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Shanghai Shang Shaft Bearing Quality Inspection Institute Co., Ltd.

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Bearing manufacturing technology-heat treatment

2021-05-26 15:21

To meet the bearing performance requirements,quenching hardeningfor the purposeThe heat treatment is an indispensable process.The energy consumed in the heat treatment process accounts for about 25% of the total energy consumption of the bearing manufacturing process.. Therefore, from the perspective of energy saving and environmental protection, the heat treatment in the bearing manufacturing process is analyzed, and the main countermeasures include:High efficiency of heat treatment processSmall deformation heat treatment technology, etc..

1 heat treatment method of bearing

Bearing heat treatment methods for the purpose of quenching and hardening areMany, in most cases can be roughly dividedOverall quenching,infiltrationcarbon quenching,high frequency quenching,carbonitridingWait.

 

Overall quenchingIs the most common bearing heat treatment process,Most of the materials used high carbon chromium bearing steel (GCr15). After heating to about 840 ° C. in a heat treatment furnace, the carbide formed by spheroidizing annealing is moderately and uniformly solid-dissolved in the matrix, quenching and cooling is further performed, and then low-temperature tempering is performed. ByUndissolved carbides and tempering precipitated carbides to obtain uniform precipitated tissue, can be used to requestWear resistanceof the bearings.

 

carburizing and quenchingIs in the heat treatment furnace will be low-carbon alloy steel at about 930 degrees C insulation, management furnace carburizing gas, from the surface to the inside to make carbon infiltration diffusion, quenching heat treatment, due to the gradient of carbon concentration, quenching surface hardening, because.Low internal specific surface hardness, for the requirementImpact resistancesexual bearings.

 

high frequency quenchingIs the use of induction heating heat treatment, the main.To be used for medium carbon steel,Applied to complex shapesthe car wheelwith bearings, etc., only hardenedNecessary parts.

 

carbonitridingIs to add 5% left in the carburizing gas.The right ammonia precipitates nitrides near the surface layer.In consideration of the difference in solubility between carbon and nitrogen in austenite, the treatment is usually performed at 750 to 850 ° C. which is lower than the carburizing temperature, and in particular, it is most effective for prolonging the life of a bearing used in an environment in which foreign matter is mixed.

2   Heat treatment efficiency (compression carburizing time)

In the heat treatment of carburizing and quenching, it is especially required to pass high.In order to realize energy saving, compared with the treatment time of overall quenching, the treatment time of carburizing quenching is several to tens of times as long as the former (the treatment time varies with the bearing size, but large-size bearings sometimes need to be treated for nearly 100 h in the furnace). Therefore, more energy is consumed. In order to reduce energy consumption, shortening carburizing time is a major issue to be solved.

 

gas carburizingWidely used in mass production. Usually,According to the mechanism of carburization, it is generally believed that there are two stages.In the first stage, the carbon moves to the surface of the steel due to the carburizing reaction between the carburizing gas and the steel, and in the second stage, after the carbon potential of the carburizing gas and the steel surface is roughly equal, the carbon on the steel surface diffuses to the inside and the surface carbon potential decreases.To shorten the carburizing time, it is necessary to increase the speed of the first and second stages.

 

When using ordinary RX gas for gas carburization,In order to accelerate the carburization of its first stage, it is generally necessary.increase carburizingCO, H in gas2of the pressure. In recent years, isFocus on changing the CO, H contained in the carburizing gas2the pressure,Development aims to increase the carburizing speed and compress the carburizing time.the technologyin addition,As an alternative to gas carburizing.Art, through incalledvacuum carburizingoflow pressure atmosphere, makeA small amount of hydrocarbon gas in direct contact with the workpiece, to enterline carburizing,Increasing the penetration rate of carbon on the steel surface.

 

In the second stage of carbon diffusion, due to the diffusion of carbonBy increasing the carburizing temperature, it is possible to significantly increase the temperature.Shorten the carburizing time. However, due to the high carburizing temperature,Crystalgrain coarseningand other important reasons, will lead to mechanical performance degradationlow, so,carburizing temperatureExistenceCan not exceed grain coarseningLimit temperatureof the problem. Development can solve the above problemsThe material is also a way to solve the problem.

 

In terms of materials, it is necessary.Development of new materials, I .e.Materials that readily allow carbon to penetrate into the surface of steel. In addition, throughUsehigh hardenabilityThe material, so that the effective hardening layer depth plus.Deep, can shorten the carburizing time.But at present, the price of such new materials is higher.

3   Small deformation heat treatment technology

Deformation occurs during heat treatment, after heat treatment.The amount of deformation determines the grinding surplus of the subsequent grinding process (Figure 1).Amount. In the manufacturing process of bearings, the energy consumption is second only to heat treatment.is the grinding, assembly process, throughDecrease grinding allowanceCompressible grinding cycle time to reduce heat treatment deformation.to cut a lot of energy consumption.

The dimensional change of bearing steel during heat treatment (Figure 2) isFor example, as the temperature rises during heating, thermal expansion occurs with Ao.body, and produce heat shrinkage when cooling,martensitic transformationHeat treatment deformation. In this way, there is a heat treatment.Many of the factors that cause dimensional change, due to the dimensional changedeviation, and heat treatment deformation occurs. TheDimensional changeThe deviation of the chemical is the temperature deviation during heating and cooling.up,Uniform heating and cooling can reduce heat treatment deformation..

 

To this end,Prevent heat treatment deformationthe countermeasuresThere are the followingspecies:Reduce the temperature during heating is not the purpose of multi-stage heating;Reduce the temperature during cooling is not the purpose of multi-stage cooling; There are also measures such as the type of quenching oil and the optimization of the stirring speed of the oil tank.. In recent years, it has also been adopted.Other countermeasures: For example, by simulating the oil flow in the oil tank during heat treatment, analysis is carried out to develop an oil tank with uniform oil flow; by improving the heat treatment fixture, uniform heating and cooling can be achieved.

 

In addition, especially in the prone to heat treatment deformation.Thinwall bearingaspects, accordingRelationship between temperature and dimensional change during heat treatment(Fig. 2) Quenching(Die quenching). In the case of bearing steel, the phase change caused by overall quenching leads to dimensional expansion. After the Ms point, the outer diameter side is constrained, and the quenching is completed while the up and down direction of the workpiece is controlled by a press. Since the size is almost unchanged after the completion of carburizing and quenching, thermal shrinkage occurs during the quenching process, and quenching is carried out while controlling and constraining the inner diameter side with a press, thus greatly reducing the heat treatment deformation (Figure 3).

 

 

When the size of the heat treatment changes, the change is used.Restrained quenching is one of the effective countermeasures to reduce heat treatment deformation.. Bearings are not as complex as gears, because of the simple shape, has been the implementation of constrained quenching, however, the cost of heat treatment of constrained quenching is higher than the cost of ordinary heat treatment is a problem to be solved.

Therefore, the development of noHeat treatment technology that produces heat treatment deformation is a meaningful research.research work. For the factors that affect steel deformation, it can also betake appropriate countermeasures, for example,Reduce material segregation, in the notIn the forging process, the residual stress of the material is reduced.

 

Summary

Several typical heat treatment methods for bearings are described above,High efficiency and low deformation heat treatment technology for energy saving purposes.technology, but in order to meet the requirements of reducing the burden on the environment, the hotWe will play a greater role in the management of processes and further improve the technology.level is indispensable, it is necessary to take energy saving as the core,Further realize low cost.