The Influence of Frictional Treatment and Low-Temperature Plasma Carburizing on the Structure and Phase Composition of Metastable Austenitic Steel

The features of the structure and phase composition of corrosion-resistant austenitic chromium–nickel steel (16.80 wt % Cr, 8.44 wt % Ni) subjected to carburizing in electron beam plasma at temperatures of 350 and 500°C, frictional treatment with a sliding indenter, and a combination of frictional treatment and plasma carburizing have been considered. It has been established that plasma carburizing results in the formation of a modified surface layer consisting of carbon-saturated austenite and carbides (Cr23C6, Fe3C); in this case, the formation of γC-phase occurs only at a temperature of 350°C. The depth of a modified layer increases with an increase in the carburizing temperature. It has been shown that it is useful to perform combined frictional treatment and plasma carburizing at a carburizing temperature of 350°C, since in this case the deformation-induced structure formed as a result of frictional treatment is preserved, and the precipitated carbides remain highly dispersed. In this case, frictional treatment should provide the formation of the deepest possible diffusion-active layer with a dispersed structure.


INTRODUCTION
There are many ways to harden the surface of austenitic steels. Surface deformation treatments and various types of chemical surface modification are the most widely used. Frictional treatment [1][2][3][4][5][6][7][8] can be highlighted among the methods of strain hardening of austenitic steels that are prone to adhesion upon contact interaction. Together with efficient hardening, frictional treatment of austenitic steels makes it possible to obtain high-quality low-roughness surfaces with negligible amount of material-continuity defects. In this case, the depth of the hardened layer can reach 500 μm. Among the known methods of chemical modification, low-temperature (below 550°C) plasma surface treatments are of great interest, in particular, plasma carburizing [9][10][11][12][13][14][15][16] and plasma nitriding [17][18][19]. These plasma treatments make it possible to obtain austenite in the steel structure supersaturated with carbon (γ C ) or nitrogen (γ N ) with increased hardness, which contributes to effective hardening and improving the service characteristics of austenitic chromium-nickel steels. However, as a rule, the depth of the hardened layer does not exceed 100 μm. It should be noted that plasma carburizing and nitriding are usually performed using glow discharge facilities. However, there are alternative ways to generate plasma. For example, the use of low-energy electron beams [16,17] makes it possible to efficiently generate high-density plasma (10 10 -10 12 cm -3 ) and obtain a required temperature of treated objects with no use of external heating, which is a significant advantage.
The attention of researchers to combined treatments including strain hardening and chemical modification of the steel surface has significantly increased in recent years [20][21][22][23][24][25][26][27][28][29]. In particular, this makes it possible to obtain a higher depth of hardening with a higher quality of the hardened surface. For example, the use of frictional treatment before plasma nitriding makes it possible to reduce the roughness parameter Ra of the formed surface of austenitic steel by several times [26]. It should be emphasized that there is no information in the literature on the use of combined treatment of austenitic chromium-nickel steels, which includes carburizing in electron beam plasma with preliminary frictional treatment. Thus, the use of this combined treatment is not only practically substantiated, but is also of considerable scientific interest.
The aim of this work was to study the structure and phase composition of austenitic AISI 321 steel subjected to carburizing in electron beam plasma at temperatures of 350 and 500°C, to frictional treatment with a sliding indenter, and to combined frictional treatment and plasma carburizing.

EXPERIMENTAL
The material under study was commercial corrosion-resistant austenitic AISI 321 steel with the following chemical composition (wt %): 0.05 С, 16 The studied samples of 40 × 25 × 10 mm were cut out from sheet steel using electroerosive cutting on a FANUC Robocut α-0iE machine. Before subsequent treatment, the samples were subjected to quenching from 1100°C with water cooling, mechanical grinding, and electrolytic polishing in sulfur-phosphorus electrolyte with the composition 100 mL H 2 SO 4 + 400 mL H 3 PO 4 + 20 g CrO 3 . The prepared samples were subjected to frictional treatment, plasma carburizing, or combined treatments, as a result of which a set of studied samples was obtained ( Table 1).
Carburizing of samples was performed in argonacetylene (Ar + C 2 H 2 ) plasma of a low-energy electron beam. A two-stage source of a wide electron beam (D = 100 mm) with a meshed plasma cathode was used. Figure 1 shows the experimental setup for carburizing. At the initial stage, a glow discharge was ignited in an argon atmosphere (30 cm 3 /min), then an accelerating voltage (U 2 ) was applied between the mesh and discharge chamber. A bias voltage (-350 V relative to the discharge chamber) was applied to the table with the samples, and the samples were subjected to ion purification and heating for 30 min. Next, acetylene (1.5 cm 3 /min) was let into the chamber and the beam parameters were set (current I 2 , voltage U 2 ), which provided heating of the samples to the required temperature (T = 350 and 500°C). The samples were held in the steady state for 6 h. The main technological parameters of carburizing are presented in Table 2.
The frictional treatment of samples was performed using a laboratory setup in an oxidation-free argonflow medium with reciprocating sliding of a semispherical indenter made of synthetic diamond with semi-sphere radius R = 3 mm along the steel surface at the average sliding speed V = 0.065 m/s under the load P = 392 N and single scanning of the sample surface with indenter displacement d = 0.1 mm for each double pass.
The structure of AISI 321 steel after different treatment modes was studied using a Tescan VEGA II XMU scanning electron microscope (SEM). X-ray dif-fraction (XRD) analysis was performed on a PANalytical Empyrean diffractometer in CuKα radiation. The phase composition, angular position 2θ of lines, and integral width B of lines were determined.

RESULTS AND DISCUSSION
The microstructure of quenched AISI 321 steel is fully austenitic with the titanium carbide TiC inclusions [2,7,30,31]. XRD analysis showed that there is no α phase in the structure of quenched steel (Fig. 2а, Table 3). After plasma carburizing at a temperature Т = 350°C, a 25 μm-thick modified layer is formed on the surface of AISI 321 steel, which is clearly seen on the transverse section (Fig. 3a). The presence of this layer is commonly related to the formation of carbonsupersaturated austenite γ C [11], which is confirmed by the XRD data (Fig. 2b, Table 3). These data show that after carburizing at Т = 350°C, the maxima of austenite lines are shifted towards smaller diffraction angles, i.e., the γ C phase with an increased lattice parameter a γ is formed. The determination of carbon content X C in the γ C phase using the dependence a γ = a 0 + 0.044X C [32] (where a 0 = 3.607 Å, a γ = 3.668 Å) resulted in X C = 1.39 wt %. Carburizing at Т = 350°C also results in the formation of chromium carbide Cr 23 C 6 . Precipitation of carbides leads to depletion of neighboring austenite regions of carbon, which is confirmed by a weak peak of the γ phase in the diffraction pattern (Fig. 2b). It is important to emphasize that the precipitation of secondary phases in the AISI 321 steel was previously observed upon chemical modification of the surface with similar parameters (temperature and holding time) [23,28]. In particular, the formation of Cr-N and Fe-N chemical bonds after plasma nitriding at Т = 350°C was shown for the first time in [28] using X-ray photoelectron spectroscopy (XPS).
An increase in the plasma carburizing temperature to T = 500°C results in a depth of the modified layer of at least 45-50 μm due to enhanced diffusion of carbon into the steel surface at a higher treatment temperature (Fig. 3b); however, the layer is not as pronounced as after carburizing at T = 350°C (Fig. 3a). A large number of dispersed particles are observed within this layer ( Fig. 3b), which, according to XRD, are chromium carbide Cr 23 C 6 and cementite Fe 3 C (Fig. 2c). The formation of carbides leads to a shift of the maxima of the austenite lines towards larger diffraction angles (Table 3) and to a decrease in the carbon content in austenite, as a result of which the γ C phase is not formed. We note that the integral width В (111)γ of the X-ray line of austenite increases with an increase in the plasma carburizing temperature. The increase in the line width is caused by crystal lattice microdistortions, which can be caused by both the increased carbon content in the lattice and the growth in the density of crystal structure defects due to relaxation of thermal stresses by deformation upon cooling after carburizing [30,31]. An increase in the carburizing temperature leads to an increase in thermal stresses; therefore, after carburizing at Т = 500°C, the width В (111)γ is slightly larger than after carburizing at Т = 350°C (Table 3), even despite the lower carbon content in austenite after carburizing at Т = 500°C.
The frictional treatment results in the formation of a 20-25 μm-thick deformed layer on the surface of AISI 321 steel containing elongated crystals (Fig. 4). At a depth of more than 25 μm, a structure of deformed austenite is observed with a large number of slip bands within the initial austenite grains (Fig. 4). According to the data of X-ray diffraction analysis, frictional treatment is accompanied by the formation of deformationinduced martensite in the surface layer of the AISI 321 steel in the amount of V α = 72 vol %, as well as a sharp increase in the width В (111)γ from 16.0 to 100.7 min and a shift of the maxima of austenite lines towards larger diffraction angles as a result of an intensive deformation effect on the treated surface (Fig. 2d, Table 3).   20 The combined treatment, including frictional treatment and plasma carburizing at a temperature of Т = 350°C, is accompanied by the precipitation of dispersed particles in the surface layer of steel (Figs. 5a, 5b), which are chromium carbide Cr 23 C 6 (Fig. 2e). We note that carbides are observed at a depth less than 25 μm, i.e., the highest diffusion activity is exhibited by the layer with a highly dispersed structure. Therefore, frictional treatment should provide the formation of the deepest possible diffusion-active layer. The amount of the α phase after such a combined treatment does not decrease and is preserved at a level of V α = 72 vol %, but the width В (110)α increases from 37.3 to 40.9 min (see Table 3). Taking the defect structure of deformation-induced martensite into account, the indicated increase in the width В (110)α can be due to the saturation of the α phase with carbon in the course of carburizing. The width В (111)γ , on the contrary, decreased from 100.7 to 78.0 min (Table 3), which may be due to the recovery in cold-worked austenite when heating in the course of carburizing.
The combined treatment including frictional treatment and plasma carburizing at a temperature of Т = 500°C was accompanied by the precipitation of a higher amount of larger particles in a 20-25 μm thick layer; the number of these particles in the underlying layers was considerably less (Figs. 5c, 5d). According to the XRD analysis, these particles are chromium carbide Cr 23 C 6 and cementite Fe 3 C (Fig. 2f). In this case, there is no α phase in the steel structure, and the width В (111)γ sharply decreased from 78.0 to 16.6 min and became almost equal to the width of the quenched-steel line (Fig. 2f, Table 3), which indicates that recrystallization occurred. We note that the combined treatment does not significantly affect the posi-tion of the maxima of the α-phase lines; the maxima of the austenite lines shift towards smaller angles with increasing carburizing temperature and come close to the positions of the maxima of the quenched-steel lines (Table 3) due to the recovery in cold-worked austenite and recrystallization.

CONCLUSIONS
The structure and phase composition of the austenitic AISI 321 steel subjected to frictional treatment and carburizing in electron beam plasma at temperatures T = 350 and 500°C have been studied.
It has been established that plasma carburizing results in the formation of a modified surface layer consisting of the carbon-enhanced austenite and carbides. The phase composition after carburizing at T = 350 and 500°C can be determined as γ C + γ + Cr 23 C 6 and γ + Cr 23 C 6 + Fe 3 C, respectively. The depth of the modified layer grows with an increase in the carburizing temperature and is 25 μm at T = 350°C and no less than 40-45 μm at T = 500°C.
Frictional treatment results in the formation of a deformation-induced austenitic-martensitic structure in the surface layer of the AISI 321 steel. The phase composition after the combined treatment including frictional treatment and carburizing at T = 350 and 500°C can be determined as α' + γ + Cr 23 C 6 and γ + Cr 23 C 6 + Fe 3 C, respectively.
It has been shown that it is useful to perform the combined frictional treatment and plasma carburizing at a carburizing temperature Т = 350°C, since in this case the deformation-induced structure formed as a result of frictional treatment is preserved and the precipitated carbides remain highly dispersed. In this case, frictional treatment should provide the formation of the deepest possible diffusion-active layer with a dispersed structure.

CONFLICT OF INTEREST
The authors declare that they have no conflicts of interest.

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