Surface Modification of Chromium–Nickel Steel by Electrolytic Plasma Nitriding Method

Abstract

Electrolytic plasma nitriding is an attractive chemical heat treatment used to improve the surface properties of steel by implementing nitrogen saturation. This method is widely applied to steel and iron-based alloys operating under various operating conditions. In this work, using liquid-phase plasma nitriding technology, a nitrided layer was obtained on the surface of 40CrNi steel in electrolytes of different concentrations. The microstructure and phase composition of the nitrided layer were investigated and analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD), and we performed Vickers hardness and wear resistance tests using the ball-on-disc method. The microhardness and wear resistance of nitrided 40CrNi steel were significantly improved due to the lubricating properties of the ε-Fe₂N phase formed on its surface.

1. Introduction

Nowadays, alloyed and special steels are chosen for manufacturing machine parts operating under conditions of high mechanical and thermal effects accompanied by impact loads [1]. However, surface layers often need additional hardening. Currently, there is already an array of scientific research on various technological methods of surface hardening of metal alloys to improve the functional properties of surface layers, such as heat treatment [2], including the use of concentrated energy flows of different nature [3], and chemical–thermal treatment [4]. However, the potential of their efficiency has been exhausted. Therefore, recently, numerous studies have been conducted to develop duplex technologies combining known processes [4,5]. But the empirical approach and the associated costs of numerous experiments to establish regularities between technological parameters and the resulting structure and properties do not allow to control structure formation and make them uncompetitive for production.

Also, in mechanical engineering technology, when modifying the surface layers of steels and alloys, multicomponent chemical heat treatment (CHT) can be effectively used, which consists of simultaneous or sequential diffusion saturation of the surface with several chemical elements [6]. Complex methods of chemical heat treatment such as nitriding and carbonitriding can significantly increase the resistance to wear, as well as increasing the corrosion resistance and a number of other properties of the surface layers of machine parts [7]. Nevertheless, the problem of developing a theory for the formation of nitrided and carbonitrided layers with a complex of valuable properties—such as high wear and corrosion resistance—remains unsolved [8]. Traditional chemical heat treatment in a gas environment (nitriding) and solid carburizer (cementation) has disadvantages in the complexity of using a diffusion-active environment and complex non-standard expensive equipment, and chemical heat hardening often requires repeated heat treatment for the final formation of product properties [9,10].

A promising method for improving the resource and operational properties of steel parts is considered to be chemical heat treatment by electrolytic plasma treatment (diffusion saturation), which is used to saturate the surface of materials with light elements (nitrogen and carbon) [11,12,13,14,15]. When a voltage in the range of 200–350 V is applied to the electrochemical cell, local boiling of liquid occurs around the workpiece due to the release of Joule heat [16]. Under these conditions, the electrolyte near the surface of the workpiece heats up to the boiling point, and the workpiece, being separated from the bulk of the electrolyte, heats up to temperatures of 400–1100 °C [17]. High temperatures of the workpiece will allow saturation of the surface with atoms of light elements contained in donor substances dissolved in the electrolyte. The presence of nitrogen-containing components in the electrolyte causes a certain nitrogen potential of the vapour-gas shell, which makes chemical–thermal treatment possible [18,19]. Nitrogen-containing electrolytes include aqueous solutions of nitrogen donor chemicals, mainly ammonium sulphate and chloride, and urea [20]. Ammonium salts, the anions of which exhibit more sharply expressed oxidizing properties, decompose irreversibly: a redox reaction occurs, during which the ammonium ion is oxidized and the anion is reduced. Under conditions of electrolyte–plasma heating diffusion processes are intensified, which will significantly reduce the time of chemical–thermal treatment up to 5–10 min [21]. This technology has a number of advantages over other methods of surface modification of steel parts, the main ones of which are high treatment speed, low cost, and the possibility of obtaining promising structures and combining diffusion saturation with hardening in one technological process [22]. The disadvantages of electrolyte–plasma nitriding include the relatively small depth of nitrogen saturation on the surface, which may be insufficient for some applications requiring deeper hardening. In addition, the treatment of complex-shaped parts can lead to uneven distribution of nitrogen, especially in hard-to-reach areas [23,24].

During electrolytic plasma nitriding technology, a sample is placed in an electrolyte and treated with an electric arc discharge to form a layer with high hardness, corrosion, and wear resistance in a few minutes. The main parameters that determine the performance of the process include voltage, processing time, current density, frequency, and electrolyte composition. The advantages of this method over other existing chemical heat treatment methods include the high workpiece heating rate (up to 250 °C/s) and high diffusion saturation rate (up to 20 μm/min) [25]. In addition, the use of electrolytic plasma heating allows for the formation of a protective oxide layer on the surface, which further improves corrosion resistance. Nevertheless, the tribological characteristics of this process remain insufficiently studied [26].

This paper focuses on the influence of the concentration of urea used as a nitriding source on the microstructure and properties of plasma–electrolytic nitriding of 40CrNi steel. The aim of this study is to optimize the electrolyte ratio, to create a basis for further studies of electrolyte composition, and to identify the mechanism of wear of steel products after their electrolytic plasma nitriding under a wide range of conditions.

2. Materials and Methods

Structural alloy steel 40CrNi (AISI 5140 analogue) was selected as the object of this study in accordance with the objective. Selection of the research material is justified by the fact that the steel is widely used for manufacturing critical loaded parts subjected to vibration and dynamic loads, which are subjected to requirements of increased strength and toughness. Before electrolytic plasma nitriding, the surface of steel 40CrNi samples of size 2 × 2 × 1 cm³ was grinded on grinding paper with grit from P100 to P2000 and polished with diamond pastes of 0.25–0.50 μm, then cleaned with alcohol.

Electrolytic plasma nitriding (EPN) of steel samples was carried out on the installation, which consists of a power source, a chamber for the electrolytic plasma treatment of materials, and a personal computer. The scheme of EPN of samples is shown in Figure 1. The chemical composition of the studied steel 40CrNi (AISI 5140) is as follows: C: 0.36–0.44%; Si: 0.17–0.37%; Mn: 0.5–0.8%; Ni: 1.0–1.4%; Cr: 0.45–0.75%; Cu: up to 0.3%; S: up to 0.035%; P: up to 0.035% (GOST 4543-71).

An electrolyte based on sodium carbonate and urea was chosen for EPN. The choice of urea is associated with the fact that this substance is a source of nitrogen and carbon, as well as being characterized by cost-effectiveness and environmental safety [27]. The electrolyte’s environmental safety is crucial, given that commonly used chemicals often include hazardous compounds, such as cyanides. Ammonium chloride was chosen as a source of nitrogen. Thus, EPN was carried out in electrolytes, the composition of which is presented in

Table 1. Electrolyte composition.

Sample	Electrolyte Composition	Pressures	Time
40CrNi-1	7% ammonium chloride (NH4Cl) 14% urea ((NH2)2CO) 7% sodium carbonate (Na2CO3) 72% distilled water (H2O)	320B 220B	7 s 7 min
40CrNi-2	10% sodium carbonate (Na2CO3) 20% urea ((NH2)2CO) 70% distilled water (H2O)	320B 220B	7 s 7 min

.

Electrolytic plasma nitriding was carried out as follows: The bath (4) was filled with the electrolyte; with the help of the pump (6), the electrolyte flows into the electrolytic cell (2) and then, through the edge, is drained back into the bath. Thus, the electrolyte is in a circulation regime. The rate of electrolyte supply is adjustable. The sample to be treated is fixed by means of a special fixing device (3). The electrolytic cell consists of a cone-shaped nozzle and a stainless-steel partition as the anode. Through the opening of the cone-shaped nozzle, a jet of electrolyte is fed to the treated area. The anode is connected to the positive pole of the power source and the treated product (cathode) to its negative pole. To obtain the effect of nitriding in electrolyte plasma, a necessary condition is the existence of a stable vapour-gas shell around the treated material, which contains nitrogen ions that penetrate deep into the surface of the material to form nitrided layers. A stable discharge was obtained using a two-stage electrolytic plasma heating method, which involves heating the treated product (cathode) to 400–600 °C in an anomalous discharge at a voltage of 320 V, followed by a transition to the film boiling regime by sharply decreasing the voltage to 220 V. Positive nitrogen and metal ions create a layer adjacent to the heated surface of the product—the cathode. Electric discharges pass through this layer and intensify mass transfer processes. The introduction of the appropriate water-soluble salt into the electrolyte provides doping of the product surface with elements that form positive ions.

An X’PertPro X-ray diffractometer (PANalytical, Almelo, Netherlands) was used to determine the phase composition. Copper (Cu Kα emission, wavelength 0.15418 nm) was used as a radiation source. The analyses were performed at a voltage of 40 kV and a current of 30 mA. Phase identification was carried out using the ICDD PDF-2 database. The volume content of crystalline phases was calculated in the PowderCell 2.4 programme using the Rietveld method. The microstructure of the samples was revealed by chemical etching with a 4% solution of nitric acid (HNO₃) in ethyl alcohol. The structure of the samples was investigated using a TESCAN MIRA3 scanning electron microscope with an attachment for X-ray spectral analysis (EDS, TESCAN MIRA3, Brno, Czech Republic). The hardness of the nitrided layer (distribution of microhardness along the depth) was determined using a Metolab 502 (Metolab, St. Petersburg, Russia) microhardness tester. The tribological characteristics of steel were measured in the sliding friction regime according to the “ball-on-disc” scheme on an Anton Paar TRB³ tribometer (CSM Instruments, Anton Paar, Graz, Austria). The sample rotation speed was 2 cm/s, with a load of 6 N; a 6 mm diameter Si₃N₄ (silicon nitride) ball was used as the counterbody. The wear volume was determined using a Surtronic S-100 profilometer (Taylor Hobson, Leicester, UK).

3. Results

The diffractogram of the surface of the initial sample of 40CrNi steel shows the crystal structure characteristic only for α-Fe (BCC, ferrite) (Figure 2). Other reflexes are not detected, or their intensity is at the level of background “noise”. The lattice parameter measured in the direction normal to the investigated surface is a = 2.8690Å (

Table 2. XRD data.

Sample	Detected Phases	Phase Content, Mas. %	Lattice Parameters, Å
40CrNi initial	Fe_229_BCC	100	a = 2.8690
40CrNi-1	Fe_229_BCC	91	a = 2.8688
	Fe2N_164_trigonal	9	a = 2.6648 c = 4.3371
40CrNi-2	Fe2N_164_trigonal	30	a = 2.6650 c = 4.3369
	Fe2N_60_orthorombic	50	a = 4.3200 b = 5.5793 c = 4.6916
	Fe3O4_166_rhombohedral	20	a = 5.8514 c = 14.5110

). According to X-ray analysis, the surface layer after nitriding of steel 40CrNi in ammonium chloride and urea electrolytes includes Fe₂N nitrides and Fe₃O₄ oxides and martensite and ferrite of the original structure. According to the state diagram of the Fe-N system, the second nitride phase ε- Fe₂N is formed when the limit of saturation with nitrogen is reached. The simplest method of obtaining a layer of this protective phase is high-temperature nitriding at 650–700 °C. Increasing the temperature also accelerates the diffusion process [28]. It is revealed that in carbamide electrolytes, the intensity of Fe₂N peaks increases, which indicates the intensification of nitrogen diffusion. The formation of Fe₃O₄ phases as a result of high-temperature oxidation of steel, as well as Fe₂N iron nitrides as a result of nitrogen diffusion and martensite as a result of quenching, is assumed.

The structure of the cross-sectional layer of steel after nitriding in the electrolyte containing ammonium chloride and urea on the surface revealed a continuous oxide layer with characteristic pores and a network of cracks and is presented in Figure 3. The addition of ammonium chloride in the nitriding composition led to an increase in the oxide layer and a decrease in the thicknesses of the outer nitrided and inner diffusion layer (Figure 3) [29]. This character is related to the difference in cathodic processes for chloride ion. The oxygen formed at the cathode in the latter case will additionally oxidize the sample cathode material and promote the growth of the oxide layer.

In the 40CrNi-2 sample, multilayer diffusion layers are formed as a result of diffusion saturation with nitrogen (Figure 4). An oxide–nitride zone formed on the surface layer with a thickness of 25–30 μm, containing, according to X-ray analysis, iron nitride ε-Fe₂N and iron oxide Fe₃O₄. Under this zone is located the nitride–martensitic sublayer, often referred to as the white layer. This is followed by a third zone of internal nitriding [30]. This zone is a heterophase region based on a highly nitrided α-solid solution with excessive γ′-phase nitrides or special nitrides. In this case, martensite formation occurs only in the nitrogen penetration region, which reduces the quenching temperature. The presence of residual austenite indicates incomplete martensitic transformation. After a continuous martensitic sublayer with dispersed nitrides ε-Fe₂N, grains of martensite and pearlite are observed, which confirms incomplete quenching.

According to the X-ray energy-dispersive analysis of nitrogen, its concentration is maximum directly under the oxide layer and further decreases into the depth of the sample (Figure 5), which fully reflects the alternation of phases in the surface layer after nitriding. After nitriding in urea electrolyte for 7 min, an oxide–nitride layer (30–35 μm thick, containing 4% oxygen and 3.6% nitrogen) is formed on the steel surface. Cathodic nitriding of steel in urea electrolyte leads to the formation of Fe₂N nitrides and Fe₃O₄ iron oxides; the microstructure of the nitrided layer contains a nitride zone and an internal nitriding zone.

At the next stage of work, tribological tests of model samples before and after nitriding were carried out. Nitriding of 40CrNi steel had a positive effect on the increase in wear resistance (Figure 6). When nitriding steel in urea solutions, the coefficient of dry friction with nitride beads decreased from 0.82 in the untreated sample to 0.63 after nitriding for 7 min, with an increase in wear resistance of 1.5 times. These improvements are explained by the lubricating properties of the ε-Fe₂N phase formed on its surface.

The main indicators characterizing the diffusion layer are surface hardness and the hardness distribution over the thickness of the nitride zone and internal nitriding zone. The hardness of the diffusion zone is lower than that of the nitride zone and monotonically decreases from the boundary of the two zones of the layer to the hardness of non-nitrided steel. Nitriding regimes and the amount of alloying elements significantly affect the hardness of the diffusion zone (Figure 7).

The analysis of the results of microhardness measurement along the depth of the diffusion layer shows that with the addition of ammonium chloride in the electrolyte (sample 40CrNi-1), the distribution of microhardness along the thickness of the diffusion layer becomes smoother, without sharp transitions to the core. The drop in the microhardness value of the material along the depth of the diffusion layer is associated with a decrease in the concentration of dissolved nitrogen in the α-phase. In the sample 40CrNi-1 in urea nitriding electrolytes, the hardness maximum is displayed in the surface at a depth of ~60 μm, which is associated with the appearance of a layer of high-nitrogen phases of ε-Fe₂N in the surface zone. Further hardness decreases occur to 700–500 HV with the appearance of nitride–martensite structures. When the austenite is cooled at a rate higher than the critical quenching rate, transformation begins when the martensitic transformation onset temperature is reached. This process results in the formation of a highly supersaturated solid solution of nitrogen in α-iron, which is called a nitride–martensitic layer.

4. Conclusions

Thus, taking into account the analysis of the obtained research results, the following conclusions can be drawn:

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On the basis of metallographic and X-ray diffraction analyses, it was determined that after electrolyte–plasma nitriding, multilayer diffusion layers are formed. In the surface layer, the formed oxide–nitride zone 25–30 μm thick, containing iron nitride Fe₂N and iron oxide Fe₃O₄. Under this zone is located a nitride–martensitic sublayer. This is followed by the third zone of internal nitriding.

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Tribological tests showed that the hardened samples have high wear resistance compared to the original samples. After nitriding steel in solutions of carbamide, the coefficient of dry friction decreases from 0.82 in the untreated sample to 0.63 in the sample after nitriding within 7 min, and the wear resistance increases by 1.5 times.

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It is determined that after electrolyte–plasma nitriding, the surface hardness increases significantly. Nitriding regimes and the amount of alloying elements significantly affect the hardness of the diffusion zone.