bearing life and vibration noise. The steel ball is subjected to periodic variable loads on the whole surface, and the contact area is small and the contact stress is large, so it is easy to produce fatigue damage. The processing technology of steel ball is very unique and specialized in the machinery industry, which has high requirements for raw materials, process technology, processing equipment, tooling, grinding fluid and so on.
1. Processing process
At present, there are two main processing methods:
Cold heading processing: used for mass production of steel balls, the wire is formed into a ball by the cold heading machine.
Grinding process: Precision machining of the steel ball through the ball grinding machine and the ball lapping machine to improve the dimensional accuracy and surface quality.
2. Raw Materials
GCr15 wire is widely used due to its cost-effective and easy heat treatment, accounting for more than 85% of the market. For occasions where corrosion resistance is required, stainless steel balls are usually used; For special purposes, such as aircraft, materials such as high-speed tool steel (tungsten series steel) or heat-resistant steel (such as M50) may be required. Special purpose steel balls usually require complex heat treatment processes, such as high temperature quenching, multiple tempering, cold treatment, etc., which makes the material cost and processing cost relatively high.
3. Process equipment
China has formed a set of professional steel ball processing technology equipment, including:
Cold heading machine: such as Z32, Z32G series steel ball cold heading machine and high-speed cold heading machine.
Light ball machine: such as 3M49, 3M79 series.
Ball mill: such as 3M46, 3M76 series.
Ball lapping machine: such as 3M47, 3M77 series.
Heat treatment furnace: used for heat treatment of steel balls.
Surface strengthening machine: used to improve the surface hardness and wear resistance of steel balls.
Cleaning machine: Brush based cleaning and high strength sponge based rotary extrusion cleaning.
Inspection equipment: such as surface quality appearance inspection instrument based on vision system, eddy current photoelectric appearance inspection instrument based on eddy current detection principle.
4. Quality control methods
Domestic steel ball quality control has formed a complete method system, mainly including raw material quality, size and size accuracy, heat treatment hardness and metallographic structure, surface defects and other aspects of control. With the continuous improvement of processing technology and equipment, most companies can meet the standard requirements in terms of dimensional accuracy and heat treatment hardness. However, the following issues still need attention:
Raw material quality control: to ensure the purity of raw materials.
Dimensional stability detection and control: Improve dimensional stability.
Surface strengthening and stress control: Increase surface hardness and reduce indentation formation.
Surface topography detection and control: Ensure surface quality.
Surface ripple monitoring and control: reduce ripple.
Shipment quality confirmation: ensure the quality of finished products.
Contact fatigue life test and control: improve durability.
5. Contact fatigue failure
Rolling fatigue can be divided into two types: internal origin type spalling and surface origin type spalling. Internal origin type spalling is usually caused by non-metallic inclusions and needs to be addressed by improving the purity of raw materials. Surface origin stripping is directly related to the environment of use, the lubrication state, and the surface state (such as surface roughness and scars), and its lifetime is usually shorter than that of internal origin stripping. In order to deal with surface origin spalling, carbonitriding treatment can improve the surface hardness of steel balls, reduce indentation formation and improve durability.
6. Summary
The improvement of the quality of the steel balls is the primary goal, requiring strict quality management from the billet stage, and the development of higher function automated appearance inspection meters to prevent unqualified products from leaving the factory. In addition, in order to improve durability, in addition to requiring steel mills to further improve the purity of materials, it is also necessary to develop steel ball heat treatment technology to adapt to different application scenarios, adding more added value to the traditional GCr15 steel ball.
In recent years, with the diversification of demand for bearing applications, especially applications under special conditions such as high speed and high temperature, the trend of using ceramic balls (such as Si3N4 balls) as substitutes is rising. Despite the high demand for ceramic balls, their high cost is still a problem that needs to be solved.
