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Influence of surface roughness and phosphate coating on galling resistance of medium-grade carbon steel
Journal Pre-proof Influence of surface roughness and phosphate coating on galling resistance of medium-grade carbon steel B. Podgornik, F. Kafexhiu, A. Nevosad, E. Badisch PII:
S0043-1648(19)31007-5
DOI:
https://doi.org/10.1016/j.wear.2019.203180
Reference:
WEA 203180
To appear in:
Wear
Received Date: 21 June 2019 Revised Date:
16 December 2019
Accepted Date: 31 December 2019
Please cite this article as: B. Podgornik, F. Kafexhiu, A. Nevosad, E. Badisch, Influence of surface roughness and phosphate coating on galling resistance of medium-grade carbon steel, Wear (2020), doi: https://doi.org/10.1016/j.wear.2019.203180. 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.
CRediT author statement
Bojan Podgornik: Conceptualization, Methodology, Validation, Writing, Editing. Fevzi Kafexhiu: Investigation – load scanner testing. Andreas Nevosad: Investigation – SEM microscopy. Ewald Badisch: Supervising, Validation, Writing – Reviewing.
Influence of surface roughness and phosphate coating on galling resistance of mediumgrade carbon steel B. Podgornik1*, F. Kafexhiu1, A. Nevosad2, E. Badisch2 1 Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia 2 AC2T research GmbH, Viktor-Kaplan-Straße 2/C, 2700 Wiener Neustadt, Austria
Abstract The aim of this research work was to investigate the influence of surface roughness and presence of phosphate coating on the galling resistance of medium-grade carbon steel. Galling resistance was evaluated in terms of coefficient of friction level and stability, critical loads for galling initiation, surface quality, and volume of adhered material, analysed by load-scanner test rig as a function of load. Results of the investigation show that best galling resistance is not provided by very smooth surfaces but with semi rough ones (Ra = 0.21 – 0.28 µm), obtained by fine turning. Furthermore, by phosphating and formation of low-friction Mn-P coating superior galling resistance for medium-grade carbon steel is provided, at the same time diminishing effect of surface roughness. Keywords: medium-grade carbon steel, phosphating, surface roughness, galling resistance, friction
1. Introduction Fluid or gas leakage in thread connections occurs under the lack or absence of thread compound, but mainly by wear of the contact surfaces. Because of a shrink fit, pressure in the thread connector gradually increases during tightening, finally providing tight integration with high contact pressure and high-pressure gas sealing even at high temperatures [1]. However, relative movement of thread contact surfaces during connection assembly and disassembly as well as eventual vibrations and temperature dilatations lead to wear, mainly being identified as galling wear [2]. According to the ASTM standard G40 - Standard Terminology Relating to Wear and Erosion, galling is defined as “A form of surface damage arising between sliding solids, distinguished by macroscopic, usually localized, roughening and creations of protrusion rising above the original surface. It often includes material transfer, or plastic flow, or both”. Thread galling also leads to non-uniform load distribution and reduced resistance of the connection, thus determining the connection life. In order to improve thread galling resistance the entire surface of the connection is normally coated by grease or treated by solid lubricants [3]. In this respect, a lot of thread connection investigations have been made, which focused on the sealing mechanisms and sealability studies [1,4,5], effect of grease degradation under high shear stress and temperature [6] and thread compound rating for improved galling resistance [3]. As long known by the industry and shown by the experimental results [3], inclusion of heavy metal particles, graphite and anti-wear additives in the grease and thread compounds provides superior anti-galling properties, otherwise not guaranteed by pure grease or non-doped thread compounds. However, environmental concerns and legislations dictate the need to use environmentally friendly lubricants and thread compounds normally used to provide proper sealing, lubrication and anti-rusting properties of the connection, or even to eliminate their use. Removing dopants and compounds from the makeup process of threaded connection during casing and tubing not only reduces the risk of the environment contamination but also provides a cleaner and safer working area [6]. Conversely, the removal of the dopants and compounds greatly increases the risk of galling damage to connections during power-tight assembly makeup. For undoped or even compound-free connections the antigalling properties need to be provided by other surface treatments [7], such as electrodeposited copper [4], phosphating or phosphate conversion coating [8]. Phosphatizing is an inexpensive and well-developed process, defined as the treatment of a metal surface to give a reasonably hard, electrically non-conducting surface coating of insoluble phosphate. The coating is formed as a result of a topochemical reaction by employing a solution of phosphoric acid and salt. Phosphate coating is a type of conversion coating, which is highly adherent to the underlying metal and is considerably more absorptive than the metal [9]. Due to its economic availability, ease of application and ability to afford excellent corrosion resistance, wear protection, adhesion and lubricative properties it plays a significant role in many industries [8]
*
Corresponding author, tel.: +38614701930, e-mail: [email protected]
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and mechanical applications. Furthermore, surface structure of phosphate coating also acts as a base and carrier for lubricants, thus making these coatings interesting for tribological applications with high galling risk. The main types of phosphate conversion coatings in usage are zinc phosphate and manganese phosphate coatings [10]. Especially manganese phosphate coatings are preferred in sliding tribological applications since they can be used both in dry and lubricated conditions for protection against scuffing, seizure and galling. In this respect scuffing is classified as a sudden catastrophic surface failure mainly caused by extreme local surface temperatures, which are dictated by high contact pressures and sliding velocity. Typical for scuffing is solidphase welding and material transfer between sliding surfaces with parts of the surfaces becoming rough and heavily distorted, thus loosing integrity and functionality [11]. It was shown that manganese phosphate coatings can reduce abrasive wear by one third, eliminate adhesive wear at elevated temperature [12] and show reduced friction at increased loads [13,14]. Another important aspect in terms of adhesive wear and providing high galling resistance are surface roughness and surface topography. As shown in [15-17] surface topography has a decisive influence on galling initiation, material transfer and transfer layer build up. In general, polishing of the surface, especially in the case of hard and wear resistant surfaces is known to result in reduced friction when sliding against softer steel. Polishing removes irregularities and asperities from the surface, thus eliminating potential sources for initiation of material transfer and substantially improving galling resistance [15]. However, investigations performed on textured surfaces indicate that even with “rougher” surfaces which are cheaper to produce, selection of proper topography and roughness parameters can result in improved galling resistance [17]. Therefore, the aim of this research work was to investigate the influence of surface roughness, combined either with phosphate coating or without it on the galling resistance of medium-grade carbon steel. Galling resistance was evaluated in terms of coefficient of friction and surface quality and analysed as a function of load.
2. Experimental 2.1 Material and treatments Material used in this investigation was commercial medium-grade carbon steel used in oil and gas industry with the following composition (in wt. %): 0.3% C, 0.3% Si, 0,5% Mn, 0.9% Cr, 0.5% Mo. Material was delivered in the form of pre-hardened hot rolled bars with a hardness of 200 HB. From the delivered bars, specimens in the shape of cylinders (∅10 × 100 mm) were machined using classical turning process. Reference specimens (sample C) were obtained by fine turning, resulting in an average surface roughness (Ra) of 0.28 µm. By changing turning parameters (cutting speed, depth of cut, feed rate) specimens with higher surface roughness were prepared; sample D (Ra = 0.32 µm) and sample E (Ra = 0.46 µm). On the other hand, combining fine turning with grinding (sample B) and polishing (sample A) smoother surfaces with an average surface roughness of 0.21 µm and 0.10 µm were obtained, respectively. Rz roughness values varied in the range of 0.55 µm for the polished surface and 2.14 µm for the coarse turning process. The list of specimens and designations is given in Table 1 and the 3D images of different technical surfaces of the medium-grade carbon steel prior to testing shown in Fig. 1. The device used to acquire 3D images was Alicona InfiniteFocus G4 specified in Chapter 2.3. Table 1: Specimens designation, preparation and resulting surface roughness prior to testing Sample A Sample A*
Coating Mn-P
Sample B Sample C Sample D Sample E
-
Process fine turning + polishing fine turning + polishing phosphating fine turning + grinding fine turning turning coarse turning
* roughness before coating/phosphating
2
+
Ra [µm] 0.10 µm 0.10 µm*
Rz [µm] 0.55 µm 0.55 µm*
0.21 µm 0.28 µm 0.32 µm 0.46 µm
0.96 µm 1.22 µm 1.38 µm 2.14 µm
Figure 1: 3D topography images (false colour illustration) of technical surfaces of medium-grade carbon steel prior to testing: a) sample A – fine turning and polishing, b) sample B – fine turning and grinding, c) sample C – fine turning, d) sample D – turning, e) sample E – coarse turning. In order to determine the effect of phosphating one set of specimens prepared by fine turning and polishing (sample A) was coated in a commercial manganese phosphating process. Specimens were first cleaned, activated in a Mn-phosphate solution (1.5 g/l) at 40 °C for 4 min [18] and then coated at a temperature of 90 °C for 14 min, resulting in 5-10 µm thick homogeneous and dense crystal-like Mn-P coating. Coating consisted of abundant small manganese phosphate crystals and relatively few large ones, as shown in Fig. 2, acquired by JEOL JSM-6500F Field Emission Scanning Electron Microscope. Similar surface morphology of such Mn-P coatings has been shown by various authors [10,19,20]. Coating XRD analysis confirmed the presence of hureaulite phase.
(a)
(b)
Figure 2: SEM micrographs of the Mn-P coating surface morphology deposited on the medium-grade carbon steel; a) 500x magnification and b) 3,000x magnfication
2.2 Galling testing Galling resistance of the investigated steel was evaluated in a load-scanning test rig (Fig. 3) found as a very suitable method for comparing metallic materials in terms of galling resistance [21]. There are also other galling test methods available, including ASTM G98 and G196 Standard Test Methods for Galling Resistance of Materials and Material Couples, the Twist Compression Test, and Block on Cylinder (Hummel) test, among others. However, cross-cylinder configuration and spring-based loading mechanism of the load-scanner allow gradual increase in normal load during a forward sliding stroke, with each point along the wear scar corresponding to a specific load and displaying an unique contact history [21,22]. Results can be analysed in terms of coefficient of friction level, friction increase as a function of load, material transfer intensity and critical loads for galling initiation and transfer layer build up. Besides being very simple it also provides results representing a whole range of loads during one single test run. In the current case, the test configuration involved self-mated medium-grade carbon steel specimens of the selected surface roughness. However, for tests involving Mn-P phosphate coating, coated specimen was set as a moving cylinder, forced to slide against stationary
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uncoated one of a chosen surface roughness. Before each test the test specimens were ultrasonically cleaned in ethanol and dried in air.
FN = 200 - 1500 N
FS vs = 0.01 m/s Moving specimen holder
moving cylinder (uncoated/ coated)
stationary cylinder
Loading spring
(a)
(b)
Figure 3: Load-scanning test rig configuration; (a) schematic and (b) actual photo
All tests were performed under dry single-stroke sliding conditions at room temperature, a constant sliding speed of 0.01 m/s and normal load in a range 200 – 1500 N, corresponding to the nominal pure elastic Hertzian contact pressure of over 2 GPa. However, when exceeding yield stress of the carbon steel plastic deformation will occur resulting in much lower contact stress. After reaching the maximum load, test was stopped, specimens unloaded and wear tracks on both contact specimens analysed. During testing coefficient of friction was measured as a function of load and analysed in terms of friction stability. 2.3 Surface analysis After galling tests wear tracks were analysed including optical microscopy (OM), Scanning Electron Microscopy (SEM) and 3D topography analyses. Analyses included measurement of surface roughness and volume of adhered material at 350 N, 700 N and 1000 N, as well as critical galling loads determination performed by wear track microscopy. Four critical loads were identified. Critical load LC1 when first single scratch extending across the entire wear track is observed (Fig. 4a), critical load LC2 indicating single galling event when first macroscopic evidence of the material adhesion and transfer is observed (Fig. 4b), appearance of more severe galling with denser scratching pattern as critical load LC3 (Fig. 4c) and critical load LC4 for extensive galling (Fig. 4d). Critical loads were ratified by analyzing coefficient of friction curves, with changes in coefficient of friction (friction spikes) corresponding to galling and scratching events. OM microscopy involved Nikon Microphot FXA metallographic microscope equipped with Hitachi 3CCD camera, while SEM microscopy was performed by JEOL JSM-6500F Field Emission Scanning Electron Microscope. Alicona InfiniteFocus G4 3D mini coordinate measurement machine was used to do topography analysis over the uniform area of 1 mm2. Finally, the resulting surface topography in the wear tracks was measured employing Leica DCM3D 3D confocal microscope.
a)
b)
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d)
c)
Figure 4: Example for critical load identification; (a) single scratch extending across the entire wear track (LC1), (b) single galling event (LC2), (c) denser scratching pattern (LC3) and (d) extensive galling (LC4)
3. Results and discussion 3.1 Effect of surface roughness Influence of surface roughness on friction behaviour of self-mated uncoated medium-grade carbon steel is shown in Fig. 5. In the case of fine turned and polished surface (sample A) initial coefficient of friction was ∼0.12 and except of occasional friction spikes remained at that level up to the load of 1000 N. Friction spikes correspond to individual galling events, as confirmed by post-test microscopic analysis of the wear track (Fig. 6a). When exceeding load of 1000 N sudden abrupt increase in friction, indicating severe galling was observed, as shown in Fig. 5 and confirmed by wear track analysis (Fig. 6b). Average coefficient of friction over the whole load range investigated (200 – 1400 N) was 0.14 (Fig. 7). Switching from fine turned and polished surface to fine turned and grinded one (sample B), improved friction and galling performance of the investigated steel was obtained. Although the same level of initial friction of about 0.12 was shown, it remained low and stable more or less over the entire load range. Exception were few friction spikes observed above 1000 N load (Fig. 5) with the average coefficient of friction remaining at the level of 0.12. Stable friction with only few galling events observed at high loads and indicated by friction spikes was found also for fine turned surface without any further grinding or polishing post-processing (sample C). However, in this case friction level has been increased with the average coefficient of friction reaching level of 0.13 (Fig. 7). Further increase in surface roughness (samples D and E) led to increase in initial and average coefficient of friction, as well as to more unstable friction behaviour and appearance of friction spikes already at low loads. However, although the same level of average coefficient of friction of about 0.14, as found for the smoothest fine turned and polished surface was obtained there was no abrupt change or increase in friction, as shown in Fig. 5.
(a)
(b)
Figure 5: Coefficient of friction vs. normal load for uncoated medium-grade carbon steel with different surface roughness; (a) samples A, B and C, and (b) samples C, D and E
5
b)
a)
Figure 6: OM micrographs of sample A wear track; a) first galling events (FN = 350 N) and b) severe galling (FN = 1000 N); arrows indicate the direction of sliding Summarizing all the average coefficient of friction measured and displayed in Fig. 7, a decrease of 10-15 % is observed for sample B and sample C compared to others.
Figure 7: Average coefficient of friction for uncoated medium-grade carbon steel with different surface roughness By performing microscopic analysis of the wear tracks, four critical loads were determined. As shown in Fig. 8 the highest critical load for the appearance of first galling events or scratches was shown by Sample B and followed by using only fine turning process (sample C). As compared to other specimens and types of surface preparation critical load LC1 was increased from 200 N to 500 N and critical load LC2 from 350 N to 1100 N. In terms of more severe galling the best results were shown by fine turned surface (sample C), with critical load LC3 exceeding 1500 N and not showing any signs of extensive galling (Fig. 9). Fine turned surface (sample C) was closely followed by fine turned and grinded surface (sample B; LC3 ≈ 1300 N) still not displaying any extensive galling. However, making the surface smoother (sample A) or rougher (samples D & E) had similar effect on galling resistance of medium-grade carbon steel, achieving lower critical loads and also showing extensive galling between 900 N and 1100 N load. Critical load for the first galling events (LC2) was reduced to ∼400N, for more severe galling (LC3) to ∼550 N.
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Figure 8: Critical loads for different galling steps in the case of uncoated medium-grade carbon steel 3D images of the wear tracks at maximum load level after load-scanner test are given in Fig. 9. In general, plastic deformation of the surface regions can be observed for all samples investigated which can be correlated to the high load-scanner stresses exceeding yield strength of the carbon steel. For the smoothest sample A, pronounced wear marks are visible which is in good agreement with the fluctuation in the coefficient of friction and its high level in Fig. 5. For samples B and C there are almost no signs of galling and scratching present. When the surface roughness is increasing (samples D and E) some galling events occur at the highest loaded surface.
Figure 9: 3D topographies (false colour illustration) of after load-scanner testing at maximum normal load: a) sample A – fine turning and polishing, b) sample B – fine turning and grinding, c) sample C – fine turning (1), d) sample D – turning (2), e) sample E – turning (3). Finally, the specimens and the effect of surface roughness was evaluated also in terms of volume of adhered material, measured by 3D optical microscope at the positions corresponding to load of 350 N, 700 N and 1000 N (Fig. 10). In the case of fine turned and polished surface extensive amount of material has been adhered to the contact surface, ranging from 3·10-5 – 7·10-5 mm3, as shown in Fig. 10. Switching to fine turning and grinding (sample B) volume of adhered material has been reduced down to 1·10-5 – 2·10-5 mm3. However, the best results with volume of adhered material being below 0.5·10-5 mm3 even above 1000 N load were shown by simply fine
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turned surface (sample C). Further increase in surface roughness obtained by changing turning parameters had no positive effect. On contrary, it led to deteriorated results, as shown in Fig. 10. Volume of adhered material has increased for surface roughness above 0.3 µm, especially at high loads, exceeding fine turned and grinded surface (sample B) but still beating polished surface (sample A).
Figure 10: Volume of adhered material for uncoated medium-grade carbon steel with different surface roughness
Results of this investigation indicate that very smooth surfaces do not provide the most beneficial galling resistance of the investigated steel. The presence of machining grooves leads to contact area reduction and redistribution thus postponing adhesion and formation of patches of adhered material. Furthermore, machining grooves also allow trapping of released wear particles and patches of adhered material and prevent scratching of the contact surfaces and re-adhesion of the patches (Fig. 11). However, too high surface roughness with very distinctive machining grooves will result in considerable contact area reduction and contact pressure increase, which has negative effect and will again accelerate material adhesion and consequently ploughing and scratching of contact surfaces. As shown in Figs. 7 and 8, the lowest friction and the highest galling resistance of self-mated medium-grade carbon steel is obtained when using machined/fine turned surfaces with average surface roughness in the range between 0.21 µm and 0.28 µm.
(a)
(b)
Figure 11: Surface scratching and ploughing caused by patches of adhered material; sample D, load 400 N; (a) 500x magnification and (b) 2,000x magnification 3.2 Effect of Mn-P coating
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The effect of Mn-P coating was evaluated by phosphating fine turned and polished specimen (sample A*) and testing it against uncoated medium-grade carbon steel with three different surface roughness values; 0.10 µ m (sample A), 0.28 µm (sample C) and 0.32 µm (sample D). Sliding against Mn-P coating gave almost complete galling protection, up to the maximum load of 1500 N, as indicated by low and very stable friction (Fig. 12), with the average coefficient of friction for sample C (Ra = 0.28 µm) being reduced to about 0.11. Furthermore, although some scratches were observed on the wear track of sample C, found at similar loads as observed for the contact of non-treated medium-grade carbon steel (LC = 300 – 500 N; Fig. 13), no galling events or severe galling could be detected, as shown in Fig. 14. This was confirmed also by the measurement of the volume of adhered material being below 0.5·10-5 mm3 for the whole load range investigated (Fig. 15).
Figure 12: Coefficient of friction vs. normal load for medium-grade carbon steel tested against Mn-P coated one.
Figure 13: Critical loads for the appearance of first scratches (LC1) and denser scratching pattern (LC1’) for medium-grade carbon steel tested against Mn-P coated one
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a1)
b1)
a2)
b2)
Figure 14: OM micrographs of the wear track for sample C tested against Mn-P coated medium-grade carbon steel; a1) uncoated sample C and a2) Mn-P coated counter-sample at the position of first scratches (∼350 N), and b1) uncoated sample C and b2) Mn-P coated counter-sample at 1000 N load; arrows indicate the direction of sliding
Figure 15: Volume of adhered material for medium-grade carbon steel tested against Mn-P coated one.
In terms of the combined effect of phosphating (Mn-P coating) and surface roughness, using very smooth (sample A) or medium rough surface (sample C) had no evident effect, neither in terms of coefficient of friction stability and average value, nor critical loads for the appearance of scratches and volume of adhered material, as shown in Figs. 12, 13 and 15. However, further increase in surface roughness (sample D), when combined with phosphated and Mn-P coated counter-surface resulted in more unstable and higher coefficient of friction (Fig. 12), especially at lower loads, but at the same time in higher critical loads for the appearance of scratches (Fig.
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13) and reduced volume of adhered material (Fig. 15). At higher surface roughness values Mn-P coating transfer is accelerated and is trapped within machining grooves (Fig. 16), as confirmed by the EDS analysis of the trapped particles (point P3) shown in Fig. 7. This transferred coating material then provides improved galling resistance. On the other hand, positive effect of the phosphate coating might be limited to fewer sliding cycles (assembly/disassembly) due to increased coating removal rate.
(a)
(b)
Figure 16: OM micrographs of the wear track for sample D tested against Mn-P coated medium-grade carbon steel; a) position at the load of 300 N and b) 1000 N; arrows indicate the direction of sliding
a)
b) • P2 • P3
• P1
Spectrum Unworn coating Uncoated steel Wear track P1 Wear track P2 Wear track P3
C 9.34 0.45 2.53 3.19 29.7
O 43.52
F 2.49
P 15.4
2.55
0.44
11.71
2.31
Cr 0.85 0.92 0.88 0.66
Mn 20.5 1.28 1.67 1.86 3.01
Fe 8.75 97.42 91.89 94.07 52.61
All results in wt. %
Figure 17: SEM micrographs at (a) 500x and (b) 2,000x magnification with the corresponding EDS analysis of the wear track for sample D tested against Mn-P coated medium-grade carbon steel (FN = 700 N); white square in Fig. 17a represents area of Fig. 17b, P1-P3 are three different spots of the wear track analysed, while white arrows indicate the direction of sliding
Results of the load-scanner tests performed against Mn-P coated steel indicate that through the formation of lowfriction coating with good anti-adhesive properties, phosphating successfully protects medium-grade carbon steel against galling at the same time nullifying or at least greatly diminishing the effect of surface roughness. By preventing material sticking and adhesion Mn-P coating protects contact surfaces at high loads while sufficient surface roughness promotes coating transfer, thus providing superior galling resistance.
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4. Conclusions Results of this investigation can be summarized as follows: - Very smooth surfaces (fine turning + polishing) do not provide the most beneficial galling resistance of the medium-grade carbon steel. The best performance characterized by low and stable coefficient of friction, high critical loads for galling initiation and material transfer, as well as the lowest volume of adhered material is obtained by semi rough machined surfaces (Ra = 0.21 – 0.28 µm), prepared by fine turning/grinding. Machining grooves lead to contact area redistribution and trapping of wear particles and patches of adhered material, thus postponing adhesion and scratching of the contact surfaces to higher loads. - Through the formation of low-friction Mn-P coating with good anti-adhesive properties, phosphating of one of the contact surfaces successfully protects medium-grade carbon steel against galling at the same time diminishing the effect of surface roughness and providing superior galling resistance. However, for coated contact increase in surface roughness, although increasing friction has positive effect on galling resistance. It promotes coating transfer and protection against galling.
Acknowledgement This work was funded by the Austrian COMET Program (Project K2 XTribology, no. 849109) and carried out at the “Excellence Centre of Tribology” in collaboration with the Institute of Metals and Technology in Ljubljana, Slovenia (ARRS Research programme P2-0050). The authors thank to H. Zacharias for the management of the testing specimens.
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Highlights • • • • •
Effect of surface roughness and presence of phosphate coating on the galling resistance of medium grade carbon steel has been investigated. Very smooth surfaces do not provide the best galling resistance of the medium grade carbon steel. The best performance is obtained by semi rough machined surfaces. Machining grooves lead to contact area redistribution and trapping of wear particles. Phosphating successfully protects medium grade carbon steel against galling and diminishes roughness effect.
Conflict of Interest and Authorship Conformation Form
As corresponding author, I Prof. Bojan Podgornik , hereby confirm on behalf of all authors that:
o
All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
o
This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.
o
The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript
S0043-1648(19)31007-5
DOI:
https://doi.org/10.1016/j.wear.2019.203180
Reference:
WEA 203180
To appear in:
Wear
Received Date: 21 June 2019 Revised Date:
16 December 2019
Accepted Date: 31 December 2019
Please cite this article as: B. Podgornik, F. Kafexhiu, A. Nevosad, E. Badisch, Influence of surface roughness and phosphate coating on galling resistance of medium-grade carbon steel, Wear (2020), doi: https://doi.org/10.1016/j.wear.2019.203180. 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.
CRediT author statement
Bojan Podgornik: Conceptualization, Methodology, Validation, Writing, Editing. Fevzi Kafexhiu: Investigation – load scanner testing. Andreas Nevosad: Investigation – SEM microscopy. Ewald Badisch: Supervising, Validation, Writing – Reviewing.
Influence of surface roughness and phosphate coating on galling resistance of mediumgrade carbon steel B. Podgornik1*, F. Kafexhiu1, A. Nevosad2, E. Badisch2 1 Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia 2 AC2T research GmbH, Viktor-Kaplan-Straße 2/C, 2700 Wiener Neustadt, Austria
Abstract The aim of this research work was to investigate the influence of surface roughness and presence of phosphate coating on the galling resistance of medium-grade carbon steel. Galling resistance was evaluated in terms of coefficient of friction level and stability, critical loads for galling initiation, surface quality, and volume of adhered material, analysed by load-scanner test rig as a function of load. Results of the investigation show that best galling resistance is not provided by very smooth surfaces but with semi rough ones (Ra = 0.21 – 0.28 µm), obtained by fine turning. Furthermore, by phosphating and formation of low-friction Mn-P coating superior galling resistance for medium-grade carbon steel is provided, at the same time diminishing effect of surface roughness. Keywords: medium-grade carbon steel, phosphating, surface roughness, galling resistance, friction
1. Introduction Fluid or gas leakage in thread connections occurs under the lack or absence of thread compound, but mainly by wear of the contact surfaces. Because of a shrink fit, pressure in the thread connector gradually increases during tightening, finally providing tight integration with high contact pressure and high-pressure gas sealing even at high temperatures [1]. However, relative movement of thread contact surfaces during connection assembly and disassembly as well as eventual vibrations and temperature dilatations lead to wear, mainly being identified as galling wear [2]. According to the ASTM standard G40 - Standard Terminology Relating to Wear and Erosion, galling is defined as “A form of surface damage arising between sliding solids, distinguished by macroscopic, usually localized, roughening and creations of protrusion rising above the original surface. It often includes material transfer, or plastic flow, or both”. Thread galling also leads to non-uniform load distribution and reduced resistance of the connection, thus determining the connection life. In order to improve thread galling resistance the entire surface of the connection is normally coated by grease or treated by solid lubricants [3]. In this respect, a lot of thread connection investigations have been made, which focused on the sealing mechanisms and sealability studies [1,4,5], effect of grease degradation under high shear stress and temperature [6] and thread compound rating for improved galling resistance [3]. As long known by the industry and shown by the experimental results [3], inclusion of heavy metal particles, graphite and anti-wear additives in the grease and thread compounds provides superior anti-galling properties, otherwise not guaranteed by pure grease or non-doped thread compounds. However, environmental concerns and legislations dictate the need to use environmentally friendly lubricants and thread compounds normally used to provide proper sealing, lubrication and anti-rusting properties of the connection, or even to eliminate their use. Removing dopants and compounds from the makeup process of threaded connection during casing and tubing not only reduces the risk of the environment contamination but also provides a cleaner and safer working area [6]. Conversely, the removal of the dopants and compounds greatly increases the risk of galling damage to connections during power-tight assembly makeup. For undoped or even compound-free connections the antigalling properties need to be provided by other surface treatments [7], such as electrodeposited copper [4], phosphating or phosphate conversion coating [8]. Phosphatizing is an inexpensive and well-developed process, defined as the treatment of a metal surface to give a reasonably hard, electrically non-conducting surface coating of insoluble phosphate. The coating is formed as a result of a topochemical reaction by employing a solution of phosphoric acid and salt. Phosphate coating is a type of conversion coating, which is highly adherent to the underlying metal and is considerably more absorptive than the metal [9]. Due to its economic availability, ease of application and ability to afford excellent corrosion resistance, wear protection, adhesion and lubricative properties it plays a significant role in many industries [8]
*
Corresponding author, tel.: +38614701930, e-mail: [email protected]
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and mechanical applications. Furthermore, surface structure of phosphate coating also acts as a base and carrier for lubricants, thus making these coatings interesting for tribological applications with high galling risk. The main types of phosphate conversion coatings in usage are zinc phosphate and manganese phosphate coatings [10]. Especially manganese phosphate coatings are preferred in sliding tribological applications since they can be used both in dry and lubricated conditions for protection against scuffing, seizure and galling. In this respect scuffing is classified as a sudden catastrophic surface failure mainly caused by extreme local surface temperatures, which are dictated by high contact pressures and sliding velocity. Typical for scuffing is solidphase welding and material transfer between sliding surfaces with parts of the surfaces becoming rough and heavily distorted, thus loosing integrity and functionality [11]. It was shown that manganese phosphate coatings can reduce abrasive wear by one third, eliminate adhesive wear at elevated temperature [12] and show reduced friction at increased loads [13,14]. Another important aspect in terms of adhesive wear and providing high galling resistance are surface roughness and surface topography. As shown in [15-17] surface topography has a decisive influence on galling initiation, material transfer and transfer layer build up. In general, polishing of the surface, especially in the case of hard and wear resistant surfaces is known to result in reduced friction when sliding against softer steel. Polishing removes irregularities and asperities from the surface, thus eliminating potential sources for initiation of material transfer and substantially improving galling resistance [15]. However, investigations performed on textured surfaces indicate that even with “rougher” surfaces which are cheaper to produce, selection of proper topography and roughness parameters can result in improved galling resistance [17]. Therefore, the aim of this research work was to investigate the influence of surface roughness, combined either with phosphate coating or without it on the galling resistance of medium-grade carbon steel. Galling resistance was evaluated in terms of coefficient of friction and surface quality and analysed as a function of load.
2. Experimental 2.1 Material and treatments Material used in this investigation was commercial medium-grade carbon steel used in oil and gas industry with the following composition (in wt. %): 0.3% C, 0.3% Si, 0,5% Mn, 0.9% Cr, 0.5% Mo. Material was delivered in the form of pre-hardened hot rolled bars with a hardness of 200 HB. From the delivered bars, specimens in the shape of cylinders (∅10 × 100 mm) were machined using classical turning process. Reference specimens (sample C) were obtained by fine turning, resulting in an average surface roughness (Ra) of 0.28 µm. By changing turning parameters (cutting speed, depth of cut, feed rate) specimens with higher surface roughness were prepared; sample D (Ra = 0.32 µm) and sample E (Ra = 0.46 µm). On the other hand, combining fine turning with grinding (sample B) and polishing (sample A) smoother surfaces with an average surface roughness of 0.21 µm and 0.10 µm were obtained, respectively. Rz roughness values varied in the range of 0.55 µm for the polished surface and 2.14 µm for the coarse turning process. The list of specimens and designations is given in Table 1 and the 3D images of different technical surfaces of the medium-grade carbon steel prior to testing shown in Fig. 1. The device used to acquire 3D images was Alicona InfiniteFocus G4 specified in Chapter 2.3. Table 1: Specimens designation, preparation and resulting surface roughness prior to testing Sample A Sample A*
Coating Mn-P
Sample B Sample C Sample D Sample E
-
Process fine turning + polishing fine turning + polishing phosphating fine turning + grinding fine turning turning coarse turning
* roughness before coating/phosphating
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+
Ra [µm] 0.10 µm 0.10 µm*
Rz [µm] 0.55 µm 0.55 µm*
0.21 µm 0.28 µm 0.32 µm 0.46 µm
0.96 µm 1.22 µm 1.38 µm 2.14 µm
Figure 1: 3D topography images (false colour illustration) of technical surfaces of medium-grade carbon steel prior to testing: a) sample A – fine turning and polishing, b) sample B – fine turning and grinding, c) sample C – fine turning, d) sample D – turning, e) sample E – coarse turning. In order to determine the effect of phosphating one set of specimens prepared by fine turning and polishing (sample A) was coated in a commercial manganese phosphating process. Specimens were first cleaned, activated in a Mn-phosphate solution (1.5 g/l) at 40 °C for 4 min [18] and then coated at a temperature of 90 °C for 14 min, resulting in 5-10 µm thick homogeneous and dense crystal-like Mn-P coating. Coating consisted of abundant small manganese phosphate crystals and relatively few large ones, as shown in Fig. 2, acquired by JEOL JSM-6500F Field Emission Scanning Electron Microscope. Similar surface morphology of such Mn-P coatings has been shown by various authors [10,19,20]. Coating XRD analysis confirmed the presence of hureaulite phase.
(a)
(b)
Figure 2: SEM micrographs of the Mn-P coating surface morphology deposited on the medium-grade carbon steel; a) 500x magnification and b) 3,000x magnfication
2.2 Galling testing Galling resistance of the investigated steel was evaluated in a load-scanning test rig (Fig. 3) found as a very suitable method for comparing metallic materials in terms of galling resistance [21]. There are also other galling test methods available, including ASTM G98 and G196 Standard Test Methods for Galling Resistance of Materials and Material Couples, the Twist Compression Test, and Block on Cylinder (Hummel) test, among others. However, cross-cylinder configuration and spring-based loading mechanism of the load-scanner allow gradual increase in normal load during a forward sliding stroke, with each point along the wear scar corresponding to a specific load and displaying an unique contact history [21,22]. Results can be analysed in terms of coefficient of friction level, friction increase as a function of load, material transfer intensity and critical loads for galling initiation and transfer layer build up. Besides being very simple it also provides results representing a whole range of loads during one single test run. In the current case, the test configuration involved self-mated medium-grade carbon steel specimens of the selected surface roughness. However, for tests involving Mn-P phosphate coating, coated specimen was set as a moving cylinder, forced to slide against stationary
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uncoated one of a chosen surface roughness. Before each test the test specimens were ultrasonically cleaned in ethanol and dried in air.
FN = 200 - 1500 N
FS vs = 0.01 m/s Moving specimen holder
moving cylinder (uncoated/ coated)
stationary cylinder
Loading spring
(a)
(b)
Figure 3: Load-scanning test rig configuration; (a) schematic and (b) actual photo
All tests were performed under dry single-stroke sliding conditions at room temperature, a constant sliding speed of 0.01 m/s and normal load in a range 200 – 1500 N, corresponding to the nominal pure elastic Hertzian contact pressure of over 2 GPa. However, when exceeding yield stress of the carbon steel plastic deformation will occur resulting in much lower contact stress. After reaching the maximum load, test was stopped, specimens unloaded and wear tracks on both contact specimens analysed. During testing coefficient of friction was measured as a function of load and analysed in terms of friction stability. 2.3 Surface analysis After galling tests wear tracks were analysed including optical microscopy (OM), Scanning Electron Microscopy (SEM) and 3D topography analyses. Analyses included measurement of surface roughness and volume of adhered material at 350 N, 700 N and 1000 N, as well as critical galling loads determination performed by wear track microscopy. Four critical loads were identified. Critical load LC1 when first single scratch extending across the entire wear track is observed (Fig. 4a), critical load LC2 indicating single galling event when first macroscopic evidence of the material adhesion and transfer is observed (Fig. 4b), appearance of more severe galling with denser scratching pattern as critical load LC3 (Fig. 4c) and critical load LC4 for extensive galling (Fig. 4d). Critical loads were ratified by analyzing coefficient of friction curves, with changes in coefficient of friction (friction spikes) corresponding to galling and scratching events. OM microscopy involved Nikon Microphot FXA metallographic microscope equipped with Hitachi 3CCD camera, while SEM microscopy was performed by JEOL JSM-6500F Field Emission Scanning Electron Microscope. Alicona InfiniteFocus G4 3D mini coordinate measurement machine was used to do topography analysis over the uniform area of 1 mm2. Finally, the resulting surface topography in the wear tracks was measured employing Leica DCM3D 3D confocal microscope.
a)
b)
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d)
c)
Figure 4: Example for critical load identification; (a) single scratch extending across the entire wear track (LC1), (b) single galling event (LC2), (c) denser scratching pattern (LC3) and (d) extensive galling (LC4)
3. Results and discussion 3.1 Effect of surface roughness Influence of surface roughness on friction behaviour of self-mated uncoated medium-grade carbon steel is shown in Fig. 5. In the case of fine turned and polished surface (sample A) initial coefficient of friction was ∼0.12 and except of occasional friction spikes remained at that level up to the load of 1000 N. Friction spikes correspond to individual galling events, as confirmed by post-test microscopic analysis of the wear track (Fig. 6a). When exceeding load of 1000 N sudden abrupt increase in friction, indicating severe galling was observed, as shown in Fig. 5 and confirmed by wear track analysis (Fig. 6b). Average coefficient of friction over the whole load range investigated (200 – 1400 N) was 0.14 (Fig. 7). Switching from fine turned and polished surface to fine turned and grinded one (sample B), improved friction and galling performance of the investigated steel was obtained. Although the same level of initial friction of about 0.12 was shown, it remained low and stable more or less over the entire load range. Exception were few friction spikes observed above 1000 N load (Fig. 5) with the average coefficient of friction remaining at the level of 0.12. Stable friction with only few galling events observed at high loads and indicated by friction spikes was found also for fine turned surface without any further grinding or polishing post-processing (sample C). However, in this case friction level has been increased with the average coefficient of friction reaching level of 0.13 (Fig. 7). Further increase in surface roughness (samples D and E) led to increase in initial and average coefficient of friction, as well as to more unstable friction behaviour and appearance of friction spikes already at low loads. However, although the same level of average coefficient of friction of about 0.14, as found for the smoothest fine turned and polished surface was obtained there was no abrupt change or increase in friction, as shown in Fig. 5.
(a)
(b)
Figure 5: Coefficient of friction vs. normal load for uncoated medium-grade carbon steel with different surface roughness; (a) samples A, B and C, and (b) samples C, D and E
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b)
a)
Figure 6: OM micrographs of sample A wear track; a) first galling events (FN = 350 N) and b) severe galling (FN = 1000 N); arrows indicate the direction of sliding Summarizing all the average coefficient of friction measured and displayed in Fig. 7, a decrease of 10-15 % is observed for sample B and sample C compared to others.
Figure 7: Average coefficient of friction for uncoated medium-grade carbon steel with different surface roughness By performing microscopic analysis of the wear tracks, four critical loads were determined. As shown in Fig. 8 the highest critical load for the appearance of first galling events or scratches was shown by Sample B and followed by using only fine turning process (sample C). As compared to other specimens and types of surface preparation critical load LC1 was increased from 200 N to 500 N and critical load LC2 from 350 N to 1100 N. In terms of more severe galling the best results were shown by fine turned surface (sample C), with critical load LC3 exceeding 1500 N and not showing any signs of extensive galling (Fig. 9). Fine turned surface (sample C) was closely followed by fine turned and grinded surface (sample B; LC3 ≈ 1300 N) still not displaying any extensive galling. However, making the surface smoother (sample A) or rougher (samples D & E) had similar effect on galling resistance of medium-grade carbon steel, achieving lower critical loads and also showing extensive galling between 900 N and 1100 N load. Critical load for the first galling events (LC2) was reduced to ∼400N, for more severe galling (LC3) to ∼550 N.
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Figure 8: Critical loads for different galling steps in the case of uncoated medium-grade carbon steel 3D images of the wear tracks at maximum load level after load-scanner test are given in Fig. 9. In general, plastic deformation of the surface regions can be observed for all samples investigated which can be correlated to the high load-scanner stresses exceeding yield strength of the carbon steel. For the smoothest sample A, pronounced wear marks are visible which is in good agreement with the fluctuation in the coefficient of friction and its high level in Fig. 5. For samples B and C there are almost no signs of galling and scratching present. When the surface roughness is increasing (samples D and E) some galling events occur at the highest loaded surface.
Figure 9: 3D topographies (false colour illustration) of after load-scanner testing at maximum normal load: a) sample A – fine turning and polishing, b) sample B – fine turning and grinding, c) sample C – fine turning (1), d) sample D – turning (2), e) sample E – turning (3). Finally, the specimens and the effect of surface roughness was evaluated also in terms of volume of adhered material, measured by 3D optical microscope at the positions corresponding to load of 350 N, 700 N and 1000 N (Fig. 10). In the case of fine turned and polished surface extensive amount of material has been adhered to the contact surface, ranging from 3·10-5 – 7·10-5 mm3, as shown in Fig. 10. Switching to fine turning and grinding (sample B) volume of adhered material has been reduced down to 1·10-5 – 2·10-5 mm3. However, the best results with volume of adhered material being below 0.5·10-5 mm3 even above 1000 N load were shown by simply fine
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turned surface (sample C). Further increase in surface roughness obtained by changing turning parameters had no positive effect. On contrary, it led to deteriorated results, as shown in Fig. 10. Volume of adhered material has increased for surface roughness above 0.3 µm, especially at high loads, exceeding fine turned and grinded surface (sample B) but still beating polished surface (sample A).
Figure 10: Volume of adhered material for uncoated medium-grade carbon steel with different surface roughness
Results of this investigation indicate that very smooth surfaces do not provide the most beneficial galling resistance of the investigated steel. The presence of machining grooves leads to contact area reduction and redistribution thus postponing adhesion and formation of patches of adhered material. Furthermore, machining grooves also allow trapping of released wear particles and patches of adhered material and prevent scratching of the contact surfaces and re-adhesion of the patches (Fig. 11). However, too high surface roughness with very distinctive machining grooves will result in considerable contact area reduction and contact pressure increase, which has negative effect and will again accelerate material adhesion and consequently ploughing and scratching of contact surfaces. As shown in Figs. 7 and 8, the lowest friction and the highest galling resistance of self-mated medium-grade carbon steel is obtained when using machined/fine turned surfaces with average surface roughness in the range between 0.21 µm and 0.28 µm.
(a)
(b)
Figure 11: Surface scratching and ploughing caused by patches of adhered material; sample D, load 400 N; (a) 500x magnification and (b) 2,000x magnification 3.2 Effect of Mn-P coating
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The effect of Mn-P coating was evaluated by phosphating fine turned and polished specimen (sample A*) and testing it against uncoated medium-grade carbon steel with three different surface roughness values; 0.10 µ m (sample A), 0.28 µm (sample C) and 0.32 µm (sample D). Sliding against Mn-P coating gave almost complete galling protection, up to the maximum load of 1500 N, as indicated by low and very stable friction (Fig. 12), with the average coefficient of friction for sample C (Ra = 0.28 µm) being reduced to about 0.11. Furthermore, although some scratches were observed on the wear track of sample C, found at similar loads as observed for the contact of non-treated medium-grade carbon steel (LC = 300 – 500 N; Fig. 13), no galling events or severe galling could be detected, as shown in Fig. 14. This was confirmed also by the measurement of the volume of adhered material being below 0.5·10-5 mm3 for the whole load range investigated (Fig. 15).
Figure 12: Coefficient of friction vs. normal load for medium-grade carbon steel tested against Mn-P coated one.
Figure 13: Critical loads for the appearance of first scratches (LC1) and denser scratching pattern (LC1’) for medium-grade carbon steel tested against Mn-P coated one
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a1)
b1)
a2)
b2)
Figure 14: OM micrographs of the wear track for sample C tested against Mn-P coated medium-grade carbon steel; a1) uncoated sample C and a2) Mn-P coated counter-sample at the position of first scratches (∼350 N), and b1) uncoated sample C and b2) Mn-P coated counter-sample at 1000 N load; arrows indicate the direction of sliding
Figure 15: Volume of adhered material for medium-grade carbon steel tested against Mn-P coated one.
In terms of the combined effect of phosphating (Mn-P coating) and surface roughness, using very smooth (sample A) or medium rough surface (sample C) had no evident effect, neither in terms of coefficient of friction stability and average value, nor critical loads for the appearance of scratches and volume of adhered material, as shown in Figs. 12, 13 and 15. However, further increase in surface roughness (sample D), when combined with phosphated and Mn-P coated counter-surface resulted in more unstable and higher coefficient of friction (Fig. 12), especially at lower loads, but at the same time in higher critical loads for the appearance of scratches (Fig.
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13) and reduced volume of adhered material (Fig. 15). At higher surface roughness values Mn-P coating transfer is accelerated and is trapped within machining grooves (Fig. 16), as confirmed by the EDS analysis of the trapped particles (point P3) shown in Fig. 7. This transferred coating material then provides improved galling resistance. On the other hand, positive effect of the phosphate coating might be limited to fewer sliding cycles (assembly/disassembly) due to increased coating removal rate.
(a)
(b)
Figure 16: OM micrographs of the wear track for sample D tested against Mn-P coated medium-grade carbon steel; a) position at the load of 300 N and b) 1000 N; arrows indicate the direction of sliding
a)
b) • P2 • P3
• P1
Spectrum Unworn coating Uncoated steel Wear track P1 Wear track P2 Wear track P3
C 9.34 0.45 2.53 3.19 29.7
O 43.52
F 2.49
P 15.4
2.55
0.44
11.71
2.31
Cr 0.85 0.92 0.88 0.66
Mn 20.5 1.28 1.67 1.86 3.01
Fe 8.75 97.42 91.89 94.07 52.61
All results in wt. %
Figure 17: SEM micrographs at (a) 500x and (b) 2,000x magnification with the corresponding EDS analysis of the wear track for sample D tested against Mn-P coated medium-grade carbon steel (FN = 700 N); white square in Fig. 17a represents area of Fig. 17b, P1-P3 are three different spots of the wear track analysed, while white arrows indicate the direction of sliding
Results of the load-scanner tests performed against Mn-P coated steel indicate that through the formation of lowfriction coating with good anti-adhesive properties, phosphating successfully protects medium-grade carbon steel against galling at the same time nullifying or at least greatly diminishing the effect of surface roughness. By preventing material sticking and adhesion Mn-P coating protects contact surfaces at high loads while sufficient surface roughness promotes coating transfer, thus providing superior galling resistance.
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4. Conclusions Results of this investigation can be summarized as follows: - Very smooth surfaces (fine turning + polishing) do not provide the most beneficial galling resistance of the medium-grade carbon steel. The best performance characterized by low and stable coefficient of friction, high critical loads for galling initiation and material transfer, as well as the lowest volume of adhered material is obtained by semi rough machined surfaces (Ra = 0.21 – 0.28 µm), prepared by fine turning/grinding. Machining grooves lead to contact area redistribution and trapping of wear particles and patches of adhered material, thus postponing adhesion and scratching of the contact surfaces to higher loads. - Through the formation of low-friction Mn-P coating with good anti-adhesive properties, phosphating of one of the contact surfaces successfully protects medium-grade carbon steel against galling at the same time diminishing the effect of surface roughness and providing superior galling resistance. However, for coated contact increase in surface roughness, although increasing friction has positive effect on galling resistance. It promotes coating transfer and protection against galling.
Acknowledgement This work was funded by the Austrian COMET Program (Project K2 XTribology, no. 849109) and carried out at the “Excellence Centre of Tribology” in collaboration with the Institute of Metals and Technology in Ljubljana, Slovenia (ARRS Research programme P2-0050). The authors thank to H. Zacharias for the management of the testing specimens.
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Highlights • • • • •
Effect of surface roughness and presence of phosphate coating on the galling resistance of medium grade carbon steel has been investigated. Very smooth surfaces do not provide the best galling resistance of the medium grade carbon steel. The best performance is obtained by semi rough machined surfaces. Machining grooves lead to contact area redistribution and trapping of wear particles. Phosphating successfully protects medium grade carbon steel against galling and diminishes roughness effect.
Conflict of Interest and Authorship Conformation Form
As corresponding author, I Prof. Bojan Podgornik , hereby confirm on behalf of all authors that:
o
All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
o
This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.
o
The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript