RU2109080C1 - Plant for gas low-temperature thermochemical treatment of steels and alloys - Google Patents

Abstract

FIELD: devices for gas low-temperature thermochemical treatment of steels and alloys, in particular, devices for gas nitriding and gas carbonitriding of metal articles. SUBSTANCE: plant has electric furnace with or without muffle, vessel with ammonia, pipelines for supply and discharge of gases, device for gas mixing and batching. Vessel with catalyst is installed on pipeline for gas supply to electric furnace. EFFECT: provision of perfectioning of existing methods of low-temperature thermochemical treatment of steels and alloys and development of novel processes for enhancing durability of machine components and tools. 4 cl, 2 dwg, 1 tbl

Description

 The invention relates to devices for gas low-temperature chemical-thermal treatment (NTHTO) of steels and alloys, in particular to devices for gas nitriding (GA) and gas nitrocarburizing (SSC) of metal products.

 The methods of NTHTO (GA and SSC) of steels and alloys are widely known in mechanical engineering technology as methods of increasing the surface hardness, wear resistance and chemical resistance of various metal products, machine parts and apparatuses.

The most common GA and SSC methods in practice are based on the use of ammonia-containing furnace atmospheres in the process of NTTC. With all the variety of these methods, devices for their implementation are characterized by the commonality of the design of their structural schemes. The schematic diagrams of such devices for the GA and SSC for many decades of their history, which have not undergone qualitative changes, contain the following main elements in their structure (see, for example, [1, 2]): a furnace (as a rule, an electric furnace with or without a muffle) for heating metal products to temperatures of 500-700 ^o C; a tank with ammonia for the formation of ammonia-containing gas streams that create the necessary nitriding furnace atmosphere; highways supplying working gas flows to the furnace and exhaust from it exhaust gases; devices for mixing and portioning gas streams sent to the furnace.

 These features of the devices for GA and SSC are inherent in the analogue closest to the present invention (analogue selected for the prototype - [3]).

In FIG. 1 shows a structural diagram of this installation: a furnace for heating metal products to temperatures of 500-700 ^o C 1; a tank with ammonia for the formation of ammonia-containing gas streams creating the necessary nitriding furnace atmosphere 2; highways supplying working gas flows to the furnace and exhaust from it exhaust gases 3, 4; devices for mixing and portioning gas streams sent to the furnace 5.

The above-mentioned analog devices (including the prototype device) are characterized by significant disadvantages, which are manifested when the GA and SSC processes are carried out in these devices. The most significant of these disadvantages are as follows:
a) poor control of the NTXTO process due to the uncertainty in choosing the GA and SSC modes, which depend not only on the material and type of product, but also on such random factors as the type of furnace and its design;
b) high ammonia consumption (of the order of several cubic meters of ammonia per hour per one cubic meter of furnace volume) and a long duration of products in the furnace (of the order of several tens of hours) during GA and SSC operations, which negatively affects the cost of finished products;
c) the limited range of materials and types of products that can be subjected to NTHTO (for example, the inability to nitrate high-speed steels and powder metallurgy products).

 The aim of this invention is the creation of a new device for HA and SSC steels and alloys, which would have eliminated the above disadvantages inherent in analogues and prototype.

 The goal is achieved when carrying out the GA and SSC processes in the proposed device, which is made in the form of an installation for NTHTO, containing in its circuit (Fig. 2), as well as in the prototype installation, an electric furnace (with or without a muffle) 1, a tank with ammonia 2, a gas supply and exhaust pipe 3, 4, a gas mixing and batching device 5, but different from the prototype in that a container with a catalyst 6 is installed on the gas pipe to the electric furnace.

When conducting experimental studies of the characteristics of the NTHTO process of steels and alloys on the units constituting the subject of the proposed technical solution, in which ammonia-containing gas streams, when fed into the furnace, are subjected to catalytic treatment in a container with a catalyst, it was found:
a) a significant expansion of the capabilities for controlling the NTXTO process, expressed in the implementation of a wide area of alpha-solid solution with anomalously large lattice parameters, in the availability of regulation of the final nitrogen content in steel in a wide range of values, in an increase in the depth of the surface layer of the product to 1 mm with specified parameters hardness, in increasing the corrosion resistance of structural steels, in significantly suppressing the negative process of surface decarburization of steels during nitriding;
b) a significant reduction in the consumption of ammonia during the HA process (4-8 times compared with traditional nitriding in ammonia-air media without their catalytic treatment) with a significant reduction in the duration of processing of steel products in a nitriding furnace atmosphere (2-3 times);
c) expanding the range of materials and products that lend themselves to NTXTO processes, in particular, nitriding high-speed steels and powder metallurgy products.

 In the experiments, various schemes of the arrangement of the container with catalyst 6 relative to furnace 1 were examined. It was found that the best results of NTXTO carried out in the proposed device are achieved by placing the container with catalyst 6 inside the furnace space 1.

 In the experiments, catalysts of various shapes were used, loaded into a container 6: catalysts in the form of granules, powder, fabrics, nets.

 In the experiments, the possibility was established of using a wide range of different chemical composition of the catalysts loaded into the tank 6 of the proposed device for NTHTO: catalysts made of materials based on aluminum oxides, silicon oxides and / or metals and their alloys based on iron, nickel, chromium , aluminum, copper, containing in their composition catalytically active elements from a number of platinum group metals (platinum, palladium, rhodium), chromium, nickel, cobalt, manganese, lead, zinc.

 The authors cannot yet formulate a unique scientific explanation of the nature of the above features of NTTHO, which are realized during the process in the proposed device. It is indisputable only that when a tank with a catalyst is introduced into the NTCHO installation scheme, the furnace atmosphere subjected to catalytic treatment in this tank acquires special activity with respect to the nitriding effect on steel and alloy products.

 To fully understand the mechanisms of these unexpected effects, a special cycle of fundamental scientific research will be required. At this stage, the authors propose only some hypothetical notions for interpreting the profound changes in NTXTO processes during the catalytic activation of the furnace atmosphere.

 The starting position, which is the starting point of this hypothesis, is a concept that is unconventional for the existing theory of nitriding. It is based on a consideration, according to which it is argued that, when NTCHT metals, the active components in the gas medium are not stable, equilibrium products of the interaction of the components of the furnace atmosphere (in the most typical case, these are products of ammonia oxidation by atmospheric oxygen), but intermediate, labile, highly chemically active formations (nitrogen, hydrogen, oxygen-containing radicals, ions, ion radicals). It is these nonequilibrium, but long enough living "fragments" that have high activity, small size, and the ability to penetrate into the solid metal matrix and react with it, make a decisive contribution to the effect of surface nitriding of metal and alloy products. The introduction of a catalytic factor (in accordance with the proposed technical solution) in the NTHTO process, which specifically affects the conversion of gas reagents in the furnace atmosphere during its interaction with metal surfaces, allows targeted and selective control of possible parallel routes of the ammonia oxidation reaction, i.e., the whole spectrum resulting in the process of final and intermediate products. It is this factor that, in the authors' opinion, is the reason for the implementation of the positive features of the NTXTO process, carried out under the conditions of the proposed device.

 The furnace atmosphere formed as a result of these processes ensures the saturation of steels and alloys with nitrogen according to a predominantly solid-solution mechanism with the maximum approximation of the result to the iron-nitrogen state diagram. It is well known that real HA in ordinary furnace atmospheres is accompanied by the formation of highly nitrogenous iron nitrides from the very first stages of saturation. Highly nitrous iron nitrides are present in this case at any nitrogen content in iron, being nonequilibrium in the binary diagram. This fact causes many problems of NTHTO: diffusion damping in the process time, excessive layer porosity, limited applicability to various materials, increased fragility of products, low degree of repeatability of technological processes.

To verify the proposed technical solution, a study was conducted of the mechanisms of diffusion of nitrogen into iron from the gas phase formed by devices similar to the prototype and the proposed device. Armco-iron samples were processed in the prototype installation and in the layout of the proposed installation. Saturation modes included heating to 540 ^o C, holding at different values of saturation ability of the furnace atmosphere (potential) in each case until the nitrogen concentration in iron was in equilibrium with the atmospheric potential and cooling in a nitrogen stream. On the samples thus treated, the nitrogen content in iron was studied by vacuum melting and the phase composition by X-ray phase analysis. The results were evaluated by the degree of approximation to the binary iron - nitrogen diagram. The research results are shown in the table.

 As can be seen from the table, the furnace atmosphere formed in the proposed installation provides the process of saturation of iron with nitrogen under conditions more approximate to the binary diagram of iron - nitrogen. This opens up great opportunities for improving the existing NTXTO technologies and developing new technological processes with the aim of increasing the durability of machine parts and tools, as well as optimizing serial engineering technologies.

Claims (4)

 1. Installation for gas low-temperature chemical-thermal treatment of steel and alloys, in particular for gas nitriding and nitrocarburizing of metal products, containing an electric furnace with a muffle or without a muffle, a tank with ammonia, gas supply and exhaust lines, gas mixing and batching devices, characterized in that on the gas supply line to the electric furnace, a container with a catalyst is installed.  2. Installation according to claim 1, characterized in that the tank with the catalyst on the gas supply line is located inside the furnace space.  3. Installation according to claim 1 or 2, characterized in that the container with the catalyst is loaded with catalyst in the form of granules, powder, fabrics, nets.  4. Installation according to claim 1 or 2, characterized in that the container with the catalyst is loaded with a catalyst made of materials based on aluminum oxides and / or silicon oxides and / or metals and their alloys based on iron, nickel, chromium, aluminum , copper, while the catalyst contains catalytically active components from the following series of elements: platinum group metals (platinum, palladium, rhodium), chromium, nickel, cobalt, manganese, copper, zinc.

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