Abstract: Aiming at the problem of early failure of a three-point contact ball bearing in use, the appearance inspection, metallographic analysis, hardness test and material inspection were carried out, and the possibility of failure caused by material problems was ruled out. Through failure analysis, the measurement of the axial load on the bearing and the calculation of the maximum contact stress between the steel ball and the outer ring, it is found that the excessive axial load is the main reason for the failure of the bearing.
The front bearing of the high-pressure compressor rotor of a certain type of gas turbine No. 3 is a three-point contact ball bearing, and the material is high-temperature bearing steel GCr4Mo4V. The bearing failed in advance before reaching the design life (98 000 h) after running for less than 2 years. The reasons for the failure are analyzed below.
1. Working condition of high pressure rotor bearing
The high-pressure rotor is supported by the front and rear bearings to support its operation. The front and rear bearings are fixed by the outer ring and the inner ring rotates. The front bearing is a three-point contact ball bearing (fig. 1); the rear bearing is a cylindrical roller bearing with no ribs on the outer ring (fig. 2). When the gas turbine is working, the rotation of the high-pressure turbine drives the high-pressure compressor to work, so that the air flowing from the low-pressure compressor accelerates and flows to the combustion chamber again, so only the front bearing bears the axial load. Due to the high temperature ejected from the combustion chamber
The pressurized gas acts directly on the high-pressure turbine, so the axial load on the high-pressure rotor is consistent with the direction of the airflow. Only the outer ring and the left half of the inner ring of the front bearing bear the axial load, while the right half is basically unaffected.
The structural dimensions of the front bearing are: outer diameter D=240 mm, inner diameter d=160 mm, width B=38 mm, ball diameter Dw =23.81 mm, ball number Z=22, ball group pitch circle diameter Dpw =201 mm, design limit The rotational speed is 12 000 r/min. The lubrication and cooling of the bearing is mainly realized by spraying oil on the end face of the bearing, and the lubricating oil is injected into the gap between the inner ring of the bearing and the cage in an equal amount from 3 places. Six round holes are machined at the joints of the two inner half rings of the bearing, and the lubricating oil in the holes is sputtered into the bearing under the action of centrifugal force, so as to realize additional oil supply to the bearing radially. When the gas turbine is in the 0.1 working condition, the high-pressure rotor speed is 7 600 r/min; when the 0.35 working condition, the rotating speed is 8 500 r/min; when the 0.6 working condition, the rotating speed is 8 900 r/min; when the 0.8 working condition, the rotating speed is 9 200 r/min; 1.0 working condition, the speed is 9 900 r/min.
2. Bearing inspection
2.1 Visual inspection
The inspection of the disassembled bearing found that the mating surface of the outer ring and the bearing seat, the mating surface of the inner ring and the journal installation were free of wear and other abnormal phenomena. The surface of the outer ring channel has different degrees of pitting corrosion, especially the continuous flake spalling on the surface of the outer ring channel, and its length is about 1/4 of the circumference of the channel. The pitting corrosion of the inner ring occurred on the left half-ring channel, and no pitting corrosion and other failure phenomena were found on the right half-ring channel. Due to the serious failure of the outer ring (Figure 3), a more detailed inspection and analysis of the outer ring was carried out.
2.2 Metallographic analysis
The characteristic sample was cut along the longitudinal direction from the bright part of the outer ring channel, the cut surface of the sample was ground flat, and treated with an etchant, and the surface was magnified 500 times for observation under a microscope. The microstructure was martensite, primary carbide, secondary Secondary carbides and retained austenite. According to the thickness of the matrix martensite structure and the degree of carbide dissolution, the structure is rated as 3 grades. According to the relevant regulations in JB/T2850-1993, its microstructures meet the standard requirements. There are a lot of fatigue cracks in the spalling part of the channel, most of the cracks extend along the surface of the channel, and some extend to the core, as shown in Figure 4, the crack depth is about 2.2 ~ 6 μm
2.3 Hardness testing
According to the relevant regulations of JB/T2850-1993, on the end face of the outer ring along the circumference every 120°, use a Rockwell hardness tester to test the hardness of 3 points on the outer ring. The hardness of the ferrule after the fire is 60 ~ 64HRC, and the hardness difference is not more than 2HRC when the diameter of the outer ring is greater than 100 mm).
2.4 Material inspection
Take a sample from the outer ring for chemical analysis, and its chemical composition and content (mass fraction) are shown in Table 1. The results show that the content of P is slightly beyond the standard, and the contents of other elements are within the standard requirements, but the excessive amount of P is not enough to affect the quality and overall performance of the bearing, so the chemical composition and content of the bearing meet the requirements of the relevant standards
3. Failure analysis
Through metallographic analysis and material inspection, the problem of bearing material is ruled out. Since one side of the outer ring channel surface is peeled off, the other side is not peeled off, and the left half channel of the inner ring also has obvious pitting corrosion, while the right half channel has no obvious pitting corrosion, peeling, scratching and wear. Therefore, it can be preliminarily judged that the bearing was subjected to a large axial load during the working process, resulting in excessive stress at the contact part of the bearing channel and the steel ball, and premature fatigue failure occurred under the action of cyclic load.
In order to determine whether the bearing is subjected to excessive axial load during the working process, a force measuring elastic ring (the force measuring range of the elastic ring is less than or equal to 30 kN) is installed in the position as shown in Figure 5, and the first axial load is carried out. The measurement results are shown in Figure 6. by Tuco
It is known that the axial load of the high-pressure rotor exceeds the specified range (18kN) on the drawing. From the trend of the curve, the axial load has exceeded the normal measurement range of the elastic ring (30kN), and its direction is towards the turbine. After the axial load test, the bearing and elastic ring were removed for inspection, and it was found that there were 18 very clear contact marks on the bearing surface of the bearing seat before high pressure, which indicated that there was a large axial load during the operation of the gas turbine. , causing the elastic ring to deform and contact the bearing seat.
In order to determine the magnitude of the axial load, the axial load of the high-pressure rotor was measured for the second time. This time, an elastic ring with a measurement range of 50 kN was replaced. The measurement results are shown in Figure 7. It can be seen from the figure that the axial load of the high-pressure rotor increases first and then decreases, and the axial load reaches the maximum at 0.45, which is 38.5 kN, and at 0.23 to 0.8, its value is greater than The axial load has been reduced to 20.5 kN at 30kN, 1.0 working condition. The data provided by the bearing manufacturer shows that for cylindrical roller bearings and ball bearings, when the actual contact stress P≤2 000 MPa, it will not cause changes in the material structure, so an infinitely long life can be obtained; the actual contact stress is higher than 2 At 400 MPa, beyond the elastic deformation range of the material, permanent deformation will occur, eventually leading to material failure; when the stress is higher than 4 200 MPa, the bearing life will be greatly shortened, but the ultimate strength of the material has not been exceeded.
According to the results of the second axial load measurement, when the gas turbine works at 0.35, the axial load of the high-pressure rotor is 37 kN, and the rotational speed is 8 500 r/min. If the axial load acting on the bearing is not considered, and Only by calculating the effect of the centrifugal force generated by the high-speed operation of the steel ball on the bearing, it can be obtained that the maximum contact stress between the steel ball and the outer ring is 1 834 MPa; when the centrifugal force and the axial load of 37 kN are taken into account at the same time, the maximum contact stress between the steel ball and the outer ring can be obtained. The maximum contact stress of the ring is 2 526 MPa; when the gas turbine works under the working condition of 0.8, the axial load of the high-pressure rotor is 30 kN, and the rotating speed is 9 200 r/min. The maximum contact stress between the ball and the outer ring is 2 500 MPa. Therefore, if the gas turbine works at or near 0.35 to 0.8 for a long time, the contact stress between the steel ball and the outer ring will exceed the elastic deformation range of the bearing material and cause permanent deformation, and the bearing will fail before reaching the design life.
From the failure analysis and axial load test, it can be known that the premature failure of the bearing is caused by the excessive axial load on the bearing. To solve the problem of bearing failure, the most fundamental way is to find the cause of the excessive axial load of the gas turbine, and solve it, and finally keep the axial load on the bearing within a reasonable range. If the cause of the excessive axial load cannot be determined, the bearing with higher carrying capacity can be replaced, or the bearing can be optimized to meet the requirements of use.