AGMA 13FTM16-2013 The Anatomy of a Lubrication Erosion Failure - Causation Initiation Progression and Prevention.pdf

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1、13FTM16 AGMA Technical Paper The Anatomy of a Lubrication Erosion Failure - Causation, Initiation, Progression and Prevention By R.J. Drago and R.J. Cunningham, Drive Systems Technology, Inc., and W. Flynn, Chalmers clean with no evidence of corrosion products observed and deep in size. DST also not

2、ed that the erosion was observed in the root area on either side of the pitted surface. No clear evidence of electrical arc type damage or any other type of metallurgical defect was observed on any of the tooth defects metallurgically examined. 15 13FTM16 Figure 14. View of cavitation erosion defect

3、 within tooth contact pattern (Note: The very light contact pattern on the coast flanks is a normal condition due to back driving as the machine is shut down) Hardness, chemistry and microscopic examination of pinion shaft and defects Three (3) cross sections through the teeth defects from Pinion Se

4、gment B were removed for determination of certain metallurgical properties. These samples were cut on a perpendicular to the Pinion axis and were prepared such that the defects could be polished into. This was done in order to determine the microscopic characteristics of the cavitation erosion defec

5、ts. Each sample was then evaluated in the unetched condition for microhardness traverse data. Subsequently, the samples were chemically etched for microstructural features both at the surface and below the surface. This included evaluation for any evidence of reharden and/or retemper burns. The thre

6、e test samples were labeled A-1, A-2 and A-3. The areas removed for evaluation contained three teeth each as shown in Figure 15. Figure 15. View of four (4) areas to be removed from tooth segment B for further metallurgical evaluation 16 13FTM16 Also shown in Figure 15 is a fourth test sample, label

7、ed B-1. This test sample was removed from an area which was defect free as illustrated in the figure. It was prepared by removing a cross section perpendicular to the direction of the helical teeth. This sample was used to determine the actual metallurgical characteristics of the Pinion component. I

8、t included case and core hardness properties, nitride case depth and microstructural features of the Pinion shaft teeth. After removal of the all four (4) areas by EDM machining, each was metallurgically mounted and polished. A polished cross-section through one of the A test samples is illustrated

9、in Figure 13c. All of the A samples looked similar to that shown in the figure. After metallurgical preparation of all four test samples, a small additional area was removed for the purpose of determining the chemical makeup and quality level of the Pinion Shaft material. A brief summary of the test

10、 samples is given in Table 1. a. Hardness evaluation 1. Each of the four (4) tooth segments was carefully removed from Tooth Segment B by electro discharge machining. Subsequent mounting and polishing was accomplished using standard metallurgical techniques. Samples were water cooled during cutting

11、and polishing to assure that properties of the material would not be altered. Each sample was evaluated for microhardness characteristics at the surface as well as the core. All microhardness traverses were accomplished using a Leitz Microhardness Test Machine. Hardness traverse evaluation was accom

12、plished at depths below the surface of 0.003, 0.005, 0.010 inch and then every 0.010 inch up to 0.050 inches. DPHN values obtained were converted to Rockwell C values using standard conversion tables. A polynomial fit to the hardness vs. depth curves was used for all test samples to determine the ni

13、tride case depth. The nitride case depth was determined as the depth where the hardness dropped to 110% of the core hardness. This hardness value was determined as R/C 36. In addition, each sample was evaluated for surface and core hardness and decarburization if present. 2. The microhardness result

14、s for test samples A-1, A-2 and A-3 are illustrated schematically in Figures 16a, 16b and 16c respectively. Data obtained from these curves are illustrated in Table 2. The results for test sample B-1 is also illustrated in Table 2 and shown schematically in Figure 16d. 3. Based on the hardness vs. d

15、epth curves, each tooth surface had been nitrided. Hardness values obtained on each test sample were consistent with each other and typical of heat treated and hardened material. Table 1. Test samples removed from pinion tooth segment B for further metallurgical evaluation Test sample label Area eva

16、luated Sample removed for Evaluation purpose A-1 Cross section through 3 tooth pit defects Erosion/pitting features, other defects and microhardness studies Confirm failure mode A-2 Cross section through 3 tooth pit defects Erosion/pitting features, other defects and microhardness studies Confirm fa

17、ilure mode A-3 Cross section through 3 tooth pit defects Erosion/pitting features, other defects and microhardness studies Confirm failure mode B-1 Cross section through 2 tooth pit defects Hardness, microstructure nitride case depth around the tooth profile Confirm Pinion heat treat properties Unla

18、beled Cross section in core area of pinion tooth segment Chemical analysis major elements + carbon, sulfur and phosphorus Determine material chemistry and quality level of steel used for Pinion 17 13FTM16 a) Schematic representation of hardness data from test sample A-1 b) Schematic representation o

19、f hardness data from test sample A-2 c) Schematic representation of hardness data from test sample A-3 d) Schematic representation of hardness data from test sample B-1 Figure 16. Hardness vs. depth curves for all test samples Table 2. Microhardness vs. depth below surface data (All R/15N and BHN re

20、quirement values converted to R/C values) Coupon sample Coupon test location Surface R/C evaluated at 0.005 inch Nitride case depth at R/C 361), inch Core hardness range, R/C See figureA-1 Damaged flank 48.5 0.030 31 - 33 16a A-1 Undamaged flank 45.5 0.022 16a A-2 Damaged flank 49.0 0.030 31 - 33 16

21、b A-2 Undamaged flank 47.0 0.023 16b A-3 Damaged flank 47.0 0.032 31 - 33 16c A-3 Undamaged flank 47.0 0.024 16c B-1 Left flank 47.0 0.024 31 - 33 16d B-1 Right flank 47.0 0.023 16d B-1 Root 47.0 0.022 16d Requirements 47 - 552)0.018 - 0.023 33 - 37.53)NOTES: 1)Determined at hardness drop to 110% of

22、 core hardness 2)Values converted from R15N 84 88. 3)Converted from BHN 320 - 360 18 13FTM16 4. All nitride case depth values (obtained at R/C 36 depth) met or exceeded the requirements given in Reference G - shown in the Background section of this DST report and listed in Table 2. The nitride case

23、depth values obtained on all of the samples are considered consistent with heat treated nitrided 4340 steel. Surface and core hardness values also met the requirements. 5. Comparison of the data shown schematically for test samples A-1 to A-3 with test sample B-1 shows that the values obtained from

24、the latter sample was more consistent than that of the former. The reason for this is the cut angle of each test sample. Sample B-1 was cut on a perpendicular to the angle of the helical tooth resulting in consistent values on either side of the tooth profile. Samples A-1 to A-3 were cut on a perpen

25、dicular to the axis of the Pinion shaft and as such were not cut on a perpendicular to the helical tooth angle. These samples had a skewed cross section angle through the nitride layer and thus showed one side of the layer being somewhat greater than the other. This was characteristic on all three s

26、amples. Again, the reason for the cut/mounting angle on these latter samples was not for nitride layer evaluation but rather to obtain a clear cross section through the pit type defects present on each tooth. 6. Core hardness values were slightly lower than the requirement. However, this most likely

27、 is a function of the carbon level or the base material and the tempering temperature used during the hardening cycle for the 4340 through hardened steel. 7. Surface hardness values met the minimum hardness requirement and were considered typical for nitrided 4340 steel with low core hardness. 8. Ov

28、erall, the hardness data obtained from samples was consistent with through hardened and nitrided 4340 steel. 9. Chemical analysis. Chemical analysis was obtained by Direct Reading Atom Emissions Spectroscopy. The data disclosed that the material conformed to the requirements of AISI 4340, UNS G43400

29、. This material is a typical through hardened steel which is acceptable for nitriding. The results are illustrated in Table 3. Brief review of the chemical data disclosed that the quality of the material was consistent with vacuum degassed steel. This was indicated by the relatively low phosphorus a

30、nd sulfur contents. b. Microstructural evaluation. Microstructural evaluation of each test sample was accomplished in the unetched and etched condition. The etchant used for this work was 2 % Nital. All of the cross sections detailed above were evaluated. The following characteristics were observed:

31、 1. A typical test sample cross section is illustrated in Figure 13c. Examination of test samples A-1 to A-3 disclosed that the tip of each tooth on the coast side only was severely eroded. In many areas, extending from the eroded surface was a relatively deep cavity as shown in Figure 17a. The dept

32、h of this cavity was estimated to be at least 1/3 of the overall tooth height dimension ( 0.25 inch). It was apparent that the erosion to the tooth tips occurred on about 80% of the tip land on the coast side. None of the teeth showed any damage on the tip land on the load side of the tooth. In most

33、 cases, the tooth tip on the coast side of the Pinion tooth had been completely eroded away as shown in Figure 17b. The erosion damage extended from the tip of the tooth down along the coast side of the tooth, thus destroying any evidence of the addendum tooth profile or the corner of the tooth tip.

34、 This is illustrated in Figures 17b and 17c. Closer examination of a cavity formed in the eroded area revealed that it extended from the tooth tip down in towards the center thickness of the tooth. At the bottom of the cavity, secondary cavities along with cracks extending from these were observed.

35、This is shown in Figure 17d. This cracking reveals the method of the continuation of the growth of the cavity. Growth of the cavity through small, sharp cracks extending from the base of the large/primary cavity is most likely part of the cavitation erosion failure mechanism. Had service life contin

36、ued the growth of the large cavity most likely would have progressed completely through the tooth. 2. Evaluation of the root area between the eroded teeth disclosed evidence of further erosion and the formation of additional cavities. This is illustrated in Figure 18a. The surface topography of the

37、eroded area was characterized by somewhat rounded features with small crack type defects in between. This is illustrated in Figure 18b which shows a relatively large cavity which has formed at the base/center of the eroded root. These photographs confirm the presence of cavitation erosion in the too

38、th root areas as shown in Figures 12b and 12c. 19 13FTM16 Table 3. Chemical analysis of LS helical pinion (all values listed in percentage) Element Minimum requirement Maximum requirement Actual Carbon 0.38 0.43 0.40Chromium 0.70 0.90 0.81Manganese 0.60 0.80 0.73 Molybdenum 0.20 0.30 0.25Nickel 1.65

39、 2.00 1.80Phosphorus - - 0.035 0.010 Sulfur - 0.040 0.009Silicon 0.15 0.35 0.26 Iron Balancea) View of typical metallurgical sample showing depth of pit b) Magnified view of erosion damage observed on tooth tip c) Significant erosion damage on another tooth with large, deep cavity partially shown d)

40、 View of large cavity shown in Figure 16c showing growth characteristics - not erosion topographical profile Figure 17. Metallurgical unetched cross sections through defects 20 13FTM16 a) Erosion present in root area shows formation of cavity b) Large cavity present in root area - note erosion profi

41、le Figure 18. Magnified views of cavitation erosion in root area 3. The cavitation erosion profile in the root was similar to that observed on the tooth coast side and tip of the Pinion teeth as pointed out in Figures 17b, 17c and 17d. The pattern is also shown in Figure 20a. Microscopic examination

42、, using higher magnification of the profile is shown in Figures 19b and 19c. Clearly the profile resembles a somewhat spherical, honeycomb appearance. The honeycomb profile observed on the affected teeth of the LS Pinion is further confirmation of a cavitation erosion mode of failure. a) Typical too

43、th profile showing erosion profile b) Honeycomb features of erosion profile - typical of cavitation erosion c) Magnified view of honeycomb profile present in eroded areas Figure 19. Magnified profile characteristics of erosion 21 13FTM16 a) View of eroded tip with cavity formation present (no rehard

44、ening present) b) Typical erosion characteristics on flank-tip erosion - area shown is not typical of electrical arc damage c) Magnified view of flank erosion shown in Figures 19a and 19b - no rehardening present d) Additional magnification view of tooth tip corner eroded area - arrows show rough ed

45、ge with no rehardening present e) View of root area erosion pattern - similar to that in Figure 19 Figure 20. Metallurgical etched cross sections through defects 4. In addition to the above, microscopic examination of the unetched test samples disclosed the following: i. Cleanliness evaluation (incl

46、usion characterization) of each of the test sample appeared to be consistent with vacuum degassed melted material. ii. No evidence of intergranular oxidation (IGO) was present along the gear tooth profile of any test sample. iii. No other metallurgical anomalies were observed on the test samples tha

47、t could contributed to the cavitation erosion failures observed. 22 13FTM16 5. Three of the four test samples were chemically etched using 2 per cent Nital etchant. Microscopic examination of the eroded areas on the teeth showed no evidence of rehardening or retempering (see Figures 20a and 20b). Re

48、hardening, which would appear white in color with no evidence of structure within it, would indicate that the defects could have been a result of an electrical discharge, i.e., arc burn. No evidence of arc burns was observed in any of the eroded areas. In order to confirm the lack of rehardening in

49、the eroded areas, magnified views of the tooth damage shown in Figure 19b were metallurgically evaluated. These are given in Figures 20c and 20d. Each Figure shows the same area as in Figure 20b at higher magnifications. Clearly no evidence of rehardening is present. The microstructural characteristics in the eroded root areas yielded similar results in that no evidence of rehardening was present. A typical root area is illustrated in Figure 20e. 6. Review of test sample B-1 was accomplished after etching in 2% Nital etchant. Microscopic examination of the nitrided case micro