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White Etching Formation Due to Surface Stress

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One of the major causes of premature failures of wind turbine gearing systems is the formation of axial cracks in wind turbine bearings. Increased surface stress results in increased subsurface stress, which in turn leads to a phase change in steel known as white etching areas (WEA). Cracks form in the steel within or at the edge of the WEA and these cracks lead to fatigue damage.

Afton Chemical has conducted research on the causes of white etching formation and the role of lubricants in controlling WEA. Afton observed that lubricants that can control surface stress by forming the proper tribofilm on a surface can control fatigue.

Afton Technical Advisor Mark Devlin presented on this topic during the 2015 AWEA O&M and Safety Seminar.

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White Etching Formation Due to Surface Stress

  1. 1. AftonChemical.com White Etching Formation Due to Surface Stress 2015 AWEA O&M and Safety Seminar Drivetrain Improvements & Failure Prevention Session Presented by Mark Devlin Technical Advisor Afton Chemical Corporation
  2. 2. AftonChemical.com Overview of Presentation Discussion on Detection of White Etching Cracks (WEC)  Can We See WEC in a…..  Bearing that Failed in Field  Bearing Tested at High Stress Conditions in FE8 Rig Discussion of Role of Lubricant in Preventing WEC  What Do We Already Know About Fatigue Mechanisms • Effect of friction and tribofilm formation  Friction and Tribofilm Properties of Commercial Lubricants • There are different lubricants for different applications • They may not all be suitable to prevent WEC Summary 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
  3. 3. AftonChemical.com Introduction to White Etching Cracks (WEC) Can Occur in High Speed, Intermediate and Planetary Bearings  Where torque converted to electrical current Etch Subsurface and White Cracks Appear Leads to Axial Cracks on Surface and Premature Failure 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
  4. 4. AftonChemical.com Many Possible Reasons for White Etching Cracks Issue is Complex and Still being Debated  Operational Factors: • Fluctuating Torque; Stop/Start Operation; Torque Reversals; Misalignments; Wind Gust Loading  Environmental Drivers • Water Contamination; Corrosion; Slip; Vibration; Bending Stress; Impact Loading  Effects on System • H diffusion into steel; Crack creation due to Corrosion; Traction and Impact shear stress All of the Factors / Drivers / Effects are Interconnected Two Major Mechanisms  Stress induced formation of WEC  Hydrogen induced formation of WEC 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
  5. 5. AftonChemical.com Failed Inner Race from High Speed Bearing in Wind Turbine Gear Box Can We Observe White Etching Crack below Axial Crack? 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
  6. 6. AftonChemical.com Wind Turbine Bearing Race Failed in Field Where are we looking for White Etching Cracks 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
  7. 7. AftonChemical.com Sample Preparation to Detect White Etching Cracks 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
  8. 8. AftonChemical.com White Etching Cracks Below Macro-pit Etched Steel has Areas that Appear White White Areas have Undergone Phase Transition and are Near Cracks 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation. 2.5 mm 10 mm 50 mm Over-Rolling Direction
  9. 9. AftonChemical.com White Etching Cracks Which are About to Form a Macro-pit Phase Transition in Steel Observed which is White When Etched 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
  10. 10. AftonChemical.com FE8 Bearing Fatigue Rig to Create WEC Standard FE8 Bearing Wear Test Conditions  100kN; 80 ⁰C; 7.5 RPM; 80 hrs (DIN 51819-3) Run FE8 Bearing Rig at Higher Stress  60 kN; 100⁰ C; 750 RPM; Until Failure  Rougher bearing race surfaces 0.5-0.6 mm Ra  15% sliding (roller bearing without thrust) Fluid Tested Forms Pits Repeatable Before 20 hrs 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
  11. 11. AftonChemical.com FE8 Roller with Macro-pits Cross-cut Surface where Surface Damage Exists on Roller 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation. Cross-Cut Plane Look for WEC Between Macro-pits
  12. 12. AftonChemical.com White Etching Crack in the FE8 Roller 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
  13. 13. AftonChemical.com Generalized Stress Induced Fatigue Mechanism Increase in Surface Stress Causes Increase in Subsurface Stress Increase in Subsurface Stress Causes Phase Transition in Steel  Hardness change observed in subsurface Cracks Form at Interface Between Phases 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation. Rolling Fatigue 5.1 GPa 120C 4000 RPM 8 Lubricants with Different Friction Characteristics Run 15 times to Failure “Fatigue Performance of Low Viscosity Energy Efficient Lubricants”, 9th International Symposium on Fuels and Lubricants, April 15-17, 2014, New Delhi, India “Effects of Gear Oil Properties on Pitting Life in the Rolling Four-ball Test Configuration” Tribology Transactions 57 (1), p 104-113, 2014 “Lubricant Additives for Improved Pitting Performance Through a Reduction of Thin-film Friction” Tribology International 80, p. 122-130, 2014 “Investigations into the Occurrence of Pitting in Lubricated Rolling Four-ball Tests” Lubrication Science Online 12/2013 “Pit Formation Mechanisms in Lubricated Rolling Four-Ball Tests”, Proceedings of the 15th Nordic Symposium on Tribology, Trondheim , June 2012.
  14. 14. AftonChemical.com Generalized Stress Induced Fatigue Mechanism Increase in Surface Stress Causes Increase in Subsurface Stress Increase in Subsurface Stress Causes Phase Transition in Steel  Hardness change observed in subsurface Cracks Form at Interface Between Phases 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation. Subsequent Tests Stopped Prior to Failure Observed Subsurface Phase Change with Subsequent Change in Material Hardness “Fatigue Performance of Low Viscosity Energy Efficient Lubricants”, 9th International Symposium on Fuels and Lubricants, April 15-17, 2014, New Delhi, India “Effects of Gear Oil Properties on Pitting Life in the Rolling Four-ball Test Configuration” Tribology Transactions 57 (1), p 104-113, 2014 “Lubricant Additives for Improved Pitting Performance Through a Reduction of Thin-film Friction” Tribology International 80, p. 122-130, 2014 “Investigations into the Occurrence of Pitting in Lubricated Rolling Four-ball Tests” Lubrication Science Online 12/2013 “Pit Formation Mechanisms in Lubricated Rolling Four-Ball Tests”, Proceedings of the 15th Nordic Symposium on Tribology, Trondheim , June 2012.
  15. 15. AftonChemical.com Role of Lubricant in Reducing Stress-Induced Fatigue Lower Friction to Lower Surface Stress  Maintain low friction under extreme conditions Lubricants that Form Tribofilms which Contain High Amounts of P and Low Amounts of S Protect Surfaces 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation. Additional Tests Performed Effect of Tribofilm on Fatigue Life Increase P and Reduce S in Tribofilm and Fatigue Life Improves 0 50000 100000 150000 200000 250000 GO-E GO-I GO-J L50Life Roller From Test With Fluid GO-J Secondary Electron Image Phase Map/Image Overlay 0 10 20 30 Atomic% C O P S • Fe is also detected • Normalize P and S to amount of Fe, since Tribofilm thickness inversely related to Fe Tribofilms Formed on Surface Before Failure were Characterized Fatigue Life Improved when Films are High in P and Low in S Normalized Amount of P and S in Tribofilms 0.0 1.0 2.0 3.0 4.0 5.0 6.0 GO-E GO-I GO-J Atomic% P S With Changes in Anti-wear and Extreme Pressure Additives Can Raise Amount of P and Lower Amount of S in Tribofilm “Fatigue Performance of Low Viscosity Energy Efficient Lubricants”, 9th International Symposium on Fuels and Lubricants, April 15-17, 2014, New Delhi, India “Effects of Gear Oil Properties on Pitting Life in the Rolling Four-ball Test Configuration” Tribology Transactions 57 (1), p 104-113, 2014 “Lubricant Additives for Improved Pitting Performance Through a Reduction of Thin-film Friction” Tribology International 80, p. 122-130, 2014 “Investigations into the Occurrence of Pitting in Lubricated Rolling Four-ball Tests” Lubrication Science Online 12/2013 “Pit Formation Mechanisms in Lubricated Rolling Four-Ball Tests”, Proceedings of the 15th Nordic Symposium on Tribology, Trondheim , June 2012.
  16. 16. AftonChemical.com Oils Examined Different Lubricants for Different Applications  Lubricants are made for different applications  Some do not control WEC formation but are not designed to do so Oils Tested  Automotive Gear Oil (Oil A) • Oil that causes WEC in FE8 test  Industrial Gear Oils (Oils B and C)  Wind Turbine Gear Oils (Oils D, E and F) 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
  17. 17. AftonChemical.com Friction Measurement and Tribofilm Formation 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
  18. 18. AftonChemical.com Low Friction Results in Decrease in Surface Stress Fluid Should Keep Friction Low as Sliding Increases 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
  19. 19. AftonChemical.com Low Friction Results in Decrease in Surface Stress Fluid Should Keep Friction Low as Load Increases 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
  20. 20. AftonChemical.com Tribofilm Composition Lubricant Should Form Tribofilm High in P and Low in S Past Research has Shown that P and S content of Lubricant IS NOT Related to the Amount of P and S in the Tribofilm  Tribofilm formation is complex process involving chemical transformation of additives and pressure induced film formation 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation. 0.0 1.0 2.0 3.0 4.0 5.0 6.0 Oil A Oil B Oil C Oil D Oil E Oil F PercentPorSinTribofilm P S
  21. 21. AftonChemical.com Summary of Lubricant Effects on WEC Formation Issue is Complex and Still being Debated  Operational Factors; Environmental Drivers; Effects on System All of the Factors / Drivers / Effects are Interconnected Two Major Mechanisms  Stress induced formation of WEC  Hydrogen induced formation of WEC Lubricant can be Good at Several Things to Control WEC Formation  Being “great” at one thing may not prevent WEC  Friction Reduction : E, F better than A, D, C better than B  Tribofilm Structure : C, D, E, F similar and better than B better than A  Control of H induced mechanism – Still needs to be assessed but formation of Tribofilm can control H ingress into steel 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
  22. 22. AftonChemical.com Summary White Etching Cracks (WEC) are Detected in Bearings from Field That Have Axial Cracks  WEC are beneath axial cracks and in places where axial cracks have not formed White Etching Cracks (WEC) are Detected in Bearings from FE8 Rig Operated Under High Stress Conditions  Tests performed with lubricant with moderate friction and tribofilm which contains very low amount of P Reduction in Stress-Induced Fatigue by Lubricant  Lower friction under extreme conditions  Form tribofilms that contain high amounts of P and low amounts of S Lubricants Designed for Wind Turbines  Do have low friction under extreme conditions  Do form the proper tribofilms 2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.

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