RU2792514C1 - Method for thermal diffusion chromium plating of parts made of steel or iron-based alloys with addition of emitter to saturating powder mixture - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to chemical-thermal treatment (CHT) by the method of thermal diffusion saturation and can be used in any mechanical engineering industry for coating products made of steels and iron-based alloys. According to the method, fat-free parts are loaded into a container and covered with a saturating powder mixture containing the following components: metal powder based on chromium 35 wt.% fraction not more than 150 microns, aluminium oxide 45 wt.% fraction 125-140 microns, granulated ammonium chloride 3 wt.% fraction of 2-3 mm, and calcium tungstate 17 wt.% fraction of not more than 100 microns as emitters of electrons and anions O^2-, the container is sealed and heated to a temperature of 900-1000°C.

EFFECT: keeping the increased depth of the reinforcing diffusion layer.

1 cl, 3 dwg, 3 tbl, 2 ex

Description

The invention relates to metallurgy, in particular to chemical-thermal treatment (CHT) by the method of thermal diffusion saturation, and can be used in any branch of mechanical engineering for coating products made of steels and iron-based alloys.

A known method of chromium plating, including heating, saturation in a powder mixture containing a chromium-containing component CrCl ₃ (0.001-30 wt.%), a copper-containing component (copper oxide 0.001-2 wt.%) and corundum (68-99.998 wt.%), and heating and saturation is carried out in a hydrogen atmosphere, and also in the process of heating at a temperature of the saturating mixture of 350-500°C, exposure is carried out for 5-25 min [1].

The disadvantage of this method is the complication of the technological process by the need to evacuate air from the container and pump hydrogen into it, increase the duration of the process due to the need for isothermal holding of the charge at 350-500°C, the danger of overheating of the metal since the chemical treatment is carried out at temperatures of 900-1300°C.

Also known is a method of surface hardening of metal products in a container filled with coal powder. A layer of liquid or gel-like electrically conductive adhesive material is applied to the surface of the product, and then a layer of powder of a material containing an alloying element, or a mixture of these components is applied. The product is heated by passing an electric current using the container and the product as electrodes. As a result, the formation of microarc discharges around the sample is observed, then a heating region appears in the same place and the subsequent occurrence of an exothermic combustion reaction of coal powder. The total holding time in the process of diffusion saturation is 3 min. [2].

The disadvantage of this method is the high complexity of the process due to the additional operation of coating parts with technological mixtures.

The closest analogue, taken as a prototype, is a method of contact process of chemical-thermal treatment of steels and iron-based alloys, in which it is proposed to introduce tungsten powder with a purity of 99.95% as electron emitters into the composition of the saturating mixture. The saturation diffusion process is accelerated due to the contribution of the internal thermionic field [4]. The disadvantage of this method is the high cost of the proposed addition of pure tungsten powder.

The invention is based on the task of maintaining the increased depth of the strengthening diffusion _{layer by adding CaWO 4} ^oxide powder in the amount of 15-20 wt. %, fraction less than 100 microns.

The task is achieved in that the method of thermal diffusion chromium plating of parts made of steels or iron-based alloys, in which fat-free parts are loaded into a container, filled with a saturating powder mixture containing chromium, ammonium chloride, aluminum oxide and an electron emitter, the container is sealed and heating is carried out up to a temperature of 900-1000°C, according to the invention, calcium tungstate powder is used as an emitter of electrons and anions O ^2- , while the specified saturating powder mixture consists of the following components: metal powder based on chromium 35 wt. %, fraction not more than 150 microns, aluminum oxide 45 wt. %, fraction 125-140 microns, granulated ammonium chloride 3 wt. %, fraction 2-3 mm, calcium tungstate 17 wt. %, fraction not more than 100 microns.

The essence of the invention is illustrated by the images in figures 1, 2 and tables 1-3, which show:

- Fig 1. The results of measurements of the current strength between two iron electrodes in technological backfill for chromium plating, containing as emitters of electrons and anions O ^{2 -} CaWO ₄ powder when the container is heated from 500 to 1000°C;

- Table 1. Values of the depth of the diffusion layer of chromium;

- Fig. 2. Area for spot chemical analysis of the sample surface after chromium plating without adding an electron emitter to the mixture;

- Table 2. The results of point X-ray microchemical analysis (at the points indicated in figure 2 of the sample after chromium plating without the addition of electron emitters and anions O ^2- , wt.%;

- Fig.3. Area for point chemical analysis of the sample surface after chromium plating with the addition of CaWO ₄ powder to the emitter mixture and O ^2- anions;

- Table 3. The results of point X-ray microspectral chemical analysis (at the points indicated in figure 3 of the sample after chromium plating with the addition of an electron emitter and O ^2- anions, wt. %.

The method is carried out as follows.

The sequence of operations of the proposed method includes: loading defatted parts into a container, filling them with a saturating mixture, sealing the container and heating the container to a temperature of 900-1000°C. The lower temperature limit of heating is due to the need to ensure sufficient diffusion mobility of chromium atoms in α- and γ-iron and the fact that in iron-based steels and alloys at temperatures above 900°C a single-phase austenitic structure is formed, which at these temperatures has the highest chromium solubility. The upper limit of heating is due to the fact that when heated above 1000°C, the metal overheats, accompanied by an undesirable growth of austenite grains.

The proposed method differs from the prototype composition of the mixture. In its composition, the saturating mixture, as in the prototype, includes chromium, ammonium chloride, aluminum oxide, but the essential distinguishing feature is the presence of CaWO ₄ oxide powder with a fraction of not more than 100 μm in an amount of 15-20 wt. % of the total mass of technological backfill.

The saturating mixture, unlike the prototype, has the following composition, wt. %: metal powder based on chromium (35 wt.%, fraction not more than 150 microns); aluminum oxide powder (45 wt.%, fraction 125-140 microns); ammonium chloride granulated (3 wt.%) with a fraction of 2-3 mm; the emitter of electrons and oxygen anions - CaWO ₄ oxide with a fraction of not more than 100 μm, in the amount of 17 wt. %.

CaWO ₄ oxide powder with a fraction of not more than 100 μm is generally available, can be obtained both from the natural mineral scheelite (mineral composition: 19.48 wt.% - CaO; 80.52 wt.% - WO ₃ , may additionally contain up to 10 wt. % impurity MoO ₃ ), deposits of which are also in Russia (Middle Urals, Chukotka, Eastern Transbaikalia, North Caucasus), and from products of processing of waste from machine-building and electrical production. The use of such an additive in technological filling, in comparison with tungsten powder with a purity of 99.95%, will significantly reduce the cost of the process.

At a CTO temperature of 900-1000 C, well-studied processes of evaporation of chlorides or other halides of saturating elements, deposition of atomic metals on the surface of parts and their diffusion into the surface layer of parts, formation of the structure and the desired physical and mechanical properties take place. The thickness of the diffusion layer is determined by the holding time. This description includes all known processes of contact chemical-thermal metallization of the surface of steels and alloys based on iron, regardless of the saturating metal [5]. The driving force of such a process of surface saturation is diffusion, described by the well-known two Fick's laws [5-9].

In the theory of transport in solids, two fundamental postulates are formulated, from which it follows [6-8]:

1. Particles of each type (cations, anions, electrons) move regardless of whether particles of other types are moving or at rest.

2. There are two driving forces for charged particles of each kind: diffusion, due to the gradient of their chemical potential, and electric, due to the electric field, and the action of each of these driving forces does not depend on the action of the other driving force.

The correctness of these postulates is confirmed by all the results of the thermodynamics of irreversible processes taken together. In accordance with these postulates, the flux of charged particles of type k (J _k ) can be written as follows:

,

,

и

- вклад в общий поток диффузионных и электрических сил, соответственно.Where

And

- contribution to the total flow of diffusion and electric forces, respectively.

Thus, the problem is reduced to the fact that in order to accelerate the saturation diffusion process, it is necessary to add the work of the electric field.

When additional sources of electrons and anions are added to the saturating mixture (emitters of anions and electrons - for example, CaWO ₄ oxide), an electric field is formed in addition to the concentration field, which occurs during the heating of the mixture. Heating of emitters of electrons and anions to a temperature above the lower threshold for the beginning of emission (450°C - 500°C) and further to saturation temperatures of 900-1000°C determines the appearance of thermoelectrons in the working space. Iron and iron-based alloys in the heated state are the positive electrode [10]. Therefore, the appearance of thermoelectrons generates an electric field directed towards the positive electrode, which is the workpiece.

The results of measuring the current in the electrical circuit, the proposed backfill when heated to 1000°C - shown in Fig.1.

The fixed current indicates the appearance of a directed flow of electrons from the emitters to the surface of the saturable product. This directed flow is an additional (except diffusion flow) carrier of atomic chromium to the surface of the saturated product and an intensifier of the chromium diffusion process.

To test the claimed method of thermal diffusion chromium plating in comparison with the method taken as a prototype, laboratory tests were carried out.

Examples of the implementation of the method.

Samples of steel 35Kh2N3 of the same size were subjected to chromium plating for 24 hours at a temperature of 1000°C in containers of the same type, differing only in the composition of the saturating mixture. Taking into account the established optimal content of the mixture components, the following compositions were chosen for the experiment.

Example 1 In one case: chromium-based metal powder (47 wt.%, fraction not more than 150 microns); aluminum oxide powder (50 wt.%, fraction 125-140 microns); granulated ammonium chloride (3 wt. %) with a fraction of 2-3 mm;

Example 2. Secondly - chromium-based metal powder (35 wt.%, fraction not more than 150 microns); aluminum oxide powder (45 wt.%, fraction 125-140 microns); powder CaWO ₄ 17 wt. % and a fraction of not more than 100 microns; ammonium chloride (3 wt.%) granular, fraction 2-3 mm.

It has been established that in the process of saturation of the surface of parts in containers with an emitter additive (in the form of 17 wt. % calcium tungstate powder - CaWO ₄ ), the depth of the diffusion layer increases by a factor of 2.0 compared to a sample saturated without an emitter additive (Table 1 ).

In addition, the positive effect of the additional action of the electric field, which manifests itself in an increase in the depth of the diffusion layer that occurs during heating, can be determined from a comparison of Tables 2 and 3 of the results of a point chemical analysis of the composition of samples saturated without emitters and samples with an organized introduction of electron and anion emitters About ^2- .

The depth of the diffusion layer was determined on a JEOL JSM-6460 LV electron microscope on thin sections made across the saturable surface. In FIG. Figures 2 and 3 show photographs of samples saturated with pure chromium and with additions of electron emitters and oxygen anions. The results of point X-ray microanalysis of the points indicated in Figs. 2, 3 are given in table. 2, 3. Each number of the spectrum in Fig. 2, 3 corresponds to the composition of the spectrum with the same number in the corresponding table. The sample surface corresponds to point 8 in FIG. 2, v. 4 in Fig. 3.

An analysis of the technical results shows that the proposed method for intensifying the process of chemical-thermal treatment of steels and iron-based alloys makes it possible to obtain a diffusion layer depth comparable to that of the prototype, at a significantly lower cost of the process than in the prototype.

Information sources

1. RU 2212470, publ. 09/20/2003 "Method of diffusion chromium plating of metal materials in a fluidized bed".

2. RU 2555320, publ. 07/10/2015 "Method of surface hardening of metal products".

3. RU 2048605, publ. 07/10/1995 "Composition for diffusion hardening of iron-carbon alloys".

4. RU 2778388, publ. 08/18/2022 "Method of the contact process of chemical-thermal treatment of steels and iron-based alloys".

5. Minkevich A.N. Chemical-thermal treatment of metals and alloys. - M.: Mashinostroenie, 1965. - 491 p.

6. Hauffe, K. Reactions in solids and on their surfaces. Per. with him. - M.: Publishing House of Foreign Literature. Ch1. 1962, 415 p., Ch2, 1963. 275 p.

7. Darken L.S., Gurri R.V. Physical chemistry of metals: Per. from English. - M.: Metallurgizdat, 1960. - 582 p.

8. Chebotin V.N. Physical chemistry of a solid state. - M.: Chemistry, 1982. - 320 p.

9. Umansky Ya.S., Finkelstein B.N., Blanter M.E. and other Physical basis of metallurgy. - M.: Metallurgizdat, 1955. - 721 p.

10. Alloys for thermocouples / I. L. Rogelberg, V. M. M. Beylip: Metallurgy, 1983, 360 p.

Claims (1)

A method for thermal diffusion chromium plating of parts made of steel or iron-based alloys with the addition of an emitter to a saturating powder mixture, in which fat-free parts are loaded into a container, filled with a saturating powder mixture, the container is sealed and heated to a temperature of 900-1000 ° C, characterized in that that the specified powder saturating mixture perform containing the following components: metal powder based on chromium 35 wt. % fraction not more than 150 microns, aluminum oxide 45 wt. % fraction 125-140 microns, granulated ammonium chloride 3 wt. % fraction of 2-3 mm, and as emitters of electrons and anions O ^2- - calcium tungstate 17 wt. % fraction not more than 100 microns.

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