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Influence of the metalworking fluid on the micropitting wear of gears
Journal Pre-proof Influence of the metalworking fluid on the micropitting wear of gears Christian Brecher, Christoph Löpenhaus, René Greschert PII:
S0043-1648(18)31173-6
DOI:
https://doi.org/10.1016/j.wear.2019.202996
Reference:
WEA 202996
To appear in:
Wear
Received Date: 19 September 2018 Revised Date:
10 July 2019
Accepted Date: 31 July 2019
Please cite this article as: C. Brecher, C. Löpenhaus, René. Greschert, Influence of the metalworking fluid on the micropitting wear of gears, Wear (2019), doi: https://doi.org/10.1016/j.wear.2019.202996. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Influence of the metalworking fluid on the micropitting wear of gears Christian Brecher, Christoph Löpenhaus, René Greschert* WZL of RWTH Aachen University, Steinbachstraße 19, 52074 Aachen, Germany *corresponding author: [email protected], Phone: +49 (0)241 / 80-23620 Abstract Boundary layers can protect tooth flanks from metal-to-metal contacts. The characteristics of boundary layers and the resulting frictional and wear behavior are determined by the processes of manufacturing and running-in. This paper focuses on the influence of the metalworking fluid on the boundary layer formation of gear surfaces and the resulting wear behavior of the gears in terms of micropitting. Therefore, test gears were manufactured with different metalworking fluids consisting of mineral oil, ester oil and additives. The test gears were measured regarding their boundary layer properties, e.g. microstructure, reaction layers and residual stresses, which correlated with the choice of metalworking fluids in the respective grinding processes. In the subsequent tests of the wear behavior, repeatable differences in the degree of micropitting were observed. Especially the use of ester oil or chemically active sulfur additives in the grinding oils led to better tribological performances of the gears in the later application. Carry-over effects of the grinding oil from manufacturing into application as well as the need of a certain degree of mechanical and chemical energy to improve the precondition process of the gears are possible explanations for the observations made in this study. Keywords gears; grinding; metalworking fluid; micropitting; oil; wear
Test gear design
1 Introduction Boundary Layers develop on the surface of workpieces during manufacturing, running-in and application. They differ from the basic material regarding structure, chemical composition and mechanical properties [1]. At the very surface, contaminations, adsorption and reaction layers are observed, containing elements of the surrounding air and fluid [2-5]. Underneath, tribomutation and deformation layers can be found, showing different microstructures, e.g. regarding grain size and orientation, than the basic material [5-8]. Also in case of gear flanks, the formation of boundary layers is influenced by the manufacturing process [9-11] and by the running-in procedure [12-13]. It has been shown that the composition of the metalworking fluid (here: grinding oil) influences the boundary layer and the friction and wear behavior of gear analogy test parts on the twodisk test rig [9]. In this report the influence of the metalworking fluid on the resulting boundary layer and on the micropitting wear of gears is investigated. For applications in automotive engine construction, the relationship between the manufacturing-induced boundary layer and the running behavior has already been investigated [5-7]. If friction and wear are traced back to boundary layer properties also for gear transmissions, resource-saving gear use is made possible by optimized gear production.
2 Materials and Methods 2.1 Test gear design and manufacturing machine The specimen geometry was designed on the basis of the 17/18 gear geometry (Fig. 1) [13] with modifications regarding micro geometry. The material 16MnCr5 (1.7131) and the case hardening process were based on standard specifications from gear manufacturing and made it possible to compare the component surfaces with the two-disk test specimens of the first project phase [9, 14]. The grinding wheel specification was 91DA 80/80 F 15 V. The grinding wheel contained a mixed grain of corundum and CubitronTM II in ceramic bond. For each gear, the grinding wheel was dressed once for rough grinding (grinding stock ∆s = 0,04 mm) and once for finishing grinding. Regarding gear geometry, a gear quality of IT4 was achieved. The test gears were finished by discontinuous profile grinding on a Klingelnberg Höfler Viper 500W machine. Due to a modification of the grinding machine, which was not related to the project, the grinding spindle was changed during the course of the project. The change and its effects are documented and evaluated in section 2.2. The metalworking fluid was provided by a Hoffmann HSF 100 SE/K lubricating oil unit.
Grinding machine
Material
16MnCr5 (1.7131) case hardened Hardness = 61 HRC Macro Geometry
mn z1/2 b αn β
= = = = =
5 mm 17 / 18 10 mm 20° 0°
Micro Geometry
Tip relief: Ca1/2 = 120 µm Lead crowning: Cβ1/2 = 2 / 12 µm
©WZL/ Winandy
Grinding Machine
Klingelnberg Höfler Viper 500 W Grinding Oil Unit
Hoffmann HSF 100 SE/K VTank = 2000 l poil,max = 27 bar Qoil = 120 l/min
Fig. 1: Test gear design and grinding machine 2.2 Metalworking fluid variation The metalworking fluid was the subject of the test variation. Six different metalworking fluids were applied in total and sets of test gears were ground with each fluid. The metalworking fluid base oils and additives were identical to the materials used in the first project phase for external cylindrical grinding [9]. All metalworking fluids corresponded to standard market products and were provided by Fuchs. Hydrocrack and ester oils were applied as base oils because they are used in industrial gear grinding and differ in their chemical mechanisms of action [4, 9]. On the molecular level the ester oil, in comparison to hydrocrack oil, is assigned a polarity and thus an active effort for covering metal surfaces [4]. The base oils were used in the additive-free state to test their natural lubricating effect. Subsequently, the base oils were supplemented by additives. Commercially available sulfur and phosphor additives were added as further fluid variants. Both additives were classified as "active" in their lubricating film formation, so that a lubricating effect was to be expected even at low temperatures and short contact times [4]. The interactions at the simultaneous use of two different additives were also considered. In the last step, a fully formulated commercially available product of the type Ecocut HFN 13 LE UNI was considered. The product consisted of hydrocrack oil and the components of the metalworking fluid variants tested before as well as other additives for increasing performance and improving secondary properties such as heat and oxidation resistance, low foaming or minimizing odor. As a stitch test, some gears were manufactured without fluid supply during the very last finishing stroke and were also examined on the test rig. The test plan overview is given in Fig. 2. Apart from
Variant
Base oil
Additives [vol-%]
H
None Hydrocrack oil
HS
5% Sulfur
(ISO VG 8)
HSP
5% Sulfur + 1% Phosphor
E
Ester oil
2% Schwefel 5% None ohne
ES
(ISO VG 8)
5% Sulfur
Eco
Ecocut HFN 13 LE UNI (commercial product)
Dry
Ecocut HFN 13 LE UNI, last finishing cut without fluid
Fig. 2: Metalworking fluid variation For every fluid variant, nine sets of test gears were ground, while all other grinding parameters remained constant. Since roughness influences [16] were not in the focus of this project, the grinding parameters were chosen in a way that the roughness values of the ground gears scattered in a small range. To induce the occurrence of micropitting in the later tests, a surface roughness of Ra = 0.6 µm was aimed for.
3.2 Residual stresses A residual stress depth curve was determined on one pinion flank per fluid variant. However, residual stresses are mainly considered to have an effect on pitting, but not on micropitting strength [14, 19]. The residual stress measurements were carried out using X-ray diffraction. Since axial and tangential residual stresses showed the same tendencies with regard to their course over the depth profile, only the tangential residual stresses are shown in Fig. 4. All variants investigated show qualitatively comparable residual stress depth curves. For the measuring point 5 µm below the surface, variants H and Dry show compressive stress amounts reduced by up to 200 MPa, i.e. 30%. This reduction is greater than the measuring error of ∆ = ± 15 MPa. It can be assumed that in the absence of additives a non-polar base oil (H) or a dry grinding process (Dry), the heat input into the workpiece during grinding is critical and therefore causing a decay of compressive stresses [19]. As soon as the metalworking fluid contained additives (HS, HSP…), the residual stress depth profiles are assumed to be unaffected by exaggerated heat input due to the metalworking fluid. The residual stress depth profiles of the other grinding oil variants do not allow any further differentiation. 0 σRS,tang [MPa]
the metalworking fluid, the other process parameters of the grinding machine as well as the conditions and the transmission oil of the test rig remained constant. Thermal microstructure damage to the test parts was ruled out with the aid of nital etching [15]. For this reason, the first ground pinion of each variant was etched. In order to avoid etch effects on the wear test results, the etched pinions were separated from the others and only used for the residual stress measurements, where their chemical constitution was of no importance.
Z [µm]
Modification of the grinding machine
H
HS
HSP1 HSP2
E
ES
Eco
Dry
H
HS
HSP1 HSP2
E
ES
Eco
Dry
2
2 0
0
-2
-2
0
Lm [mm]
Rill width Rsm
60
80
100
120
t [µm]
-200 Fluid variants H HS HSP1 HSP2
-400
-800
4 = 0.42 mm
0
Lm [mm] Rsm = 0.20 mm
Fig. 3: Roughness values of the gear flanks
4
0 σRS,tang [MPa]
3.1 Surface roughness After grinding, the surface profiles of the gear flanks were investigated by tactile profiling acc. to VDI guideline 2612-5 [17] of all gears and all variants, except for those after nital etching. For each gear flank side, measurements were conducted on three circumferentially distributed teeth and summarized in average values, Fig. 3. These averaged measurement values of all gears and variants are within the scatter band of Ra = 0.6 ± 0.1 µm. To increase the tribological loading in the test rig and thus to increase the micropitting risk, a groove structure acc. to [18] was superimposed on the profile by a defined overlap and speed ratio during the dressing process. Due to a modification of the grinding machine, which was not related to the project and took place after grinding the gears of fluid variant HSP1 (spindle change), the profile rill width, but not the Ra and Rz values were changed. This difference was taken into account in the tests of the micropitting wear. For this reason, a second series of test gears was ground using the same grinding oil variant, labelled as HSP2 to establish comparable test data to the fluid variants E, ES, Eco and dry grinding.
3.5 Rz 3.0 [µm] 2.5
40
-600
3 Test gear analysis
0.7 Ra 0.6 [µm] 0.5
20
20
40
60
80
100
120
t [µm]
-200 -400 -600
Fluid variants E ES Eco Dry
-800
Fig. 4: Residual stress depth profiles 3.3 Structure and hardness After grinding, cross sections of pinion teeth were prepared for all variants and examined by light and electron microscopy to characterize the microstructure. The surface hardness was determined for all variants to a value of HRC 61±1 by conducting three measurements per gear and analysing one gear per variant. In addition, hardness depth profile measurements were carried out on the cross sections. Neither the appearance of the microstructure in the light microscopic observation nor the hardness profiles allow a differentiation of the different metalworking fluid variants. The analysis results for variant HSP are shown as an example in Fig. 5. To identify further distinguishing features, an additional analysis of the boundary layer structure was carried out using FIB-SEM for this variant. The orientation of the cross section is orthogonal to the direction of the grinding grooves, so that the roughness profile is visible in the image. Below the FIB protective strip, which appears white in the SEM image, the boundary layer can be seen. The top surface bears an amorphous, black layer of varying thickness, which is likely to be a reaction or oxidation layer [2]. Since the contact time for chemical additive reaction is very short in the grinding process, the origin of the layer is assigned to oxidative processes due to the heat generated during the grinding process [4].
700 600 500 400 1.0
t[mm]
2.0
500 µm
Protective strip
200 µm
1 µm FIB-SEM: NMI Tübingen
Fig. 5: Microstructure of variant H gears after manufacturing Since such a layer was not observed for the two-disk test specimens of the first project phase despite the same materials and metalworking fluids, it is assumed that the process heat during the profile grinding of the gears is greater than the process heat during the external cylindrical grinding of the disks [9]. The process kinematics, the metalworking fluid injection and the component volume are possible causes for this difference [14]. Due to the oxidation of the surface layer, an influence on the mechanisms of action of the metalworking fluid components is possible [4]. Below the thin, black layer is the martensitic structure, which appears significantly fine-grained in depths down to 1 µm, before the underlying coarse-grained basic structure is reached. The layer thickness of the finegrained area and the characteristic of the transition to the coarsegrained area were used in the first project phase as distinguishing features between different variants of specimen finishing [9]. However, in this study, all test gear variants showed surface microstructures and hardness profiles which were very similar to the ones shown in Fig. 5.
4 Experimental tests 4.1 Test setup The tests of the micropitting capacity were carried out on a standardized back-to-back gear test rig acc. to DIN ISO 14635-1 [20] with an axial distance of a = 91.5 mm. The test rig consists of two gearboxes (test and slave transmission) which are driven by an electric motor and distorted against each other to load the test gears with a certain amount of torque. For the test gear variants, which were ground with different metalworking fluids, two tests were carried out for each variant under constant conditions and the micropitting damage was evaluated on the basis of the mean profile variation due to micropitting ffm [21]. After grinding, the gear flanks were cleaned of metalworking fluid residues using compressed air and stored in VCI bags. The use of chemically more aggressive agents or preservatives was avoided to emphasize carry-over effects. Volatile corrosion inhibitors (VCI) protect workpieces by outgassing amine-based corrosion inhibitors (containing the elements N, C, H) and building up a temporary reaction-inhibiting film on the metal surface. The VCI film volatilizes after removal of the VCI bag [22]. The lubrication in the test rig was applied as splash lubrication using an industrial gear oil (CLP), which was also used in the first project phase in the two-disk contact [9]. Compared to the two-disk test, the oil was used in an increased viscosity class of VG 220 instead of VG 68 to avoid scuffing damages to the gear flanks. The degree of micropitting damage was determined by profile measurements of the mean profile deviation ffm at the test pinion flanks on a gear measuring centre of the type Klingelnberg P16. For this purpose, the profile shape before and after testing was measured an compared for five teeth evenly distributed over the circumference. In the new condition,
the test gears showed profile deviations up to ffm,new = 3 µm. The average profile deviations after testing are shown in Fig. 6. Due to different rill widths Rsm of the tooth flanks resulting from the grinding machine modification (see Fig. 3), the torque had to be reduced to T1 = 315 Nm for the tests of variant HSP2 and later to avoid other damages such as scuffing. Therefore, the variants ground before the modification of the grinding machine cannot be compared to the variants ground after the modification, except for the HSP1 and HSP2 variants, which were ground using the same metalworking fluid, but different machine settings. However, in between the series H, HS, HSP1 and in between the series of HSP2, E, ES, Eco, Dry relative comparisons are admissible. 4.2 Test results and discussion The test results (Fig. 6) show different degrees of micropitting damage depending on the metalworking fluid composition of the grinding process. For the hydrocrack grinding oils, a reduction of 50% in mean profile deviation due to micropitting occurs if a sulfur additive (HS) is added to the additive-free hydrocrack oil (H). The amount of micropitting damage increases when the phosphor additive is added to the grinding oil (HSP). Since the average roughness profiles of the test gears are all at the same level, the tendency of the amount of micropitting damage cannot be explained by the average surface roughness. However, it is noticeable that the variant H with the highest micropitting damage after grinding showed reduced residual compressive stress amounts compared to the variants HS and HSP, see Fig. 4. When changing from variant HSP2 to the additive-free ester oil (E), a reduction in the amount of micropitting damage can be observed. In contrast to the hydrocrack oil, the addition of the sulfur additive does not improve the micropitting load capacity (ES). The test gears ground with the market product (Eco) show a similar micropitting wear behavior as the gears of the variants E and ES. As an individual study without a repeat test, a gear set was ground dry in the last finishing stroke and also examined once in the micropitting test. The micropitting damage occurring for this variant lies in between the HSP2 and the E, ES and Eco variants. Mean profile deviation ffm [µm]
HV1
T1=500 Nm
14
Modification of the grinding machine T1=315 Nm
12 10 8 6 4 2 H
HS
HSP1 HSP2 E ES Metalworking fluid variant
Test conditions n1 = 1500 min-1 T1 = 500 / 315 Nm T = 90°C N1 = 2.2·106 cycles Ntests = 2 tests
Eco
Dry
Run-In T1 = 61 Nm N1 = 135 000 cycles Damage criteria Micropitting HSP1
2 mm
Fig. 6: Mean profile deviation ffm due to micropitting
5 Discussion The improvement of the wear behavior of parts ground with ester oil compared to those parts which were ground with hydrocrack oil has already been observed in the two-disk tests in the first period of the project [9]. Furthermore, both the gear tests and the two-disk tests indicate better performance for those parts that had been ground using metalworking fluids with only one active component (e.g. HS or E) instead of fluids without
active component (H) or more than one active component (HSP, ES). As the test gear analyses after grinding (section 3) show, the different micropitting capacities cannot be attributed to the average surface roughness, residual stresses and hardness profiles of the gears. However, the test results of the gear tests and the two-disk tests [9] indicate that the metalworking fluid influences the wear behavior of the workpiece in some way. Possible ways of information transfer from manufacturing to application are discussed in Fig. 7. On the one hand it has been shown that the inner boundary layer, mainly concerning the micro- or nanostructure, can be preconditioned by manufacturing processes in a favorable way as proposed by Berlet [5] and Scherge [6]. However, this change in microstructure has not yet been observed in the FIB-SEM analyses of the test gears. Therefore, the analyses should be extended to measurements of the nanotopography and the nanohardness of the gears to find indications to their frictional and wear behavior. Energy input
Wear in application
Thesis: The energy input influences the workpiece microstructure insufficient preconditioning
high defect density
Chemical surface coverage Thesis: The metalworking fluid remains on the surface and influences the transmission oil metallic surface (simplified)
δ+ H
production optimum
O δ− δ+
Fe
Power densitiy in grinding
Preconditioning of the microstructure (inner boundary layer)
δ+
C δ−
δ+
C ester oil (simplified)
Preconditioning of the chemical surface coverage (outer boundary layer)
Fig. 7: Possible ways of information transfer from manufacturing to application [4, 5, 6, 23] On the other hand, the outer boundary layer is chemically preconditioned by the metalworking fluid as well. Analyses of the chemical surface coverage show that the metalworking fluid molecules are carried over into the final application despite intensive cleaning e.g. in hexane and acetone [9]. Since the additives of the transmission oil in the are chemically related to the additives of the metalworking fluid, they aim at the same "docking points" on the metal surface to form lubrication films or build up protective layers [4]. If those points are still covered by the molecules of the metalworking fluid, the transmission oil additives might be blocked or decelerated in their working mechanisms. With regard to the test results on gear flanks and two-disk contacts, homogenous films of metalworking fluid components of only one type (e.g. HS or E) and of lower binding energies (Ester) proved to be favorable [4, 13, 23].
6 Summary and Conclusion The subject of this work was to investigate the influence of the metalworking fluid (here: grinding oil) on the micropitting wear of the gears in their later application. For this purpose, several variants of test gears were produced, which differed in terms of the composition of the metalworking fluid. All other process parameters, including the transmission oil in the gear test rig, remained constant. The test results indicate that the micropitting wear in the test is dependent of the metalworking fluid composition in the manufacturing process. Those gears which were ground with additive-free hydrocrack oil show the most progressive wear after the test, while the gears which were ground using ester-based grinding oils show smaller wear amounts. These findings are in accordance with pitting tests [24] and two-disk tests [9], which also show improved wear
resistance for parts ground with ester-based grinding oils. Regarding the boundary layer, low wear resistance correlates with oxide layers, reduced amounts of compressive stresses or lack of surface-active additives in the grinding oil, which are all indications of higher heat flux into the work pieces during the grinding process (e.g. variants H and Dry). These observations show that a certain additive content in the metal working fluid is necessary to support the tribological properties of the microstructure even below the specification of grinding burn [15]. Furthermore, chemical analyses provide evidence that even after thorough cleaning of the test parts there are still constituents of the metalworking fluid taken over into the final application [24]. In case of gears, the additives of the transmission oil are designed to build protection layers on the gear flanks. Residues of the metalworking fluid, which remain on the gear flanks after grinding, interfere with the buildup of such protection layers if they cannot be removed from the surface. For this reason, metalworking fluids containing ester oil might lead to better wear behavior, since ester oil is characterized as surface-active, but of low binding energy [4]. Acknowledgements This work was supported by the German Research Foundation (DFG) [Project Numbers BR2905/44-1 and BR2905/44-2]. Furthermore, we thank Fuchs Europe Schmierstoffe GmbH and Fuchs Wisura GmbH for their consulting in terms of oil choice.
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Stachowiak G, Batchelor AW (2013) Engineering Tribology, Butterworth-Heinemann, Oxford
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Reichelt M, Mayer J et al (2006) TEM and nanomechanical studies on tribological surface modifications formed on roller bearings under controlled lubrication conditions. Journal of Materials Science 41:4543-4553
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Schulz J, Brinksmeier E, Meyer D (2013) On the interactions of additives in metalworking fluids with metal surfaces. Lubricants 1:75-94
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Berlet P, Dienwiebel M, Scherge M (2010) The effect of sample finishing on the tribology of metal/metal lubricated contacts. Wear 268:1518-1523
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Scherge M, Linsler D, Schlarb T (2015) The running-in corridor of lubricated metal-metal contacts. Wear 342-343: 60-64
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Brink A, Lichtenberg K, Scherge M (2016) The influence of the initial near-surface microstructure and imposed stress level on the running-in characteristics of lubricated steel contacts. Wear 360361: 114-120
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Büscher R, Fischer A (2005) The pathways of dynamic recrystallization in allmetal hip joints. Wear 259: 887-897
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Brecher C, Löpenhaus C, Greschert R (2015) Influence of the metalworking fluid on the running behavior of gear analogy test parts. Prod. Eng. Res. Devel. 9:425-431
10. Brecher C, Brumm M, Hübner F, Henser J (2013) Influence of the manufacturing method on the running behavior of beveloid gears. Prod. Eng. Res. Devel. 7:265-274 11. Jao T C, Inacker O, Beckmann P, Yatsunami K (2007) Effects of viscosity and surface roughness on gear contact tribological layers. Lubrication Science 19:57-70 12. Lohner T, Mayer J, Michaelis K, Höhn BR, Stahl K (2017) On the running-in behavior of lubricated line contacts. Proc IMechE Part J: J Engineering Tribology, 231/4:441-452. 13. Stahl K, Kopnarski M, Lohner T, Emrich S et al (2017) Einfluss triboinduzierter Schichten auf Schäden und Reibungsverhalten von Zahnrädern unter besonderer Berücksichtigung des Einlaufvorgangs – experimentelle und analytische Untersuchungen. DFG Abschlusskolloquium SPP 1551. 58.
Tribologie-Fachtagung Göttingen. Gesellschaft für Tribologie e.V., Aachen, pp. 167-186. 14. Klocke F, Brecher C (2017) Zahnrad- und Getriebetechnik. Auslegung - Herstellung - Untersuchung – Simulation. Carl Hanser, Munich 15. ISO 14104 (2017) Gears - Surface temper etch inspection after grinding, chemical method, ISO, Geneva 16. Höhn BR, Michaelis K, Kreil O (2006) Influence of surface roughness on pressure distribution and film thickness in EHLcontacts. Tribology International 12:1719-1725 17. Guideline VDI 2612-5 (2015) Surface roughness measurement of cylindrical gears and bevel gears by means on stylus-type instruments. Beuth, Berlin 18. Klocke F, Löpenhaus C, Staudt J (2017) Performance of gears manufactured by 5-axis milling. Gear Technology 34/2: 58-65 19. Klocke F, Gomes J, Löpenhaus C, Rego R (2016) Assessing the heterogeneity of residual stress for complementing the fatigue performance comprehension. Journal of Strain Analysis for Engineering Design 51/5: 347-357 20. ISO 14635-1: Gears - FZG test procedures - Part 1: FZG test method A/8,3/90 for relative scuffing load-carrying capacity of oils. ISO, Geneva, 2000 21. Höhn BR, Winter H, Schönnenbeck G, Emmert S (1993) Test procedure for the investigation of the micro-pitting capacity of gear lubricants. FVA Information Sheet 54/I-IV, FVA, Frankfurt a.M. 22. Rammelt U, Koehler S, Reinhard G (2011) Efficiency of Vapor Phase Corrosion Inhibitors for Ferrous Metals in Neutral and Alkaline Solutions.Corrosion 67/4: 045001-1 - 045001-7 23. Klocke F, Maßmann T, Gerschwiler K (2005) Combination of PVD tool coatings and biodegradable lubricants in metal forming and machining. Wear 259:1197-1206 24. Brecher C, Löpenhaus C, Greschert R (2018) Influence of the metalworking fluid on the grinding process and on the running behaviour of the workpiece. In: Proceedings of the 8th Congress of the German Academic Association for Production Technology (WGP), Aachen, November 19-20, pp. 522-531
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Reproducible differences in the degree of micropitting wear were observed for gears which had been ground using different metalworking fluids Ways of information transfer are discussed to combine the molecular effects of the metalworking fluid with the wear behavior in the final application The interactions between metalworking fluid and transmission oil are of great importance for the wear behavior of the ground surface
S0043-1648(18)31173-6
DOI:
https://doi.org/10.1016/j.wear.2019.202996
Reference:
WEA 202996
To appear in:
Wear
Received Date: 19 September 2018 Revised Date:
10 July 2019
Accepted Date: 31 July 2019
Please cite this article as: C. Brecher, C. Löpenhaus, René. Greschert, Influence of the metalworking fluid on the micropitting wear of gears, Wear (2019), doi: https://doi.org/10.1016/j.wear.2019.202996. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Influence of the metalworking fluid on the micropitting wear of gears Christian Brecher, Christoph Löpenhaus, René Greschert* WZL of RWTH Aachen University, Steinbachstraße 19, 52074 Aachen, Germany *corresponding author: [email protected], Phone: +49 (0)241 / 80-23620 Abstract Boundary layers can protect tooth flanks from metal-to-metal contacts. The characteristics of boundary layers and the resulting frictional and wear behavior are determined by the processes of manufacturing and running-in. This paper focuses on the influence of the metalworking fluid on the boundary layer formation of gear surfaces and the resulting wear behavior of the gears in terms of micropitting. Therefore, test gears were manufactured with different metalworking fluids consisting of mineral oil, ester oil and additives. The test gears were measured regarding their boundary layer properties, e.g. microstructure, reaction layers and residual stresses, which correlated with the choice of metalworking fluids in the respective grinding processes. In the subsequent tests of the wear behavior, repeatable differences in the degree of micropitting were observed. Especially the use of ester oil or chemically active sulfur additives in the grinding oils led to better tribological performances of the gears in the later application. Carry-over effects of the grinding oil from manufacturing into application as well as the need of a certain degree of mechanical and chemical energy to improve the precondition process of the gears are possible explanations for the observations made in this study. Keywords gears; grinding; metalworking fluid; micropitting; oil; wear
Test gear design
1 Introduction Boundary Layers develop on the surface of workpieces during manufacturing, running-in and application. They differ from the basic material regarding structure, chemical composition and mechanical properties [1]. At the very surface, contaminations, adsorption and reaction layers are observed, containing elements of the surrounding air and fluid [2-5]. Underneath, tribomutation and deformation layers can be found, showing different microstructures, e.g. regarding grain size and orientation, than the basic material [5-8]. Also in case of gear flanks, the formation of boundary layers is influenced by the manufacturing process [9-11] and by the running-in procedure [12-13]. It has been shown that the composition of the metalworking fluid (here: grinding oil) influences the boundary layer and the friction and wear behavior of gear analogy test parts on the twodisk test rig [9]. In this report the influence of the metalworking fluid on the resulting boundary layer and on the micropitting wear of gears is investigated. For applications in automotive engine construction, the relationship between the manufacturing-induced boundary layer and the running behavior has already been investigated [5-7]. If friction and wear are traced back to boundary layer properties also for gear transmissions, resource-saving gear use is made possible by optimized gear production.
2 Materials and Methods 2.1 Test gear design and manufacturing machine The specimen geometry was designed on the basis of the 17/18 gear geometry (Fig. 1) [13] with modifications regarding micro geometry. The material 16MnCr5 (1.7131) and the case hardening process were based on standard specifications from gear manufacturing and made it possible to compare the component surfaces with the two-disk test specimens of the first project phase [9, 14]. The grinding wheel specification was 91DA 80/80 F 15 V. The grinding wheel contained a mixed grain of corundum and CubitronTM II in ceramic bond. For each gear, the grinding wheel was dressed once for rough grinding (grinding stock ∆s = 0,04 mm) and once for finishing grinding. Regarding gear geometry, a gear quality of IT4 was achieved. The test gears were finished by discontinuous profile grinding on a Klingelnberg Höfler Viper 500W machine. Due to a modification of the grinding machine, which was not related to the project, the grinding spindle was changed during the course of the project. The change and its effects are documented and evaluated in section 2.2. The metalworking fluid was provided by a Hoffmann HSF 100 SE/K lubricating oil unit.
Grinding machine
Material
16MnCr5 (1.7131) case hardened Hardness = 61 HRC Macro Geometry
mn z1/2 b αn β
= = = = =
5 mm 17 / 18 10 mm 20° 0°
Micro Geometry
Tip relief: Ca1/2 = 120 µm Lead crowning: Cβ1/2 = 2 / 12 µm
©WZL/ Winandy
Grinding Machine
Klingelnberg Höfler Viper 500 W Grinding Oil Unit
Hoffmann HSF 100 SE/K VTank = 2000 l poil,max = 27 bar Qoil = 120 l/min
Fig. 1: Test gear design and grinding machine 2.2 Metalworking fluid variation The metalworking fluid was the subject of the test variation. Six different metalworking fluids were applied in total and sets of test gears were ground with each fluid. The metalworking fluid base oils and additives were identical to the materials used in the first project phase for external cylindrical grinding [9]. All metalworking fluids corresponded to standard market products and were provided by Fuchs. Hydrocrack and ester oils were applied as base oils because they are used in industrial gear grinding and differ in their chemical mechanisms of action [4, 9]. On the molecular level the ester oil, in comparison to hydrocrack oil, is assigned a polarity and thus an active effort for covering metal surfaces [4]. The base oils were used in the additive-free state to test their natural lubricating effect. Subsequently, the base oils were supplemented by additives. Commercially available sulfur and phosphor additives were added as further fluid variants. Both additives were classified as "active" in their lubricating film formation, so that a lubricating effect was to be expected even at low temperatures and short contact times [4]. The interactions at the simultaneous use of two different additives were also considered. In the last step, a fully formulated commercially available product of the type Ecocut HFN 13 LE UNI was considered. The product consisted of hydrocrack oil and the components of the metalworking fluid variants tested before as well as other additives for increasing performance and improving secondary properties such as heat and oxidation resistance, low foaming or minimizing odor. As a stitch test, some gears were manufactured without fluid supply during the very last finishing stroke and were also examined on the test rig. The test plan overview is given in Fig. 2. Apart from
Variant
Base oil
Additives [vol-%]
H
None Hydrocrack oil
HS
5% Sulfur
(ISO VG 8)
HSP
5% Sulfur + 1% Phosphor
E
Ester oil
2% Schwefel 5% None ohne
ES
(ISO VG 8)
5% Sulfur
Eco
Ecocut HFN 13 LE UNI (commercial product)
Dry
Ecocut HFN 13 LE UNI, last finishing cut without fluid
Fig. 2: Metalworking fluid variation For every fluid variant, nine sets of test gears were ground, while all other grinding parameters remained constant. Since roughness influences [16] were not in the focus of this project, the grinding parameters were chosen in a way that the roughness values of the ground gears scattered in a small range. To induce the occurrence of micropitting in the later tests, a surface roughness of Ra = 0.6 µm was aimed for.
3.2 Residual stresses A residual stress depth curve was determined on one pinion flank per fluid variant. However, residual stresses are mainly considered to have an effect on pitting, but not on micropitting strength [14, 19]. The residual stress measurements were carried out using X-ray diffraction. Since axial and tangential residual stresses showed the same tendencies with regard to their course over the depth profile, only the tangential residual stresses are shown in Fig. 4. All variants investigated show qualitatively comparable residual stress depth curves. For the measuring point 5 µm below the surface, variants H and Dry show compressive stress amounts reduced by up to 200 MPa, i.e. 30%. This reduction is greater than the measuring error of ∆ = ± 15 MPa. It can be assumed that in the absence of additives a non-polar base oil (H) or a dry grinding process (Dry), the heat input into the workpiece during grinding is critical and therefore causing a decay of compressive stresses [19]. As soon as the metalworking fluid contained additives (HS, HSP…), the residual stress depth profiles are assumed to be unaffected by exaggerated heat input due to the metalworking fluid. The residual stress depth profiles of the other grinding oil variants do not allow any further differentiation. 0 σRS,tang [MPa]
the metalworking fluid, the other process parameters of the grinding machine as well as the conditions and the transmission oil of the test rig remained constant. Thermal microstructure damage to the test parts was ruled out with the aid of nital etching [15]. For this reason, the first ground pinion of each variant was etched. In order to avoid etch effects on the wear test results, the etched pinions were separated from the others and only used for the residual stress measurements, where their chemical constitution was of no importance.
Z [µm]
Modification of the grinding machine
H
HS
HSP1 HSP2
E
ES
Eco
Dry
H
HS
HSP1 HSP2
E
ES
Eco
Dry
2
2 0
0
-2
-2
0
Lm [mm]
Rill width Rsm
60
80
100
120
t [µm]
-200 Fluid variants H HS HSP1 HSP2
-400
-800
4 = 0.42 mm
0
Lm [mm] Rsm = 0.20 mm
Fig. 3: Roughness values of the gear flanks
4
0 σRS,tang [MPa]
3.1 Surface roughness After grinding, the surface profiles of the gear flanks were investigated by tactile profiling acc. to VDI guideline 2612-5 [17] of all gears and all variants, except for those after nital etching. For each gear flank side, measurements were conducted on three circumferentially distributed teeth and summarized in average values, Fig. 3. These averaged measurement values of all gears and variants are within the scatter band of Ra = 0.6 ± 0.1 µm. To increase the tribological loading in the test rig and thus to increase the micropitting risk, a groove structure acc. to [18] was superimposed on the profile by a defined overlap and speed ratio during the dressing process. Due to a modification of the grinding machine, which was not related to the project and took place after grinding the gears of fluid variant HSP1 (spindle change), the profile rill width, but not the Ra and Rz values were changed. This difference was taken into account in the tests of the micropitting wear. For this reason, a second series of test gears was ground using the same grinding oil variant, labelled as HSP2 to establish comparable test data to the fluid variants E, ES, Eco and dry grinding.
3.5 Rz 3.0 [µm] 2.5
40
-600
3 Test gear analysis
0.7 Ra 0.6 [µm] 0.5
20
20
40
60
80
100
120
t [µm]
-200 -400 -600
Fluid variants E ES Eco Dry
-800
Fig. 4: Residual stress depth profiles 3.3 Structure and hardness After grinding, cross sections of pinion teeth were prepared for all variants and examined by light and electron microscopy to characterize the microstructure. The surface hardness was determined for all variants to a value of HRC 61±1 by conducting three measurements per gear and analysing one gear per variant. In addition, hardness depth profile measurements were carried out on the cross sections. Neither the appearance of the microstructure in the light microscopic observation nor the hardness profiles allow a differentiation of the different metalworking fluid variants. The analysis results for variant HSP are shown as an example in Fig. 5. To identify further distinguishing features, an additional analysis of the boundary layer structure was carried out using FIB-SEM for this variant. The orientation of the cross section is orthogonal to the direction of the grinding grooves, so that the roughness profile is visible in the image. Below the FIB protective strip, which appears white in the SEM image, the boundary layer can be seen. The top surface bears an amorphous, black layer of varying thickness, which is likely to be a reaction or oxidation layer [2]. Since the contact time for chemical additive reaction is very short in the grinding process, the origin of the layer is assigned to oxidative processes due to the heat generated during the grinding process [4].
700 600 500 400 1.0
t[mm]
2.0
500 µm
Protective strip
200 µm
1 µm FIB-SEM: NMI Tübingen
Fig. 5: Microstructure of variant H gears after manufacturing Since such a layer was not observed for the two-disk test specimens of the first project phase despite the same materials and metalworking fluids, it is assumed that the process heat during the profile grinding of the gears is greater than the process heat during the external cylindrical grinding of the disks [9]. The process kinematics, the metalworking fluid injection and the component volume are possible causes for this difference [14]. Due to the oxidation of the surface layer, an influence on the mechanisms of action of the metalworking fluid components is possible [4]. Below the thin, black layer is the martensitic structure, which appears significantly fine-grained in depths down to 1 µm, before the underlying coarse-grained basic structure is reached. The layer thickness of the finegrained area and the characteristic of the transition to the coarsegrained area were used in the first project phase as distinguishing features between different variants of specimen finishing [9]. However, in this study, all test gear variants showed surface microstructures and hardness profiles which were very similar to the ones shown in Fig. 5.
4 Experimental tests 4.1 Test setup The tests of the micropitting capacity were carried out on a standardized back-to-back gear test rig acc. to DIN ISO 14635-1 [20] with an axial distance of a = 91.5 mm. The test rig consists of two gearboxes (test and slave transmission) which are driven by an electric motor and distorted against each other to load the test gears with a certain amount of torque. For the test gear variants, which were ground with different metalworking fluids, two tests were carried out for each variant under constant conditions and the micropitting damage was evaluated on the basis of the mean profile variation due to micropitting ffm [21]. After grinding, the gear flanks were cleaned of metalworking fluid residues using compressed air and stored in VCI bags. The use of chemically more aggressive agents or preservatives was avoided to emphasize carry-over effects. Volatile corrosion inhibitors (VCI) protect workpieces by outgassing amine-based corrosion inhibitors (containing the elements N, C, H) and building up a temporary reaction-inhibiting film on the metal surface. The VCI film volatilizes after removal of the VCI bag [22]. The lubrication in the test rig was applied as splash lubrication using an industrial gear oil (CLP), which was also used in the first project phase in the two-disk contact [9]. Compared to the two-disk test, the oil was used in an increased viscosity class of VG 220 instead of VG 68 to avoid scuffing damages to the gear flanks. The degree of micropitting damage was determined by profile measurements of the mean profile deviation ffm at the test pinion flanks on a gear measuring centre of the type Klingelnberg P16. For this purpose, the profile shape before and after testing was measured an compared for five teeth evenly distributed over the circumference. In the new condition,
the test gears showed profile deviations up to ffm,new = 3 µm. The average profile deviations after testing are shown in Fig. 6. Due to different rill widths Rsm of the tooth flanks resulting from the grinding machine modification (see Fig. 3), the torque had to be reduced to T1 = 315 Nm for the tests of variant HSP2 and later to avoid other damages such as scuffing. Therefore, the variants ground before the modification of the grinding machine cannot be compared to the variants ground after the modification, except for the HSP1 and HSP2 variants, which were ground using the same metalworking fluid, but different machine settings. However, in between the series H, HS, HSP1 and in between the series of HSP2, E, ES, Eco, Dry relative comparisons are admissible. 4.2 Test results and discussion The test results (Fig. 6) show different degrees of micropitting damage depending on the metalworking fluid composition of the grinding process. For the hydrocrack grinding oils, a reduction of 50% in mean profile deviation due to micropitting occurs if a sulfur additive (HS) is added to the additive-free hydrocrack oil (H). The amount of micropitting damage increases when the phosphor additive is added to the grinding oil (HSP). Since the average roughness profiles of the test gears are all at the same level, the tendency of the amount of micropitting damage cannot be explained by the average surface roughness. However, it is noticeable that the variant H with the highest micropitting damage after grinding showed reduced residual compressive stress amounts compared to the variants HS and HSP, see Fig. 4. When changing from variant HSP2 to the additive-free ester oil (E), a reduction in the amount of micropitting damage can be observed. In contrast to the hydrocrack oil, the addition of the sulfur additive does not improve the micropitting load capacity (ES). The test gears ground with the market product (Eco) show a similar micropitting wear behavior as the gears of the variants E and ES. As an individual study without a repeat test, a gear set was ground dry in the last finishing stroke and also examined once in the micropitting test. The micropitting damage occurring for this variant lies in between the HSP2 and the E, ES and Eco variants. Mean profile deviation ffm [µm]
HV1
T1=500 Nm
14
Modification of the grinding machine T1=315 Nm
12 10 8 6 4 2 H
HS
HSP1 HSP2 E ES Metalworking fluid variant
Test conditions n1 = 1500 min-1 T1 = 500 / 315 Nm T = 90°C N1 = 2.2·106 cycles Ntests = 2 tests
Eco
Dry
Run-In T1 = 61 Nm N1 = 135 000 cycles Damage criteria Micropitting HSP1
2 mm
Fig. 6: Mean profile deviation ffm due to micropitting
5 Discussion The improvement of the wear behavior of parts ground with ester oil compared to those parts which were ground with hydrocrack oil has already been observed in the two-disk tests in the first period of the project [9]. Furthermore, both the gear tests and the two-disk tests indicate better performance for those parts that had been ground using metalworking fluids with only one active component (e.g. HS or E) instead of fluids without
active component (H) or more than one active component (HSP, ES). As the test gear analyses after grinding (section 3) show, the different micropitting capacities cannot be attributed to the average surface roughness, residual stresses and hardness profiles of the gears. However, the test results of the gear tests and the two-disk tests [9] indicate that the metalworking fluid influences the wear behavior of the workpiece in some way. Possible ways of information transfer from manufacturing to application are discussed in Fig. 7. On the one hand it has been shown that the inner boundary layer, mainly concerning the micro- or nanostructure, can be preconditioned by manufacturing processes in a favorable way as proposed by Berlet [5] and Scherge [6]. However, this change in microstructure has not yet been observed in the FIB-SEM analyses of the test gears. Therefore, the analyses should be extended to measurements of the nanotopography and the nanohardness of the gears to find indications to their frictional and wear behavior. Energy input
Wear in application
Thesis: The energy input influences the workpiece microstructure insufficient preconditioning
high defect density
Chemical surface coverage Thesis: The metalworking fluid remains on the surface and influences the transmission oil metallic surface (simplified)
δ+ H
production optimum
O δ− δ+
Fe
Power densitiy in grinding
Preconditioning of the microstructure (inner boundary layer)
δ+
C δ−
δ+
C ester oil (simplified)
Preconditioning of the chemical surface coverage (outer boundary layer)
Fig. 7: Possible ways of information transfer from manufacturing to application [4, 5, 6, 23] On the other hand, the outer boundary layer is chemically preconditioned by the metalworking fluid as well. Analyses of the chemical surface coverage show that the metalworking fluid molecules are carried over into the final application despite intensive cleaning e.g. in hexane and acetone [9]. Since the additives of the transmission oil in the are chemically related to the additives of the metalworking fluid, they aim at the same "docking points" on the metal surface to form lubrication films or build up protective layers [4]. If those points are still covered by the molecules of the metalworking fluid, the transmission oil additives might be blocked or decelerated in their working mechanisms. With regard to the test results on gear flanks and two-disk contacts, homogenous films of metalworking fluid components of only one type (e.g. HS or E) and of lower binding energies (Ester) proved to be favorable [4, 13, 23].
6 Summary and Conclusion The subject of this work was to investigate the influence of the metalworking fluid (here: grinding oil) on the micropitting wear of the gears in their later application. For this purpose, several variants of test gears were produced, which differed in terms of the composition of the metalworking fluid. All other process parameters, including the transmission oil in the gear test rig, remained constant. The test results indicate that the micropitting wear in the test is dependent of the metalworking fluid composition in the manufacturing process. Those gears which were ground with additive-free hydrocrack oil show the most progressive wear after the test, while the gears which were ground using ester-based grinding oils show smaller wear amounts. These findings are in accordance with pitting tests [24] and two-disk tests [9], which also show improved wear
resistance for parts ground with ester-based grinding oils. Regarding the boundary layer, low wear resistance correlates with oxide layers, reduced amounts of compressive stresses or lack of surface-active additives in the grinding oil, which are all indications of higher heat flux into the work pieces during the grinding process (e.g. variants H and Dry). These observations show that a certain additive content in the metal working fluid is necessary to support the tribological properties of the microstructure even below the specification of grinding burn [15]. Furthermore, chemical analyses provide evidence that even after thorough cleaning of the test parts there are still constituents of the metalworking fluid taken over into the final application [24]. In case of gears, the additives of the transmission oil are designed to build protection layers on the gear flanks. Residues of the metalworking fluid, which remain on the gear flanks after grinding, interfere with the buildup of such protection layers if they cannot be removed from the surface. For this reason, metalworking fluids containing ester oil might lead to better wear behavior, since ester oil is characterized as surface-active, but of low binding energy [4]. Acknowledgements This work was supported by the German Research Foundation (DFG) [Project Numbers BR2905/44-1 and BR2905/44-2]. Furthermore, we thank Fuchs Europe Schmierstoffe GmbH and Fuchs Wisura GmbH for their consulting in terms of oil choice.
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Reproducible differences in the degree of micropitting wear were observed for gears which had been ground using different metalworking fluids Ways of information transfer are discussed to combine the molecular effects of the metalworking fluid with the wear behavior in the final application The interactions between metalworking fluid and transmission oil are of great importance for the wear behavior of the ground surface