Inﬂuence of electrolytic-plasma surface quenching on the structure and strength properties of ferritic-pearlite class wheel steel

This paper examines the inﬂuence of electrolyte-plasma surface hardening on the structure and micro- hardness of wheel steel mark 2. In the work electrolyte-plasma surface quenching was carried out in an electrolyte made from an aqueous solution of 10% carbamide (NH 2 ) 2 CO + 20% sodium carbon- ate Na 2 CO 3 . The work investigated the strength limit, ﬂuidity and wear intensity of the wheeled steel after electrolyte-plasma surface quenching. After electrolytic-plasma surface quenching, a batch, high-temperature plate and low-temperature plate martensit is formed on the surface of the sample. Investigations have been carried out on microhardness determination on cross-section of wheel steel samples after quenching in aqueous solution of electrolyte. It is found that after electrolytic-plasma surface quenching, the microhardening values of this hardened surface layer increased on ≈ 3 times compared to the steel matrix, and the thickness of the hardened layer is 1000-1500 µ m. According to the results of the scanning transmission electron microscopy, the electrolyte-plasma surface quenching caused a change in the morphological constituents of steel mark 2. In the initial state, the matrix of steel is a α -phase, the morphological components of which are fragmented unfragmented ferrite and pearlite.


Introduction
The development and introduction of new resource-efficient technologies that increase the quality of products largely determine the effective development of industrial sectors. The stable and efficient operation of the machine-building industry is impossible without the use of new technologies, which provide the necessary complex of strength and plastic properties of the structural steels. This requires an understanding of the nature of the processes in steels. The elucidation of the physical mechanisms of formation and evolution of structural-phase states in steels is one of the important tasks of modern physics of condensed state and material science, because it underpins the development and creation of effective ways to improve performance. In most cases, the durability of machines and mechanisms depends on the durability of the parts, many of which are subject to significant impact variables. In these cases, the parts shall be principally of high strength and hardness of the surface layer combined with sufficient core viscosity. This can be achieved through the use of various surface hardening techniques. Surface hardening is one such method. Surface hardening is achieved by the short-term heating of the surface layer of metal to the temperature of quenching and subsequent rapid cooling. Nowadays, various methods of surface quenching are used in industrial production [1][2][3][4][5], one of which is an electrolyte-plasma surface quenching [6][7][8][9][10][11], characterized in that the entire surface of the part is heated in solution. At the same time, heating takes place fairly quickly and depending on the time of heating it is possible to adjust the depth of the quenching layer. However, the main attention was paid to the influence of surface hardening on mechanical and tribological properties, and there was almost no analysis of the effect on the structural-phase state of the material. The purpose of this work is to investigate the influence of electrolyte-plasma surface injection on the structure and strength characteristics of wheeled steel.

Experimental procedure
In this work EPSQ was subjected to samples of steel mark 2, used for the manufacture of locomotive wheel sets in accordance with the requirements of GOST 398-96. The chemical composition of steel: 0.55-0.65% C; 0.5-0.9% Mn; 0.22-0.45% Si; No more 0.1% V; No more 0.03% S; No more 0.035% P; the rest of Fe, also the permissible mass fraction (%): Ni ≤ 0.25, Cr=0.20, Cu=0.30 to GOST 398-96.
Blanks of steel samples of mark 2 for study were cut out of a bandage in the form of a parallelepiped measuring 15 × 15 × 10 mm 3 . The technology of preliminary heat treatment of steel wheels provides for their hardening and subsequent tempering. In this work, in its initial state, steel mark 2 was heattreated by quenching at 890-920 • C (exposure 2 h) with subsequent tempering at 580-620 • C (exposure 2.5 h, cooling in warm water at 30-60 • C) [12]. EPSQ was carried out at the electrolyte-plasma treatment facility developed by the authors of works [12][13]. A schematic diagram of the installation for the EPSQ is shown in (Figure 1). The power source was a powerful rectifier, giving a maximum of 360V/60A output in the form of direct current. Samples were processed by rapid heating for 2 s and then cooling in a flow-through electrolyte. A water solution of urea and sodium carbonate was chosen as the electrolyte. Technological parameters of electrolytic-plasma surface quenching: the processing time was 2 seconds, T max =850-900 • C; U= 300V; I=40A; electrolyte composition (%, mass): 10% carbamide (NH 2 ) 2 CO+20% sodium carbonate Na 2 CO 3 +70% water.
Electrolyte-plasma surface quenching of the steel was carried out in the cathode mode in electrolyte-plasma treatment machine, the scheme ( Figure 1a) and processing ( Figure 1b) shown in Figure 1. The microstructure of the surface was studied on an optical microscope "ALTAMI-MET-1M". The microhardness of the surface layers of the samples before and after processing was measured by the method of pressing the diamond indenter using a PMT-3M instrument at a load of 1 N and a holding time at this load of 10 s. Tribological characteristics were studied on the THT-S-BE-0000 tribometer. The wear tracks were investigated using the MICROMEASURE 3D station contactless 3D profilometer. The phase composition of the samples was studied by X-ray diffraction analysis on an X'PertPro diffractometer using CuK α radiation. Test samples for abrasive wear was performed on an experimental setup for testing for abrasive wear according to the scheme "rotating roller -flat surface" in accordance with GOST 23.208-79. 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 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 GOST 23.208-79 which coincides with the American standard ASTM C 6568.

Results and Discussion
A study of the structure and properties of wheel steel mark 2 before and after EPPI showed that after electrolyte-plasma surface hardening, the mechanical properties of wheel steel increase. The structural-phase conditions of the toughened surface layers of steel of mark 2 were investigated to determine the structural factors affecting wear, hardness and other characteristics of the steels. Table 1 shows the testing results of steel mark 2. It has been determined that the strength limit and the yield stress of steel mark 2 are increased after the EPSQ, although only the surface layer of 1-1.5 mm thickness has been modified. Symbols in this table: σ B -tensile strength (temporary resistance); σ 0.2 -yield strength; δelongation after rupture; ψ -relative narrowing at break. Studies have shown that, in the initial state, the matrix of steel is a α -phase, a solid solution of carbon and alloying elements in α -Fe with an OCC crystalline lattice. The morphological components of the α -phase are the plate pearlite and ferrite Figure 2 (a-c). The plate pearlite, which has a 35% volumetric fraction of PV, which is almost perfect (Figure 2a), is present in the form of a non-fragmented ( Figure 2b) and fragmented (Figure 2c). The bulk portion of non-fragmented ferrite is ≈ 10%, and the fragmented is ≈ 55%. Volume fractions were calculated using the method based on the Cavalieri-Aker-Glagolev principle [11]. It postulates the relationship between the shares of area ( P S ) and volume ( P V ): This is one of the fundamental relations of stereology postulated by Saltykov [11]. Surface quenching has led to martensitic transformation, that is to the formation of packet-plate martensite: packet and plate (low temperature and high temperature). Packaged martensite (Figure 3a) is a structural formation consisting of a set of nearly parallel crystals (ribs) of elongated shape forming a packet. The formation of the packet martensite appears to have come from a fragmented ferrite. This is indicated by the fact that the bulk fraction of the packet martensite is 60%, that is, almost as much as the original fragmented ferrite was. The low-temperature plate martensite, which has a volume fraction of 10%, is sufficiently large, separately arranged, the martensite (plate) crystals with a dispersion structure (Figure 3b). A comparison with the initial state of steel shows that the laminated low-temperature martensite was most likely formed from non-fragmented ferrite, which also had a 10% volume fraction. High temperature plate martensite, with a volume of 30%, represents large, separate martensite crystals (plates), often extending through all grains as well as crystals of arbitrary shape, which have no clear cut and no separate partition boundaries (Figure 3c). It's probably formed from a lamellar pearlite. The tribological characteristics of the specimens before and after EPSQ were characterized by wear and tear ( Table 2). After the EPSQ of the samples, there is an increase in wear resistance under dry friction conditions. Studies have shown that surface hardening of steel 2 subjected to electrolyteplasma surface hardening in an electrolyte plasma, structural changes were detected. Figure 4 (a-c) shows the microstructure of the steel cross section after treatment in an electrolyte containing an aqueous solution of 10% carbamide (NH 2 ) 2 CO and 20% sodium carbonate Na 2 CO 3 with a treatment time of 2 seconds at a temperature of 860 • C. As can be seen from Figure 4, the electrolytic-plasma surface quenching led to a change in the microstructure of the cross section, where the zoning of structures typical of electrolyte-plasma treatment is visible. The cross-sectional structure consists of 3 zones: 1 zone -a zone of surface hardening with a thickness of 1000-1500 µm, 2 zone -a zone of thermal influence, 3 zone -a matrix.
As is known, one of the most important properties of the surface layer, which significantly affects the strength characteristics, is microhardness, the value of which in the initial state (matrix) of steel mark 2 is ≈ 140 HV. In this work, we studied the changes in the microhardness of a specimen of steel mark 2 over a cross section after electrolytic plasma surface quenching. According to the obtained results, shown in Figure 5, the average microhardness in the surface quenching zone is ≈ 420 HV, in the heat-affected zone it is ≈ 260 HV and, accordingly, in the steel mark 2 matrix, the microhardness remains unchanged.

Conclusion
Analyzing the results obtained on the phase composition, fine structure and mechanical properties of the wheel steel mark 2 before and after electrolytic-plasma surface quenching, the following main conclusions can be drawn: -It is established that in its initial state, matrix of steel 2represents α -phase the volume ratio of unfragmented ferrite is ≈ 10% and 55% of fragmented, perlite with a volume of ≈ 35%; -It was found that the morphological components of the structure of steel mark 2 after EPSH on T max = 850-900 • C and the time of exposure 2 s: martensite in the form of packet martensite with a share of volume 60%, Lamellar cold and lamellar temperature martensites with volumetric fractions ≈ 10% and ≈ 30% respectively; -It was found that the process of electrolytic-plasma surface quenching leads to a change and hardening of the surface layer of grade 2 steel, the thickness of the hardened layer is ≈ 1000-1500 µm, and the microhardness increases by 3 times.