Effect of Electrofriction Treatment on Microstructure, Corrosion Resistance and Wear Resistance of Cladding Coatings

Abstract

In recent years, the issue of increasing the wear resistance of the working bodies of agricultural machinery designed for cutting and breaking the soil has received special attention. The surface layers of working bodies of agricultural machinery during operation are subjected to intensive abrasive wear, which leads to rapid wear of equipment and a reduction in its service life. The induction cladding method using materials such as Sormait-1 is widely used to increase the wear resistance of tool working surfaces. However, after coating, additional heat treatment is required to improve the physical and mechanical properties of the material and increase its durability. In electrofriction technology (EFT) hardening, the surfaces of the parts are subjected to melting under the influence of electric arcs. In this work, three types of surface treatment of L53 steel have been investigated: induction cladding using Sormait-1, electrofriction treatment, and a combination of induction cladding followed by electrofriction treatment. The microstructure was analyzed using optical microscopy and scanning electron microscopy. Erosion and abrasion tests were carried out in accordance with ASTM G65 and ASTM G76-04 international standards to evaluate the wear resistance of the materials under mechanical stress. A dendritic structure was formed after the induction cladding of the Sormait-1 material, but subsequent electrofriction treatment resulted in a reduction of this dendritic structure, which contributed to an increase in the hardness of the material. However, the highest hardness, reaching 965 HV, was recorded after electrofriction treatment of L53 steel. This is explained by needle martensite in the structure, which is formed as a result of quenching. Further, the influence of structural characteristics and hardness on erosion and abrasion wear resistance was examined. The analysis showed that the material microstructure and hardness have a decisive influence on the improvement of wear resistance, especially under conditions of intensive erosion and abrasive friction.

1. Introduction

The surface layers of many working bodies of agricultural machinery are subjected to intensive abrasive wear during operation, resulting in premature failure of the equipment. The working parts of agricultural soil-tilling equipment are made from medium-carbon structural steel, which is sensitive to abrasive wear. To enhance wear resistance, protective layers of varying thickness are applied to the surfaces of these parts using various methods. This improves abrasion resistance and extends the equipment’s service life. To protect against abrasive wear, the methods of hardening [1,2,3,4,5], thermodiffusion treatment [6], gas-thermal spraying [7], the application of ceramic and bimetallic coatings [8,9], and surfacing [10] are used. The cladding method is widely used due to several advantages, such as the continuity of the process, versatility for parts of different sizes, and the possibility of cladding in any position [11]. Yu and Zhang [12] studied the process of induction cladding of alloys and composite materials, focusing on its mechanisms, parameters, and materials used. Their research highlights the method’s industrial applications and benefits, such as energy efficiency, eco-friendliness, and controllable coating thickness. Additionally, they addressed challenges and future developments, emphasizing the need for special materials and process optimization for diverse industrial applications. Article [13] examines the physico-mechanical properties of gray cast iron for use in soil-tilling agricultural machinery. It describes arc and electro-spark deposition methods to enhance wear resistance, as well as heat treatment to increase hardness and resistance to abrasive wear.

In agricultural engineering, plasma and electrode cladding, as well as induction cladding of wear-resistant powders (Sormait-1, Sormait-2), are often used. However, these methods involve heating the clad item, which leads to thermal deformations and a reduction of material hardness. The clad layer does not provide a self-sharpening of the tool blade during operation and quickly wears out due to the destruction of the tempered base of the product. The existing methods of extending the service life of the working bodies of agricultural machinery are either too costly or do not significantly increase their durability.

The high cost of surfacing materials and electricity, as well as the need to ensure the self-sharpening of the tool blade in the process of its wear, led to the development of electrofriction technology (EFT) hardening [14]. The method of electrofriction tool hardening is carried out by electromechanical impact on the surface of the tool blade at high electric current density and formation of hardened layers with thickness up to 3 mm on the cutting edge of the blade with the width of the hardened layer 10–20 mm. The impact on the modified surface of the tool occurs through friction on the surface of the electrode-anode, with periodic opening of the friction surfaces and the formation of contact-arc discharges in the gap. These discharges heat the surfaces of the tool and electrode to melt and introduce alloying elements from the electrode material into the melt on the tool surface.

The task of creating metallic materials with qualitatively new properties is usually solved on the basis of a complex approach, combining the principles of forming the chemical composition of the material and then the structure by developing technological processes for its hardening treatment. Among hardening technologies, a special place is occupied by physical and mechanical methods that influence the surface of the material, as its condition largely determines the level of strength and operational properties of parts of agricultural machinery. Obtaining hardened surface layers is achieved by the purposeful formation of a given structural state of the metal by heat treatment methods, including the use of highly concentrated energy sources such as ionic, laser, ultrasonic, high-frequency induction, and others [15,16,17,18,19,20]. As a result of such treatment, either structural changes in the original surface occur, i.e., a modifying process, or the formation of a coating on the surface. Processes of modifying influence on the surface cause changes in the structure and phase composition of the surface layer, which, in turn, is a prerequisite for obtaining new properties [21].

The pairing of induction cladding and electrofriction hardening techniques is particularly valuable from a performance improvement standpoint. In connection with the above, this work aims to evaluate the effectiveness of electrofriction treatment on the microstructure, corrosion resistance, and wear resistance of clad coatings.

2. Materials and Methods

L53 alloy steel with chemical composition of 0.45% C, 0.7% Mn, 0.3% Si, 0.9% Cr, 0.3% Ni, 0.035% S, and 0.035% P was chosen as the substrate material. The sample size of L53 steels was 100 × 20 × 6 mm³. Prior to the experiments, the surface of the steel samples was mechanically treated using P100 grit sandpaper. The surfaces of the L53 steel were examined after three types of treatments: induction cladding with Sormite-1, electrofriction treatment, and a combination of induction cladding followed by electrofriction treatment.

Coatings were obtained by induction cladding using specialized cladding materials ‘Sormit-1’. The chemical composition of ‘Sormit-1’ material is shown in

Table 1. Chemical composition of ‘Sormite-1’ material.

Fe	Cr	Mn	Ni	Si	C
rest	25.0–31.0	1.5	3.0–5.0	2.8–4.2	2.5–3.0

. The composition of Sormite-1 fully corresponds to that of hypereutectic cast iron.

On the basis of the Research Centre ‘Surface Engineering and Tribology’ of S. Amanzholov East Kazakhstan University (Ust-Kamenogorsk, Kazakhstan) under the guidance of Dr. Y.N. Tyurin (E.O. Paton Institute of Electric Welding of the National Academy of Sciences of Ukraine) an installation for electrofriction hardening was developed. Figure 1 shows the scheme of the unit for EFT hardening of a flat product with a disc electrode. Grey cast iron with a diameter of 170 mm was chosen as an electrode (anode). The friction of the electrode against the workpiece is accompanied by the formation and breaking of electrical contact between them. The contact between the workpiece and the electrode is carried out under a layer of cooling liquid (water), which causes heating up to melting of only the contacting surfaces. The power source was a 75 kW welding rectifier providing a maximum output power of 70 V/1000 A in direct current. The electric current was limited to 450 A. The voltage of the electric current was 70 V. The electrode rotation speed was 160 rpm while moving the sample at a speed of 5 deg/s.

The microstructure of the investigated samples was studied using a TESCAN MIRA3 LMH (TESCAN, Brno, Czech Republic) scanning electron microscope. Mechanical processing, including grinding and polishing, as well as chemical etching, was carried out to prepare the samples for the analysis of steel microstructure. Etching of the surface of the samples was carried out for 10 s in a solution of 4.0% nitric acid (HNO₃) in ethanol, which allowed revealing structural features of steel and cast iron. To determine the hardness of steels according to the Vickers method, a microhardness measuring device, Metolab 502 (Metolab, Moscow, Russian), was used. At induction cladding and electrofriction treatment, hardness measurements along the cross-section of hardened samples were carried out in the form of three tracks parallel to each other at a distance of 200 µm, with a step of 50 µm. The abrasion test was carried out in accordance with ASTM G65 (Figure 2a) [22]. Prior to testing, the samples were ultrasonically cleaned to remove foreign particles. A cylindrical rubber roller pressed radially against the flat surface of the test sample with a force of 44 N was rotated at a frequency of 1 s⁻¹. The feed rate of abrasive particles between the rubber wheel and the sample, i.e., into the test zone, was 41–42 g/min. Electrocorundum with grain size 200–250 µm was used as abrasive particles. The samples were tested for ten minutes. Erosion wear tests at room temperature were performed on a special bench in accordance with ASTM G76-04 (Figure 2b) [23]. The tests used a 3 mm diameter tubular nozzle positioned 15 mm from the sample; the angle of the nozzle relative to the sample was 90°. Quartz sand with a grain diameter of 50 µm was used as an abrasive. The duration of the tests was three minutes. Before and after the abrasive and erosive wear tests, the samples were mass measured using CRYSTAL 100 analytical scales with an accuracy of 0.0001 g.

The corrosion study used a potentiodynamic polarisation test, as shown in Figure 3. The scan rate was 0.5 mV/s with a scan range of −0.8 V to +0.8 V in 3.5% NaCl solution. The results obtained from the current-potential measurements were used to construct a polarisation curve, which typically provides information on corrosion potential, corrosion current, and passivation behavior. For each batch, at least three samples were used to ensure the reliability of the measurement results.

3. Results

In the induction cladding process of Sormait-1 material, a fine-grained dendritic structure is usually formed, which can be equiaxed dendrites, columnar dendrites, or mixed morphology. These characteristics depend on cladding parameters such as cladding current, cladding speed, cladding thickness, and preheating temperature. These parameters affect the thermal conditions of the process and the solidification rate of the molten material, which determines the shape and size of the dendritic structure. Slow cooling results in a larger dendritic structure, while high cooling rates result in a finer dendritic structure [24].

Figure 4 shows the cross-sectional microstructure of a cross-sectional layer of Sormait-1 clad material on the surface of L53 steel. In the initial state of steel, L53 has a ferrite-perlite structure (Figure 4d). Ferritic areas tend to appear lighter and more homogeneous, whereas perlite is represented by darker zones with a characteristic lamellar structure. The interface between the base metal and the cladding shows a high-quality bond between the cladding and the substrate (Figure 4c). The structure of the deposited metal includes dendrites (alloyed austenite-carbides) and rosette-type eutectics consisting of austenite and carbides, as well as decomposition products of austenite (troostite) [25]. The enlarged image (Figure 4b) shows that the dendrites have a columnar shape, indicating directional cooling of the metal in this area.

A distinctive feature of the used technology (electrofriction hardening) of EFH is the zonality of the formed structures along the thickness of the modified layer, which can be divided into the following zones: hardening zone, heat affected zone, and the base of the treated material. After EFH, needle martensite was found in the near-surface layer (Figure 5b). The transformation of austenite to martensite is diffusionless, and only lattice rearrangement occurs without changing the concentration of reacting phases. Martensite is a solid carbon solution in α-iron with the same concentration as the original austenite [26]. Since the solubility of carbon in the α-phase is only ~0.01%, martensite is a supersaturated solid solution. The heat-affected zone (Figure 5c) contains martensite and highly dispersed pearlite (troostite). Homogeneous (homogeneous austenite) always transforms into highly dispersed pearlite (troostite). Consequently, heating to high temperatures leads to a more homogeneous structure that favors the formation of lamellar structures.

Figure 6 shows the local microstructure changes when combining induction cladding with subsequent electrofriction treatment on the surface of L53 steel. Figure 6a shows the general structure of the material, where three zones are distinguished: the bottom layer is a substrate consisting of steel in the initial state with a pearlitic-ferritic structure. This is followed by a coating layer obtained using induction technology from the surfacing material Sormait-1, characterized by a larger dendritic structure. Fine columnar and equiaxed dendrites are observed on the coating surface in the hardened layer formed as a result of the subsequent electrofriction treatment. During the subsequent electrofriction treatment, the upper layers may be subjected to a higher cooling rate than the intermediate layers, which leads to changes in the microstructure of the dendrites: in the lower layer of the coating, the width of the dendrites is about 35–45 μm, whereas in the upper layer, due to more intense cooling, the dendrites are reduced to about 4–9 μm. This effect is due to the fact that the upper layers cool faster during subsequent electrofriction processing, resulting in the formation of a finer dendritic structure. As a result of these changes in microstructure, the coating acquires different physical and mechanical properties on a layer-by-layer basis. In particular, the finer dendrites in the top layer contribute to its hardness and wear resistance, while the larger dendrites in the middle layer provide the necessary ductility and strength of the material. This results in an optimum combination of characteristics such as high surface hardness and good resistance to mechanical stresses.

Figure 7 shows the results of microhardness measurement along the cross-section of the sample. After induction cladding of Sormait-1 material, the average cross-sectional hardness was 485 HV_0.1. The hardness values are homogeneous over the entire cross-section, with no sharp changes in the upper and lower layers. However, after electrofriction hardening, the graph of microhardness distribution over the depth of the sample cross-section shows that microhardness gradually decreases from the surface of the hardened layer to the base (base metal), starting from 1024 HV_0.1 and up to 256 HV_0.1. This reflects the change in the microstructure of the material: from martensite at the surface (average 965 HV_0.1) to a depth of 100 µm (top layer), low-tempered martensite and bainite-martensite (average 755 HV_0.1) at a depth of 300 µm (second layer), and troostite (average 375 HV_0.1) in the third layer at a depth of 450 µm. Further, the microhardness reaches the initial state of the base metal. This may be due to the fact that during electrofriction hardening, the surface layer undergoes rapid cooling using inflow water, resulting in high hardness. The rapid cooling favors the formation of a martensitic structure, which explains the increase in the hardness of the surface layer. Hardness changes when combining induction cladding with subsequent electrofriction treatment on the surface of L53 steel showed that the combined hardening increases the surface hardness up to 646 HV_0.1. Further, there is a decrease in hardness deep into the material, associated with the transition to a coarse-grained dendritic structure.

Based on the results of wear tests on the fixed abrasive (Figure 8a), the highest wear resistance was demonstrated by the coating obtained by induction cladding followed by electrofriction treatment (IC+EFT), with a mass loss of 0.127 g. The lowest wear resistance was shown by electrofriction treatment with a mass loss of 0.1359 g. The wear resistance of the IC+EFT hardened coating was three times higher than that of L53 steel and two times higher than that of the induction cladding coating (0.2693 g).

The results of the erosion tests presented in the graph show the comparative behavior of the four types of coatings in terms of mass loss and wear area after exposure to erosion factors. The most interesting results were obtained for the coating treated by induction cladding followed by electrofriction treatment (IC+EFT). This coating showed the lowest mass loss of about 0.0582 g and a wear area of 0.395 cm², indicating its high resistance to erosion. Compared to other coatings, such as L53 steel, which lost 0.0953 g mass, and IC, with a loss of 0.0837 g, the combination of induction cladding and electrofriction treatment significantly improves the durability of the material. However, it is worth noting that the wear area of the IC+EFT coating was found to be larger than that of the coating treated with electrofriction treatment (EFT) alone. This may indicate that in the case of the combined treatment, the erosion process spreads over a wider surface area despite the reduced mass loss. This wear distribution may indicate a more uniform removal of material from the surface, which may be related to the peculiarities of the coating structure after the combined treatment. Induction cladding combined with the electrofriction treatment forms a tough but slightly more susceptible surface to widespread erosive wear, while the reduced mass loss indicates improved mechanical performance.

The data in

Table 2. Results of corrosion tests.

	L53 Steel	IC	EFT	IC+EFT
A (cm2)	0.785	0.785	0.785	0.785
I corr. (A)	0.69239	0.65645	0.6832	0.63123
i corr. (A/cm2)	0.88202548	0.83624204	0.87031847	0.80411465
Corrosion rate (mm/a)	0.00331716	0.00314498	0.00327314	0.00302415

and the potentiodynamic plot (Figure 9) show a significant difference in the corrosion resistance of the different types of coating used for steel. The table shows key parameters such as corrosion current density (Icorr) and corrosion rate (C.R.), which reflect the behavior of the materials in corrosion tests. The coating obtained by induction cladding followed by electrofriction treatment (IC+EFT) shows the best performance with its Icorr value of 0.63123 and corrosion rate of 0.00302415, which is significantly lower compared to other coatings, including L53 steel and induction cladding or electrofriction treated coatings separately. In the potentiodynamic curve plot, the curve for L53 steel goes into a more negative region on the potential axis, which confirms its high corrosivity. This is reflected in the table data, where the corrosion current density and corrosion rate for the steel are maximum. On the contrary, the IC and EFT coatings show improved results, although their corrosion resistance is still inferior to the combined IC+EFT coating. This combination of treatments appears to create a denser and more corrosion-protected surface, which prevents the formation of micro-cracks and moisture retention, significantly slowing down the corrosion process. Overall, the combined IC+EFT coating demonstrates the best corrosion resistance of all the options studied, making it the most promising for applications requiring protection from aggressive environments.

4. Conclusions

Three types of surface treatment for L53 steel were studied: induction cladding using Sormite-1, electrofriction treatment, and a combination of induction cladding followed by electrofriction treatment:

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Induction cladding using Sormite-1 forms a dendritic structure with an average hardness of 485 HV;

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The combination of induction cladding followed by electrofriction treatment refines the dendritic structure and increases the material hardness to 646 HV, significantly improving the corrosion resistance of the coating compared to both the original L53 steel and the coatings obtained only by induction cladding or electrofriction treatment;

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The maximum hardness (965 HV) was achieved after electrofriction treatment of L53 steel, which is associated with the formation of needle-like martensite;

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The influence of microstructure and hardness on erosion and abrasive wear resistance confirmed that these parameters are crucial for increasing the durability of working tools.

Thus, the combination of induction cladding with subsequent electrofriction treatment provides a comprehensive improvement in performance characteristics. This can find wide application in improving the wear resistance of working tools in agricultural machinery designed for cutting and breaking soil.