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Increase of bearing life of rolling mill

Increase of bearing life of rolling mill

2020-04-30

Characteristics of rolling mill bearing

The roller and raceway are line contact or modified line contact, the radial bearing capacity is large, suitable for bearing heavy load and impact load. 

Small friction coefficient, suitable for high speed, limit speed close to deep groove ball bearings. 

Can move axially, can adapt to thermal expansion or installation error caused by the relative position of the shaft and shell changes, can be used as a free end support. 

The machining requirements of the shaft or seat hole are high. The relative deflection of the outer ring axis should be strictly controlled after the bearing is installed to avoid the concentration of contact stress. 

The inner or outer ring can be separated for easy installation and disassembly. 

A solid cage is used for steel plate stamping retainers, high-speed bearings or bearings requiring smooth operation.Retainers with fiberglass reinforced plastic can also be designed and supplied according to the usage and user requirements. 

Maintenance, overhaul and abnormal handling of two mill bearings

In order to maintain the original performance of the bearing in good condition for as long as possible, maintenance, maintenance, in order to prevent accidents, to ensure the reliability of operation, improve productivity, economy. 

Maintenance of the best corresponding mechanical operating conditions of the operating standards, regular.This includes monitoring the operation status, supplementing or changing lubricants, and regular disassembly inspections. 

As a maintenance item in operation, there is the bearing rotation sound, vibration, temperature, lubricant state and so on. 

Factors affecting bearing life of rolling mill and their control

Material factors affecting bearing life

The early failure forms of rolling bearings mainly include cracking, plastic deformation, wear, corrosion and fatigue.In addition to service conditions, the failure of bearing parts is mainly restricted by the hardness, strength, toughness, wear resistance, corrosion resistance and internal stress state of steel.The main underlying factors that affect these performance and state are the following. 

1.1 martensite in hardened steel

When the original structure of high carbon chromium steel is granular pearlite, the carbon content of quenched martensite obviously affects the mechanical properties of steel under quenching and tempering condition.The strength and toughness are about 0.5%, the contact fatigue life is about 0.55%, and the crushing resistance is about 0.42%. When the carbon content of quenched martensite of GCr15 steel is 0.5% ~ 0.56%, the comprehensive mechanical properties with the strongest anti-failure ability can be obtained. 

It should be pointed out that the martensite obtained in this case is cryptocrystalline martensite and the carbon content measured is the average carbon content.In fact, the carbon content in martensite is not uniform in the microzone, and the carbon concentration near the carbide is higher than that far away from the carbide proferritic part, so they begin to undergo martensite transformation at different temperatures, thus inhibiting the growth of martensite grain and the display of microform and becoming cryptocrystalline martensite.It can avoid the microcracks which are easy to appear when high carbon steel is quenched, and its substructure is dislocation plate strip martensite with high strength and toughness.Therefore, only when the medium carbon cryptocrystalline martensite is obtained when the high carbon steel is quenched can the bearing parts obtain the matrix with the best anti-failure ability. 

1.2 residual austenite in hardened steel

After normal quenching, high carbon chromium steel can contain 8% ~ 20%Ar (residual austenite).There are advantages and disadvantages of Ar in bearing parts, in order to promote the advantages and disadvantages, the content of Ar should be appropriate.Since Ar amount is mainly related to austenitizing condition of quenching heating, its amount will affect the carbon content of quenching martensite and the amount of undissolved carbides, so it is difficult to reflect the influence of Ar amount on mechanical properties.Therefore, the austenitic condition was fixed and the austenitic thermal stabilization process was used to obtain different Ar quantities. The influence of Ar content on the hardness and contact fatigue life of GCr15 steel after quenching and tempering was studied.With the increase of austenite content, hardness and contact fatigue life both increased, and then decreased after reaching the peak, but the Ar content of the peak was different. The hardness peak appeared at about 17%Ar, while the contact fatigue life peak appeared at about 9%.When the test load decreases, the influence of increasing Ar on contact fatigue life decreases.This is because when the amount of Ar is not too much, it has little effect on the strength reduction, while the toughening effect is more obvious.The reason is that when the load is small, a small amount of Ar deformation occurs, which not only reduces the stress peak, but also strengthens the deformed Ar processing and the stress-strain induced martensitic transformation.However, if the load is large, the large plastic deformation of Ar and the matrix will locally produce stress concentration and fracture, thus reducing the life.It should be pointed out that the beneficial effect of Ar must be under the stable state of Ar. 

1.3 undissolved carbides in hardened steel

The number, morphology, size and distribution of undissolved carbide in quenched steel are affected by the chemical composition of steel and the original structure before quenching as well as the austenitizing conditions.Carbide is a hard and brittle phase, which is not only beneficial to wear resistance, but also can reduce the toughness and fatigue resistance due to the stress concentration caused by non-spherical carbide and matrix.In addition to its own effect on the properties of steel, quenched undissolved carbides also affect the carbon content and Ar content and distribution of quenched martensite, thus producing additional effects on the properties of steel.To explain not dissolve the performance impact of carbide, using different carbon content of steel, the martensite after quenching carbon content is the same as the Ar content without dissolving carbide content in different state, after 150 ℃ tempering, due to the martensitic carbon content is the same, and the hardness is higher, and therefore did not dissolve a small amount of carbide increased to higher hardness value is not big, reflect the strength and toughness of crushing load decreases, sensitive to stress concentration of the contact fatigue life is significantly reduced.So it is harmful to the comprehensive mechanical properties and failure resistance of steel to quench too much undissolved carbide.Reducing the carbon content of bearing steel is one of the ways to improve the service life of bearing steel. 

The size, morphology and distribution of hardened undissolved carbides also affect the properties of materials.In order to avoid the harm of undissolved carbides in bearing steel, it is required that the undissolved carbides should be less (in quantity), smaller (in size), uniform (with small difference in size and uniform distribution), and round (each carbide is spherical).It should be pointed out that it is necessary for bearing steel to have a small amount of undissolved carbide after quenching, not only to maintain sufficient wear resistance, but also to obtain fine grain cryptocrystalline martensite. 

1.4 residual stress after quenching and tempering

Bearing parts still have greater internal stress after quenching and tempering.There are two states of residual internal stress in parts: advantage and disadvantage.After heat treatment, the fatigue strength of steel increases with the increase of residual compressive stress on the surface, while the fatigue strength of steel decreases when the residual compressive stress on the surface is tensile stress.This is due to the large parts of the fatigue failure appeared under tensile stress, when the surface has larger residual compressive stress, tensile stress of the same numerical, and make the actual values under tensile stress of steel is reduced, the higher fatigue strength limit, when the surface has larger residual tensile stress, and under tensile stress superposition of load and the tensile stress in the steel actual bear increases obviously, even reduce fatigue strength limit.Therefore, it is also one of the measures to improve the service life of bearing parts after quenching and tempering the surface residual compressive stress, (of course, excessive residual stress may cause deformation or even cracking of parts, should be given enough attention). 

1.5 impurity content of steel

Impurities in steel include nonmetallic inclusions and harmful element (acid soluble) content, which are often mutually reinforcing to steel properties, such as the higher the oxygen content, the more oxide inclusions.The effect of impurities in steel on mechanical properties and failure resistance of parts is related to the type, nature, quantity, size and shape of impurities, but it usually reduces the toughness, plasticity and fatigue life. 

With the increase of inclusion size, the fatigue strength decreases, and the higher the tensile strength of steel, the greater the decreasing trend.As the oxygen content in steel increases (oxide inclusion increases), bending fatigue and contact fatigue life also decrease under the action of high stress.Therefore, it is necessary to reduce the oxygen content of manufacturing steel for bearing parts under high stress.Some studies have shown that MnS inclusions in steel, because of their ellipsoidal shape and the ability to wrap harmful oxide inclusions, have little impact on fatigue life reduction and may even be beneficial. 

Control of material factors affecting bearing life

In order to make the above material factors affecting the bearing life in the best state, it is necessary to control the original structure of steel before quenching, the technical measures can be taken: high temperature (1050℃) austenitic rate of cooling to 630℃ isothermal normalizing to obtain pseudo-eutectoid fine pearlite structure, or cold to 420℃ isothermal treatment to obtain bainite structure.It is also possible to obtain fine granular pearlite structure by rapid annealing of forging mill waste heat to ensure the fine and uniform distribution of carbides in steel.When the original tissue in this state is austenitized by quenching and heating, in addition to the carbide dissolved in austenitic, the undissolved carbide will aggregate into fine particles. 

When the original organization must be in the steel, the carbon content of quenched martensite (that is, the austenite after quenching heating carbon content), the residual austenite and the body not dissolve carbide mainly depends on the amount of quenching heating temperature and holding time, with higher quenching heating temperature (a), not dissolve the decrease in the number of carbide in steel (quenching martensite increased carbon content), retained austenite, hardness first increases with the increasing of quenching temperature, peak and decreased and then increased with the temperature.When the quenching heating temperature is constant, with the extension of austenitizing time, the amount of undissolved carbide decreases, the amount of residual austenitic increases, the hardness increases, and the time is long, this trend slows down.When the carbides in the original tissues are small, because the carbides are easy to dissolve into austenite, the hardness peak after quenching is moved to a lower temperature and appears in a shorter austenitizing time. 

In summary, the best structure composition of GCrl5 steel after quenching is about 7% undissolved carbide and 9% residual austenite (the average carbon content of cryptocrystalline martensite is about 0.55%).Moreover, when the carbides in the original tissues are small and evenly distributed, it is beneficial to obtain high comprehensive mechanical properties and have high service life when the microstructure composition at the above level is controlled reliably.It should be pointed out that in the original tissues with fine dispersed carbides, the fine undissolved carbides will aggregate and grow and make it coarsening when the quenching and heat preservation is carried out.Therefore, the quenching heating time of bearing parts with this kind of original structure should not be too long, and the adoption of rapid heating austenitizing quenching process can obtain higher comprehensive mechanical properties. 

In order to make the bearing parts after quenching and tempering the surface residual compressive stress, can be in the quenching heating through the atmosphere of carburizing or nitriding, a short time of surface carburizing or nitriding.Since the actual carbon content of austenite at the time of quenching heating of this steel is not high, far lower than the equilibrium concentration shown on the phase diagram, carbon (or nitrogen) can be absorbed.When the austenite contains higher carbon or nitrogen, its Ms decreases. During quenching, the surface layer has a martensitic transformation compared with the inner layer and the core, resulting in greater residual compressive stress.GCrl5 steel was treated with carburizing atmosphere and non-carburizing atmosphere after heating and quenching (both by low temperature tempering).The reason is that the surface of carburized parts has a large residual compressive stress. 

3 conclusion

The main material factors and control degree that affect the service life of high carbon chromium steel rolling bearing parts are as follows:

(1) the carbide in the original structure of steel before quenching needs to be fine and diffuse.High temperature austenitizing 630℃ or 420℃ can be adopted, and the process of rapid annealing of residual heat of forging rolling can also be used. 

(2) after quenching of GCr15 steel, it is required to obtain the microstructure of cryptocrystalline martensite with an average carbon content of about 0.55%, undissolved carbides in uniform and circular states with an average carbon content of about 9% Ar and about 7%.The microstructure can be controlled by quenching temperature and time. 

(3) after parts are quenched and tempered at low temperature, the residual compressive stress on the surface is required to be large, which is conducive to the improvement of fatigue resistance.The surface can be carburized or nitriding for a short time during the quenching heating process, so that the surface residual has a large compressive stress. 

(4) the manufacture of steel for bearing parts requires a high degree of purity, mainly to reduce the content of O2, N2, P, oxide and phosphide.Electroslag remelting, vacuum smelting and other technical measures can be adopted to make the oxygen content of the material ≤15PPM.


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