APPLYING SURFACE PLASMA HARDENING FOR IMPROVINGTHE TRIBOLOGICAL CHARACTERISTICS OF STEEL PARTS

Abstract

This work presents the results of experimental studies on the application of surface plasma hardening to improve the tribological characteristics of steel marks of 40CrNi, 20Cr2Ni4A, and 34CrNi1Mn. According to the obtained results, it was established that, after plasma treatment, a modified layer with a thickness of 1–1.2 mm with high hardness and wear resistance is formed, consisting of a hardened layer of fine-grained martensite and, an intermediate layer of perlite and martensite. It was determined that, after treatment with a heating time of 3 min, the microhardness of steels 40CrNi and 20Cr2Ni4A doubles, and the steel 34CrNi1Mn increases 1.6 times, depending on the initial state, and the wear resistance of all steel samples increases, on average, 30 times. słowa kluczowe: utwardzanie plazmowe, mikrotwardość, odporność na zużycie, modyfikowana warstwa powierzchniowa, martenzyt, austenit szczątkowy, współczynnik tarcia, zużycie. Streszczenie W niniejszej pracy przedstawiono wyniki eksperymentalnych badań nad stosowaniem utwardzania plazmy powierzchniowej w celu poprawy charakterystyki tribologicznej próbek stalowych 40CrNi, 20Cr2Ni4A i 34CrNi1Mn. Zgodnie z uzyskanymi wynikami ustalono, że po obróbce plazmowej powstaje zmodyfikowana warstwa o grubości 1–1,2 mm o dużej twardości i odporności na ścieranie, składająca się z utwardzonej warstwy drobnoziarnistego martenzytu i warstwy pośredniej z perlitu i martenzytu. Stwierdzono, że po obróbce z czasem ogrzewania 3 min mikrotwardość stali 40CrNi i 20Cr2Ni4A podwaja się, a stal 34CrNi1Mn wzrasta 1,6 razy, w zależności od stanu początkowego, i odporność na zużycie wszystkich próbek stali wzrasta średnio 30 razy. * S. Amanzholov East Kazakhstan State University, Ust-Kamenogorsk, Kazakhstan, ** D. Serikbaev East Kazakhstan State Technical University, Ust-Kamenogorsk, Kazakhstan, a [email protected], b [email protected], c [email protected] INtRODUctION For increasing the service life of loaded steel parts (for example, gear wheels, etc.) operating under the action of cyclic loads and subjected to heavy wear in operation, it is necessary to radically change the approach to creating the required set of properties [L. 1–3]. The implementation of such a set of properties is possible when applying the method of surface heat treatment. At the present time, along with metallurgical methods and heat treatment under the conditions of manufacturers, local surface hardening of wear surfaces with the use of various technologies is also being considered to increase the service life of gear wheels. Surface thermal hardening of steel parts is one of the most effective and efficient ways to increase the service life of loaded elements of machines and mechanisms. In this case, only the most loaded working surface of the part is strengthened, leaving the core intact [L. 4]. At the same time, the progress in improving the quality of heat treatment (hardening) of the working surfaces of parts is associated with the use of concentrated energy sources: electron and laser beams, plasma jets. Such methods allow achieving higher performance properties and quality of hardening. At present, high-frequency, gas-flame, plasma, electronbeam, and laser processing are widely used for surface thermal hardening of gear wheels [L. 5–8]. At the same time, of all the existing methods of hardening in terms of 50 ISSN 0208-7774 T R I B O L O G I A 1/2019 their technical and economic indicators and the results of a comparative analysis, plasma surface hardening is recommended. The main distinguishing feature of the plasma surfacehardening method is the possibility of obtaining heating and cooling rates of materials that are several orders of magnitude higher than the values typical of traditional hardening methods (furnace hardening, high-frequency hardening, flame hardening, etc.), which contributes to obtaining hardened layers with previously unattainable levels operational properties [L. 9, 10]. The resulting quench-type structures have high hardness, wear resistance, and fracture resistance. In connection what was mentioned above, the purpose of this work is the use of surface plasma quenching technology, providing a given structure of the surface layer to increase the hardness, wear resistance, and strength characteristics of steel parts. mAtERIAL AND mEtHODS In accordance with the goal, 40CrNi, 20Cr2Ni4А, and 34CrNi1Mn steel samples were chosen as the objects of study. The choice of materials research is justified by the fact that these steels are widely used for the manufacture of hard-loaded gears. The chemical compositions of steels 40CrNi, 20Cr2Ni4А, and 34CrNi1Mn are presented in table 1. Plasma surface hardening of steel samples was carried out at the facility, which constructively consists of a power source, an electrolyte-plasma material processing chamber, and a personal computer. Figure 1 shows a setup diagram for plasma surface hardening (PSH) materials. PSH steel samples were treated as follows. Before starting work, the working bath is filled with electrolyte. Then the electrolyte, through a pump installed at the bottom of the working bath, enters the electrolytic cell. In this case, the electrolyte exits through the opening of the cone-shaped partition in the form of a jet and fills the electrolytic cell. Then the electrolyte is drained over the edge of the electrolytic cell into a pan, and then back into the working bath. Thus, the electrolyte is in a circulation mode [L. 10, 11]. To study the general nature of the structure, an optical microscope was used –NEOPHOT-21 of the National Research Laboratory for Collective Use of S. Amanzholov EKSU. The preparation of metallographic sections of steel samples was carried out according to the methods described in [L. 12]. The morphology and elemental composition of the sample treated in plasma were investigated in the engineering laboratory of D. Serikbayev EKSTU on a JSM-6390LV raster electron microscope – made by JEOL (Japan), with an INCAEnergy energy dispersive microanalyser from OXFORD Instruments. The microhardnesses of steel samples were measured at the National Research Laboratory for Collective Use of S. Amanzholov EKSU on the device MTC-3 in accordance with Industry Standard 9450-76, with the load on the indenter P = 1 N and the dwell time at this load of 10 s. Tribological sliding friction tests were carried out on a THT-SBE-0000 high-temperature tribometer in the laboratory of Tomsk polytechnic university (Tomsk, Russia) using the standard “ball-disk” technique (figure 2a) (international standards ASTM G 133-95 and ASTM G 99). The tribological characteristics of the modified layer were characterized by wear intensity and friction coefficient [L. 13, 14]. Abrasive wear samples were tested at the National Research Laboratory for Collective Use of S. Amanzholov EKSU, on the experimental setup for table 1. the chemical composition of the investigated steels Tabela 1. Skład chemiczny badanych stali Steel marks C Si Mn Ni S P Cr Cu Мо 40CrNi 0.36–0.44 0.17–0.37 0.5–0.8 1.0–1.4 < 0.04 <0.04 0.45–0.75 < 0.3 20Cr2Ni4А 0.16–0.22 0.17–0.37 0.3–0.6 3.25–3.65 < 0.02 < 0.02 1.25–1.65 < 0.3 34CrNi1Мn 0.3–0.4 0.17–0.37 0.5–0.8 1.3–1.7 < 0.04 < 0.03 1.3–1.7 0.2–0.3 fig. 1. Installation diagram for electrolytic-plasma surface hardening: 1 – processed sample (cathode), 2 – anode from stainless steel with holes, 3 – conical partition, 4 – working chamber – bath with electrolyte, 5 – tray, 6-pump, 7 – heat exchanger Rys. 1. Schemat instalacji do hartowania powierzchni elektrolitycznie plazmy: 1 – próbka (katoda), 2 – anoda ze stali nierdzewnej z otworami, 3 – stożkowa przegroda, 4 – komora robocza – wanna z elektrolitem, 5 – taca, 6 – pompa, 7 – wymiennik ciepła 51 ISSN 0208-7774 T R I B O L O G I A 1/2019 testing abrasive wear when rubbing with not rigidly fixed abrasive particles according to the “rotating roller – flat surface” scheme in accordance with Industry Standard 23.208-79, which coincides with the American ASTM C 6568 standard (figure 2 b). The durability of the treated sample was evaluated by comparing its wear with the wear of the reference sample (not the treated sample). Wear was measured by the gravimetric method on an ADB-200 analytical balance with an accuracy of up to 0.0001 g. The wear resistance of the test material was estimated by the weight loss of the samples during the test according to Industry Standard -23.208-79. fig.2. Diagram of instruments for tribological testing of samples: a) according to the scheme ball–disk, b) according to the scheme rotating roller–flat surface Rys. 2. Schemat przyrządów do badania tribologicznego próbek: a) zgodnie ze schematem „kulka–dysk”, b) zgodnie ze schematem obracający się wałek–pła-