What are the common forms of fatigue failure and anti-fatigue methods of bearings?
A large number of application practices and life experiments have shown that bearing failure is mostly contact surface fatigue. ISO 25243-2004 lists fatigue at the top of the six common failure modes of bearings, and the fracture that is listed in sixth place is also called fatigue fracture because of fatigue in the formation process. Typical fatigue failure is divided into sub-surface origin type and surface origin type.
I. Sub-surface origin type fatigue
The maximum contact stress of rolling contact occurs somewhere under the surface at a certain depth, and under the repeated action of alternating stress, a fatigue source (microcrack) is formed at that location. The crack source gradually expands to the surface under cyclic stress, forming an open flake crack, which is then torn into flake particles peeling off from the surface, producing pockmarks and pits. If there is some kind of weak point in the bearing steel, or defects (common such as non-metallic inclusions, air gaps, coarse carbide crystal interface), will accelerate the formation of fatigue sources and the expansion of fatigue cracks, greatly reducing fatigue life.
II. Surface origin type fatigue
Contact surface damage, these injuries may be the original, that is, the manufacturing process of the formation of scratches, bruises, may also be in use, such as hard particles in the lubricant, bearing parts of the relative movement of tiny abrasions; damage may be poor lubrication, such as lubricant poverty, lubricant failure; poor lubrication state intensify the relative sliding between the rolling body and the raceway, resulting in surface damage at the Micro crack at the root of micro-convex body; crack expansion leads to micro-convex body off, or form a flaky spalling area. This kind of spalling depth is shallow, sometimes easy to confuse with the dark gray etching.
III. Fatigue fracture
The origin of fatigue fracture is excessive tight fit assembly stress and cyclic alternating stress formation of fatigue yielding, assembly stress, alternating stress and yield limit of the balance between once lost, it will produce fracture along the direction of the collar axis, the formation of penetrating cracks.
In practice, the normal use of failure of the bearing, the damage is mostly as described above, that is, contact surface fatigue, and three types of fatigue failure and the most common type of sub-surface origin fatigue, ASO281 and ISO281/amd.2 recommended bearing life calculation method is based on the sub-surface origin fatigue.
Commonly used anti-fatigue methods are:
A. Heat treatment technology
Heat treatment is commonly used to improve the mechanical properties of the material process, in order to adapt to the different requirements of the use of different materials parts, the need to choose a different heat treatment process, pre-heat treatment organization, quenching heating temperature, heating rate, cooling mode (medium and speed), tempering temperature and time have a significant impact on the mechanical properties, to optimize many heat treatment parameters, combination, in order to find to adapt to the use of conditions To optimize and combine many heat treatment parameters to achieve the best performance to meet the conditions of use, thus extending the fatigue life of the parts. Build a virtual production platform for heat treatment, and promote the transformation of heat treatment technology to high-tech knowledge-intensive. The optimization of heat treatment process parameters and the development of digital heat treatment technology are important prerequisites for fatigue-resistant manufacturing.
B. Surface chemical heat treatment
Surface chemical heat treatment of the modified role mainly in the surface, according to the different requirements of use, the choice of infiltration of chemical elements, such as carburizing after quenching and tempering to improve the surface hardness, but the workpiece distortion is not easy to control: nitriding after the formation of metal nitride can obtain higher surface hardness and wear resistance, corrosion resistance and fatigue resistance, and the workpiece distortion is small, but not high efficiency; co-infiltration process to make hardness, wear resistance, corrosion resistance, fatigue resistance Performance is better, and quenching distortion less, but the hardening layer is thin, not suitable for heavy-duty workpiece. The development direction of surface chemical heat treatment is to expand the application of low-temperature chemical treatment, improve the quality of the seepage layer, accelerate the treatment process, the development of environmentally friendly processes, composite seepage process and simulation of digital processing technology.
C. Application of surface strengthening technology
Traditional surface hardening technology derived from the principle of cold hardening, such as shot blasting, sandblasting, shot blasting, etc., new surface hardening technologies such as laser surface hardening, laser shot blasting surface hardening, ultrasonic roller hardening, chemical methods of surface hardening, compounding various processes of surface hardening new technologies have been successfully applied in many fields, such as laser a shot blasting process (laser impact treatment), the use of high-energy pulse laser on the surface of the part to form Shock wave, so that the surface materials produce compression and plastic deformation, the formation of surface residual compressive stress, thereby enhancing the fatigue resistance (such as resistance to stress cracks, corrosion fatigue, etc.).
D. Surface modification techniques
The main surface modification techniques commonly used are ion implantation and surface coating.
Ion injection is a non-high-temperature process without the constraints of metallurgy and equilibrium phase diagram, and different injection elements and dosages can be selected according to different needs to obtain the desired surface properties. For example, chromium ions are injected to enhance the corrosion and fatigue resistance of the substrate material; boron ions are injected to enhance the wear resistance of the substrate.
Surface coating techniques include physical vapor deposition (PVD), chemical vapor deposition (CVD) radio frequency sputtering (RF) ion plating (PSC), chemical plating, etc.
In addition, the ion infiltration process uses high-voltage direct current under a certain vacuum to put the infiltrated elements in an ionic state, so that the resulting ion stream bombards the surface of the workpiece and forms a compound on the surface to reduce friction and improve wear resistance.
E. Microfabrication and finishing technology
As an advanced manufacturing technology, high-precision microfabrication, blending and finishing technologies also play an important role in improving the fatigue resistance of basic parts. The ultra-precision grinding processing and eddy current light finishing processing aim to reduce the surface roughness of the workpiece, and the physical and chemical characteristics of the processed surface, mechanical characteristics, and the contour shape at the contact are beneficially changed, which can correct the contact stress distribution, facilitate the formation of power lubricant film, and improve the fatigue life.
F. Coordinated hardness matching
The hardness matching relationship of different parts can also coordinate the stress and strain transfer state at the rolling contact, which has obvious effect on extending the fatigue life of the parts.
Source: China Bearing Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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