Effect of Electrolytic-Plasma Hardening on the Microstructure and Tribological Properties of Low-Alloy Steels

Abstract

This study investigates the effect of electrolytic-plasma hardening (EPH) on the structure, phase composition, and tribological properties of the low-alloy steels 20Cr2Ni4A and 37Cr4 (1.7034) (EN). Hardening was carried out at a voltage of 320 V for 7 s in an aqueous solution containing 20% soda ash and 10% carbamide. Using scanning electron microscopy, the formation of a zonal structure with a hardened surface layer characterized by a needle-like martensitic morphology was revealed. X-ray diffraction analysis revealed the presence of Fe, Fe₃C, Fe₂C, and FeO phases. Microhardness measurements confirmed a significant increase in the hardness of the hardened layer. Tribological tests showed a reduction in the coefficient of friction to 0.574 for 20Cr2Ni4A steel and to 0.424 for 37Cr4 (1.7034) (EN) steel, indicating an improvement in wear resistance after EPH.

1. Introduction

Low-alloy structural steels are widely used in mechanical engineering and other industries due to their combination of strength, manufacturability, and relatively low cost. However, resource-determining damages to components—such as wear, fatigue cracks, and others—originate mainly in the surface layers, where maximum contact stresses and friction occur [1,2]. To enhance the durability and reliability of heavily loaded components, it is necessary to create a gradient of properties: a hard, wear-resistant surface layer combined with a tough, impact-resistant core. In traditional practice, various technologies are used: through hardening with tempering, carburizing and nitrocarburizing, gas or ion nitriding, as well as localized heating by high-frequency currents (induction hardening) or by a high-energy beam (laser hardening) [3,4,5]. These methods have proven effective in increasing the hardness and fatigue strength of steels, significantly extending the service life of components such as gears and shafts [6,7]. Nevertheless, traditional technologies also have drawbacks. Through hardening requires prolonged heating of the entire component followed by cooling, which leads to significant thermal and structural deformations. Induction and laser hardening provide more localized strengthening but are associated with high equipment costs and energy consumption [8,9]. One of the promising methods is electrolytic-plasma hardening (EPH), which enables the formation of a hard quenched layer on the steel surface within seconds through concentrated plasma heating in an aqueous electrolyte followed by rapid cooling [10,11].

The electrolytic-plasma hardening (EPH) method is a form of technology involving contact arc discharge in a liquid electrolyte. In a typical setup, a steel specimen is immersed in an aqueous electrolyte solution and connected as the cathode to a source of constant or pulsed high voltage [12,13]. When a certain voltage threshold is reached, a stable gaseous envelope (vapor-gas film) forms at the metal–solution interface, and microarc discharges (plasma) occur directly at the metal surface [14]. The cathodic mode (negative potential on the workpiece) leads to intense heating of the surface layer due to the energy of ions and plasma microdischarges. In effect, the surface zone of the metal is rapidly heated to hardening temperatures (typically 900–1000 °C) within just 1–5 s. This is followed by extremely rapid cooling (self-quenching) due to thermal conduction into the bulk material and direct contact with the cooling electrolyte [11,15]. The thickness of the hardened layer depends on process parameters—voltage, heating time, electrolyte concentration, and hydrodynamic conditions—and typically ranges from tens of micrometers to several millimeters [16,17,18].

Over the past two decades, the EPH method has been actively studied, and a substantial body of data has been accumulated on the structure, phase composition, and properties of the hardened layers in low-alloy steels. The hardened layers produced by EPH on low-alloy steels typically consist of fine-dispersed martensite with carbide particles (Fe₃C or alloyed M₂₃C₆, M₇C₃) or nitrides, inherited from the initial pearlite or formed during rapid transformations [19,20]. It has been established that after electrolytic-plasma hardening, steels with a ferrite–pearlite structure develop a surface martensite with a fine or lath-like/needle-like morphology [21,22]. The initial structure (ferrite and pearlite) is almost completely transformed into hardened martensite with a small fraction of retained austenite and dispersed carbide inclusions [23,24]. For example, in steel 45 (0.45% C), EPH results in the transformation of the surface ferrite–pearlite structure into hardened martensite with cementite precipitates, while the microhardness increases by 2.5–3.5 times (from ~200 HV to 500–700 HV) [25]. Similar results have been obtained for the low-alloy steels 40X; (0.4% C, Cr, Mn) and 30CrMnSi, where the microhardness of the hardened zone reaches 600–800 HV—two to four times higher than the initial value—due to the formation of fine-needle martensite [26]. In some cases, combined saturation leads to multiple improvements: it has been reported that plasma boriding of 45 steel increased its wear resistance by 16 times due to the formation of superhard boride phases (FeB, Fe₂B) [27]. A number of studies have demonstrated the possibility of localized restoration of worn surfaces: for example, combining EPH with the deposition of a thin coating allows for the recovery of the original hardness and surface smoothness of components [28]. Duplex treatment is of great interest for low-alloy steels: first, EPH is performed, followed by the deposition of a hard coating (such as nitride, DLC, etc.). These dual technologies enable the achievement of outstanding wear and corrosion resistance [29]. However, despite the significant number of studies, several aspects remain insufficiently explored: the effect of EPH on medium-carbon alloy steels containing multiple alloying elements; the relationship between structural-phase transformations, treatment duration, and tribological characteristics; and the potential technological limitations of the method during short-term processing.

Thus, the novelty of this study lies in the comprehensive investigation of the effects of short-term EPH on the microstructure, phase composition, microhardness, and tribological properties of low-alloy steels 20Cr2Ni4A and 37Cr4 (1.7034) (EN). Unlike most studies, this work focuses on the comparison of two steels with different alloying levels under identical treatment conditions, which allows for a deeper understanding of the mechanisms underlying the formation of hardened layers. The results of the present study may contribute significantly to the advancement of EPH as a resource-efficient and rapid technology for localized surface strengthening of components operating under frictional conditions, such as splined shafts, bushings, pins, and other machine elements. The high processing speed, simplicity of equipment implementation, and the absence of a need for a protective atmosphere make EPH an attractive alternative to conventional heat treatment methods.

2. Materials and Methods

Low-alloy steels 20Cr2Ni4A and 37Cr4 (1.7034) (EN), which are analogs of 3316H and 5140H steels, respectively, were selected for EPH. Samples made of the specified steels were prepared in the form of cubes with dimensions of 20 × 20 × 20 mm. Before hardening, the surfaces of the samples were prepared using a HYMP-2 grinding and polishing machine (Shenzhen Ebpu Technology Co., Wenling, China) to achieve a surface roughness of Ra 1.0 ± 0.1 µm. After the polishing operations were completed, the samples were thoroughly rinsed with running water to remove abrasive particles and contaminants. The chemical composition of the steel is presented in

Table 1. Chemical composition of the steels.

Steel	C	Si	Mn	Ni	Cr	Cu	P	S
20Cr2Ni4A	0.16–0.22%	0.17–0.37%	0.3–0.6%	3.25–3.65%	1.25–1.65%	up to 0.3%	up to 0.025%	up to 0.025%
37Cr4 (1.7034) (EN)	0.36–0.44%	0.17–0.37%	0.5–0.8%	up to 0.3%	0.8–1.1%	up to 0.3%	up to 0.035%	up to 0.035%

.

An aqueous electrolyte containing 20% sodium carbonate and 10% carbamide was used in the experiment. The experiment was carried out using an electrolytic-plasma hardening (EPH) setup equipped with a 40 kW DC power rectifier and a specialized treatment chamber housing a plasma torch, as shown in Figure 1.

The hardening process of the samples was carried out according to the following scheme: First, the electrolyte was poured into the working bath (5). Using a pump (4) located at the bottom of the bath, the electrolyte was circulated through pipelines (8) into the plasma torch (3), which was installed in the treatment chamber (2). After passing through the plasma torch, the electrolyte was drained back into the working bath through specially designed outlets, ensuring continuous liquid circulation. The electrolyte flow rate was maintained within the range of 4–7 L/min, while the cooling water circulating through the heat exchanger flowed at 3–6 L/min, allowing the electrolyte temperature to be kept at 50–60 °C. The samples (6) were fixed in a holder and immersed in the electrolyte in such a way that the treated area was located 2–3 mm from the opening in the conical partition. Through this opening, a jet of electrolyte was directed to ensure cooling and temperature control of the samples. The anode (7) was connected to the positive terminal of the power supply (1), while the samples, acting as the cathode, were connected to the negative terminal. Then, a voltage of 320 V was applied to the samples for 3, 4, 5, or 7 s to rapidly reach the hardening temperature, after which they were cooled in the same electrolyte [30].

X-ray phase analysis was carried out using an X’Pert PRO PANalytical diffractometer (PANalytical BV, Almelo, The Netherlands) equipped with a copper anode tube operating at 40 kV and 30 mA. Cu-Kα radiation (λ = 1.541 Å) was recorded in the 2θ range from 30° to 90°, with a scanning step of 0.02° and a counting time of 0.5 s per step. The acquired diffraction patterns were processed and interpreted using the HighScore Plus (version 3.0e) software package. The surface microstructure and cross-sectional morphology of the coatings were examined using scanning electron microscopy (SEM) on a SEM3200 instrument (CIQTEK Co., Ltd., Hefei, China) after etching in a solution consisting of 4% nitric acid and 96% alcohol. Microhardness measurements of the samples were performed using a Metolab 502 device by the Vickers method (HV) in accordance with GOST R ISO 6507-1-2007 [31] (Metolab LLC, Krasnoiarsk, Russia). The following parameters were used during the tests: a load of 10 g and a dwell time of 10 s. The coefficient of friction was evaluated using a universal tribological tester TRB3 (Anton Paar, Graz, Austria) based on the ball-on-disk method in accordance with ASTM G133 [32]. The tests were conducted under dry sliding conditions at an ambient temperature of 25 ± 1 °C and a sliding speed of 0.05 m/s. Contact occurred between the samples and a 100Cr6 steel counterbody under a vertical load of 10 N. Friction coefficient values were recorded after a sliding distance of 60 m.

3. Results and Discussion

Based on the analysis of microhardness of 20Cr2Ni4A and 37Cr4 (1.7034) (EN) steel samples after electrolytic-plasma hardening at 320 V for 3, 4, 5, and 7 s in an electrolyte containing 20% soda ash and 10% urea, as shown in Figure 2, it was established that the samples treated for 7 s are the most promising for further research. For 20Cr2Ni4A steel (Figure 2a), this regime resulted in the formation of the deepest hardened layer with a consistently high level of microhardness—values exceeded 600 HV at depths up to 100–120 µm. This indicates the high efficiency of surface saturation with active components, uniform thermal exposure, and the possible formation of strong nitride phases. While shorter treatment times (3–5 s) resulted in a less pronounced hardened layer characterized by a sharp decrease in hardness with depth, the 7 s treatment demonstrated the best combination of depth and hardness of the modified layer.

A similar pattern was observed for 37Cr4 (1.7034) (EN) steel (Figure 2b): samples treated for 3–5 s exhibited high surface hardness, but the hardened layer rapidly diminished beyond 100 µm. The sample treated for 7 s exhibited a more uniform hardness distribution, although with slightly lower maximum values. This may indicate structural stabilization due to recrystallization or phase transformations resulting from prolonged thermal exposure. Such a hardening profile may be beneficial for applications where not only surface hardness but also the ductility of the surface layer is important. Thus, the selection of samples treated for 7 s for both steel grades is justified, as they exhibit the best balance between depth, uniformity, and microhardness level, making them the most promising candidates for subsequent comprehensive investigation of structure, phase composition, and tribological properties.

X-ray diffraction analysis of the surface layers of 20Cr2Ni4A steel (Figure 3a) and 37Cr4 (1.7034) (EN) steel (Figure 3b), subjected to EPH at 320 V for 7 s, revealed the formation of a complex phase composition comprising a ferritic matrix, carbide, and oxide components. For 20Cr2Ni4A steel (Figure 3a), intense diffraction peaks corresponding to α-iron (Fe), as well as the carbides Fe₃C (cementite) and Fe₂C, were identified, indicating active surface saturation with carbon under high-temperature plasma conditions. The presence of the FeO phase confirms partial oxidation of the surface layer. In the case of 37Cr4 (1.7034) (EN) steel (Figure 3b), a similar phase composition is observed; however, the diffraction peaks corresponding to Fe₂C carbide exhibit higher intensity, indicating more pronounced carbide formation. Additionally, an increase in the number and intensity of peaks corresponding to the FeO phase is recorded, indicating a greater tendency of this steel grade toward oxidation processes under EPH conditions.

The results of structural analysis of the cross-section of 20Cr2Ni4A steel after EPH at 320 V for 7 s revealed the formation of a well-defined zonal structure. The micrograph (Figure 4) reveals three structural zones: I—hardened surface layer, II—transition zone, and III—unaffected core. The structural features of each zone were further analyzed using SEM at a magnification of ×2000. The upper layer (Zone I) is characterized by a fine needle-like microstructure typical of martensite, indicating that high temperatures were achieved during the short-term plasma discharge exposure followed by rapid cooling. Such a structure is the result of phase transformations under the high cooling rates typical of EPH and indicates the effectiveness of surface hardening. The carbide phases (Fe3C, Fe2C) identified in this layer on the X-ray diffraction pattern confirm the presence of thermal decomposition products and subsequent crystallization under rapid cooling conditions. The transition zone (II) exhibits a heterogeneous morphology characteristic of regions with a temperature and stress gradient. Signs of partial recrystallization and changes in grain shape and size are observed, reflecting the gradual decrease in thermal influence with increasing depth from the surface. This is consistent with the reduced intensity of carbide phases observed in the X-ray diffraction analysis and indicates incomplete phase transformations in this region. The deeper zone (III) retains the original ferrite–pearlite structure typical of 20Cr2Ni4A steel after standard heat treatment. The absence of microstructural changes in this region indicates that the thermal effect of EPH is localized within the upper layer, approximately 100–120 µm thick, which is also confirmed by the results of microhardness profiling.

Microstructural analysis of the cross-section of 37Cr4 (1.7034) (EN) steel subjected to EPH at 320 V for 7 s revealed the formation of a typical three-zone structure (Figure 5). The SEM image obtained at ×35 magnification clearly shows the division into I—the hardened surface zone, II—transition region, and III—unaffected core. SEM images at ×2000 magnification (a–c) allowed for a detailed characterization of the morphological features of each zone. The surface zone I (a) is characterized by a well-defined needle-like structure typical of the martensitic phase. This morphology indicates the formation of a hardened layer as a result of rapid cooling caused by the steep temperature gradient generated during EPH discharge exposure. The characteristic appearance of the structure indicates a high degree of austenite undercooling, which is also confirmed by X-ray diffraction data showing intense peaks of α-iron and carbide phases (Fe₃C, Fe₂C). The probable presence of retained austenite in this zone may be attributed to the high carbon content and the intensity of thermal exposure. The transition zone II (b) exhibits a heterogeneous microstructure, containing regions of tempered martensite as well as elements of a ferrite–carbide mixture. The formation of such a structure is due to the reduced intensity of thermal exposure at a certain depth and the decreased cooling rate. It is likely that partial decomposition of martensite and precipitation of carbide phases occurred in this region, which is also consistent with the gradual decrease in the intensity of carbide peaks observed in the X-ray diffraction pattern. The deeper zone III (c) corresponds to the original microstructure of 37Cr4 (1.7034) (EN) steel and is represented by a typical ferrite–pearlite matrix with uniformly distributed pearlite colonies. The absence of signs of thermal and phase changes in this region confirms the localized nature of EPH, confined to the upper layers of the material. Overall, the results of the microstructural analysis of 37Cr4 (1.7034) (EN) steel demonstrate the formation of a clearly defined zonal structure as a result of EPH. The surface hardened layer formed by high-speed cooling exhibits a needle-like martensitic structure with a high density of dispersed carbides. The transition zone indicates a gradual attenuation of thermal effects and phase transformations, whereas the core retains its original structure.

Figure 6 shows the dependence of the coefficient of friction on the sliding distance for 20Cr2Ni4A steel samples in the initial state and after EPH. The obtained results indicate a significant influence of EPH on the tribological characteristics of the investigated material. The untreated sample exhibits a higher coefficient of friction, stabilizing at an average value of 0.748. The elevated friction values indicate intense interaction between the contact surfaces in the absence of a hardened layer. The characteristic instability of the curve may also be attributed to localized seizure and adhesive wear. At the same time, the sample subjected to EPH shows a reduction in the coefficient of friction to an average value of 0.574, which represents a decrease of approximately 23% compared to the initial state. A smoother and more stable curve is observed throughout the entire sliding distance, indicating improved anti-friction properties. Such behavior is attributed to the formation of a hardened surface layer with a fine-dispersed martensitic structure, as well as the presence of carbide phases previously confirmed by X-ray phase analysis. These structural features contribute to reduced contact resistance and limited wear intensity. The reduction in the coefficient of friction after EPH may also be associated with the increased microhardness of the surface layer, which hinders plastic deformation upon contact and reduces the contribution of the adhesive wear mechanism. Moreover, the formed structure demonstrates good stability under dry friction conditions, which is particularly important for heavily loaded friction units in mechanical engineering.

Figure 7 presents the dependence of the coefficient of friction on the sliding distance for 37Cr4 (1.7034) (EN) steel in the initial state and after EPH. Analysis of the obtained curves indicates a pronounced reduction in the coefficient of friction as a result of EPH, confirming the positive effect of this technology on the tribological properties of the steel. The average coefficient of friction for the untreated sample is 0.609, whereas for the EPH sample it is 0.424, corresponding to a reduction of approximately 30%. This reduction indicates a significant improvement in the anti-friction properties of the material. The curve of the hardened sample shows a gradual increase in the coefficient of friction at the initial stage, followed by stabilization at a lower level compared to the untreated state. Such stabilization indicates stable performance of the contact pair and a reduction in wear intensity. The elevated friction level and high amplitude fluctuations of the coefficient in the untreated sample indicate the presence of unstable surface interaction processes, including seizure, microscopic scuffing, and material degradation. In contrast, the hardened sample is characterized by smaller fluctuations and a smoother curve profile, reflecting stable friction with a reduced level of adhesive and abrasive wear. The improvement in the tribological characteristics of 37Cr4 (1.7034) (EN) steel after EPH is associated with the formation of a hardened surface layer consisting of a martensitic structure with a high density of dispersed carbide phases, as confirmed by X-ray phase analysis and microstructural investigations. The increase in microhardness and homogeneity of the surface layer contributes to a reduction in plastic deformation in the contact zone, which in turn decreases the coefficient of friction.

4. Conclusions

The results of the conducted research have shown that EPH at a voltage of 320 V for 7 s has a significant effect on the phase composition, microstructure, microhardness, and tribological characteristics of 20Cr2Ni4A and 37Cr4 steels. Microstructural analysis revealed the formation of a well-defined hardened surface layer with an acicular martensitic structure and carbide phases such as Fe₃C and Fe₂C. X-ray phase analysis confirmed the presence of thermally induced carbide and oxide compounds, while microhardness measurements showed a significant increase in the hardness of the hardened layer compared to the base material. The conducted tribological tests showed a reduction in the average coefficient of friction after EPH by 23% for 20Cr2Ni4A steel and by 30% for 37Cr4 steel, indicating improved wear resistance and operational stability under dry friction conditions. Despite the positive results obtained, the study has a number of limitations. In particular, heterogeneity in the thickness and phase composition of the hardened layer was observed, along with a lack of data on residual stresses. Future work is planned to address these issues, as well as to optimize the parameters of EPH through numerical modeling of thermal and electrical processes, and to expand the range of investigated steels and alloys.