1、13FTM15 AGMA Technical Paper White Structure Flaking in Rolling Bearings for Wind Turbine Gearboxes By H. Uyama and H. Yamada, NSK Ltd. 2 13FTM15 White Structure Flaking in Rolling Bearings for Wind Turbine Gearboxes Hideyuki Uyama and Hiroki Yamada, NSK Ltd. The statements and opinions contained he
2、rein are those of the author and should not be construed as an official action or opinion of the American Gear Manufacturers Association. Abstract Bearing failures in wind turbine gearboxes were investigated and rolling contact fatigue tests to reproduce them using a hydrogen-charge method were cond
3、ucted. Two main failure modes in wind turbine gearbox bearings were white structure flaking and axial cracking, which were involving a microstructural change. Both failure modes can be reproduced by using specimens charged with hydrogen. Operating conditions, which can induce hydrogen generation fro
4、m lubricant and penetration of the bearing steel were discussed. Effects of bearing material on white structure flaking life were suggested as one of the countermeasures. Copyright 2013 American Gear Manufacturers Association 1001 N. Fairfax Street, Suite 500 Alexandria, Virginia 22314 September 201
5、3 ISBN: 978-1-61481-072-8 3 13FTM15 White Structure Flaking in Rolling Bearings for Wind Turbine Gearboxes Hideyuki Uyama and Hiroki Yamada, NSK Ltd. Introduction Premature failures of rolling bearings occasionally occur in wind turbine gearboxes 1. One of the main failure modes is flaking involving
6、 a microstructural change. This type of flaking is called white structure flaking (WSF) or white etching crack (WEC) because the area of the microstructural change observed in the flaking cross sections looks white after etching. Therefore, understanding the mechanism of white structure flaking is i
7、mportant for wind turbine gearbox reliability. Flaking in rolling bearings occurs due to rolling contact fatigue and it is a similar phenomenon as spalling in gears. Flaking is generally classified to subsurface originated flaking, which is initiated at nonmetallic inclusions in materials and surfac
8、e originated flaking, which occurs under contaminated or poor lubrication conditions 2. However, recently white structure flaking can be seen in several applications, which is a different type of flaking from the subsurface and surface originated flaking mentioned above. For example, it is known tha
9、t white structure flaking sometimes occurs in bearings for automotive electrical accessories as shown in Figure 1 3. There are many studies about the failure mechanism and the countermeasure for white structure flaking in automotive bearings. Some of them suggested that this type of flaking is induc
10、ed by hydrogen generated by decomposition of the lubricating oil, grease, or water in the lubricant and that this phenomenon is concerned with hydrogen embrittlement 4 5 6 7 8 9 10 11. Axial cracks are also observed in failed bearings for wind turbine gear boxes 12. This failure mode is very unique
11、and it is seldom found in other applications. The same microstructural change as seen in white structure flaking is often observed in the cross sections around the axial cracks. However, it is unclear whether the mechanisms of white structure flaking and axial cracking are the same or not. In this s
12、tudy, rolling contact fatigue tests were performed in order to reproduce white structure flaking and axial cracking by using specimens charged with hydrogen. From the view of hydrogen theory, influencing factors in operating conditions were discussed and effects of materials on bearing life were sug
13、gested as the countermeasure. Observation results of failed bearings for wind turbine gearboxes Failed bearings used in wind turbine gearboxes have been observed and two types of failures were mainly observed, which are classified as white structure flaking and axial cracking. Figure 1. An example o
14、f the cross section of white structure flaking in an automotive electrical accessory bearing 3 4 13FTM15 Figure 2 shows the observation results of a failed cylindrical roller bearing, which were used on the high speed shaft in wind turbine gearboxes. A small flaking was seen in the raceway surface a
15、s shown in Figure 2a. Figure 2b shows the cross section of the flaking area at the dotted line in Figure 2a. A microstructural change called white structure was observed at the flaking. Flaking morphology of failed bearings in wind turbine gearbox and automotive electrical accessories seem to be ver
16、y similar as shown in Figure 1 and Figure 2b although bearing types and size are quite different. Namely, small size ball bearings are used for automotive electrical accessory and large size roller bearings are used for wind turbine gearboxes. Figure 2c shows the cross section of an area without fla
17、king in the same bearing as shown in Figures 2a and 2b. White structure was observed even in this area, which is most likely to be the prior stage to flaking. Therefore it is presumed that this type of flaking in wind turbine gearboxes is initiated at the white structure. Figure 3 shows the observat
18、ion results of the other failed bearing, which is also a cylindrical roller bearing and used on the high speed shaft in wind turbine gearboxes. There were several large cracks longer than 10 mm and many small cracks around 1-3 mm in the axial direction on the raceway surface of the inner ring. Figur
19、e 3a shows two small cracks chosen of many axial cracks, which were observed on the raceway surface. The small cracks seem to be an early stage of crack propagation. A small axial crack was chosen for the cross section observation because small cracks are easier than large cracks to find the locatio
20、n of the crack initiation. Figure 3b shows the cross section including the small axial crack area. White structure was seen and it is seemed that a crack propagated along the white structure and reached the raceway surface. This crack is seen as the axial crack on the raceway surface. a) Raceway sur
21、face of flaking area b) Cross section of the dotted line in Figure 2a c) Cross section of no flaking area Figure 2. The raceway and the cross section of a failed bearing with white structure. 5 13FTM15 a) Axial cracks on the raceway surface b) The cross section through the cracks Figure 3. The racew
22、ay surface and the cross section of a failed bearing with axial cracks. Rolling contact fatigue tests to reproduce white structure flaking and axial cracking Reproduction of the bearing failure mode is important to know the failure mechanism and to find the most appropriate countermeasure. We carrie
23、d out two kinds of rolling contact fatigue tests in order to reproduce white structure flaking and the axial cracks. Hydrogen is utilized in these tests because microstructural changes called white structure were seen in both of these failure modes. Experiment to reproduce white structure flaking Fl
24、at disk type specimens with a diameter of 65 mm and a thickness of 6 mm were used in rolling contact fatigue test. The specimens were made of JIS-SUJ2 bearing steel, equivalent to SAE52100 and DIN-100Cr6. The specimens were quenched and tempered to produce a final hardness of 740 HV and the surface
25、was ground and then lapped. Before rolling contact fatigue testing, the specimens were charged with hydrogen by immersing them in NH4SCN aqueous solution at 323 K for 24 h. The specimens were immediately assembled into the thrust bearing test machine after having been charged with hydrogen as shown
26、in Figure 4. The upper race was a 51305 thrust bearing ring and the lower race was the specimen mentioned above. The rolling elements were 6 balls with a diameter of 9.525 mm. The retainer used was made of brass. The lubricating oil used was ISO-VG68. The maximum contact pressure was 3.8 GPa and the
27、 rotating speed was 1000 min-1. Test result of rolling contact fatigue to reproduce white structure flaking Figure 5 shows the result of thrust type rolling contact fatigue tests using the hydrogen-charged specimen and uncharged specimen. Flaking occurred in the hydrogen-charged specimens, and the r
28、olling contact fatigue life was much shorter than in the uncharged specimen. Figure 6a shows the microstructure of the flaking cross section in the hydrogen-charged specimen. White structure was observed around the flaking area. White structure was observed also in the cross section of an area witho
29、ut flaking as shown in Figure 6b. Therefore, it is presumed that this flaking was initiated from white structure formed subsurface. On the other hand, flaking did not occur and the tests were suspended in the uncharged specimen. There was no microstructural change in the uncharged specimen. Therefor
30、e, it is presumed that hydrogen induced microstructural change and decreased rolling contact fatigue life. 6 13FTM15 Figure 4. Schematic of the thrust type rolling contact fatigue test machine Figure 5. The results of thrust type rolling contact fatigue tests a) Flaking area b) No flaking area Figur
31、e 6. The cross section of hydrogen-charged specimen 7 13FTM15 It seems that these microstructural changes observed in the rolling contact fatigue tests using hydrogen-charged specimens are the same microstructure as seen in failed bearings of wind turbine gearboxes and automotive electrical accessor
32、ies. It is reported that hydrogen enhances localized plasticity and this mechanism is known as the HELP theory 13. Therefore, it is supposed that white structure represents a localized microstructural change by interaction between cyclic plasticity and hydrogen in the rolling contact fatigue process
33、 11. Experiment to reproduce the axial cracks Cylindrical roller bearings are used for the experiment to reproduce the axial cracks, because they are often used for wind turbine gearboxes and the axial cracks have not been seen in ball bearings. Although, white structure flaking has been observed in
34、 ball bearings. Bearing number of N308 made of JIS-SUJ2 bearing steel, equivalent to SAE 52100, were used as the test bearings with a bore diameter of 40 mm and an outside diameter of 90 mm. Only the outer ring was separated and charged with hydrogen by the same method mentioned previously and the i
35、nner ring and the rollers were uncharged, and then the test bearing was set on the radial type bearing test machine as shown in Figure 7. The reason why the outer ring was chosen for hydrogen charge is that hydrogen in the outer ring is more difficult to diffuse out of the steel than the inner ring
36、as the temperature of outer rings are normally lower than of inner rings. The lubricating oil used was ISO-VG150. The maximum contact pressure on the outer raceway was 2.1 GPa and the rotating speed was 3000 min-1. Test result of bearing life test to reproduce axial cracks Bearing life test of the h
37、ydrogen-charged bearing was stopped by detecting the vibration at the testing time of 280 h. On the other hand, the test of the uncharged bearing was suspended at the testing time of more than 1000 h because there was no sign of bearing failure. Figure 8 shows the outer ring raceway surface which wa
38、s charged with hydrogen. One large crack and two small cracks were observed. These cracks propagated straight in the axial direction and were identical to the axial cracks in the failed bearings of wind turbine gearboxes. Figure 7. A schematic of radial type bearing test machine 8 13FTM15 a) Large a
39、xial crack b) Magnification of position 1 in Figure 8a c) Magnification of position 2 in Figure 8a Figure 8. The raceway surface of the hydrogen-charged outer ring Figure 9a shows the cross sections of the cracking area including the position 2 in Figure 8a. The large crack propagated in the depth d
40、irection. White structure was observed independently in Figure 9a position 1 to the left of the large crack magnified in Figure 9b. And also, Figure 9c is the magnification of the position 2 in Figure 9a and including the small axial crack in Figure 8c. The small crack connected with white structure
41、. Therefore it is supposed that white structure was formed first such as in Figure 9b and then a small crack initiated from the white structure and propagated to the raceway surface such as Figure 9c and finally the crack propagated in the axial and depth directions such as is visible in Figure 8a a
42、nd Figure 9a. Figure 9d shows the cross section including the small axial crack in Figure 8b. White structure was observed also in this area and it seems that the crack initiated at the white structure and propagated to the surface. However, white structure was not observed on the cross section of t
43、he large axial cracks. This reason is supposed that the initiation of the large axial crack would be white structure, but it is difficult to observe the cross section pinpointing the crack initiation. It is much easier to observe the cross section of the crack initiation in the small axial cracks. 9
44、 13FTM15 a) The cross section of position 2 in Figure 8a b) Magnification of position 1 in Figure 9a c) Magnification of position 2 in Figure 9a d) The cross section of the small crack in Figure 8b Figure 9. The cross section of the cracked area of a hydrogen-charged outer ring 10 13FTM15 Failure mo
45、de of the axial cracks is very unique and is seldom seen in other applications except for wind turbine gearboxes. However, it seems that the hydrogen-charge method does reproduce it. This method is very simple and the other effects on rolling contact fatigue are small. Therefore, it is supposed that
46、 axial cracks seen in wind turbine gearbox bearings are also caused by hydrogen. The patterns of white structure due to hydrogen are random, so that cracks along the white structure can propagate in various directions. It is supposed that some cracks mainly propagate in a horizontal direction to the
47、 rolling elements translational direction and finally cause flaking, and that other cracks mainly grow in the vertical direction to the rolling elements translational direction and results in the axial cracks on the raceway surface as shown in Figure 10. Operating condition inducing white structure
48、flaking The bearing failures in wind turbine gearboxes are more likely to be caused by hydrogen as shown in the rolling contact fatigue tests to reproduce white structure flaking and axial cracking. Therefore, it is important to know the causes of hydrogen generation and penetration into the bearing
49、 steel, although there is no direct evidence that hydrogen is generated and penetrated into the steel in wind turbine gearbox bearings. It is reported that hydrogen is generated by decomposition of lubricant and it is enhanced by the type of lubricant, water in the lubricant, slip, vibration, and electric current 3 4 5 6 7 8 9 14. These previous studies are mainly conducted for automotive bearings. However, influencing factors are basically common also to wind turbine gearbox bearings. Type of lubricant It is reported that l
