Influence of the parameters of hydrogen nitrogen nitrogen in a glow discharge on tribological and physico

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Khmelnytsky National University

Influence of the parameters of hydrogen nitrogen nitrogen in a glow discharge on tribological and physico-chemical properties of steel 40X

N.M. Stechishina, M.S. Stechishin,

A.V. Martynyuk, N.V. Lukianyuk,

V.V. Lyukhovets, Yu.M. Bilyk

Abstract

metal ammonia nitriding physicochemical

One of the modern and effective methods of hardening metals is nitriding in a glow discharge in ammonia or in an anhydrous medium (nitrogen + argon) - BATR. This paper presents the results of experimental studies comparing the results of tribological and physicochemical properties of hardened surfaces obtained by nitriding with autonomous and interconnected BATR modes. The complex of traditionally fixed values of operating parameters (temperature, composition of the gas mixture, pressure and saturation time) without taking into account energy characteristics (voltage, current density and specific discharge power) significantly reduces the technological capabilities of BATR to achieve the necessary physicochemical properties of metal surfaces specified by conditions exploitation. Taking into account the energy characteristics of BATR, a significant reduction in the energy consumption of the nitriding process is achieved. The energy levels of the main subprocesses are significantly different: the formation of nitrides occurs at low energies, surface sputtering occurs at high voltage values, and nitrogen diffusion occurs at increased current density values. In cases where the energy of the flow is insufficient, either a glow discharge may not occur at all, or with a lack of voltage, the nitride ball on the surface is not sprayed and it acts as a barrier that prevents the diffusion process into the inner layers of the metal, which leads to low physicochemical and, correspondingly, tribological indicators of nitrided balls. The quantitative ratio between them and the required operational properties of the metal, respectively, can be achieved only through an independent combination of the energy and operating characteristics of BATR.

Key words: non-hydrogen nitriding in a glow discharge (BATR); wear resistance of nitrided balls; phase composition; voltage; current density; specific power of the discharge.

Анотація

Стечишина Н.М., Стечишин М.С., Мартинюк А.В., Лукьянюк М.В., Люховець В.В., Білик

Ю.М. Вплив параметрів водню азоту азоту у тліючому розряді на трибологічні та фізико-хімічні властивості сталі 40Х.

Одним із сучасних та ефективних методів зміцнення металів є азотування у тліючому розряді в аміаку або в безводному середовищі (азот + аргон) - БАТР. У цій статті представлені результати експериментальних досліджень, у яких порівнюються результати трибологічних та фізико -хімічних властивостей затверділих поверхонь, отримані азотуванням з автономними та взаємопов'язаними модами BATR. Комплекс традиційно фіксованих значень робочих параметрів (температура, склад газової суміші, тиск і час насичення) без урахування енергетичних характеристик (напруги, щільності струму та питомої потужності розряду) значно знижує технологічні можливості БАТР для досягнення необхідних фізико - хімічні властивості металевих поверхонь, визначені умовами експлуатації. Враховуючи енергетичні характеристики БАТР, досягається значне скорочення споживання енергії в процесі азотування. Рівні енергії основних підпроцесів істотно відрізняються: утворення нітридів відбувається при низьких енергіях, поверхневе розпилення відбувається при високих значеннях напруги, а дифузія азоту відбувається при збільшених значеннях щільності струму. У випадках, коли енергія потоку недостатня, або тліючий розряд може взагалі не виникати, або при нестачі напруги нітридна кулька на поверхні не розбризкується, і вона діє як бар'єр, що перешкоджає процесу дифузії в внутрішніх шарів металу, що призводить до низьких фізико -хімічних і, відповідно, трибологічних показників азотованих кульок. Кількісне співвідношення між ними та необхідними експлуатаційними властивостями металу відповідно може бути досягнуто лише шляхом незалежного поєднання енергетичних та робочих характеристик BATR.

Ключові слова: безводневе азотування у тліючому розряді (БАТР); зносостійкість азотованих кульок; фазовий склад; напруга; густина струму; питома потужність розряду.

Introduction

One of the modern and effective methods of hardening metals is nitriding in a glow discharge in ammonia or in an anhydrous medium (nitrogen + argon) - BATR. Nitriding in a hydrogen-free environment excludes the possibility of explosion of the installation and hydrogen embrittlement of surfaces due to the diffusion of hydrogen formed during the decomposition of ammonia into the depth of the metal. In addition, the BATR process is absolutely environmentally friendly. Also, at present, most studies are devoted to nitriding processes with interdependent parameters, i.e., each combination of operating parameters (temperature, composition of the gas mixture and its pressure) automatically corresponds to a combination of energy (voltage, current density), as a result of which the latter are not considered as controlled. process factors [1].

Among all the operating characteristics of the process, the most significant parameter is the preset surface temperature, since to achieve and maintain it throughout the entire nitriding time, a certain specific combination of voltage and current density is required. Providing a given surface temperature due to factors alternative to a glow discharge allows changing the energy parameters of BATR in a wider range. In world practice, ensuring the independence of temperature from the energy characteristics of BATR is achieved by using chambers with "hot walls" [2]. Providing a given surface temperature due to factors alternative to a glow discharge allows not only to reduce the nitriding time, but also opens up fundamentally new possibilities for improving the controllability of the BATR process and obtaining the properties of surfaces of metallic materials specified by operating conditions, depending on the conditions of their operation. Despite the obvious new capabilities of BATR with independent energy parameters, it has not actually been investigated in practical and experimental terms. This paper presents the results of experimental studies comparing the results of tribological and physicochemical properties of hardened surfaces obtained by nitriding with autonomous and interconnected bAtR modes.

Purpose of work - study of the influence of energy parameters on the tribological and physicochemical properties of the diffusion layers of steel 40X at BATR with interdependent and autonomous saturation parameters.

Materials and methods of testing

Nitriding was carried out on the UATR-1 unit designed and manufactured at the Podolsk Scientific Physics and Technology Center (PNFTTs) of the Khmelnitsky National University. The installation belongs to a diode-type model with direct current and was additionally equipped with heating elements placed in the gas- discharge chamber, which made it possible to arbitrarily change the voltage value U, and the value of the current density j was determined as the ratio of the current strength to the total area of the parts (samples) of the cage and suspension (j = I/S, A/m²). The design features of the installation for carrying out BATR processes with interdependent and autonomous nitriding modes are described in detail in [3].

The influence on the structure, phase composition and, accordingly, on the physicochemical properties of the surface nitrided layers of temperature, composition of the gaseous medium, its pressure and saturation time has been comprehensively studied, for example, in [1... 4]. Therefore, saturation was carried out in a mixture of 80 % N2 (nitrogen) and 20 % Ar (argon) at a temperature of T = 833 K for an hour. The voltage and current strength were chosen arbitrarily, based on the experience of preliminary studies. Technological modes of carrying out BATR processes are given in Table 1.

Table 1

Characteristics of BATR modes

* - modes with interdependent parameters; ** - also, but with a modified suspension shape

The specific power of the glow discharge was determined by the formula:

W ¶ю UI/S ¶ю Uj,

where S - is the total area of parts (samples) and suspension (cathode area).

Experiments 1*, 4*, 7* were carried out under interdependent nitriding regimes (the first series of experiments), i.e. each pressure of the mixture corresponded to the corresponding value of voltage and current density. The second series of experiments (1**, 4**, 7**) was carried out under the same conditions but with a changed shape of the suspension and, therefore, with a different cathode surface area. Experiments 2, 3, 5, 6, 8, and 9 correspond to the autonomous modes of the BATR. At the same time, modes 3, 5 and 8 were carried out at

U ¶ю 515В ¶ю const, and modes 2, 5 and 9 at j ¶ю 7,2A/m² ¶ю const.

Metallographic studies were carried out on a MIM-10 microscope after etching microsections with a 3 % alcohol solution of nitric acid. The microhardness was determined with a PMT-3 microhardness tester at a loading of 0.98N with fixing the hardness values on the surface and at a distance of 25, 50, 100, 200, 500 ^m.

For X-ray phase analysis, a DRON-3 diffractometer was used in the range of angles 0 from 20^o to 100° with a scanning step 0,1° and an exposure time of 10 s.

Tribological studies of the samples for wear resistance were carried out on a universal machine model 2168UMT according to the disk-finger friction scheme; friction without lubricant (dry); counterbody material - steel ШХ15 with hardness HRC61; pressure in the contact zone p = 16 MPa; controlled linear wear h, which was determined by the change in the linear size of the sample measured along the normal to the friction surface after passing the section of the friction path L in accordance with the accepted measurement step l (Table 2).

Table 2

Frequency of measurement of the results of tribological studies

The dependence of the wear resistance of the modified surfaces on the energy parameters of the discharge was confirmed by tribological studies. As a result of the experiments, it was found that under dry friction conditions for surfaces modified at higher energy values, the wear rate (Fig. 1, a) and the running-in period (Fig. 1, b) decrease, and the period of stable wear increases, and, with an increase in the content of alloying elements in steel, this pattern becomes more pronounced.

Of course, under other experimental conditions (for example, when studying the obtained samples for corrosion resistance), the dependence of the tribological properties of nitrided surfaces on the current density and voltage on the chamber electrodes may acquire a different character, but the very existence of such a dependence is beyond doubt, which refutes the provisions given in [5], according to which the energy characteristics of the discharge do not have a significant effect on the results of ATR.

Fig. 1, a - wear curves of steel; b - running-in area of wear curves of nitrided steel 40 Kh

The data presented allow us to draw a completely obvious conclusion about the effect of voltage and current density on the characteristics of the modified layer determine the wear resistance of nitrided steels in a glow discharge - but not just essential, but decisive. Moreover, in the field of the energy parameters of the regime, there is a certain limit below which the ATR process generally loses its meaning, since it leads to unacceptable results, and this is despite the fact that the values of the regime remain constant.

This means, in particular, that the set of traditionally fixed operating parameters (temperature, pressure, composition of the gas mixture and the duration of the process) does not give an unambiguous idea of the conditions for carrying out the ATR process, and therefore cannot serve as a basis for predicting its results.

This fact has been repeatedly confirmed when technological processes carried out at similar values of operating parameters, but in different installations (or even in one installation, but using a different suspension), led to completely different results.

This is due to the fact that the factors that determine the effectiveness of the nitriding process in a glow discharge are not only the parameters of the mode, but also such indicators as the cathode distance, the shape and size of the suspension and the test sample (or part), the presence of local exceptions on its surface, and many other factors.

The list of factors that are decisive for ATP, given by the American researcher David Pye in [6], including thirteen items.

Of course, direct consideration of all these indicators would lead to incredible complications in the management of the APR process. However, they can be taken into account indirectly, since the influence of all the listed factors reflect the energy parameters of the process.

It is known that the depth and phase composition of the nitride zone of the surface layer of steel determine its physicochemical properties. Thus, the nitride zone, which consists only of a phase, is characterized by a rather high plasticity, and the zone containing the phase has a lower plasticity, but a higher corrosion resistance [7, 8, 9].

The layer without a nitride zone has the highest plasticity [9, 10]. In general, the thinner the nitride zone, the more plastic the nitrided layer, but the lower its resistance to abrasive and corrosion-mechanical wear.

Thus, according to [11], for parts that are operated in corrosive environments and for wear at low contact voltages, the BATR should be carried out at the maximum possible voltage and current density, which contributes to the formation of a phase and, accordingly, we have a high corrosion resistance, as well as good running-in of friction surfaces. In this case, in order to avoid the transition of a glow discharge to an electrospark one, the condition w < Wkr should be observed, i.e. the specific power of a glow discharge should not exceed the critical specific power of the occurrence of an electric arc discharge.

A decrease in voltage and current density leads to an increase in the phase fraction, which contributes to an increase in the fatigue strength of parts operating under conditions of corrosion-mechanical wear (CMC) in corrosive environments [12].

The dependence of the wear resistance of the nitrided surfaces of steel 40Kh on the energy parameters of the discharge is confirmed by the results of testing samples with dry friction (Fig. 1, a, b) nitrided according to modes 6, 9, 3, and 5, which were nitrided at low values of U and j. in wear resistance than samples nitrided at higher values of U and j (7*, 1*, 8, 4* and 2, see Table 1). These samples (6, 9, 3, and 5) are characterized by small running-in periods at a constant wear rate and a high wear rate, determined by the tangent of the tangent to a point in the wear curve section.

The data of X-ray structural analyzes 8, у¹ - (Fig. 2) and microdurometric measurements (Table 3) confirm and explain the results of tribological experiments (Fig. 1, a, b). So, at BATR according to mode 7* (Table 1), a nitride ball of the greatest thickness (Table 3), consisting of phases (Fig. 2, a), is formed on the surface. A decrease in the values of U and j (Table 1) for mode 8 contributes to a decrease in both the thickness of the nitride zone hN (Fig. 2, b) and the thickness of the diffusion layer h (Table 3), which were identified by measuring the microhardness along the depth of the layer. The phase composition of the nitride and diffusion layers remains practically unchanged.