RU2000355C1 - Method of chromocalorizing of steel articles by diffusion under vacuum - Google Patents

Abstract

FIELD OF THE INVENTION This invention relates to a chemical thermal treatment of metals in a vacuum. The essence of the method: carry out diffusion chromoalation of steel products in vacuum with a residual pressure of not more than 1.33 Pa by applying to their surface a powder mixture consisting of 94.5-96.0% chromium and 4.0-5.5% aluminum heating to a temperature of 1100-1150 ° C with a process duration of 3.5-4.5 hours. Chromium is used in the form of a powder with an impurity content of not more than 1.0%. 1 tablet, 2 ill.

Description

The invention relates to chemical-thermal treatment of metals and alloys, in particular to diffusion chromoalitization in vacuum using chromium and aluminum powders, and can be used in the engineering and metallurgical industries for surface hardening of tools, tooling and machine parts.

A known method of chromoalitizing in vacuum products from nickel-chromium heat-resistant steels and alloys, which can be taken as a prototype, including the use of a powder mixture containing 41-47% chromium, 3-9% aluminum, 50% alumina as a separation medium . With respect to chromium, the aluminum content is 6-18 wt.%. The products of the alloys processed by the proposed method at a quenching temperature and holding time of 2-5 hours have a microhardness of 600-700 kg / mm, a layer thickness of 10 to 30 microns and high heat resistance (700- 1000 ° C).

The disadvantages of this method are: low surface microhardness and, therefore, low wear resistance; the shallow depth of the diffusion layer due to the use of alumina; cumbersomeness and laboriousness of preparatory and auxiliary works due to the need to use semi-hermetic containers for laying products with a powder mixture; the inability to perform rapid cooling-quenching of products directly in vacuum due to the large mass of the loaded container with a powder mixture containing aluminum oxide of low thermal conductivity; The method is intended to increase the durability of products only under conditions of gas corrosion.

The aim of the invention is to obtain, after chromoalitizing, a high quality saturable surface by forming a special diffusion layer on a steel product with any form of complexity, accelerating the diffusion process, reducing the complexity of the work and materials used, and also hardening the processing

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products directly with a heating temperature during chromoalitration in a vacuum furnace.

This goal is achieved by the fact that chromoaling is carried out in vacuum with a residual pressure of not more than 1.33 Pa (1 10 mm Hg) by applying a powder mixture consisting of 96 to the surface of a product before loading into a vacuum oven. .94.5 wt.% Chromium and 5.5 ... 4 wt.% Aluminum, at a process temperature of 1100-1150 ° C. Chromium was used on the form of highly pure micropowder manufactured by the calcium hydride method in accordance with TU 14-1-1474-75 NPO Tulachermet (impurity content no more than 1 wt.%), And aluminum powder - grade AP.

Before loading into a vacuum oven, the surface of the product is covered with a chromium-aluminum powder mixture to fill in the existing recesses. The powder is held onto the surface of the article due to the very shape of the saturable surface, if closed, or by pressing with steel plates and plugs (for holes). Next, the product is placed in an oven, a vacuum is created with a residual pressure of not more than 1.33 Pa and a temperature of 1100-1150 ° C for 3.5-4.5 or more hours depending on the required coating thickness.

The use of this chromium powder eliminates the welding of its particles to the surface of the saturated product when heated to 1180 ° C due to its particle size distribution and chemical purity. At the same time, the powder adheres tightly to the surface of the product. The specified method was used during chromoalithization of the forming parts of molds from 4x4 VMFS (Di22) steel at a temperature of 1120 ° C. in a vacuum with a pressure of 0.133 Pa for 4.5 hours in an electric furnace of type SEV-3.3 / 11.5. The molds processed by this method for injection molding of copper alloys provide an increase in their resistance by 2.5 ... 3 times. This increase in mold resistance was due to the formation of a special diffuse layer on the surface.

As shown by metallographic studies, as well as X-ray spectral electron probe microanalysis after processing Di22 steel in the indicated mode, the diffuse layer consists of two zones. The first, external 8 microns thick, the zone consists of chromium carbides of the composition (CrFe) 2zCb and (Cr, Fe) Ce. A second zone adjacent to the base metal having a thickness of 150 microns. is a solid solution in the iron of chromium (8-6%) and aluminum (3.5-1.5%) with a decrease in the concentration of these elements from the surface to the base metal. The microhardness of the carbide zone is 13000 MPa (1300 kg / mm), the solid solution is 2700-2300 MPa (270-230 kg / mm) with the microhardness of the base metal 5000 MPa (500 kg / mm). The surface cleanliness after chromating is practically unchanged, becoming a matte hue.

The table shows comparative data for the known and proposed methods. Moreover, for the known method

Figure 5 shows the data from both the description of the invention to the copyright certificate and the data of our experiment on Di22 steel in a known mixture with alumina. The characteristics of the diffusion layers obtained in a known manner in both cases a and b (for different grades of steel) are lower than those obtained by the proposed method: lower layer depth and the absence of a carbide zone.

5 Testing of molds for resistance showed that in the case of applying the known method, an increase in resistance is obtained insignificant. The boundary conditions for the use of just such a powder composition

0 mixtures are understood from a consideration of the graph in FIG. 1. The use of a mixture with an aluminum content of less than 4 wt.% Leads to the formation of a highly uneven diffusion layer (shown in the graph

5 dotted line). When the content of aluminum in the clay is more than 5.5 wt.%, The thickness of the carbide zone of the layer decreases to 5 microns or less, which cannot provide a significant increase in the wear and heat resistance of the surface of the mold of the product.

The use of a powder mixture with an aluminum content in the range of 4 ... 5.5 wt.% Allows one to take advantage of two diffusion processes: from chromium plating - high hardness and low thermal expansion, from aluminizing - large layer thickness and heat resistance. Figure 2 shows, by way of example, a set of mold inserts 1 for casting a control valve body prepared for loading into a vacuum oven. In the inserts, only the engraving with the gating system is to be chromo-lined, filled with a powder mixture 2, which is also retained by the plug 3. The set of inserts, consisting of two almost identical halves, is secured with wedge clamps 4 (can be replaced with a screw and nut). The hollow sign 5 has been used to reduce the number of tons of powder mixture.

as well as to improve the cooling condition of the product. With a small section of the product, the 5 mark may not be used. With sufficient insert rigidity and a simple configuration that prevents deformation during quenching, the insert is allowed to be loaded into the furnace in a horizontal position. In this case, clamps 4 are not needed to hold the powder on the engraving, but can be limited only by plugs. The latter are made of either steel or asbestos.

The above method allows: to increase the heat resistance of molds by 2.5-3 times; simplify the process by avoiding the use of a container; combine chromoalithization with quenching for steels having a high austenitizing temperature; to avoid deformation of products during hardening, which is especially important for forming tools.

Using the proposed method for processing injection molds5

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singing non-ferrous alloys at a plant with a production volume of 1,500 ... 2,000 tons. Casting copper alloys per year daot the economic effect of more than 100 tons. per year by reducing the need for molds.

Claims (1)

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