Wikipedia

White etching cracks

(Redirected from White Etching Cracks)

White etching cracks (WEC), or white structure flaking or brittle flaking, is a type of rolling contact fatigue (RCF) damage that can occur in bearing steels under certain conditions, such as hydrogen embrittlement, high stress, inadequate lubrication, and high temperature. WEC is characterised by the presence of white areas of microstructural alteration in the material, which can lead to the formation of small cracks that can grow and propagate over time, eventually leading to premature failure of the bearing. WEC has been observed in a variety of applications, including wind turbine gearboxes, automotive engines, and other heavy machinery. The exact mechanism of WEC formation is still a subject of research, but it is believed to be related to a combination of microstructural changes, such as phase transformations and grain boundary degradation, and cyclic loading.

WEC initiation at MnS inclusions in steel[1]

Cause

edit

White etching cracks (WECs), first reported in 1996,[2] are cracks that can form in the microstructure of bearing steel, leading to the development of a network of branched white cracks.[3] They are usually observed in bearings that have failed due to rolling contact fatigue or accelerated rolling contact fatigue.[4] These cracks can significantly shorten the reliability and operating life of bearings, both in the wind power industry and in several industrial applications.[5][6]

 
On overview of bearing components, the location, and appearance of a white etching crack (WEC): a) Schematic representation of a standard deep groove ball bearing; b) A low-magnification image using secondary electrons (SE) showcasing a WEC located beneath the surface in a bearing inner ring; c) A higher-magnification image using backscattered electrons (BSE) of the area emphasized in b), displaying the characteristic appearance of a WEC.[7]

The exact cause of WECs and their significance in rolling bearing failures have been the subject of much research and discussion.[8][6] Ultimately, the formation of WECs appears to be influenced by a complex interplay between material, mechanical, and chemical factors,[3] including hydrogen embrittlement, high stresses from sliding contact, inclusions,[9] electrical currents,[10] and temperature. They all also have all been identified as potential drivers of WECs.[11]

Hydrogen embrittlement

edit

One of the most commonly quoted potential causes of WECs is hydrogen embrittlement caused by an unstable equilibrium between material, mechanical, and chemical aspects,[3] which occurs when hydrogen atoms diffuse into the bearing steel, causing micro-cracks to form.[8] Hydrogen can come from a variety of sources, including the hydrocarbon lubricant or water contamination, and it is often used in laboratory tests to reproduce WECs.[12] Mechanisms behind the generation of hydrogen from lubricants was attributed to three primary factors contributing: decomposition of lubricants through catalytic reactions with a fresh metal surface, breakage of molecular chains within the lubricant due to shear on the sliding surface, and thermal decomposition of lubricants caused by heat generation during sliding.[13] Hydrogen generation is influenced by lubricity, wear width, and the catalytic reaction of a fresh metal surface.[13]

Stress localisation

edit
 
WEC initiation at MnS inclusions.

Stresses higher than anticipated can also accelerate rolling contact fatigue, which is a known precursor to WECs.[4] WECs commence at subsurface during the initial phases of their formation,[14] particularly at non-metallic inclusions. As the sliding contact period extended, these cracks extended from the subsurface region to the contact surface, ultimately leading to flaking. Furthermore, there was an observable rise in the extent of microstructural modifications near the cracks, suggesting that the presence of the crack is a precursor to these alterations.[15][12]

The direction of sliding on the bearing surface played a significant role in WEC formation. When the traction force opposed the direction of over-rolling (referred to as negative sliding), it consistently led to the development of WECs. Conversely, when the traction force aligned with the over-rolling direction (positive sliding), WECs did not manifest. The magnitude of sliding exerted a dominant influence on WEC formation. Tests conducted at a sliding-to-rolling ratio (SRR) of -30% consistently resulted in the generation of WECs, while no WECs were observed in tests at -5% SRR. Furthermore, the number of WECs appeared to correlate with variations in contact severity, including changes in surface roughness, rolling speed, and lubricant temperature.[16]

Electrical current

edit

One of the primary causes of WECs is the passage of electrical current through the bearings. Both Alternating Current (AC) and Direct Current (DC) can lead to the formation of WECs, albeit through slightly different mechanisms. In general, hydrogen generation from lubricants can be accelerated by electric current, potentially accelerating WEC formation.[17] Under certain conditions, when the current densities are low (less than 1 mA/mm2), electrical discharges can significantly shorten the lifespan of bearings by causing WECs. These WECs can develop in under 50 hours due to electrical discharges. Electrostatic sensors prove to be useful in detecting these critical discharges early on, which are associated with failures induced by WECs.[18] The analysis revealed that different reaction layers form in the examined areas, depending on the electrical polarity.[10]

In the case of AC, the rapid change in polarity involves the creation of a plasma channel through the lubricant film in the bearing, leading to a momentary, intense discharge of energy. The localised heating and rapid cooling associated with these discharges can cause changes in the microstructure of the steel, leading to the formation of WEAs and WECs.[19]

On the other hand, DC can cause a steady flow of electrons through the bearing. This can lead to the electrochemical dissolution of the metal, a process known as fretting corrosion. The constant flow of current can also cause local heating, leading to thermal gradients within the bearing material. These gradients can cause stresses that lead to the formation of WECs.[19]

Microstructure

edit
 
EDAX spectrum showing the chemical composition of MnS inclusion.[1]

WECs are sub-surface white cracks networks within local microstructural changes that are characterised by a changed microstructure known as white etching area (WEA).[3] The term "white etching" refers to the white appearance of the altered microstructure of a polished and etched steel sample in the affected areas.[20] The WEA is formed by amorphisation (phase transformation) of the martensitic microstructure due to friction at the crack faces during over-rolling,[21] and these areas appear white under an optical microscope due to their low-etching response to the etchant.[22][23][24] The microstructure of WECs consists of ultra-fine, nano-crystalline, carbide-free ferrite, or ferrite with a very fine distribution of carbide particles that exhibits a high degree of crystallographic misorientation.[25][26]

WEC propagation is mostly transgranular[27] and does not follow a certain cleavage plane.[28]

 
Electron microscopy analysis of an axial cross-section (rolling direction in/out of the page) approximately 200 μm below the raceway surface: a) BSE image of the WEC selected for further investigation with EBSD. b) Higher magnification BSE image of the ROI with the WEM and untransformed material highlighted; c) BC EBSD map of the ROI. d) IPF Z orientation map of the ROI. The crack, which, like the WEM, is also a non-indexing feature, was segmented from the BSE images and overlaid in white.[7]

Researchers observed three distinct types of microstructural alterations near the generated cracks: uniform white etching areas (WEAs), thin elongated regions of dark etching areas (DEA), and mixed regions comprising both light and dark etching areas with some misshaped carbides.[16] During repeated stress cycles, the position of the crack constantly shifts, leaving behind an area of intense plastic deformation composed of ferritic, martensite, austenite (due to austenitization) and carbides. nano-grains, i.e., WEAs.[29][26] The microscopic displacement of the crack plane in a single stress cycle accumulates to form micron-sized WEAs during repeated stress cycles. After the initial development of a fatigue crack around inclusions, the faces of the crack rub against each other during cycles of compressive stress. This results in the creation of WEAs through localised intense plastic deformation. It also causes partial bonding of the opposing crack faces and material transfer between them. Consequently, the WEC reopens at a slightly different location compared to its previous position during the release of stress.[30]

Furthermore, it has been acknowledged that WEA is one of the phases that arise from different processes and is generally observed as a result of a phase transformation in rolling contact fatigue.[26] WEA is harder than the matrix and .[29] Additionally, WECs are caused by stresses higher than anticipated and occur due to bearing rolling contact fatigue as well as accelerated rolling contact fatigue.[4]

WECs in bearings are accompanied with a white etching matter (WEM). WEM forms asymmetrically along WECs. There is no significant microstructural differences between the untransformed material adjacent to cracking and the parent material although WEM exhibits variable carbon content and increased hardness compared to the parent material. A study in 2019 suggests that WEM may initiate ahead of the crack, challenging the conventional crack-rubbing mechanism.[31]

Testing for WEC

edit

Triple disc rolling contact fatigue (RCF) Rig is a specialised testing apparatus used in the field of tribology and materials science to evaluate the fatigue resistance and durability of materials subjected to rolling contact.[32] This rig is designed for simulating the conditions encountered in various mechanical systems, such as rolling bearings, gears, and other components exposed to repeated rolling and sliding motions. The rig typically consists of three discs or rollers arranged in a specific configuration.[33] These discs can represent the interacting components of interest, such as a rolling bearing. The rig also allows precise control over the loading conditions, including the magnitude of the load, contact pressure, and contact geometry.[15][8]

PCS Instruments Micro-pitting Rig (MPR) is a specialised testing instrument used in the field of tribology and mechanical engineering to study micro-pitting, a type of surface damage that occurs in lubricated rolling and sliding contact systems. The MPR is designed to simulate real-world operating conditions by subjecting test specimens, often gears or rolling bearings, to controlled rolling and sliding contact under lubricated conditions.[16]

Impact

edit

Offshore wind turbines are subject to challenging environmental conditions, including corrosive saltwater, high wind forces, and potential electrical currents. These conditions can contribute to bearing failures and impact the reliability and maintenance of wind turbines.[6][11] Several factors that can lead to bearing failures, such as corrosion, fatigue, wear, improper lubrication, high electric currents, and the need for improved materials and designs to ensure the longevity and performance of bearings in offshore wind turbines.[34][35][36] WECs negatively affects the reliability of bearings, not only in the wind industry but also in various other industrial applications such as electric motors, paper machines, industrial gearboxes, pumps, ship propulsion systems, and the automotive sector.[37][38] 60% of wind turbines failures are liked to WEC.[39]

In October 2018, a workshop on WECs was organised in Düsseldorf by a junior research group funded by the German Federal Ministry of Education and Research (BMBF). Representatives from academia and industry gathered to discuss the mechanisms behind WEC formation in wind turbines, focusing on the fundamental material processes causing this phenomenon.[40]

Further reading

edit
 
Wikimedia Commons has media related to White etching cracks.
  • Šmeļova, Viktorija; Schwedt, Alexander; Wang, Ling; Holweger, Walter; Mayer, Joachim (2017-07-01). "Microstructural changes in White Etching Cracks (WECs) and their relationship with those in Dark Etching Region (DER) and White Etching Bands (WEBs) due to Rolling Contact Fatigue (RCF)". International Journal of Fatigue. 100: 148–158. doi:10.1016/j.ijfatigue.2017.03.027. ISSN 0142-1123. S2CID 41169654.

References

edit
  1. ^ a b Bruce, T.; Rounding, E.; Long, H.; Dwyer-Joyce, R. S. (2015-09-15). "Characterisation of white etching crack damage in wind turbine gearbox bearings". Wear. 338–339: 164–177. doi:10.1016/j.wear.2015.06.008. ISSN 0043-1648.  This article incorporates text from this source, which is available under the CC BY 4.0 license.
  2. ^ Tamada, K.; Tanaka, H. (November 1996). "Occurrence of brittle flaking on bearings used for automotive electrical instruments and auxiliary devices". Wear. 199 (2): 245–252. doi:10.1016/0043-1648(96)06990-6. ISSN 0043-1648.
  3. ^ a b c d "White Etching Cracks, what is it? | NTN SNR". www.ntn-snr.com. Retrieved 2023-04-05.
  4. ^ a b c "SKF". www.skf.com. Retrieved 2023-04-05.
  5. ^ "Prevention of white etching cracks in rolling bearings | Engineer Live". www.engineerlive.com. 25 August 2015. Retrieved 2023-04-05.
  6. ^ a b c Evans, M.-H. (2016-07-23). "An updated review: white etching cracks (WECs) and axial cracks in wind turbine gearbox bearings". Materials Science and Technology. 32 (11): 1133–1169. Bibcode:2016MatST..32.1133E. doi:10.1080/02670836.2015.1133022. ISSN 0267-0836. S2CID 113216631.
  7. ^ a b Curd, M. E.; Burnett, T. L.; Fellowes, J.; Donoghue, J.; Yan, P.; Withers, P. J. (2019-08-01). "The heterogenous distribution of white etching matter (WEM) around subsurface cracks in bearing steels". Acta Materialia. 174: 300–309. Bibcode:2019AcMat.174..300C. doi:10.1016/j.actamat.2019.05.052. ISSN 1359-6454. S2CID 189976539.  This article incorporates text from this source, which is available under the CC BY 4.0 license.
  8. ^ a b c Manieri, Francesco; Stadler, Kenred; Morales-Espejel, Guillermo E.; Kadiric, Amir (2019-03-01). "The origins of white etching cracks and their significance to rolling bearing failures". International Journal of Fatigue. 120: 107–133. doi:10.1016/j.ijfatigue.2018.10.023. ISSN 0142-1123. S2CID 139339152.
  9. ^ Gould, Benjamin; Greco, Aaron; Stadler, Kenred; Vegter, Erik; Xiao, Xianghui (2017-12-01). "Using advanced tomography techniques to investigate the development of White Etching Cracks in a prematurely failed field bearing". Tribology International. 116: 362–370. doi:10.1016/j.triboint.2017.07.028. ISSN 0301-679X.
  10. ^ a b Steinweg, F.; Mikitisin, A.; Oezel, M.; Schwedt, A.; Janitzky, T.; Hallstedt, B.; Broeckmann, C.; Mayer, J. (2022-09-15). "Formation of White Etching Cracks under electrical current flow - Influence of load, slip and polarity". Wear. 504–505: 204394. doi:10.1016/j.wear.2022.204394. ISSN 0043-1648. S2CID 249186101.
  11. ^ a b López-Uruñuela, Fernando José; Fernández-Díaz, Beatriz; Pagano, Francesco; López-Ortega, Ainara; Pinedo, Bihotz; Bayón, Raquel; Aguirrebeitia, Josu (2021-04-01). "Broad review of "White Etching Crack" failure in wind turbine gearbox bearings: Main factors and experimental investigations". International Journal of Fatigue. 145: 106091. doi:10.1016/j.ijfatigue.2020.106091. ISSN 0142-1123. S2CID 230563434.
  12. ^ a b Evans, M-H; Wang, L; Wood, Rjk (October 2014). "Formation mechanisms of white etching cracks and white etching area under rolling contact fatigue". Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology. 228 (10): 1047–1062. doi:10.1177/1350650114525363. ISSN 1350-6501. S2CID 137598797.
  13. ^ a b KOHARA, MIKA; KAWAMURA, TAKAYUKI; EGAMI, MASAKI (2006-04-01). "Study on Mechanism of Hydrogen Generation from Lubricants". Tribology Transactions. 49 (1): 53–60. doi:10.1080/05698190500486324. ISSN 1040-2004. S2CID 136944647.
  14. ^ Evans, M. -H.; Wang, L.; Jones, H.; Wood, R. J. K. (2013-09-01). "White etching crack (WEC) investigation by serial sectioning, focused ion beam and 3-D crack modelling". Tribology International. 39th LEEDS-LYON SYMPOSIUM ON TRIBOLOGY Special Issue: Great Challenges in Tribology. 65: 146–160. doi:10.1016/j.triboint.2013.03.022. ISSN 0301-679X.
  15. ^ a b Richardson, A. D.; Evans, M.-H.; Wang, L.; Wood, R. J. K.; Ingram, M.; Meuth, B. (2017-11-27). "The Evolution of White Etching Cracks (WECs) in Rolling Contact Fatigue-Tested 100Cr6 Steel". Tribology Letters. 66 (1): 6. doi:10.1007/s11249-017-0946-1. ISSN 1573-2711. PMC 6951819. PMID 31983861.
  16. ^ a b c Gould, Benjamin; Greco, Aaron (2015-10-17). "The Influence of Sliding and Contact Severity on the Generation of White Etching Cracks". Tribology Letters. 60 (2): 29. doi:10.1007/s11249-015-0602-6. ISSN 1573-2711. S2CID 138178455.
  17. ^ Tung, P.-Y.; McEniry, E.; Herbig, M. (2021-01-02). "The role of electric current in the formation of white-etching-cracks". Philosophical Magazine. 101 (1): 59–76. Bibcode:2021PMag..101...59T. doi:10.1080/14786435.2020.1819578. ISSN 1478-6435.
  18. ^ Esmaeili, Kamran; Wang, Ling; Harvey, Terry J.; White, Neil M.; Holweger, Walter (April 2023). "A Study on the Influence of Electrical Discharges on the Formation of White Etching Cracks in Oil-Lubricated Rolling Contacts and Their Detection Using Electrostatic Sensing Technique". Lubricants. 11 (4): 164. doi:10.3390/lubricants11040164. ISSN 2075-4442.
  19. ^ a b Plazenet, Thibaud; Boileau, Thierry (August 2021). "Overview of Bearing White Etching Cracks due to Electrical Currents". 2021 IEEE 13th International Symposium on Diagnostics for Electrical Machines, Power Electronics and Drives (SDEMPED). Vol. 1. pp. 440–446. doi:10.1109/SDEMPED51010.2021.9605561. ISBN 978-1-7281-9297-0. S2CID 244507729.
  20. ^ "SKF". www.skf.com. Retrieved 2023-04-05.
  21. ^ Nikolic, Ksenija; Ferreira, Vitoria Mattos; Malet, Loïc; Depover, Tom; Verbeken, Kim; Petrov, Roumen H. (2023-03-01). "Uncovering the white etching area and crack formation mechanism in bearing steel". Materials Characterization. 197: 112659. doi:10.1016/j.matchar.2023.112659. hdl:1854/LU-01GQJS6FS62GRASRW2QSCVR5MY. ISSN 1044-5803.
  22. ^ Steinweg, F.; Mikitisin, A.; Oezel, M.; Schwedt, A.; Janitzky, T.; Hallstedt, B.; Broeckmann, C.; Mayer, J. (2022-09-15). "Formation of White Etching Cracks under electrical current flow - Influence of load, slip and polarity". Wear. 504–505: 204394. doi:10.1016/j.wear.2022.204394. ISSN 0043-1648. S2CID 249186101.
  23. ^ "White etching cracks – a consequence, not a root cause of bearing failure | Evolution". Evolution Online. 2018-01-23. Retrieved 2023-04-05.
  24. ^ Stadler, Kenred; Lai, Junbiao; Vegter, Reinder (2014). "A Review: The Dilemma With Premature White Etching Crack (WEC) Bearing Failures". Bearing Steel Technologies: 10th Volume, Advances in Steel Technologies for Rolling Bearings. ASTM International: 1–22. doi:10.1520/STP158020140046. ISBN 978-0-8031-7605-8.
  25. ^ "White etching cracks – a consequence, not a root cause of bearing failure | Evolution". Evolution Online. 2018-01-23. Retrieved 2023-04-05.
  26. ^ a b c Su, Yun-Shuai; Li, Shu-Xin; Lu, Si-Yuan; Wan, Li-Biao (2018). "Phase transformation in white etching area in rolling contact fatigue". MATEC Web of Conferences. 165: 11004. doi:10.1051/matecconf/201816511004. ISSN 2261-236X.
  27. ^ Danielsen, H. K.; Hong, C.; Mishin, O. V. (2021-07-01). "Microstructural characterization of white etching cracks in bearings after long-term operation in wind turbines". Materials Letters. 294: 129754. doi:10.1016/j.matlet.2021.129754. ISSN 0167-577X. S2CID 233651419.
  28. ^ Danielsen, H. K.; Carrasco, A. J.; Fæster, S.; Dahl, K. V.; Guzmán, F. Gutiérrez; Sauvage, P.; Jacobs, G. (2019-04-01). "3D X-ray computerized tomography of White Etching Cracks (WEC)". Materials Characterization. 150: 78–87. doi:10.1016/j.matchar.2019.01.032. ISSN 1044-5803.
  29. ^ a b Wan, Li-Biao; Li, Shu-Xin; Lu, Si-Yuan; Su, Yun-Shuai; Shu, Xue-Dao; Huang, Hai-Bo (2018-02-15). "Case study: Formation of white etching layers in a failed rolling element bearing race". Wear. 396–397: 126–134. doi:10.1016/j.wear.2017.07.014. ISSN 0043-1648.
  30. ^ Morsdorf, L.; Mayweg, D.; Li, Y.; Diederichs, A.; Raabe, D.; Herbig, M. (2020-01-13). "Moving cracks form white etching areas during rolling contact fatigue in bearings". Materials Science and Engineering: A. 771: 138659. doi:10.1016/j.msea.2019.138659. ISSN 0921-5093. S2CID 210233175.
  31. ^ Curd, M. E.; Burnett, T. L.; Fellowes, J.; Donoghue, J.; Yan, P.; Withers, P. J. (2019-08-01). "The heterogenous distribution of white etching matter (WEM) around subsurface cracks in bearing steels". Acta Materialia. 174: 300–309. Bibcode:2019AcMat.174..300C. doi:10.1016/j.actamat.2019.05.052. ISSN 1359-6454. S2CID 189976539.  This article incorporates text from this source, which is available under the CC BY 4.0 license.
  32. ^ Ruellan, Arnaud; Cavoret, Jérôme; Ville, Fabrice; Kleber, Xavier; Liatard, Bernard (February 2017). "Understanding white etching cracks in rolling element bearings: State of art and multiple driver transposition on a twin-disc machine". Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology. 231 (2): 203–220. doi:10.1177/1350650116648058. ISSN 1350-6501. S2CID 113573608.
  33. ^ Kunzelmann, Björn; Rycerz, Pawel; Xu, Yilun; Arakere, Nagaraj K.; Kadiric, Amir (2023-03-01). "Prediction of rolling contact fatigue crack propagation in bearing steels using experimental crack growth data and linear elastic fracture mechanics". International Journal of Fatigue. 168: 107449. doi:10.1016/j.ijfatigue.2022.107449. ISSN 0142-1123.
  34. ^ "How large offshore wind turbines are challenging bearing designs". Windpower Engineering & Development. Retrieved 2023-04-05.
  35. ^ "Bearing the load". International Cement Review. 2019-02-20. Retrieved 2023-04-05.
  36. ^ Knight, Sara. "Solving the technical troubles in situ". www.windpowermonthly.com. Retrieved 2023-04-05.
  37. ^ "Innovative Lösungen gegen "White Etching Cracks"". www.pressebox.de (in German). 2015-02-12. Retrieved 2023-04-05.
  38. ^ "Choosing the right lubricant for today's wind turbines". Windpower Engineering & Development. Retrieved 2023-04-05.
  39. ^ "ExxonMobil shares insights from its used oil analysis service". IEN.EU - Industrial Engineering News Europe. 2020-01-09. Retrieved 2023-04-05.
  40. ^ "So entstehen Schäden an Windkraftanlagen" [This is how damage occurs to wind turbines]. Forschung für Nachhaltigkeit | FONA (in German). 2018-11-05. Retrieved 2023-04-05.
Retrieved from "https://en.wikipedia.org/w/index.php?title=White_etching_cracks&oldid=1252514655"