A large number of application practices and life tests have shown that bearing failure is mostly caused by contact surface fatigue . GB/T 24611—2020/ISO15243:2017 lists fatigue as the first of the six common failure modes of bearings . The fracture listed sixth is also called fatigue fracture due to fatigue during the formation process. Typical fatigue failures are divided into subsurface origin types and surface origin types.
1. Subsurface origin fatigue
The maximum contact stress in rolling contact occurs somewhere under the surface at a certain depth. Under the repeated action of alternating stress, fatigue sources (microcracks) are formed there. The crack source gradually expands to the surface under cyclic stress, forming open flaky cracks, which are then torn into flaky particles that peel off from the surface, causing pitting and pits. If there are some weak points or defects in the bearing steel (common ones such as non-metallic inclusions, air gaps, and grain interfaces of coarse carbides), the formation of fatigue sources and the expansion of fatigue cracks will be accelerated, greatly reducing the fatigue life.
Subsurface origin spalling on the rotating inner ring of deep groove ball bearings
2. Surface origin fatigue
There is damage on the contact surface. These damages may be original, that is, scratches and bump marks formed during the manufacturing process, or may be caused by use, such as hard particles in the lubricant and tiny scratches caused by the relative movement of bearing parts. ; There may be poor lubrication in the damaged area, such as lack of lubricant and failure of the lubricant; poor lubrication state aggravates the relative sliding between the rolling elements and the raceway, resulting in microscopic cracks at the roots of the asperities at the surface damaged area; crack expansion leads to The micro-protrusions fall off or form a flaky peeling area. This kind of peeling depth is shallow and sometimes easily confused with dark gray plaques.
Expanded spalling of subsurface origin on the stationary inner ring of a tapered roller bearing
3. Fatigue fracture
The origin of fatigue fracture is fatigue yielding caused by assembly stress and cyclic alternating stress caused by excessive tight fit. Once the balance between assembly stress, alternating stress and yield limit is lost, fracture will occur along the axis of the ferrule, forming a through-hole. shaped cracks.
In practice, the damage of bearings that fail under normal use is mostly as mentioned above, that is, contact surface fatigue. Among the three types of fatigue failure, subsurface origin fatigue is the most common. The bearing life calculation method recommended by ASO281 and ISO281/amd.2 is Based on fatigue of subsurface origin.
Commonly used anti-fatigue methods include:
A. Heat treatment technology
Heat treatment is a commonly used process to improve the mechanical properties of materials. In order to adapt to the different use requirements of parts made of different materials, different heat treatment processes need to be selected, including pre-heat treatment of the structure, quenching heating temperature, heating speed, cooling method (medium and speed), and tempering. Temperature and time have obvious effects on mechanical properties. Many heat treatment parameters must be optimized and combined to obtain the best performance suitable for use conditions, thereby extending the fatigue life of parts. Build a heat treatment virtual production platform to promote the transformation of heat treatment technology into high-tech and knowledge-intensive. The optimization of heat treatment process parameters and the development of digital heat treatment technology are important prerequisites for achieving fatigue-resistant manufacturing.
B. Surface chemical heat treatment
The modification effect of surface chemical heat treatment is mainly on the surface. The chemical elements to be infiltrated can be selected according to different use requirements, such as quenching and tempering after carburizing to improve surface hardness. However, the distortion of the workpiece is difficult to control: the metal nitride formed after nitriding can Obtain higher surface hardness and wear resistance, corrosion resistance and fatigue resistance, and the workpiece distortion is small, but the efficiency is not high; the co-infiltration process makes the hardness, wear resistance, corrosion resistance and fatigue resistance better, and the quenching distortion is Less, but the hardened layer is thin and not suitable for heavy-loaded workpieces. The development direction of surface chemical heat treatment is to expand the application of low-temperature chemical treatment, improve the quality of the infiltration layer, accelerate the treatment process, and develop environmentally friendly processes, composite infiltration processes, and analog and digital processing technologies.
C. Application of surface strengthening technology
Traditional surface strengthening technology originates from the principle of cold work hardening, such as shot blasting, sand blasting, shot peening, etc. New surface strengthening technologies such as laser surface hardening, laser shot peening surface hardening, ultrasonic roller hardening, chemical method surface hardening, composite New surface hardening technologies of various processes have been successfully applied in many fields, such as laser-shot peening process (laser shock treatment), which uses high-energy pulse laser to form shock waves on the surface of the part, causing the surface material to compress and plastically deform, forming a surface Residual compressive stress, thereby enhancing fatigue resistance (such as resistance to stress cracks, corrosion fatigue, etc.).
D. Surface modification technology
Commonly used surface modification technologies mainly include ion implantation and surface coating.
Ion implantation is a non-high-temperature process and is not limited by metallurgy and equilibrium phase diagrams. Different implanted elements and dosages can be selected according to different needs to obtain the expected surface properties. For example: chromium ions are injected to enhance the corrosion resistance and fatigue resistance of the base material; boron ions are injected to enhance the wear resistance of the base material.
Surface coating technologies include physical vapor deposition (PVD), chemical vapor deposition (CVD), radio frequency sputtering (RF), ion spraying (PSC), chemical plating, etc.
In addition, the ion infiltration process uses high-voltage direct current under a certain degree of vacuum to put the infiltrated elements in an ion state, so that the generated ion flow bombards the surface of the workpiece and forms compounds on the surface to reduce friction and improve wear resistance.
E. Micromachining and finishing technology
As an advanced manufacturing technology, high-precision micro-machining and blending, and finishing technology also play an important role in improving the fatigue resistance of basic parts. Ultra-precision grinding and eddy current finishing are used to reduce the surface roughness of the workpiece. After processing, the surface physical and chemical properties, mechanical properties, and the contour shape of the contact are all beneficially changed, which can correct the contact stress distribution and facilitate dynamic lubrication. The formation of oil film improves fatigue life.
F. Coordinated hardness matching
The hardness matching relationship of different parts can also coordinate the stress and strain transmission state at the rolling contact, which has a significant effect on extending the fatigue life of the parts.
