TRIBOLOGICAL PROPERTIES OF MEDIUM-CARBON STEEL AFTER NITRIDING COMBINED WITH SUBSEQUENT OXIDATION AND IMPREGNATION

Abstract

The paper presents the results of examinations of tribological and corrosion properties of the layers produced on a C45 medium-carbon steel surface in the gas nitriding process combined with subsequent treatment – oxidation and impregnation of corrosion inhibitor. Investigations of the microstructure of these layers and their phase composition were carried out. Hardness of the layers was measured by Vickers method. Tribological properties (linear wear) of the layers were performed by means of the three-cylinder-cone method. Resistance to corrosion was determined by electrochemical methods. The investigations showed that the linear wear of the C45 steel samples with the layers produced in the combined processes of nitriding with subsequent oxidation and impregnation was smaller than that of steel samples without layers. Moreover, the corrosion resistance of these layers in aggressive solutions containing chloride ions was very good. słowa kluczowe: azotowanie, utlenianie, warstwa azotowana, zużycie, korozja. streszczenie W pracy omówiono wyniki badań właściwości tribologicznych i korozyjnych warstw wytwarzanych na powierzchni stali średniowęglowej C45 w procesie azotowania gazowego połączonym z następną obróbką – utlenianiem i impregnacją inhibitorem korozji. Przeprowadzono badania mikrostruktury tych warstw, jak również ich składu fazowego. Twardość warstw była mierzona metodą Vickersa. Właściwości tribologiczne (zużycie liniowe) warstw oceniano metodą trzy wałeczki–stożek. Badania odporności na korozję przeprowadzono metodami elektrochemicznymi. Badania wykazały, że zużycie liniowe próbek ze stali C45 z warstwami wytworzonymi w połączonych procesach azotowania z następnym utlenianiem i impregnacją, było mniejsze niż próbek ze stali bez warstw. Ponadto odporność korozyjna tych warstw w agresywnych roztworach zawierających jony chlorkowe była bardzo dobra. * Warsaw University of Technology, Faculty of Civil Engineering, Mechanics and Petrochemistry, ul. Łukasiewicza 17, 09-400 Płock, Poland, e-mail: [email protected] ** The Institute of Precision Mechanics (IMP), ul. Duchnicka 3, 01-796 Warszawa, Poland, e-mail: [email protected] *** PhD student of full-time doctoral studies, Warsaw University of Technology, Faculty of Civil Engineering, Mechanics and Petrochemistry, ul. Łukasiewicza 17, 09-400 Płock, Poland, e-mail: [email protected] INTRODuCTION Gas nitriding of steel, based on the diffusion of nitrogen on the steel surface, serves to improve fatigue life and resistance to wear by friction tools and machine parts [L. 1–6]. The nitriding process represents a low-temperature thermo-chemical treatment (500–650oC) that is widely applied in industry to various products, mainly made of steel, both to single units and to mass or serial production [L. 7–9]. It is a cost-effective alternative to widespread high temperature technologies (800–950oC) and thermochemical treatment methods, such as carburizing or carbonitriding. The nitriding processes are most often carried out in a gas atmosphere produced from partially dissociated ammonia [L. 10–12]. As the result of nitriding, a diffusion layer is formed, the microstructure and phase composition of which depend on temperature and process time, the chemical composition of steel on which 14 ISSN 0208-7774 T R I B O L O G I A 1/2019 the layer is generated, and on atmosphere composition [L. 13, 14]. Steels for nitriding should be in the quenched and tempered condition, because nitriding takes place at a lower temperature than that of the last tempering. Lower temperatures of nitriding enable the following: significant reduction of deformations and dimensional changes of treated components, the reduction of finishing costs, and the reduction of costs connected with the wear of equipment used for heat treatment at high temperatures [L. 15, 16]. The mentioned advantages cause the range of applications of traditional and modern nitriding techniques in industry worldwide to expand systematically. A significant interest was lately focused on developing nitride layers enhancing corrosion resistance. It is well known that nitrided layers have relatively good corrosion resistance in atmospheric conditions, but it is not sufficient when exposed on severe corrosive media, e.g., containing chloride ions [L. 17–19]. There are two reasons causing weak corrosion resistance of steels with thin and porous nitrided layers on it. The first reason is porosity in the case of carbon and low-alloyed steels, and the second reason is the damage of alloyed structure by the high temperature nitriding process in the case of corrosion resistant stainless and highly alloyed steels [L. 20]. As is known, very good corrosion resistance in aggressive environments containing chloride ions are shown in the combined processes of gas nitriding with subsequent treatment – oxidation and impregnation of corrosion inhibitors [L. 21–26]. The surface zone of the nitrided layers is characterized by a particular porosity; therefore, to ensure better corrosion resistance, these layers are impregnated with corrosion inhibitors. In the oxidation process performed on the surface of the nitrided layer, thin protective oxide coatings are formed, which ensure the tightness of the porous nitrided layer. Iron forms three oxides with oxygen: Fe2O3 (hematite), Fe3O4 (magnetite), and FeO (wustite) [L. 23]. The highest corrosion resistance is demonstrated by oxide coatings produced at temperatures above 500°C, containing a large amount of iron oxides Fe3O4 [L. 22]. The article discusses the results of examinations of tribological properties and corrosion resistance of the layers produced on the steel surface in gas nitriding process with subsequent treatment – oxidation and then impregnation of corrosion inhibitor. The investigations were carried out for medium-carbon steel of the C45 grade, which is often used for machine parts exposed to severe conditions due to wear by friction and corrosion damages.