DETERMINATION OF MECHANISMS OF WEAR FOR ROLLING- -SLIDING CONTACT IN SELECTED OPERATIONAL CONDITIONS BY MEANS OF SURFACE TOPOGRAPHY AND MES STUDIES

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1 TRIBOLOGIA 3/2017 p Henryk BĄKOWSKI *, Bogusław ŁAZARZ * DETERMINATION OF MECHANISMS OF WEAR FOR ROLLING- -SLIDING CONTACT IN SELECTED OPERATIONAL CONDITIONS BY MEANS OF SURFACE TOPOGRAPHY AND MES STUDIES OKREŚLENIE MECHANIZMÓW ZUŻYWANIA SKOJARZENIA TOCZNO- -ŚLIZGOWEGO W WYBRANYCH WARUNKACH EKSPLOATACJI ZA POMOCĄ BADAŃ TOPOGRAFII POWIERZCHNI ORAZ MES Key words: Abstract Słowa kluczowe: Streszczenie rolling-sliding contact, surface topography, FEM, mechanism of wear. The paper presents the influence of selected operating factors, i.e. load, slip, and wear rate, and surface conditions. The wear mechanism between the cooperating rolling surfaces in the rolling/sliding combination varies due to the interaction of the complex stresses / deformations caused by the variable operating conditions. Surface roughness studies have allowed determining the stereological parameters that are specific to the set type of wear. In addition, simulation studies using the Finite Element Method in a roll-to-roll system have been used to determine the value and depth of maximum stresses and deformations occurring in the surface layer due to the interaction of two co-operating bodies. Geometric models have the same properties and disadvantages as the surface layer in a real object. Reflecting the operating conditions and defining the wear mechanisms in the proposed research allowed us to determine the state of the surface layer and predict its durability. skojarzenie toczno-ślizgowe, topografia powierzchni, MES, mechanizm zużycia. W pracy przedstawiono wpływ wybranych czynników eksploatacyjnych, tj. obciążenia, poślizgu i prędkości na zużycie i stan warstwy wierzchniej. Mechanizm zużywania pomiędzy współpracującymi powierzchniami tocznymi w skojarzeniu toczno-ślizgowym zmienia się na skutek oddziaływania złożonego stanu naprężeń/odkształceń wywołanego przez zmienne warunki eksploatacji. Przeprowadzone badania chropowatości powierzchni pozwoliły na określenie parametrów stereologicznych, które są charakterystyczne dla danego rodzaju zużycia. Dodatkowo przeprowadzone badania symulacyjne za pomocą metody elementów skończonych w układzie rolka-rolka pozwoliły na określenie wartości i głębokości zalegania maksymalnych naprężeń i odkształceń powstających w warstwie wierzchniej w wyniku oddziaływania wzajemnego dwóch ciał ze sobą współpracujących. Wykonane modele geometryczne miały takie same właściwości i wady, jakie ma warstwa wierzchnia w obiekcie rzeczywistym. Odzwierciedlenie warunków eksploatacji i określenie mechanizmów zużywania w zaproponowanych badaniach umożliwiło wyznaczenie stanu warstwy wierzchniej i prognozowanie jej trwałości. INTRODUCTION Railway rails are the most important element of a railroad and their wear is inevitable in world rail transport. Constant increase in the quality of produced rails allows for safe and fast rail travel. Wheel-rail wear is a very complex and difficult-to-analyse process [L. 1]. Constant change and overlap of various factors in a very short time make wear difficult to interpret. Thanks to the laboratory tests at Amsler's station in the roll-toroll combination system, it was possible to determine the mechanism and intensity of wear depending on the rolling stock, road profile and constraints existing on railway tracks, i.e. load, slide, and velocity, as well as the weather conditions. One of the wear-and-ting effects of rolling-and-sliding contact is flake wear products, whose stereological characteristics depend on the variable operating factors, materials used, the conditions, and * Silesian University of Technology, Faculty of Transport, ul. Krasińskiego 8, Katowice, Poland, henryk.bakowski@ polsl.pl.

2 6 T R I B O L O G I A 3/2017 properties of the surface layer of the material. The manufacturing of rail material that is suitable in both vertical and lateral rails, on arcs, and sections with heavy freight traffic as well as contact-fatigue damage is very difficult and even impossible. Wear will always be present, but the type and form will depend on varying operating conditions. The result of wear in the wheelrail combination is the flake wear products that peel off the top layers under the influence of variable operating factors. The formation and development of flake wear products are affected by the stresses (normal, tangential) formed during the contact of two co-operating elements (Fig. 1). In order to fully correlate the relation between wear and durability and reliability, it is important to consider all the significant types of stresses that occur in the considered friction node under variable external and internal influences [L. 2]. The completion of metallographic research is research done on light and mechanical profilographs. In order to determine the suitability of the types of rail steel examined, optimal operating conditions should be established depending on the type of rail steel, which requires additional profilographic and metallographic studies. Performing a 3-D roughness measurement on a profilographometer allowed us to develop a load-bearing curve for the samples to be tested and to determine the shape of the locations after the broken scales (flake wear products), which facilitated the explanation of the wear mechanism. In this way, it was possible to determine the wear of the rail depending on the stereological characteristics of the wear products. slide, and velocity. The study was conducted according to Hartley's polyselective experiment plan [L. 4]. In this plan, the minimum (-1), the central (0), and the maximum (+1) must first be specified. Testing conditions at the laboratory are similar to those taking place in real objects, with data taken from a selected section of the railway track. Comparative studies of rail steel were made at the Amsler laboratory station in accordance with PN-04332: In laboratory research, the most important operational parameters found in a real object that affect the durability of a wheel-rail combination are reflected. According to the operating conditions of the PKP track, the load for the passenger wagon is N/wheel (0), while for the empty freight wagon it is (-1), and N/wheel (+1), respectively [L. 5]. In this way, the force values and the stresses formed in the rolling-sliding contact in the friction node are determined. The minimum slippage value (γ = 0.3%) occurs when the train is running along a straight track, and the maximum (γ = 5%) in the arc combined with the slope and/or elevation, while the median value is determined only when the train is running through an arc. Workplace surveys were carried out at dry contact. The last parameter was velocity, and the values used were about 20 km/h, 40 km/h, and over 60 km/h, i.e. -1, 0, +1. All values are based on the rules and regulations in force [L. 6, 7]. During tribological studies at the Amsler station in a roll-to-roll system, the obtained wear products were subjected to a rophilographometric tests. In order to better understand the wear patterns of wear surfaces, profilographic measurements were made using a 3D MicroProf3000 FRT optical profilometer, with a magnification of x100 and an optical resolution of 0.15 μm. RESULTS Fig. 1. Rolling surface of rail (a), wheel-rail combination (b), wear products after friction (c) Rys. 1. Powierzchnia toczna szyny kolejowej (a), skojarzenie koło-szyna (b), produkty zużycia po odpadnięciu z powierzchni tarcia (c) MATERIAL, DEVICES AND TEST CONDITIONS Based on literature [L. 2, 3] and our own research, it has been found that operational parameters that have a significant impact on the durability of the rail are load, The preliminary study of rolling-and-sliding association was aimed at comparing the wear mechanisms in the real system, i.e. the wheel-rail and the laboratory station in the roll-to-roll system. Tribological studies have confirmed a high degree of consistency in the defects found on both associations, which demonstrates the correct choice of operating parameters (load, slide, and velocity). Metallographic studies were carried out by comparing the rolling surface of the railway track after the cooperation and the surface of samples after tribological studies. A view of the rolling surface and selected metallographic welds are shown in Figure 2. Changes that occur on the surface and below it are the result of normal and tangential stresses. At the contact area of the wheel with rail at a certain depth below the contact surface, the greatest stresses occur, which cause the fatigue wear of the material and cracks ( µm in the rail and µm in the roll and µm in the steel rail sample). After a certain number of cycles, cracks appear on the rolling surface and the resulting

3 T R I B O L O G I A 3/2017 wear products appear. Thus, the state of the surface layer of the rolling surface or railway rail is mapped onto the surface of the flake wear product. In order to better understand the mechanisms of the examined rollers, research of their profilographic surfaces and flake wear products were carried out (Fig. 3). Figure 3 shows 3D roughness profiles showing the effect of selected operating parameters on the rolling surface, and Figure 4 shows abrasive wear and fatigue wear. The smallest abrasive wear resistance is exhibited by nonheat-treated steel products. It was higher for bainitic 7 steel wear products, and the highest wear resistance of steel products was after heat treatment. Compared to the product consumed from the rolling surface of the rails used in the track, the particle in question is derived from the rail, which is subject to catastrophic wear and the rail should be subjected to thorough inspection. As a result of the profilographic survey of the flank surfaces of wear products, the curves defining the main directions characteristic for individual wear mechanisms were obtained, depending on Q load, γ slide, and n velocity (Fig. 4). a) view from the surface of the rail after fatigue wear; Place after extrusion is indicated by arrow, magnification x36 a) widok z powierzchni szyny po wystąpieniu zużycia zmęczeniowego; strzałką zaznaczono miejsce po złuszczeniu, pow. x36 b) metallographic section transverse rail along the direction of travel; Visible fatigue cracks in the surface layer at a depth of about μm b) zgład poprzeczny szyny wzdłuż kierunku jazdy; widoczne pęknięcia zmęczeniowe w warstwie powierzchniowej na głębokości około μm c) view from sample surface after abrasive wear; The arrows indicate the places after the spalling process, magnification x36 c) widok z powierzchni próbki po wystąpieniu zużycia ściernego; strzałką zaznaczono miejsca po złuszczeniu, pow. x36 d) metallographic section transverse face of the sample with visible changes in the surface layer at a depth of μm d) zgład poprzeczny czoła próbki z widocznymi zmianami w warstwie powierzchniowej na głębokości µm Fig. 2. A comparison of surface wear mechanisms: a), b) railway rails, c), d) rollers at the laboratory station Rys. 2. Porównanie mechanizmów zużywania w warstwie powierzchniowej: a), b) szyny kolejowej, c), d) rolki na stanowisku laboratoryjnym

4 8 TRIBOLOGIA 3/2017 Fig. 3. View of the wear surface a) and d), material curves (Abotta-Fireston curve), b), e), 2D surface profiles c) and f) Rys. 3. Widok powierzchni produktów zużycia a), d), krzywe udziału materiałowego (krzywa Abotta-Firestona) b), e), profile 2D c), f)

5 TRIBOLOGIA 3/ Fig. 4. Composition of surface wear stereometry: A for steel samples without heat treatment, B for steel samples after heat treatment, C for bainitic steel specimens, Spk determines surface wear resistance, with small values indicating Its high resistance, Svk is a measure of the ability of the fluid to maintain fluid through the mating surfaces Rys. 4. Zestawienie parametrów stereometrii powierzchni produktów zużycia: A dla próbek ze stali bez obróbki cieplnej, B dla próbek ze stali po obróbce cieplnej, C dla próbek ze stali bainitycznej, Spk określa odporność powierzchni na zużycie ścierne, przy czym małe jego wartości świadczą o jej dużej odporności, Svk jest miarą zdolności utrzymywania płynu przez powierzchnie współpracujące Simulation studies were conducted using the Finite Element Method in a roll-to-roll system. The geometric models presented are equipped with a gap identical to that created during operation. In order to compare the distribution and stress values, dry contact tests were performed (Fig. 5).

6 10 TRIBOLOGIA 3/2017 Fig. 5. Reduced stress distribution according to von Mises (MPa): a) crack propagation stage, b) stage of cohesion of product wear with surface, c) the step of closing the subsurface gap through the product of wear, d) one-point contact and e) wheel-rail contact Rys. 5. Rozkład naprężeń zredukowanych wg von Misesa (MPa): a) etap propagacji pęknięcia, b) etap utraty spójności produktu zużycia z powierzchnią, c) etap zamknięcia szczeliny podpowierzchniowej przez produkt zużycia, d) styk jednopunktowy oraz e) styk rozłożony w układzie koło-szyna CONCLUSIONS Based on conducted tribological, metallographic, and profilographic research, the following conclusions can be made: The destruction of the surface layer in rolling-sliding combination takes the form of a delamination wear mechanism in the form of flake wear products. Detachable products of wear have a shape and a dimension, depending on the material and operating factors influencing their formation and propagation as well as the type of wear. The dimension and shape of the wear products reflect the condition of the surface layer, and the technical condition of the object can be monitored successfully as a result of the efficient collection and identification of wear and friction surfaces. Work done under BK-254/RT1/2017.

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