Oil free bearing is a new type of lubrication bearing which has the characteristics of both metal bearing and oil-free lubrication bearing.
In the process of using oil-free bearing, it is inevitable that some conditions will appear. The early failure modes of oil-free bearing mainly include: cracking, plastic deformation, wear, corrosion and fatigue. Under normal conditions, the primary oil-free bearing is the primary internal influence factor of the fatigue: hardness, strength, resistance, wear resistance, corrosion resistance, internal stress (outside the service conditions).
Now, what are the common problems of oil-free bearing?
1. Martensite in quenched steel of oil-free bearing
Original arrangement of high carbon chromium steel: granular pearlite
Quenching + low temperature tempering: the carbon content in quenched martensite m is obviously higher than that of bearing, which affects the mechanical function of steel
When the carbon content of quenched m of GCr15 steel is 0.5% ～ 0.56%, the inductive mechanical function with the strongest failure resistance can be obtained.
M: The carbon content measured is the uniform carbon content of sliding bearing.
2. Retained austenite in quenched steel of oil-free bearing
After normal quenching, high carbon chromium steel can contain 8% ～ 20% AR (retained austenite).
The content of AR in bearing parts should be appropriate.
The hardness and the life of the steel increase with it. If it spontaneously transforms into martensite and then decreases after reaching the peak value, the favorable effect of AR is that under the stable condition of AR, the resistance of the steel will decrease sharply and become brittle.
When the axial load is small, a small amount of deformation occurs in AR, which not only reduces the stress peak, but also strengthens the deformed AR by processing strengthening and stress-strain induced martensitic transformation. The effect of increasing the amount of AR on the contact fatigue life is reduced. When the axial load is large, the larger plastic deformation of AR and the matrix will partially occur stress convergence and fracture, so that the life decreases.
3. Residual stress of oil-free bearing after quenching and tempering
The fatigue strength increases with the increase of the residual compressive stress on the surface (excessive residual stress may cause deformation of the parts). When the residual internal stress on the surface is tensile stress, the fatigue strength decreases.
After quenching and low temperature tempering, the bearing parts still have large internal stress.
4. Undissolved carbides in quenched steel
The quantity, description, size and distribution of undissolved carbides in quenched steel are affected by the chemical composition of the steel and the original arrangement before quenching, and by the austenitizing conditions.
When bearing, especially when the carbide is non spherical, the stress converges with the matrix and cracks occur, which will reduce the resistance and fatigue resistance.
Effect of quenched undissolved carbides on the function of steel
The carbon content and AR content and dispersion of quenched martensite are affected, which has additional effect on the function of steel.
Too much undissolved carbide in quenching is harmful to the mechanical function and failure resistance of steel.
It is necessary for bearing steel to have a few undissolved carbides after quenching. In order to obtain fine grain cryptocrystalline martensite, it is required that the undissolved carbides should be small (quantity is small), small (scale is small), uniform (the difference between size and size is very small, and the distribution is uniform), and round (each carbide is spherical).
Reducing the carbon content of bearing steel is one of the ways to improve the service life of parts.
5. Impurity content of oil-free bearing steel
Impurities: nonmetallic dopants harmful elements (acid soluble) if the oxygen content is higher, it is necessary to reduce the oxygen content of steel for production. Exception: the more oxide dopants, the more impurities in steel. The influence of impurities on mechanical function and anti failure ability of parts is related to the type, nature, quantity, size and shape of impurities. Generally, it has the effect of reducing resistance, plasticity and fatigue life. About working under high stress Because of the ellipsoidal shape of MNS dopant in steel, the harmful oxide can be wrapped in the bearing parts. The effect of MNS dopant on fatigue life is small and even may be beneficial.
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