RESULTS OF APPROBATION OF THE INNOVATIVE METHOD OF ION NITRIDING FOR STEELS WITH LOW TEMPERATURES OF TEMPERING

Practically all criteria of working ability are determined by the properties of surface layers – they in particular must be improved first of all [1, 2]. In this case, high surface hardness in the presence of large tensile stresses leads to brittle failure and crumbling, thus enhancing abrasive wear [3], and large compressive stresses – to plastic deformation of the base and its flow [4]. It was established [5] that in the case of surface modification by applying coatings, the destruction of the system coating-base begins essentially from the plastic deformation of substrate layer near the interface boundary [6]. In order to improve functional properties, they use in such cases creation of special phase [7] or structural [8] states, composite materials [9] and strengthening of boundaries through the distribution of “useful additives” [10]. Although this ensures, in a number of cases, improvement in the operational characteristics, it has, however, low efficiency at work under conditions of multicycle impacts and high loads. The modification of structure and surface properties by nitriding makes it possible to considerably improve their hardness, wear resistance, thermal stability and other functional characteristics, including at operation under conditions of multicycle impacts and large loads. That is why studRESULTS OF APPROBATION OF THE INNOVATIVE METHOD OF ION NITRIDING FOR STEELS WITH LOW TEMPERATURES OF TEMPERING


Introduction
Practically all criteria of working ability are determined by the properties of surface layers -they in particular must be improved first of all [1,2].In this case, high surface hardness in the presence of large tensile stresses leads to brittle failure and crumbling, thus enhancing abrasive wear [3], and large compressive stresses -to plastic deformation of the base and its flow [4].
It was established [5] that in the case of surface modification by applying coatings, the destruction of the system coating-base begins essentially from the plastic deformation of substrate layer near the interface boundary [6].In order to improve functional properties, they use in such cases creation of special phase [7] or structural [8] states, composite materials [9] and strengthening of boundaries through the distribution of "useful additives" [10].Although this ensures, in a number of cases, improvement in the operational characteristics, it has, however, low efficiency at work under conditions of multicycle impacts and high loads.
The modification of structure and surface properties by nitriding makes it possible to considerably improve their hardness, wear resistance, thermal stability and other functional characteristics, including at operation under conditions of multicycle impacts and large loads.That is why stud-

A . T e r l e t s k y
PhD, Associate Professor**

Literature review and problem statement
Depending on the operating pressure, they use at nitriding: high-vacuum technologies (for the high-voltage sources of gas ions [11]), low-vacuum conditions (for the nitriding in the glow discharge [12]) and technologies of nitriding under atmospheric pressure ("furnace nitriding" [13]).
The most widely used in industry at present is the method of "furnace nitriding" of steels with the application of ammonia at temperatures from 500 to 600 о С.The use of this technology makes it possible to attain a thickness of the nitrided layer in the range of 0.2-0.6 mm over a period from 10 to 90 the hours of treatment [14].The application of catalytic nitriding employing special catalysts for the ammonia disassociation makes it possible to decrease duration of treatment in furnace to two times [15].
To considerably increase intensity of the process of diffusion saturation of the surface with nitrogen is possible through ionic nitriding with the use of the low-temperature plasma [16].In this case, energy of the diffusion activation is greatly reduced (in comparison with the thermal "furnace nitriding" [17]), while a diffusion coefficient increases [18].
Ionic nitriding in the gas plasma of vacuum-arc installations at temperatures from 500 to 600 о С makes it possible not to reduce duration of nitriding to 1 hour only.The ionic nitriding (in comparison with the furnace nitriding) also allows improving relative wear resistance of the nitrided layer [19]; it enlarges the possibilities to regulate phase composition [20,21] (by changing energy characteristics of the process or changing the ratio of working gases in the mixture (nitrogen and argon) [22]); as well as it makes it possible to eliminate harmful effect of ammonia [23].In the process of saturation, under the action of ionic bombardment, excess concentration of tructural defects occurs in the material [24].This contributes to an increase in the intensity of diffusion processes (including the mass transfer of nitrogen [18]) and creates a possibility for the occurrence of new phases (based on the compounds with nitrogen, as an S-phase and others) [25,26].Such phases do not form undeer conditions of usual saturation with nitrogen at an elevated temperature.
Such nitriding is typically conducted after high-temperature treatment and mechanical machining [27].In this case, in the process of nitriding, the articles are exposed to negative potential from 800 to 1300 V [16], while regulating its magnitude so that to support the temperature of articles within the range from 500 to 600 о С at arc currents from 50 to 100 A. Hardness of article surface after nitriding is high and reaches the level to 11-17 GPa depending on the composition of steel.
However, by using the methods described above, it is not possible to nitride the articles made of steels with low temperatures of tempering, for example, 40KH, 9KHS, KH12MF and others, since the temperature of nitriding exceeds the temperature of tempering.
Furthermore, an analysis of the results of operational tests of nitrided steels, which work under conditions of impact-cyclic loads, revealed that the formation of nitrided high-hard layer with a sharp boundary on the surface leads to the crumbling of the surface.This is linked to the fact that formation of brittle nitride zone significantly decreases plasticity of the nitrided layer [28].That is why exploratory research has been conducted in recent years into technologies that would yield the process of nitriding with the larger uniformity of nitrogen distribution and at the larger depth [29].

The aim and objectives of the study
The aim of present study is to verify an innovation technique of nitriding steel articles with a low temperature of tempering.Innovation technique is based on a change in the sequence of regimes and number of cycles of thermochemical treatment of articles: preliminary nitriding in the vacuum gas discharge and subsequent hardening and tempering.
To accomplish the aim, the following tasks have been set: -to conduct comparative study into influence of the innovation technology of ionic nitriding on the structure and hardness of steels of different class; -to determine a phase-structural state, critical for an increase in the hardness of a near-surface region.

Materials, methods of their processing and research
We employed as the source materials of nitriding samples made of steels KH12MF, 9KHS and 40KH whose chemical composition is given in Table 1.
Field of application of steel of grade KH12MF is the dies of complex shape, gears, knurl screw dies, portages, matrices and punches of cutting dinking dies with complex configuration of working parts.Steel of grade 9KHS is used in the production of screw dies, markers, punches.Steel of grade 40KH is related to the pearlitic class and is used for manufacturing different gears, axes, critical machine parts, connecting rods, bushings and other components with enhanced strength.
In the present study, we applied a technique for nitriding steel articles in the vacuum-arc gas discharge [16], according to which the nitriding is also conducted after high-temperature treatment and mechanical machining.This technique is executed in the installation with a vacuum chamber, equipped with vacuum-arc vaporizers.Fig. 1 shows a duagram of the installation "Bulat-6" (Ukraine), designed for the nitriding in the arc gas discharge.
Vacuum-arc vaporizer 1, whose housing is electrically connected to the housing of vacuum chamber 2, is equipped with slit screen 3, which does not pass the ions of the evaporated metal of cathode but which does not impede the passage of gas particles and electrons.The housings of other two vacuum-arc vaporizers 4 and 5 are isolated from the chamber Deeper located is the operating layer of thickness to 2000 µm, which enables to carry out finishing of a part without additional strengthening, since the reinforced layer is sufficiently large.Thus, resource of the part increases considerably.
Thus, the technique verified under actual conditions is applicable for the nitriding of steels with a low temperature of tempering and consists of the ionic nitriding in the vacuum gas discharge before hardening and tempering.That is why the heating temperature during ionic nitriding influences little the process of high-temperature treatment.Hardness of the surface of articles after this sequence of operations for steels with the low temperature of tempering is at the level of 8-10 GPa.
In this case, the process of nitriding is accelerated considerably, since nitrogen atoms penetrate untempered steel more easily, and thus the duration of nitriding does not exceed 1 hour.Only one-time mechanical machining will suffice after the completion of the process of thermochemical treatment, removing a layer of about 200 µm with lowered hardness.
Results obtained in the course of study formed a basis for the directed surface modification of the industrial parties of punches of different purposes.At present, their production tests are conducted with the accumulation of statistical data.

Conclusions
The complex innovation treatment, which includes the process of ionic nitriding with the subsequent heat treatment in the form of hardening with subsequent tempering, makes it possible to substantially increase the depth of structure modification and enhance hardness, and, therefore, to improve working ability of parts made of steels with the low temperature of tempering.
1.An analysis of microstructure that we performed for different types of steels, exposed to complex treatment, revealed the generality of an increase to 2000 µm of thickness of the penetration of nitrogen atoms and the formation of region with the changed structure and enhanced hardness.In addition, general is the formation of the surface decarbonized (technological) layer.Such layer is the allowance for finishing in order to obtain the required accuracy of dimensions and surface finish.
2. We determined phase composition of the nitrided layer with high hardness, which consists of lower nitride Fe 4 N and the solution of nitrogen in α-Fe.
UDC 621.785.5:621.793.1:543.442.3DOI: 10.15587/1729-4061.2017.104179ies directed to the development of innovation technologies of nitriding are relevant and required by industry.