RU2477336C1 - Metal product cementation method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: metal product is arranged in the container with electroconductive powder medium containing an alloying element. Then, powder mixture is heated due to passing through it of electric current using electrodes; at that, as electrodes there used is container and product with the ratio of surface areas, which are equal to not less than 10:1. As electroconductive medium there used is powder of bituminous coal with grain size of 0.3-0.6 mm. Container is made in the form corresponding to the form of surface of the strengthened product that is equally spaced in the container.

EFFECT: accelerating diffusion saturation process, achieving uniform depth of the layer along the product perimetre, reducing power consumption and improving environmental friendliness of the process.

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Description

The invention relates to the field of metallurgy, namely to diffusion cementation of metallic materials, and can be used in mechanical engineering for surface hardening of machine parts and tools.

The classical method of cementation is known, which consists in diffusion saturation from powder media placed in pressurized containers using furnace heating (Voroshnin L.G. Theory and technology of chemical-thermal treatment: a training manual. Minsk: New Knowledge, 2010, p. 88) .

The method has a high energy intensity due to the length of the processing process due to the need to heat the entire container and subsequent exposure for diffusion saturation.

There is a method of diffusion saturation of metal products in powder conductive mixtures heated by passing electric current (Voroshnin L.G. Theory and technology of chemical-thermal treatment: a training manual. Minsk: New knowledge, 2010, p. 101). In the process of processing, in addition to the product, two electrodes are placed in the refractory muffle, they are filled with a powder mixture and covered with a lid. The electrodes located on opposite sides of the muffle are supplied with an electric voltage of 10 ... 59 V.

With this processing method, the diffusion saturation rate is higher than in the above-described classical method, but remains insufficiently high and energy-intensive due to the need to heat the entire volume of the conductive mixture.

The closest to implementation is a method of cementation of metal products, in which the products are placed in a furnace with a fluidized bed of graphite powder, the heating of which is carried out by passing electric current using electrodes (A.S. SU No. 375322, IPC C23C 8/64, 05/10/1973 g .).

This method is characterized by an insufficient saturation rate (on steel 25, a layer thickness of 0.8 mm in 30 minutes) and unevenness of the formed diffusion layer due to more intense heating of the sample from the sides facing the electrodes. In addition, the method involves high energy and material costs caused by the need to create a vibro-boiling layer by purging with neutral gas, air or vibration.

The technical result of the invention is to accelerate the process of diffusion saturation, achieving uniformity of the depth of the layer around the perimeter of the product, as well as reducing energy costs.

The technical result is achieved in that in the method of cementing a metal product, including placing the metal product in a container with an electrically conductive powder medium containing an alloying element, heating the powder mixture by passing electric current through it using electrodes, the difference is that the electrodes are used a container and a product with an area ratio of at least 10: 1, and, as an electrically conductive powder medium, granular coal powder 0.3-0.6 mm, and the container is made in the form corresponding to the surface shape of the hardened product, which is placed equidistantly in the container.

The implementation of the container in the form of the surface of the hardened product and the location of the product is equidistant relative to the container with a ratio of the container and product areas of at least 10: 1, which makes it possible to concentrate electric current on the surface of the product and localize microarc discharges in a limited volume around the product surface in the powder mixture layer up to 4- 6 mm, which contributes to obtaining a uniform in depth diffusion layer. When the ignition temperature is reached in the process of heating the coal powder with the powder, an exothermic combustion reaction is initiated, leading to the activation of adsorption of carbon atoms and additional heating of the product surface, which accelerates the diffusion saturation process and also reduces energy costs.

The particle size range of coal powder of 0.3-0.6 mm is due to the fact that when the size is less than 0.3 mm, a large number of fine dust particles are entrained in the gases released by heating and ignition of the powder medium and removed from the contact zone with the hardened product. With a particle size of more than 0.6 mm, the electrical resistance of the powder medium significantly decreases, the current in the electrical circuit of the container — powder medium — metal product increases significantly, which leads to the appearance of powerful electric discharges in the powder medium that also remove powder particles from the contact zone with the hardened product .

The following is an example implementation of the method.

Samples of steel grade 20X of cylindrical shape with a diameter of 12 mm and a length of 30 mm were fixed in a metal clamp, mounted vertically in the center of a metal container of cylindrical shape with a diameter of 70 mm and a height of 100 mm and filled up half the length of the sample with coal powder with a grain size of 0.3-0, 6 mm, performing the function of a conductive mixture and a source of atomic carbon for the cementation process. Then, an electric current source was connected in such a way that one electrode was a container and the other was a sample. The voltage was continuously regulated in the range of 0–50 V. As a result, the formation of microarc discharges around the sample was first observed (photo 1), and after 20 s, the region of heating and the subsequent occurrence of an exothermic reaction of coal powder combustion appeared (photo 2). The total exposure time during diffusion saturation was 90 s. The processing results were evaluated by the depth of the carburized layer with the perlite structure using microstructural analysis on a transverse microsection (photo 3). The depth of the carburized layer around the perimeter of the sample was 0.3 mm.

Comparison of the achieved value of the depth of the layer obtained in 1.5 minutes with the results of using the known method shows that the proposed method differs in accelerating the diffusion saturation process. The method allows to obtain a uniform depth of the carburized layer around the perimeter of the sample, reduce energy consumption due to the energy released during the combustion of coal powder, and does not require gas purging or container vibration.

Claims (1)

A method of cementing a metal product, comprising placing a metal product in a container with an electrically conductive powder medium containing an alloying element, heating the powder mixture by passing electric current through it using electrodes, characterized in that the container and the product with an area ratio of at least 10: 1, and as an electrically conductive medium - coal powder with a grain size of 0.3-0.6 mm, while the container is made in a form corresponding to the shape of the surface hardened product, which is placed in the container equidistantly.

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