INCREASING THE BENDING RESISTANCE AND STRENGTH OF THE STRUCTURE OF THE METALLIZED LAYER

ABSTRACT

This work examines the features of restoring worn machine parts using the metallization method. The main factors influencing the wear process of parts surfaces, such as load, temperature regime, lubrication conditions, the presence of mechanical impurities, and the nature of contacting surfaces, were analyzed. Special attention is paid to the structure and properties of metallized coatings. It has been established that the coating obtained by the metallization method has high hardness, porosity, and a good ability to retain lubricant, which contributes to a decrease in the coefficient of friction and an increase in the wear resistance of parts. The features of the microstructure of the metallized layer, the mechanical properties of the coating, and the conditions for its application in parts repair are considered. It has been shown that the metallization method is an effective method for restoring worn-out parts, ensuring increased wear resistance and extended service life.

АННОТАЦИЯ

В данной работе рассматриваются особенности восстановления изношенных деталей машин методом металлизации. Проанализированы основные факторы, влияющие на процесс износа поверхностей деталей, такие как нагрузка, температурный режим, условия смазки, наличие механических примесей и характер движения контактирующих поверхностей. Особое внимание уделено структуре и свойствам металлизированных покрытий. Установлено, что покрытие, полученное методом металлизации, обладает высокой твердостью, пористостью и хорошей способностью удерживать смазочный материал, что способствует снижению коэффициента трения и повышению износостойкости деталей. Рассмотрены особенности микроструктуры металлизированного слоя, механические свойства покрытия и условия его применения при ремонте деталей. Показано, что метод металлизации является эффективным способом восстановления изношенных деталей, обеспечивающим повышение их износостойкости и продление срока службы.

Keywords: increase, stability, bending, strength, structure, metallized layer.

Ключевые слова: повышение, устойчивость, изгиб, прочность, структура, металлизированный слой.

Introduction. Today, our Republic is contributing to the global market by producing thousands of vehicles. In recent years, the head of our state Sh.M. Mirziyoyev has adopted a number of decisions on the development of this sector. In particular, Presidential Decree PF-4947 dated February 7, 2017, "On the Action Strategy for the Development of the Republic of Uzbekistan in 2017-2021," and Presidential Decree PF-5647 dated February 1, 2019, "On Measures for the Fundamental Improvement of the Public Administration System in the Transport Sector" were adopted. The process of machine parts surface wear is complex and depends on many factors. These factors vary depending on the operating conditions of the machines.

These mainly include the following:

- load acting on the surface of components;

- connection operation temperature regime;

- presence, nature, and properties of oil;

- the degree of contamination of the lubricant with mechanical impurities, the composition and dimensions of these impurities;

- relative movement of parts (for sliding contacts);

- other operating conditions of contact pairs (vibration, corrosion, etc.).

The layer obtained by the metalization method consists of a porous and brittle metal layer with high hardness and low mechanical strength. This layer absorbs lubricant well and is resistant to wear under conditions of low relative load. However, with a high relative load from sliding and compression forces, the metallized coating quickly peels off.

The metalization method differs from other methods by the possibility of applying layers of varying thickness (from 20 μm to 8 mm). When repairing worn parts using the metallization method, the structure, hardness, wear resistance, and mechanical strength of the coating are of great importance.

Research objects and methods. Studies have shown that the microstructure of steel applied to the surface of parts by the metallization method has no resemblance to either the crystalline structure or the structure of electrolytic coatings. The main component of the metallization-formed coating structure is troostite-martensite, in which numerous thin oxide layers are formed around pores and individual particles. Because the coating metal contains voids, its density is lower than that of the cast metal. The porosity of the coating metal indicates whether the particles that make up its structure were formed in a solid or plastic state. The hardness and brittleness of the coating are significantly higher than that of the material used to metallize the part's surface.

The reason for the high hardness of steel coatings is that high-temperature metal particles sprayed onto the metallizing surface of the part quickly cool and harden under the influence of turbulent air flow. Additionally, steel particles moving at high speeds hit the part's surface or previously applied particles.

As a result, the surface is worn out, and the hardness of the coating increases even more. The idea that the coating becomes extremely hard due to the formation of oxides is not entirely accurate, since an increase in the coating's hardness is also observed when liquid steel is sprayed in an inert gas. The assumption that the increase in coating hardness occurs due to the annealing of solid particles is confirmed by the thermal treatment of the coating. During the annealing of the metallized component, the hardness of the coating decreases. The hardness of the coating depends on the metalization regime, as well as on the chemical composition of the molten wire used for spraying, in particular, on its carbon content. With an increase in carbon content, the hardness of the coating increases. The hardness of the coating can be determined using standard instruments, but not all of them allow for the determination of the true hardness of sprayed metal particles, as the coating is not uniform due to its high porosity. However, in practice, these devices are widely used to determine the average values of the hardness of individual sections of the coating. Because the coating is porous, it absorbs oil well and retains the oil film on its surface. Experimental data showed that the coating friction coefficient of the metal is 12-40% lower than that of hardened surfaces. The ability of the film to retain the oil film on its surface is especially important when starting the engine, because at this moment, semi-dry friction occurs on the moving parts of the engine. All the presented factors indicate that the wear resistance of oil-coated metallized components is significantly higher than that of non-metallized components. In an oil-free environment, i.e., during dry friction, the wear resistance of the coating decreases, therefore, it is not recommended to restore the operability of parts subjected to dry friction using metallization. Results and discussion. The mechanical strength of the coating, as shown by research conducted in this area, indicates that the tensile strength of the sprayed metal is significantly lower than the strength of the base metal. Thus, the yield strength of the layer formed by coating 0.35% carbon-containing steel wire is 18 Pa; The compressive strength of such a layer is quite high and reaches 150 Pa. In parts repaired by the metallization method, the load is perceived jointly by both metals, namely: the main metal of the part and the metal applied to it. Based on a number of studies conducted, the following conclusions can be drawn:

-since the load is distributed between the main metal and the coating metal, the mechanical properties of the part are determined exclusively by the mechanical properties of the metal;

-under variable bending loads, the fatigue limit of the coating is low, while the fatigue limit of the main metal increases slightly due to the coating;

-with an increase in the coating thickness, the bending resistance of the part (if the stress is calculated by the cross-section of the base metal) increases to some extent, while the bending stiffness practically does not depend on the coating thickness;

The bending angle of the coating is not as large as the bending angle of the base metal. Thus, when calculating the strength of coated parts, it is necessary to take into account the dimensions of the part before applying the coating. For parts subjected only to compression, the load can be increased, since the coating provides sufficient resistance to compression. Only parts with sufficient mechanical strength can be repaired using the metallization method. The metallization method is used to restore heavily worn parts to their original dimensions, allowing for the production of coatings with high wear resistance, corrosion resistance, and other desirable properties. The process of spraying molten metal particles, liquefied in a special way to a small size (3-30 μm), onto a pre-prepared surface of a part at a high speed (140-300 m/s) using compressed air is called metalization. The device used for melting and spraying metal is called a metallizer. Repairing a part by metallization is considered a modern method. In metallization, particles of molten metal with sizes of 3-300 μm are applied to the surface of a pre-prepared part in a stream of compressed air (or inert gas) at a speed of 100-300 m/s, forming a layer of this metal on it. The adhesion between the main metal of the part and the sprayed metal occurs due to mechanical and molecular bonds between them. A more advanced and effective method for preparing the surface of a part of any desired hardness for metallization is to treat the surface by spraying it with iron powder. It is recommended to spray steel powders with a particle size of 0.8-1.5 mm at an air pressure of 0.4-0.6 MPa at an angle of 25-400°. The resulting seam on the part's surface ensures good adhesion of the coating to the base. When the hardness of the part exceeds NV 325, wire winding is used. Then the workpiece is clamped in the centers of the lathe, one end of the wire is secured with a clamp on the uncovered side of the workpiece, and the other end is passed through the jaws of wooden clamps.

Conclusions. Metallization should be carried out immediately after the completion of surface preparation, i.e., before the prepared surface has time to oxidize. The interval between surface preparation and metallization should not exceed 1.5-2 hours.

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