Investigating Microstructural Alterations Leading to White Etch Cracks (WEC's) in Wind Turbine Gearbox Bearings
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This webinar outlines Sentient's development on their White Etching Crack capabilities to current materials-based prognostics life prediction models to better predict wind turbine gearbox life. The formation of white etching cracks (WEC's) in bearings are known to be one of the major failure modes associated with gearbox failure. WEC's can casue premature failures in rolling element bearings occuring as early as 1-20% of the calculated L10 life. According to the National Renewable Energy Laboratory's (NREL) gearbox failure database, 64% of gearboxes failed due to bearings and 25% due to gear failures. These failures are causing increased downtime and costly repairs. The webinar outlines microstructrual alterations detected in LSIS and HSIS bearings and how microstructural alterations impact damage in gearboxes.
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Investigating Microstructural Alterations Leading to White Etch Cracks (WEC's) in Wind Turbine Gearbox Bearings
- 1. © 2016 Sentient Science Corporation – Confidential & Proprietary
- 2. © 2016 Sentient Science Corporation – Confidential & Proprietary Host Natalie Hils Director, of Revenue Marketing nhils@sentientscience.com +1 716.807.8655
- 3. © 2016 Sentient Science Corporation – Confidential & Proprietary Webinar Instructions
- 4. © 2016 Sentient Science Corporation – Confidential & Proprietary Material Science Differentiation A Trusted Third Party Enabling Life Extension of Rotating Equipment in two ways: 1. Asset Management for Operators 2. Computational Testing & Reconfiguration for Suppliers ☑ Life Extension☑ Root Causes☑ Predict Earlier Key Industries Served WIND ENERGY • 20,000 assets under contract globally AEROSPACE • Rotorcraft, Fixed Wing, & Defense • (15 Years of R&D) RAIL • 1 Million km Global Railways • Wheel Rail Interface
- 5. SentientScienceCorp.-Proprietary/PrivateLevel1 Why Investigate White Etching Cracks? Uncertainty • Conflicting synopsis for why WEC occurs • WEC is only visible once failure is detected • Unknown whether White Etch areas are a cause or effect of cracks Risk • Currently using sensor vibration data to detect WEC • 64% of gearbox failure is due to bearings • WEC’s are known to cause premature failures in rolling element bearings occurring less than 50% of the design life Cost • HSIS bearing could cost up to $20,000 to replace • Damage to LSS & planetary systems can cost >$100,000 each • Full gearbox replacement ranges from $200,000 - $500,000
- 6. © 2016 Sentient Science Corporation – Confidential & Proprietary Presenter Dr. Harpal Singh Materials Research Scientist hsingh@sentientscience.com
- 7. © 2016 Sentient Science Corporation – Confidential & Proprietary • White Etched Areas (WEA) – Appearance of white microstructure under optical microscope after etching • White Etch Cracking (WEC) – Cracks within the microstructure of bearing steel, decorated by white areas • Irregular White Etch Areas (IrWEA) – White etches areas and cracks with irregular shape and random orientations • White Structure Flaking (WSF), Axial Cracking, Radial Cracking – Alternate names for non-classical bearing failures due to appearance of white microstructure in sub-surface • Butterfly(ies) – Small cracks with altered microstructure wings around non-metallic inclusions, voids, microcracks Microstructural Alterations in Bearings Terminology Axial Cracks Inner raceway of HSS bearing with Axial cracks
- 8. © 2016 Sentient Science Corporation – Confidential & Proprietary Classical Fatigue vs Non-classical Fatigue Voskamp, Arend Pieter. "Microstructural changes during rolling contract fatigue: " (1997). Non-classical Fatigue WECs Classical Fatigue< ~106 to 10^7 cycles Dark etch areas White etch bands
- 9. © 2016 Sentient Science Corporation – Confidential & Proprietary Wind Turbine Failures • S. Sheng, Gearbox Reliability Database: Yesterday, Today, and Tomorrow 2014, Wind Turbine Tribology Seminar 2014 • Wind Turbine Gearbox Reliability Database, condition Monitoring, and O&M Research Update, 2015 , GRC Annual Meeting HSS &HSIS - Premature Failures
- 10. © 2016 Sentient Science Corporation – Confidential & Proprietary Poll Question
- 11. © 2016 Sentient Science Corporation – Confidential & Proprietary • Axial cracks on raceways <1mm to >20 mm in length • Axial cracks with pitting or spalling • Spalling or pitting Appearance and Morphology of Premature Bearing Failures A Radial crack ASub-surface
- 12. © 2016 Sentient Science Corporation – Confidential & Proprietary Drivers for WEC Formation WEC Electric (Stray Currents) Tribo- mechanical Material (inclusions) Tribo- chemical • Considerable research has been done - long list of possible drivers and influencing factors • Tribological drivers for WEC formation are contested. Several theories are published associated with WEC formation Loading Sliding Frictional stresses Lubricant Water - Ingress Corrosion Reactions
- 13. © 2016 Sentient Science Corporation – Confidential & Proprietary Bearing Location PSB Planet shaft bearings LSIS Low Speed Intermediate speed shaft HSIS High speed Intermediate shaft HSS High speed shaft Bearing Location and Materials • Bearings investigated are sourced from 1.5 MW capacity on-shore wind turbines • Bearings from different gearbox locations are sectioned and analyzed PSB LSIS HSISHSS Bearing type Bearing Microstructure Chemical composition Contact Type Cylindrical Through hardened - Martensitic AISI 52100 Line
- 14. © 2016 Sentient Science Corporation – Confidential & Proprietary Sample Preparation LSIS HSIS Axial Direction Over- rolling direction Axial Direction • Electronic Discharge Machine (EDM) sectioning and sample preparation for surface and subsurface metallographic analysis • Metallographic preparation includes Mounting, Grinding, Polishing Etching, Optical microscopy, SEM
- 15. © 2016 Sentient Science Corporation – Confidential & Proprietary Power Flow Diagram of a 1.5 MW turbine gearbox Planet bearings LSIS bearing Planet Shaft Bearing (PS BRG) and Low-Speed Intermediate Shaft(LSIS) Bearing Characterization
- 16. © 2016 Sentient Science Corporation – Confidential & Proprietary Characterization and Analysis (PS BRG and LSIS BRG) • Alterations initiated around non-metallic inclusions • Butterfly wings oriented in the random directions to the contact surface • Wing orientations are more in agreement with -10° and - 20° • Butterflies in oxide inclusions are shorter and wider than butterflies around MnS inclusions Microstructural alteration observed in axially cross-sectioned Planet and LSIS bearings Butterfly Wing Formations
- 17. © 2016 Sentient Science Corporation – Confidential & Proprietary • Inclusion fracture • Crack propagation into bulk matrix • Butterfly wing formation Characterization and Analysis (PS BRG and LSIS BRG) Butterfly Wing Formation -Three stage process Crack Initiation Crack Extension Butterfly Wing Formation Crack Crack Butterfly
- 18. © 2016 Sentient Science Corporation – Confidential & Proprietary Characterization and Analysis (PS BRG and LSIS BRG) Crack Initiation - MnS inclusions as crack initiation sites • Majority of MnS inclusions within the Hertzian shear zone are fractured • Inclusion crack along length • Cracked inclusion is not necessary for butterfly • Free surface may also act as crack initiators MnS MnS MnS MnS Fractured MnS inclusions along the major axis observed in bearing sub-surface
- 19. © 2016 Sentient Science Corporation – Confidential & Proprietary Characterization and Analysis (PS BRG and LSIS BRG) Crack Initiation and Propagation - Dual phase inclusions (MnS + Al2O3) • Crack initiated in the free areas between matrix and oxides • In Some cases, MnS inclusion cracks • Higher likelihood of cracks around oxide inclusion compared to MnS • Dual phase are more detrimental than MnS Dual phase Dual phase Dual phase Dual phase MnS + TiN MnS Crack propagation into bulk material
- 20. © 2016 Sentient Science Corporation – Confidential & Proprietary • Most dual phase inclusions consist of Al2O3 and MnS • Few inclusions with fracture in major axis Characterization and Analysis (PS BRG and LSIS BRG) SEM and EDX analysis on dual phase inclusions
- 21. © 2016 Sentient Science Corporation – Confidential & Proprietary Characterization and Analysis (PS BRG and LSIS BRG) Crack propagation around MnS and Dual phase inclusions • Crack propagation into bulk material , whether fractured or unfractured • Crack propagation over time in both directions • Crack goes overtime to connect to another inhomogeneities in the steel matrix • Crack networks grow and reach surface to create spall Cracks Crack along major axis Through cracks Un-cracked inclusion Un-cracked inclusion Cracks cracks
- 22. © 2016 Sentient Science Corporation – Confidential & Proprietary • Butterfly around dual phase inclusions • MnS and MnS+TiN between two dual phase butterfly inclusions are un- cracked Characterization and Analysis (PS BRG and LSIS BRG) Butterfly around non-metallic inclusions
- 23. © 2016 Sentient Science Corporation – Confidential & Proprietary • Stress calculations are done based on IEC 61400-4 standard • Calculated depth of butterflies remain within the max unidirectional shear stress Characterization and Analysis (PS BRG and LSIS BRG) Depth of butterflies Bearing configuration Contact type Pmax, GPa Depth, τmax, µ𝒎 Depth, τomax, µ𝒎 Depth, σv , µ𝒎 Cylindrical Line 1.65 325 208 291 Trend in butterfly wing formation at sub-surface in LSIS bearing
- 24. © 2016 Sentient Science Corporation – Confidential & Proprietary Butterfly Mechanism Observed From Field Failures Rolling Element Stress Raiser (Inclusions) High sub- surface stresses + Rolling Element Rolling Element Cracking around stress raiser Continuous Loading cycles + PS BRG and LSIS BRG Butterflies
- 25. © 2016 Sentient Science Corporation – Confidential & Proprietary Poll Question
- 26. © 2016 Sentient Science Corporation – Confidential & Proprietary Power Flow Diagram of a 1.5 MW turbine gearbox Intermediate (HSIS) and High-Speed Shaft (HSS) bearings
- 27. © 2016 Sentient Science Corporation – Confidential & Proprietary Intermediate (HSIS) and High-Speed Shaft (HSS) bearings Nano-hardness of WEA • A total of 8 indents are made within the altered microstructure and the matrix • Average hardness measured within the altered microstructure and surrounding matrix is 13.4 GPa and 10.6 GPa, respectively Location Nano-hardness, GPa WEC 13.4 ± 0.8 Matrix 10.6 ± 0.4 SPM image highlighting indentation spots and corresponding hardness values Variation in the hardness within the altered microstructure is most likely from the variable grain size, defects and carbon composition in the localized areas
- 28. © 2016 Sentient Science Corporation – Confidential & Proprietary • Characteristic microstructural features within Irregular white etched areas. • Zones with different grain sizes are visible • Regions with areas of carbide dissolution indicates the beginning of WEA Intermediate (HSIS) and High-Speed Shaft (HSS) bearings Microstructural features within white etched area (WEA) WECs and multiple phases in the sub-surface microstructure of the HSIS bearing
- 29. © 2016 Sentient Science Corporation – Confidential & Proprietary Intermediate (HSIS) and High-Speed Shaft (HSS) bearings Dynamic Events • Sliding • loading Lubricant Chemistry • Additives • Contamination Electrical Currents • Lighting/Stray currents • Tribo-generated WECs Rapid torque reversals
- 30. © 2016 Sentient Science Corporation – Confidential & Proprietary Axial Cracking Mechanism Observed From Field Failures Rolling Element Tribo- mechanical (loading, slippage…) Tribo- chemistry (Additives, water….) + Rolling Element HH H H H H HHHH H H Rolling Element Hydrogen Diffusion Sub- surface stresses + Intermediate (HSIS) and High-Speed Shaft (HSS) bearings
- 31. © 2016 Sentient Science Corporation – Confidential & Proprietary Intermediate (HSIS) and High-Speed Shaft (HSS) bearings Sub-surface crack length • Circumferential and axial cross-sections are cut and examined to observe crack propagation • Zoom microscopy images show crack networks in circumferential and axial cross-section • About 1-5 mm deep cracks with random orientation are observed throughout the bearing sub-surface in circumferential cross-section • In axial cross-section, a horizontal crack at depth of about 7000 µ𝑚
- 32. © 2016 Sentient Science Corporation – Confidential & Proprietary Intermediate (HSIS) and High-Speed Shaft (HSS) bearings Orientations of WEA • Circumferential cross-section decorated with Ir-WECs near surface and sub-surface • Nonmetallic inclusions are seen connected through crack networks, some inclusions are without microstructural alteration • Continuous long branching of Ir-WECs extending in different directions of various length • Inclusions are encapsulated within white etched areas up to a depth of 400 µm Presence of surface damage and white etched cracks near the contact surface
- 33. © 2016 Sentient Science Corporation – Confidential & Proprietary Intermediate (HSIS) and High-Speed Shaft (HSS) bearings WEC connected non-metallic inclusions • Non-metallic inclusions connected through crack networks • One inclusion appears to be isolated from crack networks and other small inclusions are encapsulated within white etches areas (WEA) • No cracks are identified propagating within these inclusions
- 34. © 2016 Sentient Science Corporation – Confidential & Proprietary Intermediate (HSIS) and High-Speed Shaft (HSS) bearings WECs oriented parallel to the contact surface Spall WECs WECs
- 35. © 2016 Sentient Science Corporation – Confidential & Proprietary Intermediate (HSIS) and High-Speed Shaft (HSS) bearings Another observation WEC initiation and propagation Crack/WEA initiation Crack/WEA propagation Surface spall/ crack ~190 µm ~200 µm
- 36. © 2016 Sentient Science Corporation – Confidential & Proprietary Poll Question
- 37. © 2016 Sentient Science Corporation – Confidential & Proprietary • Surface treatment and coatings • Bearings with improved alloy compositions • Lubricants with low moisture adsorption • Additive degradation control in lubricants Life Extension - Preventive Measures Against White Etch Cracks Intermediate (HSIS) and High-Speed Shaft (HSS) bearings
- 38. © 2016 Sentient Science Corporation – Confidential & Proprietary Final Remarks Planet and LSIS Bearings • Butterflies form either at a location of max. shear stresses • Majority of MnS inclusions are cracked • WEC sub-surface around non- metallic inclusions • Smooth morphology of WEA • Bottom up approach • Load due to transient, deflections, etc. and number of cycles HSS and HSIS Bearings • Multiple crack networks oriented in random directions • Un cracked inclusions • WEC on surface as well sub- surface with or without cracks • Rough morphology with multiple cracks • Both top down and bottom up approach • Tribomechanical(Speed , slippage and loading condition), Tribochemical and operating cycles
- 39. SentientScienceCorp.-Proprietary/PrivateLevel1 Why Investigate White Etching Cracks? Uncertainty • Conflicting synopsis for why WEC occurs • WEC is only visible once failure is detected • Unknown whether White Etch areas are a cause or effect of cracks Risk • Currently using sensor vibration data to detect WEC • 64% of gearbox failure is due to bearings • WEC’s are known to cause premature failures in rolling element bearings occurring less than 50% of the design life Cost • HSIS bearing could cost up to $20,000 to replace • Damage to LSS & planetary systems can cost >$100,000 each • Full gearbox replacement ranges from $200,000 - $500,000
- 40. © 2016 Sentient Science Corporation – Confidential & Proprietary Meet Our Team! 1. September 12-15: Husum Wind 2. September 17-20: Railway Interchange 3. September 19-20: II International Conference on Life Extension of Wind Farms 4. September 26-28: NAWEA Conference 5. October 3-5: CanWEA Annual Conference & Expo 6. October 17-19: China WINDPOWER 7. October 25-26: AWEA Wind Energy Finance & Investment Seminar 8. November 7-9: AWEA Wind Fall Energy Symposium 2017
- 41. © 2016 Sentient Science Corporation – Confidential & Proprietary Questions? Natalie Hils Director, Revenue Marketing nhils@sentientscience.com +1 716.807.8655 Dr. Harpal Singh Materials Research Scientist hsingh@sentientscience.com
- 42. © 2016 Sentient Science Corporation – Confidential & Proprietary Thank You!
Notas do Editor
- Recording - link
- 1- We computational model rotating components to understand the life through a material science based approach 2- this allows us to understand prior to failure where a crack is occurring and provide our customers with asset actions to extend the life of their machines 3- Different industries 4-Supplier vs. operator side
- Operators are asking us if we have this capability in our model WEC are causing bearing failures that are increasing Failure risk and O&M spend No clear answer on why WEC occurs Risk: Data from an NREL report Among bearings, the highest number of failures were observed in HSS due to axial cracking and spalling
- 1- Dr. Harpal Sing will take us through the technical scope of Sentient’s work to predict WEC in their prognostic models 2-Introduce him
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- Risk: Too late in process failures are already occurring Today, using sensors to detect WEC related failures Source NREL We find a lot of operators are experiencing WEC that are leading to bearing/ gear failure and eventually to drivetrain failures Sentient is developing a computational model to understand how WEC occurs and the impact it has on the life of the GBX We will have a secondary webinar to show results once the model is more mature