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

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Surface Strengthening Technology of Steel Ball for Long Life Gas Turbine Bearing

2020-12-01 13:54

The three-point contact ball bearing for the combustion engine bears a large axial thrust load of the rotor. The steel ball bears a high contact stress and alternating load during work. The surface of the steel ball will occasionally peel off during use, which affects the service life and reliability of the bearing.There is a certain gap between the fatigue life of steel balls in internal combustion engine bearings in China and foreign countries, so it is necessary to study how to improve the fatigue resistance of steel balls.

 

The fatigue resistance of steel balls is related to the stress state of the surface and sub-surface of the steel ball. The maximum shear force on the surface of the steel ball is in the sub-surface layer, which improves the hardness of the surface of the steel ball,Increasing the surface and sub-surface compressive stress can improve the fatigue resistance and wear resistance of steel balls.

 

The surface strengthening of steel ball is in the range of plastic deformation of the surface of the steel ball, the way of mutual collision of steel balls, so that the surface of the macro elastic deformation and micro plastic deformation, resulting in the surface hardness of the steel ball to improve, the surface of the steel ball is a compressive stress state distribution. In order to meet the requirements of long life and high reliability of gas turbine bearings, surface strengthening treatment process is added during steel ball processing.

 

The size of the steel ball that is conventionally strengthened is small, and the material is ordinary bearing steel.Gas turbine bearing parts are mostly made of special steel materials such as high-temperature steel and high-speed steel, and the steel ball size is large.. Therefore, it is necessary to study the strengthening treatment technology of gas turbine bearing steel ball.

1. Surface strengthening test of steel ball

1.1 sample

The diameter of the steel ball is 22.225mm and the material is M50 steel.

1.2 strengthening test

Take 2000 grain steel balls and use the steel ball surface strengthening machine to carry out the strengthening test. Through the experiment, the process parameters are determined as follows: drum speed 15r/min, strengthening time 100~210min. The strengthening liquid (kerosene and 20# engine oil in a certain proportion) is 100~150mL.

1.3 test results

1.3.1 Stress

The surface stress of the steel ball before and after strengthening is shown in fig. 1. from the figure, it can be seen that: 1) the stress peak value of the unstrengthened ball is about-450MPa, which is 0.1mm away from the surface; 2) The peak stress value of the strengthened 2.5h ball is-900MPa, and the depth of the strengthened layer is about 0.75mm at 0.4mm from the surface. 3) The peak stress value of the strengthened 4h ball is-1100MPa, the depth of the strengthening layer is about 0.8mm. The surface stress of the strengthened steel ball is greatly increased, and the change of the surface stress of the steel ball and the depth of the strengthened layer are also different with different strengthening time.

1.3.2 Hardness

The surface hardness of the steel ball before and after strengthening is shown in Figure 2. The surface hardness of the steel ball after strengthening is obviously increased, and the average hardness is 731HV.0.1Increase to 785HV0.1 .

1.3.3 Surface metamorphic layer detection

In the failure analysis center of a company, the microstructure and hardness method were used to detect the metamorphic layer of the strengthened steel ball, and there was no obvious metamorphic layer.

1.3.4 Summary

The surface stress of the strengthened steel ball is increased by 70% ~ 88%, the surface hardness is obviously improved, and there is no obvious metamorphic layer on the surface.

2. Strengthening process

Through testing and testing, the final determination of the steel ball processing process is: forming → annealing → polishing ball → soft grinding → heat treatment → hard grinding 1 → surface strengthening treatment → hard grinding 2 → initial grinding → supplementary tempering → fine grinding → stabilization → ultra-fine grinding → final inspection → oiled packaging. The finished steel ball was tested, and the compressive stress was only reduced by about 100 MPa after stress relief treatment, which had little effect on the strengthening effect.

3. Fatigue life test

Bearing steel ball failure belongs to rolling contact fatigue failure, and its life is evaluated by the method shown in Figure 3. The test ball is in contact with four test balls at the same time under the action of a certain loading force. When the spindle fixture drives the test ball to rotate at a high speed, the four test balls will undergo pure rolling under the action of friction. The contact stress between the steel balls is 5.83GPa, and the spindle speed is 3000 r/min. Take 9 ordinary process steel balls and 6 steel balls for strengthening treatment. The fatigue life of the steel ball before and after strengthening is shown in Figure 4, and the fatigue life of the steel ball after strengthening is significantly improved.

4. Concluding remarks

The surface strengthening technology is used to strengthen the steel ball of the gas turbine bearing, and the fatigue test proves that the fatigue test life of the steel ball after strengthening is obviously improved, which shows the correctness of the surface strengthening technology of the special material steel ball.