damage [4], leading to secondary - possibly more catastrophic - damage mechanisms. In this study, a cyclic impact test is used in a lab-2-field context to simulate the contact situation between valve ball and seat [2]. With this test it is possible to reproduce the impact as well as small movements of the ball during valve closure. Results show that WC-based material pairings perform best in this cyclic impact situation, whereas Cobased materials should be avoided to extend the lifetime of the components. The comparison of wear tracks after tribological tests with results of failure analysis from worn field components validate the experimental test method. By fitting the resulting data to an empirical wear law that takes into account a higher wear rate during the wear-in phase and a lower one in the steady-state regime [5], it is possible to rank the life expectancy of ball valves for down-hole pump application featuring various mate 2 Experimental Ball-seat pairings were selected based on their presence/ importance in field application and their potential Aus Wissenschaft und Forschung 14 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 DOI 10.30419/ TuS-2019-0008 1 Introduction In down-hole pumps, multiple tribological loads act on the individual components. One of the critical parts for the oil production process is the sucker rod pump with the main valve components ball and valve seat [1]. A variety of material pairings are used to secure the functionality of this construction group. For a specific selection of appropriate materials, in-field tests are too general and time-consuming for material selection. It is assumed that impacts play a crucial role in triggering incipient From test rig to down-hole pump: Ranking material pairings for ball valves according to their impact wear resistance Christian Katsich, Ulrike Cihak-Bayr, Stefan Hönig, Stefan J. Eder* Im Rahmen dieser Arbeit wird ein Lab-2-Feld Ansatz zum Vergleich des Verschleißwiderstandes unterschiedlicher Werkstoffpaarungen von Kugelventilen bei der Rohölförderung vorgestellt. Für die Umstellung von Stellitzu Hartmetallkomponenten ist die Kenntnis der Haltbarkeit und der Lebensdauer dieser neuen Werkstoffkombinationen erforderlich. Es wurde ein Arbeitsfluss etabliert, welcher auf zyklischen Schlagversuchen basiert und die Auflösung geringer Verschleißunterschiede von Hartmetallen ermöglicht. Der Ausblick beschreibt die Prognose einer zu erwarteten Lebensdauer basierend auf einem Verschleißgesetz, welches zwischen Einlauf- und Konstantverschleiß unterscheidet. Schlüsselwörter Impact-Test, Ölförderung, Lebenszeitprognose We present a lab-2-field type approach to comparing the wear resistance of several material pairings of ball valves used in oil production. With the ongoing transition from stellite to hard metals in down-hole pump valve components, the durability and service life of these new material pairings must be known. We have established a workflow based on cyclic impact testing that allows us to resolve even slight differences in the performance of hard metals with distinct chemical compositions in a statistically meaningful way. We give an outlook into extrapolating an expected service life based on a wear law that differentiates between wear-in and steady-state wear regimes. Keywords Impact testing, oil production, life time prediction Kurzfassung Abstract * Dipl.-Ing. Christian Katsich Orcid-ID: https: / / orcid.org/ 0000-0001-8525-1105 Dipl.-Ing. Dr.mont. Ulrike Cihak-Bayr Orcid-ID: https: / / orcid.org/ 0000-0001-8858-4511 AC2T research GmbH, 2700 Wiener Neustadt, Austria Dipl.-Ing. Dr. Stefan Hönig OMV Exploration & Production GmbH, 2230 Gänserndorf, Austria Dipl.-Ing. Dr.techn. Stefan J. Eder Orcid-ID: https: / / orcid.org/ 0000-0002-2902-3076 AC2T research GmbH, 2700 Wiener Neustadt, Austria Institute of Engineering Design and Product Development, Vienna University of Technology, 1060 Vienna, Austria T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 14 benefit to extend the lifetime of the construction part. The test material pairings (ball vs. seat) are WC vs. WC, TiC-WC vs. WC and stellite vs. stellite. The chemical compositions of all tested materials are summarized in Table 1. Ball and seat materials from component parts were used to produce test samples. The seat sample dimension was given by the original seat geometry, from which the sample was cut out using metallographic preparation techniques. The sample test surface was ground and polished down to 1 µm diamond. The surface preparation method was adjusted to reproduce the measured seat surface roughness in the contact. Cleaned balls of dimension Ø 23.8 mm (for 1.75” tubing) were used without any further preparation. The experimental impact simulation of ball-seat contact was successfully established with the “CIAT” (cyclic impact abrasion test) test principle under 2-body conditions. The test setup is based on the HT-CIAT principle, which was developed at AC2T research GmbH, introduced in [2], and allows 2and 3-body cyclic impact testing at temperatures up to 800 °C. The CIAT sample holder for the ball, custommanufactured for this project by OMV E&P, is shown in Figure 1. The ball, which is fixed on the plunger, hits the sample (seat) under an impact angle of 45°. The kinematics of the test setup allow a sliding movement of approximately 1.25 mm after impact. The chosen testing parameters were determined with the help of a FEM simulation of the ball-seat system that was used to derive critical von Mises stresses beneath the contact area after impact for various impact heights. The critical von Mises stress was defined as the tensile stress of WC-Co with Co compositions matching those of used parts in the field. This combination of impact, followed by sliding, simulates the closing movement of the ball into the seat. The number of cycles for determination of ball-seat contact performance was set to 40, 200, 1000, 2500, 5000, and 25000. The impact energy is defined by the falling height as well as the plunger/ ball weight and is ~20 mJ at a frequency of 1.5 Hz. Characterization of wear behaviour was performed by measurement of 3D topography and SEM/ EDX. 3D topography measurements were carried out by Leica ® confocal white light microscopy to determine geometric properties of wear tracks on seat and ball samples. The Leica ® mountain map software was used to calculate maximum penetration, mean penetration, and wear volume of the wear tracks. All tests were performed 6 times to improve statistical evaluation. Aus Wissenschaft und Forschung 15 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 DOI 10.30419/ TuS-2019-0008 sample C W Co Cr Ni Fe Mo Ti seat WC 3.2 83.2 12.7 1.0 - - - seat stellite 1.6 14.9 53.4 22.8 2.4 2.2 1.9 ball WC 5.2 58.5 - - 12.8 - 1.7 21.9 ball TiC-WC 3.2 88.1 8.7 - - - - ball stellite 4.5 18.9 40.9 33.1 - 1.4 1.3 - Table 1: Chemical composition [wt.-%] of ball and seat materials Figure 1: Modified CIAT: plunger with mounted ball sample and seat sample in 45° position Figure 2: Mean penetration of seat samples Figure 3: Maximum penetration of seat samples T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 15 me for the TiC-WC ball is 2.3 times higher than for the WC vs. WC system with 278 ± 172 µm 3 · 10 6 at 25 k cycles. For reference, the stellite system suffers a wear volume 175 times higher than the best performer. All geometric wear parameters of the ball topography analysis are summarized in Figure 5. Due to the test setup, the contact location on the ball was not changed for an entire set of cycles (40 to 25 k cycles). As a result of this test procedure, the analyzed values represent a total sum of 33740 cycles. The diagram clearly shows the influence of the ball material on the wear of the system ball-seat. On the WC ball, the wear marks are almost negligible with very low penetration depths and wear volumes. Changing to the TiC-WC ball material in contact with the same seat material, we observe considerably more wear on the lighter hard metal ball. This effect is most pronounced for the wear volume, where it causes a difference of a factor of ~95 and can be observed even macroscopically. 4 Discussion The connection between the geometric parameters of mean penetration and maximum penetration can be explained by the observed wear mechanisms. Figure 6 presents a SEM image of the wear track on the seat sample WC vs. WC after 25 k cycles. On the surface of the WC grains, we observed wear marks in the form of scratches, abrasive marks, and surface roughening [3]. The metallic Cobased matrix indicates minor wash-out effects, but wear debris formation is also caused by the fracture of WCgrains that are still bonded in the matrix. The partially oxidized debris can be found as material adhering to the rim of the wear track. The most noticeable effect is the removal of single WC grains from the hard metal surface. This wear mechanism is responsible for the formation of maximum penetration (wear depth), while the mean penetration (mean wear depth) results mainly from grain loss, continuous wear on grains, and matrix removal. Aus Wissenschaft und Forschung 16 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 DOI 10.30419/ TuS-2019-0008 3 Results The geometric analyses of the impact areas on seat samples are presented in Figure 2 to Figure 4. Figure 2 shows the geometric mean penetration on the impact area caused by the impacting ball. For all materials, a pronounced running-in behaviour can be observed. The small increase of mean penetration between 5 k and 25 k cycles indicates a steady-state condition of wear. Best performance can be achieved with WC samples (vs. WC balls) at all measured cycles, ending up with a mean penetration of 132 ± 72 nm at 25 k cycles. Similar behaviour is exhibited by a WC sample when it is hit by the TiC- WC ball, but with an increased level of wear (195 ± 30 nm at 25 k cycles). The stellite-type system starts with 303 ± 111 nm at 40 cycles, which is a factor of ~ 5 higher than WC vs. WC. At 25 k cycles, a mean penetration of 10.8 ± 4.5 µm was calculated for the stellite seat sample. The geometric parameter of maximum penetration gives similar results compared to the mean penetration depth (Figure 3). The WC vs. WC system exhibits low wear with 0.63 ± 0.20 µm at 25 k cycles. The change of ball material from WC to TiC-WC increases the maximum wear depth to 1.00 ± 0.24 µm at 25 k cycles. In the highlevel wear regime, the maximum penetration in the stellite system was measured to be 23.81 ± 9.85 µm at 25 k cycles. The high standard deviation reveals poorly predictable wear behaviour of this material pairing for impact loading. The measured wear volumes of the seat materials are presented in Figure 4. The ranking of the material pairings is the same as that for the previously evaluated geometric parameters. At low cycle numbers (1000 to 2500), a kink in the wear volume curve can be observed for the hard metal systems. This may be attributed to the wear debris, adhering to the wear track and thus modifying the wear surface significantly. A slight increase of the standard deviation results from adhering matter offsetting the geometric analysis. The measured wear volu- Figure 4: Wear volume of seat samples Figure 5: Wear parameter of ball samples T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 16 A comparison of the wear track surface of a WC seat sample in Figure 6 with the worn contact surface of a WC seat from the field (Figure 7) indicates the validity of the experimental test setup in the lab. Similar wear mechanisms can be found despite the different atmospheric conditions to which the valve components were subjected. This includes the dry contact and the presence of oxygen in the air compared to the conditions in the field. The cumulative wear in the system ball-seat is the sum of the volume loss of ball and seat, so for a complete appraisal of the system’s resistance to impact wear, both contributions must be considered. As mentioned above, due to the design of the CIAT experiment, the orientation of the ball does not change between the six individual impact experiments on the seat that make up a batch. This means that the wear scar on the ball after 33,740 impact cycles does not have a corresponding wear scar on the seat sample, while the ball geometry is not analyzed after each of the sets of cycles for which seat data have been recorded. To be able to compare ball and seat data in a meaningful way, we have extrapolated the wear on the seat using the wear-in and steady-state contributions to the Barwell wear law [5, 7] (1) where V w is the volumetric wear loss, A denotes running-in wear volume, τ is the number of running-in cycles, k the steady-state wear rate (in the unit of volume/ cycle), and t the number of impact cycles. A glance at Figure 8 reveals that the wear volumes extrapolated using the fit function in Eq. 1 follow the timedependent trend very well, and that a purely linear approach (such as the Archard wear law [6]) assuming linear behaviour from the beginning would yield completely different results. The combined volumetric wear losses of ball and seat are shown in the bar graph in Figure 9, where the differences in performance between the WC vs. WC and the TiC-WC vs. WC systems become obvious. Aus Wissenschaft und Forschung 17 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 DOI 10.30419/ TuS-2019-0008 Figure 8: Extrapolated wear volume for seat sample Figure 9: Comparison of ball and seat sample wear volume at 33740 cycles Figure 6: Wear surface of WC seat after 25 k cycles (WC vs. WC) Figure 7: Wear surface of WC seat from the field (failure analysis) T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 17 gratefully acknowledged for financially supporting the endowed professorship tribology at the Vienna University of Technology (Grant no. WST3-F-5031370/ 001- 2017) in collaboration with AC2T research GmbH. The authors would like to thank the OMV management for the permission to present this work. Particular thanks to Christian Decker and Gerald Zehethofer for their professional support. Literature [1] M. H OY , B. K OMETER , P. B ÜRßNER , G. P USCALAU , S. E DER : SRP Equipment Customization Creating Value by Increasing Run Life in a Low Oil Price Environment, SPE-190958-MS, proceedings “SPE Artificial Lift Conference and Exhibition”, Society of Petroleum Engineers, The Woodlands TX, USA, 2018 [2] H. W INKELMANN , E. B ADISCH , M. K IRCHGASSNER , H. D ANNINGER : Wear mechanisms at high temperatures - Part 1: wear mechanisms of different Fe-based alloys at elevated temperatures, Tribology Letters 34, 155-166, 2009. [3] M.G. G EE , A. G ANT , B. R OEBUCK : Wear mechanisms in abrasion and erosion of WC/ Co and related hardmetals, Wear 263, 137-148, 2007. [4] H. W INKELMANN , H. R OJACZ , S. J. E DER , M. V ARGA , S. N UGENT : Influence of Momentum and Energy on Materials: An Experimental and Molecular Dynamics Approach for Impact Phenomena. steel research international, 88(10), 1600445, 2017. [5] S. J. E DER , G. F ELDBAUER , D. B IANCHI , U. C IHAK-BAYR , G. B ETZ , A. V ERNES : Applicability of macroscopic wear and friction laws on the atomic length scale. Physical Review Letters, 115(2), 025502, 2015. [6] J. A RCHARD : Contact and rubbing of flat surfaces. Journal of applied physics, 24(8), 981-988, 1953. [7] F.T. B ARWELL : Wear of metals. Wear, 1(4), 317-332, 1958. Aus Wissenschaft und Forschung 18 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 DOI 10.30419/ TuS-2019-0008 5 Conclusion and outlook In our contribution we have outlined a lab-2-field type workflow to predict the wear volume of different material pairings used in ball-seat valves in down-hole pumps according to their impact resistance. The material pairing stellite vs. stellite was chosen as a benchmark since it has been in widespread use until very recently. While the differences in volumetric wear loss between the benchmark system and the hard metal pairings are obvious, our workflow can also differentiate between the performances of hard metal pairings with only minor differences in chemical composition. It is planned to apply the described workflow as a tool for facilitating a lifetime ranking and prediction as well as for aiding incoming goods inspection. For the former, the wear extrapolation from section 4 was used to estimate the number of cycles required to produce a wear scar of 0.1 mm 3 , see Figure 10. By assigning the obtained valve life (in million cycles) of the best-performing material pairing a value of 100 %, one can easily read the relative service lives of the other tested material pairings from the table on the right. This kind of extrapolation assumes that the damage mechanism does not change in the steady-state wear regime. A confirmation of this claim is currently under way, as several pairings are being tested for one million cycles (with a total test time of one week per data point). The first results are reassuring that the maximum number of 25 k impact cycles in our workflow is sufficient for a reasonable extrapolation into the range of several million cycles. Acknowledgements This work was carried out at the “Excellence Centre of Tribology”, Wiener Neustadt, Austria in the framework of the Austrian COMET Program (Project K2, XTribology, no. 849109). The government of Lower Austria is Figure 10: Lifetime ranking via wear volume extrapolation T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 18