FINISHING, FLUID COMPATIBILITY AND NDT INSPECTION OF HVOF COATING Héroux Devtek Inc.- Landing Gear Division ENG/R&D - Longueuil, Québec (Canada) Les présentateurs doivent souvent transmettre des informations techniques à des auditeurs qui connaissent moins bien le sujet et le vocabulaire spécifique. Les informations peuvent être multiples ou complexes. Pour présenter des informations techniques efficacement, utilisez les conseils suivants de Dale Carnegie Training®.   Tenez compte du temps dont vous disposez et préparez la classification de vos informations. Délimitez votre sujet et découpez votre présentation en séquences claires. Suivez une progression logique et ne vous dispersez pas. Terminez la présentation par un résumé, un rappel des étapes clés ou une conclusion logique. Montrez l’intérêt que vous portez à votre assistance. Par exemple, utilisez toujours des données claires et pertinentes. Choisissez un niveau de détail et un vocabulaire adaptés à l'assistance. Utilisez des aides visuelles pour étayer vos points et étapes clés. Les auditeurs seront plus réceptifs si vous êtes à l'écoute de leurs besoins. Nihad Ben Salah, M&P

Introduction Progress Producibility: Finishing, fluid compatibility and NDT on WC-10Co-4Cr HVOF coating Pour commencer, faites ressortir l'intérêt du sujet pour l'assistance. Donnez une brève vue d'ensemble de la présentation. Tenez compte de l'intérêt de l'assistance pour le sujet ainsi que de leurs connaissances en la matière pour choisir votre vocabulaire, des exemples et des illustrations. Insistez sur l'importance du sujet pour capter l'attention des auditeurs.

Progress/ Producibility Testing Chemical stripping: 100% achieved (see previous presentations). Final report about to be released Finishing and superfinishing – 80% achieved, Narrowing the optimum grinding parameters window for a better productivity Fluid compatibility: Corrosion tests achieved – Hydrogen embrittlement in progress NDT : MPI, FPI, Barkhausen Noise Inspection on finished parts All tests should be finished by March 2003

Finishing – ”Rough” previous Results Samples: 4inch OD x 12inch L Higher wheel speed improves the finish (5000 to 7000 SFM) “High work speed with low crossfeed improves the finish and decreases the total time of grinding” BUT… Optimum infeed seems to be around 0.0002 in. Higher infeed decreases the finish and damage the coating. Lower grit (bigger abrasive particles) decreases productivity. The required finish is obtained after a longer time of grinding. For the same time, finish is better for higher grit, BUT…

Finishing –New results & Interpretation A good finish does not mean a good coating (FPI results) Most of the used grinding parameters did not damage the steel substrate (100% BNI). Concerns are raised when using higher grit wheel with inappropriate wheel speed and infeed. Higher wheel speed improves the finish but affect the coating integrity. When using higher grit, thickness control is better For a better control of the integrity of the coating (quality and thickness) it is safer to target an average finish after grinding and improve it by superfinishing

Superfinishing -Results Abrasive used : Diamond (paste, stone, ribbon) For these tests, 2µinch Ra and 95-99% Bearing Ratio were typical super-finish characteristics of WC-Co-Cr reached. The stone gives a fuzzy and “less cosmetic” finish The ribbon gives the best cosmetic finish Pressure and work speed shall be higher when using paste and ribbon, and the minimum when using the stone To have a better cosmetic finish, the two last passes should be done without vibration. No need to have very high finish as starting point for superfinishing

Abusive Grinding Grinding:*Small samples of Aermet 100, OD 0.625inch, ID0.440 inch *No cooling fluid Tests: *Metallographic preparation per ASTM E1920 and Buehler Tech-note *Micro-hardness profile *Cross section SEM observations Coating: Spalling, delamination, cracks, affected microstructure Substrate : Heat affected thickness of the substrate varies from 12 to 40 mils. Overheating could be high enough to decrease Aermet 100 hardness to 50 HRC. DO NOT UNDERESTIMATE ABUSIVE GRINDING

Abusive Grinding/SEM Observations Crack Inequal thickness due to abusive grinding

Abusive Grinding/SEM Observations Bad Surface finish Abusive grinding seems to have affected the microstructure of the coating

Abusive Grinding/SEM Observations Crack Delamination Crack preceding spalling in Area (A) Delamination observed in Area (B)

Abusive Grinding/Micro-hardness profile

Abusive Grinding/Micro-hardness profile

NDT- Barkhausen Noise Set-up for Barkhausen Noise Inspection (BNI) Calibration for maximum response of BNI with HVOF WC-Co-Cr coating and different substrates (4340, 300M and Aermet 100) Calibration for burns detection OK

NDT- Barkhausen Noise-Set up (1) Axial sensor MP decreases when HVOF coating or Cr plating thickness increases MP decreases less drastically with HVOF coating thickness than with Chrome

NDT- Barkhausen Noise-Set up (2) Different calibration is needed for different substrates. BN signal is the lowest for 300M Circumferential sensor

NDT- Barkhausen Noise-Set up (3) Burns in the substrate are easily detected through HVOF coating on calibration test pieces Gain=50 Calibration with coating is optimum for shot peening effect detection on calibration test pieces Axial sensor

NDT- Barkhausen Noise-Set up (4) Cracks in the substrate that are not linked with overheating cannot be detected by the RollScan

NDT Inspection of the coating Types of imperfections observed on the HVOF coating: cracks, pull-out, pits, porosities, “spiralling” Warning: Some defects are only detected by visual inspection of ground HVOF with Neon light - Could not be detected by FPI. These defects are more detectable in the ground condition than in the superfinished Method used for chrome (per ASTM E1417) type I, Method C, sensitivity level 3 is more sensitive than the method used for Aluminum Method A. On going: FPI, method C with higher sensitivity (level 4: Ultrahigh), MPI

NDT- Visual Visual Neon light observed defects: Pull out of HVOF coating “Indents” Pull-out  Pull-out  These defects are rarely detected by FPI Pull-out 

NDT- FPI Worst finish after grinding Ra= 19-22 µinch, tp=0% 120R75 wheel 240R75 wheel 7000/575 6500/200 4500/100 No pull-out observed Worst finish after grinding Ra= 19-22 µinch, tp=0% Visual after SF Best finish after grinding Ra=6µinch, tp=57% visual after SF After SF: Ra= 2-3 µinch, tp= 90-99%

NDT- FPI FPI detectable defects « Spiralling » not associated with pull-out FPI detectable defects Ra=7 Tp=28% Porosities? « Spiralling » associated with some pull-out not visible here « pits »

NDT What are « defects » in the HVOF after grinding? What should the FPI/MPI operators reject? Acceptance and rejection criteria should be defined

Fluid Compatibility- Tests Organic and semi-organic solutions (Immersion tests at room and high temperature) Corrosion preventive products Greases Hydraulic fluids Grease + hydraulic fluids Degreasers Déterminez quelle peut être la conclusion la plus appropriée à l'assistance et à votre présentation. Résumez la présentation, faites des propositions, recommandez une stratégie, proposez un plan ou définissez un objectif. Ne vous dispersez pas pendant la présentation pour atteindre votre objectif.

Fluid Compatibility- Tests Aqueous Solutions : Tafel plots for solutions used by immersion, Cyclic Voltametry for solutions used with applied potential to determine dissolution rates. Tests were done on freshly prepared and used (from the shop) solutions at IMI (NRC) Cleaning solutions Inspection related products (FPI, MPI, nital etch) Plating and stripping solutions Déterminez quelle peut être la conclusion la plus appropriée à l'assistance et à votre présentation. Résumez la présentation, faites des propositions, recommandez une stratégie, proposez un plan ou définissez un objectif. Ne vous dispersez pas pendant la présentation pour atteindre votre objectif.

Fluid Compatibility- Tests Aqueous Solutions : Hydrogen Embrittlement susceptibility in all previous solutions. Notched specimens for H2 embrittlement type 1a (standard) per ASTM F-519, uncoated notch (keep the test valid per Spec. + simulate any possible galvanic corrosion), Load 75% NTS Déterminez quelle peut être la conclusion la plus appropriée à l'assistance et à votre présentation. Résumez la présentation, faites des propositions, recommandez une stratégie, proposez un plan ou définissez un objectif. Ne vous dispersez pas pendant la présentation pour atteindre votre objectif.

Fluid Compatibility (Results) 1- None of the organic tested fluid in the given conditions reacted with the HVOF coating: No Thickness change No Weight change No Finish change No visual modification

Fluid Compatibility (Results) 2- Aqueous solutions: No relevant dissolution rate observed on any tested solution used by immersion (higher rate 0.008 mils/h in the Mn phosphate solution) Plating solutions are harmless in term of dissolution because the process is cathodic (0.003 mils/h) Oakite cleaning solution, anodic process could be a problem, voltametic curves show a possible high dissolution rate after 5 minutes of immersion (0.5 mils/h)

Fluid Compatibility/Hydrogen Embrittlement (Solutions tested so far: Cleaning solutions and nital etch)

Fluid Compatibility/Hydrogen Embrittlement Possible interpretation: HVOF coating could be a barrier for hydrogen removal Cleaning solutions/steel/HVOF coating: Galvanic cell? Recommandations: Increase when possible the baking temperature or the duration Process HVOF coating after any plating operation when possible On going: Tests in all plating solutions Tests with increasing baking time Repeat tests on cleaning solutions SEM observations of failed specimens

Fluid Compatibility/Hydrogen Embrittlement How the notch looks like when the whole specimen is HVOF coated