FORMATION OF A DIFFUSION LAYER ON THE WORKING SURFACE OF A REACTIVE BUSHING OPERATING UNDER CONDITIONS OF THERMO-EROSION WEAR

ABSTRACT

Studies of the structure and properties of the diffusion layer of samples after boration and borochromation have been carried out. It is revealed that during boration diffusion layers with a high content of highly porous phase are formed, which leads to a decrease in the plasticity of the layer. In this work, studies were carried out on enrichment of the working surface of reactive bush by boron by diffusion during firing, which is subjected to thermal erosion wear during dynamic loading. Obtaining brittle boride layers does not cause an increase in the operational properties of such products. It was found that alloying boride layers with chromium reduces the fragility of diffusion layers. By optimizing the borochromization process for these products, the following parameters were determined: coating composition, diffusion saturation temperature, holding time. It is proposed to combine the chemical-thermal treatment of reactive bush in the coating and heating of products for quenching. The possibility of replacing alloy steel with carbon steel with the investigated diffusion coating is shown.

АННОТАЦИЯ

Проведены исследования структуры и свойств диффузионного слоя образцов после борирования и борохромирования. Выявлено, что при борировании образуются диффузионные слои с высоким содержанием высокопористой фазы, что приводит к снижению пластичности слоя. В данной работе были проведены исследования по обогащению рабочей поверхности реактивной втулки бором путем диффузии при обжиге, которая подвергается термоэрозионному износу при динамическом нагружении. Получение хрупких боридных слоев не приводит к повышению эксплуатационных свойств таких изделий. Установлено, что легирование боридных слоев хромом снижает хрупкость диффузионных слоев. Путем оптимизации процесса борохромизации для этих продуктов были определены следующие параметры: состав покрытия, температура диффузионного насыщения, время выдержки. Предлагается сочетать химико-термическую обработку реактивной втулки при нанесении покрытия и нагрев изделий для закалки. Показана возможность замены легированной стали углеродистой сталью с исследуемым диффузионным покрытием.

Key words: diffusion metallization, borochromization, metallographic analysis, microhardness.

Ключевые слова: диффузионная металлизация, борохромирование, металлографический анализ, микротвердость.

Main part

When using diffusion boron plating as a method of surface hardening and increasing wear resistance, phases with a high boron content with very high microhardness are formed on the working surface of the reactıve bushıng.

In the literature [1], there are references to the fact that alloying boride layers with copper, chromium, nickel, aluminum, and other elements can reduce the brittleness of the boride layer. In this case, an increase in the thickness of the diffusion layer depends on the nature of the second saturating component and its position in the D.I. Mendeleev. So, for example, with simultaneous boron-nitriding, the thickness of the diffusion layer increases by 3–5%, and with simultaneous saturation with boron and aluminum, the increase in the thickness of the diffusion layer is from 5.0 to 8.5% [1,2].

In the present work, studies have been carried out on the diffusion saturation with boron of the reaction sleeve working on top of the news. The reactive bushing made of 38KhN3MFA steel experiences friction during operation under dynamic loads.

Fine methods (scanning, transmission electron microscopy (SEM, TEM) and X-ray diffraction analysis (XRD) were used to study the structure and phase composition of the resulting diffusion layer after borochromization. Detailed studies of the surface layer make it possible to determine the mechanisms of diffusion processes and predict the formation of a zone with the necessary physical and mechanical properties with sufficient quality.

Studies have shown that the surface structure in the studied steel is actually formed by three chemical elements: iron, boron and carbon. Iron is the main element, boron is the main alloying element on the surface, carbon is present in the amount introduced into the steel.

The depth of the borated layer was about 70 µm. The following layers were viewed at greater distances from the borochromization surface in order to achieve more research information; As a rule, the depths of the studied layers by TEM and XRD did not coincide.

When exposed to boron, four layers can be distinguished in the structure of the steel surface (see table).

When borochromization, the volume fraction of cementite increases. This is due to the fact that boron introduced into cementite involves an additional fraction of iron in it. The presence of borated cementite is also confirmed by the data of X-ray diffraction analysis of the phase composition of 38KhN3MFA steel at different distances (x) from the sample surface, which are given below:

Table 1.

Volume fraction, %, phases

The phase observed in a large amount in the diffusion layer is FeB boride: it has the form of columnar crystals (Fig. 1) extending from the borated surface deep into the material. As shown by TEM studies at high magnifications, FeB crystals have an irregular shape in cross section and are either defect-free or have a layered structure. The X-ray diffraction data also confirm the presence of FeB and FeB borides. Thus, a wear-resistant but very brittle diffusion layer is formed on the surface. Such layers have proven themselves well when working in friction conditions without dynamic loads.

Table 2. 

Phases observed at various depths of steel 38KhN3MFA

Figure 1. Microstructure of borochromization layer (SEM)

For the rapidly developing arms production industry, the constantly increasing aggravation of energy capacity and operating conditions of machines is a characteristic feature. One of the most effective and important methods of improving the longevity and reliability of weapons and weapon systems is thermal and chemical-thermal processing of parts. Such processing processes provide high resistance of parts to wear and fatigue breakdown.

When solving real problems occurring in parts of weapons, the complex approach to choosing the type of chemical-thermal processing of concrete is considered effective. The complex approach provides for a complex analysis of the interaction of the material of the detail and its thermal processing, the structure and properties of the reinforcing surface layer, the state of the surface in the process of wear and destruction.

The main feature of the complex approach is based on the construction of diffusion layer structure with a certain phase composition for the purpose of specific operating conditions. Thermal and chemical-thermal processing increases wear resistance of parts, and also has a positive effect on reliability and durability of weapons and weapon systems as a whole.

According to statistics, there is no universal method of chemical-thermal processing (CHP) that would meet the requirements of the required reliability depending on the operating conditions. For each specific case, it is necessary to develop its own CTO technology. In the present case, taking into account that the reactive bush is experiencing thermal-erosion wear, a method of complex boron chrome plating combined with heat treatment is proposed. At the same time, it was assumed that chromium reduces the high microhardness of the boride layer, which makes it possible to increase the working capacity of the reactive bush. Optimization of the process made it possible to establish the following parameters for boron chrome plating: the chemical composition of the mixture 60% B₄C + 35% Cr₂O₃ + 5% NaF; saturation mode: temperature - 1050 °C, holding time - 4 hours.

On fig. 2 shows the microstructure of the diffusion layer on 38KhN3MFA steel; the diffusion layer thickness was about 70 μm.

In this work, the brittleness factor was determined using a PMT-3 microscope. The total brittleness score is determined by the formula

where, , , ...,  is the relative number of prints out of their total number (usually 25 - 100) with a given brittleness score.

The average brittleness score is determined at loads of 0.2, 0.5, 1.0, 1.2 and 1.5 N. After testing, a graph of the function Z_p = f(p) is plotted, which determines the brittle fracture rate (brittleness factor γ_p) dZ_p /d(p) (tangent of the ear of the slope of the tangent to the curve of change in the brittleness score). Brittleness factor γ_p is determined for a point with a load of 1.0 N.

The microhardness (γ_p=100) of the diffusion layers of steel 38KhN3MFA and steel 50PA during boron chrome plating is 1.0 and 0.8, respectively. According to [5], at a load of 0.9 N, the brittleness factor VC is 3,23, Cr₃C₂, - 2,40, FeB – 1,70, CrB₂, - 0,10.

Obviously, the resulting diffusion layers are more plastic.

Conclusions. The temperature-time regime of diffusion hardening combined with heat treatment is determined by the growth rate of the diffusion layer of the required thickness, as well as by the conditions of heating for hardening of a particular type of steel. Taking into account the fragility of the diffusion layers, it is possible to replace a tool operating under conditions of dynamic wear made of expensive alloy steel with a tool made of carbon steel.

Figure .4. Distiribution of microhardness of diffusion layer on steels 38KhN3MFA and 50PA depending on distance to surface

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