Electrofrictional Hardening of the 40Kh and 65G Steels

Abstract

This study investigated the influence of electrofrictional treatment on the structure and hardness of the surface layers of the 40Kh and 65G steels. Based on the results of scanning electron microscopy, it was determined that during the electrofrictional hardening (EFH) of 40Kh steel, a hardened surface layer, with a microhardness of 873 ± 37 HV0.1, was formed. This layer consisted of two zones: a surface-quenched zone, with a structure of fine needle-like martensite and austenite; and a heat-affected zone (transition layer), with a structure of martensite and high-dispersion pearlite (troostite), smoothly transitioning into the original ferrite–pearlite structure. After EFH, a layer with a thickness of ~150 μm containing carbides in the martensite was formed on the surface of the 65G steel, which smoothly transitions into the heat-affected zone with a structure of needle-like martensite. The microhardness of the 65G steel in its initial state was 277 ± 20 HV0.1, and after EFH, it reached 811 ± 23 HV0.1. The results of the microstructure analysis of the 40Kh and 65G steels after EFH were consistent with the results of X-ray phase analysis. It was established that the phase composition of the 40Kh and 65G steels in their initial states consisted of an α-Fe phase with a body-centered cubic (BCC) lattice, and after EFH, both steels formed strengthening phases: residual austenite (γ-Fe) and martensite (α′-Fe). During EFH, under high temperature and pressure conditions, carbon from the cast iron electrode was alloyed with iron, contributing to the formation of cementite on the surface of the 65G steel. These obtained data allowed us to conclude that electrofrictional treatment is an effective method for the surface hardening of 40Kh and 65G steels.

1. Introduction

Modern mechanical engineering cannot be imagined without the use of medium- and high-carbon steels. The steels under consideration are most often used under heavy-duty conditions, which is why creating hardened layers on their surface will significantly increase the reliability and durability of machine parts. The formation of hardened surface layers can be achieved through purposeful formation of the required structural state of the metal through thermal and thermo-chemical treatment methods [1]. As a result of such treatment, either structural changes occur in the initial surface, i.e., the modification process, or a coating is formed on the surface. Processes of modifying the surface induce changes in the structure and phase composition of the surface layer, which, in turn, serve as a prerequisite for acquiring new properties [2].

Currently used methods for the surface hardening of components (heat treatment, spraying, laser treatment, etc.) require the use of expensive equipment, special pre-treatment of the surfaces to be hardened, and costly consumables. The application of resource-saving technologies plays an important role in surface hardening, contributing towards the reduction in resource and energy costs, as well as increasing labor productivity [3]. As already known, the plasma surface treatment of steel components represents a significant potential for saving material, labor, and energy costs [4]. In many cases, a plasma heat source can be used alongside other sources, such as lasers and electron beams, providing a high technical and economic performance of the process [5]. For instance, Rahadilov B.K. and his co-authors employed an electrolytic plasma treatment as a plasma heat source for the hardening of medium- and high-carbon steels [6,7,8,9,10]. It has been established that after electrolytic plasma hardening, a modified layer with a thickness of 0.5–2.5 mm is formed, exhibiting high hardness and wear resistance. This layer consists of a quenched layer of fine-grained martensite, an intermediate layer of pearlite and martensite [11].

In the last few years, thermomechanical treatment (TMT) has become more widely used for the hardening of mechanical engineering parts [12,13]. Thermomechanical treatment involves changing the material structure via heating and machining at different temperatures. TMT is far superior to conventional hardening and tempering [14]. The use of thermomechanical treatment on heavy duty work was not possible over the entire volume as only a localized area of the working surface needed to be strengthened. This problem was successfully solved using electrofrictional hardening. EFT is carried out by contacting a rapidly rotating electrode tool with the workpiece, and by supplying a high-density electric current to the contact zone [15].

In the present work, the electrofrictional hardening (EFH) method was applied to harden medium- and high-carbon steels. Electrofrictional technologies provide high energy density. The control of the plasma formation process was achieved through the creation and interruption of electrical contact between the surface to be hardened of the sample and the electrode beneath a layer of cooling liquid. As electrons move from the cathode to the anode, a series of collisions occur, leading to a significant increase in temperature and conductivity current. As the inter-electrode gap increases and the plasma is cooled by the liquid, it loses its electrical conductivity, and the electrical contact is broken. The energy supplied to the arc column is dissipated through heat conduction, convection, and radiation. The passing electrical current facilitates the simultaneous melting of the electrode and the product’s surface, alloying, and rapid cooling.

EFH technology is resource-efficient and environmentally friendly, and in many cases, it can replace traditional surface thermal treatments (such as induction hardening) and thermo-chemical treatments (including nitriding and carburizing). There are two main applications for electrofrictional technology: electromechanical processing [16] and electrofrictional hardening [17]. The method of electromechanical processing can be used for the surface hardening of plowshares for agricultural machinery, achieving current densities of up to 10⁹ A/m², with the formation of hardened zones up to 3 mm in depth in the form of continuous lines. One drawback of this method is the use of a complex and energy-intensive system for deforming the plowshare blade, combined with the supply of electrical energy, which limits the working life of the deforming tool and the effectiveness of the method. Additionally, electromechanical hardening is typically applied to ductile, non-quenched materials. Bogdanovich P.N. and his co-authors used electrofrictional hardening technology as an experimental method for processing the knives of cutting drums for forage harvesters. The effectiveness of using high-strength cast iron in the designs of cutting drum knives for forage harvesters was examined. The results of wear resistance tests on the hardened samples showed that depending on the testing regime, the electrofrictional hardening method increased wear resistance by 1.1–1.5 times, with a hardened zone thickness of 400 µm [17]. However, the thickness of the hardened zone (heat-affected zone) was considered a potential area for improving the adhesion wear resistance of soil-working tools and requires further research to expand the potential of electrofrictional treatment. Taking all of this into account, it can be stated that the potential of the electrofrictional hardening method has still not been fully realized.

Thus, the aim of this study was to investigate the peculiarities of the structural phase state and properties of the surface layer during the electrofrictional hardening of medium- and high-carbon steels (40Kh and 65G) and, based on this, to improve the surface hardening technology using the method of electrofrictional treatment.

2. Materials and Methods

The test samples were made of 40Kh (0.36% C; 0.8% Mn; 0.4% Si; 1.0% Cr; 0.3% Ni; 0.03% Cu; 0.035% S; 0.035% P.) and 65G (0.62% C; 0.9% Mn; 0.17% Si; 0.25% Ni; 0.25% Cr; 0.2% Cu; 0.035% S; 0.035% P.) steels. The sample sizes of the 40Kh and 65G steels were 200 × 30 × 10 mm³. The preliminary surfaces of the steel samples were machined on grinding paper with P100 grit.

On the base of the Research Center “Surface Engineering and Tribology” of S. Amanzholov East Kazakhstan University (Ust-Kamenogorsk, Kazakhstan), a unit for electrofrictional hardening was developed under the supervision of Dr. Y.N. Tyurin (E.O. Paton Institute of Electric Welding, NAS of Ukraine), as shown in Figure 1. Structurally, this unit has a body, which contains an electrical power source, a spindle assembly, a drive for rotation of the spindle and the electrode fixed at its end, a control unit, and an electrical input, as well as the assembly of the workpieces to be treated, current leads, and structural elements for collecting and supplying water to the treatment zone.

The sample is in contact with a rotating disk electrode. The friction of the electrode against the sample is accompanied by the formation and breaking of electrical contact between them. Contact between the product and the electrode was carried out using a sublayer of cooling liquid (water), which causes heating until the contacting surfaces melt. A gray cast iron with a diameter of 250 mm was used as an electrode (cathode). The power source was a welding rectifier with a power of 65 kW, giving a maximum output value of 80 V/1000 A in the form of direct current. The electric current was limited in the range of 200 A. The voltage of the electric current was 70 V. The friction of the modified surface of the sample was carried out against the periphery of the electrode with a force of 10 N. The rotation speed of the electrode was 160 rpm, while moving the sample at a speed of 5 deg/s.

X-ray phase analysis of the studied samples was performed on a SHIMADZU XRD-6000 diffractometer (monochromatic Cuα-radiation, wavelength 1.54056 Å) with the following imaging parameters: accelerating voltage 45 kV, beam current 30 mA, scanning step 0.02° in the angle range 30–85°, and signal acquisition time 0.5 s. The analysis of phase composition was performed using PDF4+ databases and the POWDER CELL 2.4 full-profile analysis program [18]. The microstructure of the investigated samples was studied on a TESCAN MIRA3 LMH scanning electron microscope. To study the microstructure of steel, mechanical treatment (grinding and polishing) and chemical treatment (etching) of the sample surface in a 4.0% nitric acid solution (HNO₃) in alcohol for 10 s were carried out. A Metolab 502 microhardness tester was used to determine the hardness of steels according to the Vickers method. The abrasion test was carried out according to ASTM G65 [19]. Before testing, the samples were ultrasonically cleaned to remove any extraneous particles from them. A cylindrical rubber roller pressed radially against the flat surface of the test specimen with a force of 44 N was rotated at a frequency of 1 s⁻¹. The rate of abrasive particles entering between the rubber wheel and the sample, i.e., into the test zone, was 41–42 g/min. Electrocorundum, with a grain size of 200–250 microns, was used as abrasive particles. The specimens were tested for 10 min. Before and after abrasive wear testing of the samples, mass measurements were performed on CRYSTAL 100 analytical scales with an accuracy of 0.0001 g.

3. Results and Discussion

Figure 2 shows the diffractograms of the initial and hardened samples of the 40Kh and 65G steels.

Table 1. Results of the X-ray phase analysis.

Steel		Detected Phases	Phase Content, Mass %
40Kh	Before treatment	α-Fe	100
	After EFH	α′-Fe	81
		γ-Fe	19
65G	Before treatment	α-Fe	100
	After EFH	α′-Fe	90
		γ-Fe	7
		Fe3C	3

shows the results of the X-ray phase analysis. The phase composition of the 40Kh and 65G steels in their initial states consisted of an α-Fe phase with a BCC lattice (Figure 2a,c). Following the EFH of 40Kh steel, residual austenite (γ-Fe) and martensite (α′-Fe) phases appeared. Martensite was formed on the steel’s surface as a result of rapid cooling during the EFH process. Residual austenite (γ-Fe), martensite (α′-Fe), and cementite (Fe₃C) appeared in the 65G steel. In the EFH process, under high temperature and pressure conditions, carbon alloying of iron from the cast iron electrode occurs, which promotes the formation of cementite on the steel’s surface. The diffractograms of the treated 40Kh and 65G steels did not show the formation of oxide compounds, since the EFH process took place in water.

During EFH, both steel and cast iron are subjected to high temperatures and pressures created via frictional forces and electrical currents. Elevated temperatures can lead to the transformation of austenite (the stable crystalline structure of iron) into more stable phases, such as martensite and cementite, but they can also contribute towards the formation of residual austenite. In the case of reverse polarity generated by the arc in the EFH process, the electrode heats up more, and the surface of the sample is alloyed with elements that are part of the cast iron (electrode). The process of heating and the influence of frictional forces lead to phase transformations in the surface layer of the material. These transformations may include recrystallization and martensitic transformation. As a result of these transformations, a layer forms on the surface that often contains phases that are characteristic of the material. In the case of the EFH of medium- and high-carbon steels, a continuous “white layer” is formed on the surface, consisting of the phases γ-Fe, α′-Fe, and the carbide phase Fe₃C. The structure and properties of EFH are primarily influenced by the temperature and duration of the liquid–solid phase interaction between the cast iron electrode and the treated material. The microstructure of the initial state of the 40Kh steel consists of a ferrite–pearlite structure (Figure 3c).

A distinctive feature of the EFH technology used is the zonality of the formed structures across the thickness of the modified layer, which can be divided into the following zones: the strengthening zone, the heat-affected zone, and the base of the treated material. The microstructure of the hardened layer of 40Kh steel consisted of a needle-like martensitic structure and residual austenite (Figure 3a). The results of the microstructure analysis of 40Kh steel are consistent with the results of the X-ray phase analysis (Figure 2b). Due to the high temperature achieved during heating, a finely dispersed, plate-like martensite and residual austenite formed in the steel structure during subsequent cooling. The heat-affected zone contained martensite and high-dispersion pearlite (troostite). The structure of the heat-affected zone and its overall width depend on the heating and cooling conditions, as well as the thermophysical characteristics of the treated material. In medium- and high-carbon steels, a fine plate-like martensite or a mixture of martensite and intermediate transformation products form in the heat-affected zone, or the same structures as in the hardened layer, but more dispersed.

From the provided microphotograph of 65G steel after EFH, it is evident that a “white layer” was present on the surface of the hard alloy, behind which lay the transitional zone, with a needle-like martensitic structure approximately 500 µm thick, followed by the unaffected material zone, with well-defined grain boundaries and a ferrite–pearlite structure (Figure 4). In the unaffected material zone, the ferrite network appeared as a flat surface, and the pearlite resembled a surface with a needle-like pattern (Figure 4d). After EFH, in the subsurface layer, with a thickness of about 150 µm, carbides were observed within the martensite (Figure 4b). The sizes of the spherical carbides, precipitating from supersaturated austenite or martensite, were less than one micron. They precipitate from the austenitic matrix when heated to high temperatures, for example, around 950 °C.

In the subsurface layer of 65G steel during EFH, cracks formed (Figure 4a). It is known that during the cooling of steel heated above the A_c1 point, phase transformations occur, the result of which depends on the cooling rate. Austenite transforms into the quenching product martensite. The A→M transformation creates large micro-stresses, leading to fragmentation and phase deposition. Internal stresses are higher, with a higher carbon content in the steel. These large stresses, in micro-volumes, sometimes lead to the formation of micro-cracks in quenched steel. Based on these data, we can explain the formation of cracks in 65G steel, which were not observed in 40Kh steel after EFH.

The structural phase composition of the material is one of the main factors determining the wear resistance of the material. The primary type of wear that the working parts of soil-tilling machines undergo during operation is abrasive wear. Figure 5a presents the results of determining the wear resistance to non-fixed abrasives for the 40Kh and 65G steels before and after EFH. The worn volume was determined based on the sample mass loss, according to the methodology outlined by the authors of [20]. A high wear resistance of the material in abrasive wear was provided with high hardness [21]. Figure 5b shows the Vickers microhardness of the 40Kh and 65G steels before and after EFH, as shown in Figure 5a. The microhardness of 40Kh steel in its initial state was 252 ± 16 HV0.1, and after EFH, it was 873 ± 37 HV0.1. The microhardness of 65G steel in its initial state was 277 ± 20 HV0.1, and after EFH, it was 811 ± 23 HV0.1. After EFH, the microhardness of the 40Kh and 65G steels increased by 3–3.5 times compared to their initial states. However, the increase in the wear resistance of the 40Kh and 65G steels during sliding friction against non-fixed abrasive particles was approximately 10%. Analyses that have been reported in the literature have shown that steels with a mixed matrix (austenite and martensite) have a significantly lower wear resistance compared to steels with a homogeneous austenitic or martensitic matrix. It is worth noting that to achieve a high wear resistance, it is necessary to apply alloying with an optimal ratio of carbide-forming elements in the chemical composition, in combination with specific cooling rates, quenching regimes, and modification processes.

The high hardness of the steels after EFH can be explained by the formation of a martensitic structure due to the extremely high heating and cooling rates, which were unattainable with traditional heat treatment methods. During EFH, the periodic interruption of the electrical contact, with an increase in the inter-electrode gap, creates conditions for the rapid cooling of the surface of the product. The Vickers hardness indentation method was used to investigate the distribution of microhardness across the depth of the samples for both the 40Kh and 65G steels (Figure 6). The distribution of microhardness along the surface depends on the location of the hardened zones. It is worth noting that with EFH, the thickness of the modified layer can be varied depending on the processing parameters [22]. Therefore, the obtained data allowed us to conclude that electrofrictional processing is an effective method for hardening the surfaces of the 40Kh and 65G steels.

4. Conclusions

• Phase transformations during the electrofrictional treatment (EFT) of 40Kh and 65G steels have been investigated. It was established that following the EFH of 40Kh steel, the phase composition consisted of residual austenite (γ-Fe) and martensite (α′-Fe). During EFH, under the conditions of a high temperature and pressure, carbon from the cast iron electrode led to the formation of cementite (Fe3C) on the surface of 65G steel, along with residual austenite (γ-Fe) and martensite (α′-Fe).
• The cross-sectional structures of the 40Kh and 65G steels after EFH were conditionally divided into three zones: the quenched layer, the zone of thermal influence, and the base of the treated material. The microstructure of the hardened layer of 40Kh steel consisted of an acicular martensitic structure and residual austenite, while in 65G steel, carbides formed in the near-surface layers. The zone of thermal influence in both steels contained martensite and highly dispersed perlite (troostite). The microstructure of the initial state of both steels consisted of a ferrite–pearlite structure.
• After EFH, the microhardness of the 40Kh and 65G steels increased by 3–3.5 times compared to the initial state. The high hardness of the steels after EFH was explained by the formation of a martensitic structure due to the extremely high heating and cooling rates, which are unattainable with traditional heat treatment methods.
• The increase in the wear resistance of the 40Kh and 65G steels during sliding friction against non-fixed abrasive particles was approximately 10%. It is worth noting that to achieve a high wear resistance, it is necessary to apply alloying with an optimal ratio of carbide-forming elements in combination with specific cooling rates, quenching modes, and modifications.

Thus, the conducted research of electrofrictional technology can serve as a basis for the creation of effective hardening technologies for tools made of medium- and high-carbon steels used in agricultural (plowshares, lancets, and coulters), road building (working bodies of earth-moving tools), mining (jackhammers and tangential cutters), and other industries.