RU2794655C1 - Method of thermal diffusion chromium plating of steels and iron-based alloys using cumulative grids - 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. The method of thermal diffusion chromium plating of iron-based steels and alloys using cumulative grids, in which degreased 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 for 24 hours, according to the invention, ceramic tubes with a diameter of 2 mm are fixed with a nichrome thread along the perimeter of the saturation sides of the surface of the part, a cumulative grid is installed on top of the tubes in the form of a rectangular plate made of nickel or molybdenum sheet, 0.3 mm thick with perforated holes of a conical shape with a diameter 0.2 mm at the top, directed by the base to the surface to be saturated and arranged with a density of 10 pieces/cm²; which, together with the part, is covered with a saturating powder mixture comprising the following components: metal powder based on chromium 32 wt.% with a fraction of not more than 150 microns, granulated ammonium chloride with 4 wt.% with a fraction of 2-3 mm, and as emitters of electrons and O^2- anions, 32 wt.% of serpentine powder with a fraction of not more than 100 microns and 32 wt.% of scheelite powder with a fraction of not more than 100 microns are used.

EFFECT: increased depth of the diffusion layer by creating an additional electron emission flow with the introduction of a mixture of scheelite and serpentine powders into it and controlling this flow by using a cumulative grid.

1 cl, 6 dwg, 4 tbl

Description

The invention relates to metallurgy, in particular to chemical-thermal treatment (HTO) 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 pump air out of the container for saturation and pump hydrogen into it.

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 exposure 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 alloys based on iron. This method includes loading defatted parts into a container, filling them with a saturating powder mixture, sealing the container and heating to a temperature of 900-1000°C for 24 hours. The saturating mixture has the following composition: chromium 32-37 wt.% with a fraction of not more than 150 microns, aluminum oxide 50 wt.% with a fraction of 125-140 microns, granulated ammonium chloride 3 wt.% with a fraction of 2-3 mm, in addition, it is proposed to introduce into the composition of the saturating mixture as electron emitters is tungsten powder with a purity of 99.95%. An increase in the depth of chromium diffusion is achieved due to the contribution of the energy of the internal thermionic field to the saturation process [3].

The disadvantage of this method is the high complexity of obtaining tungsten powder with a purity of 99.95%.

The technical task and the result of the proposed method is to increase the depth of the strengthening diffusion layer of chromium through the use of a cumulative grating to create cumulative electron-anion emission fluxes created by amorphous solid electrolytes in the saturating powder mixture: serpentine powders (fraction not more than 100 μm) and scheelite (fraction no more than 100 µm) and control of these flows.

The technical problem is achieved due to the fact that the method of thermal diffusion chromium plating of iron-based steels and alloys using cumulative gratings is characterized by the fact that fat-free parts are loaded into the container, filled with a saturating powder mixture, the container is sealed and heated to a temperature of 900-1000 ° With within 24 hours, according to the invention, along the perimeter of the sides of saturation of the surface of the part, ceramic tubes with a diameter of 2 mm are fixed with a nichrome thread, on top of the tubes establish a cumulative grating in the form of a rectangular plate made of nickel or molybdenum sheet, 0.3 mm thick, with perforated conical holes 0.2 mm in diameter at the top, directed by the base to the saturating surface and located with a density of 10 pieces/cm²; which, together with the part, is covered with a saturating powder mixture containing the following components: metal powder based on chromium 32 wt.% with a fraction of not more than 150 microns, granulated ammonium chloride 4 wt.% with a fraction of 2-3 mm, and as emitters of electrons and anions O^2- 32 wt.% of serpentine powder with a fraction of not more than 100 μm and 32 wt.% of scheelite powder with a fraction of not more than 100 μm are used.

The essence of the invention is illustrated by the images in figures 1-6 and tables 1-4 (see in the graphic part), which show:

- Fig 1. The results of measurements of the current strength between two iron electrodes in a saturating powder mixture for chromium plating, containing as emitters of electrons and anions O ^{2 -} serpentine powder (fraction not more than 100 microns) and scheelite powder (fraction not more than 100 microns) during heating container from 500 to 1000°C;

- Fig 2. Layout of the cumulative grating: a) the general layout of the installation of the cumulative grating at the part and the view of the conical perforated hole in the section; b) the location of the perforated holes on the cumulative grid; c) section A-A; where are indicated: 1 - saturable part with a size of 24×10×10; 2 - ceramic tube with a diameter of 2 mm; 3 - grating (made of nickel or molybdenum) 30×15×0.3 mm in size; 4 - nichrome thread for attaching a ceramic tube to the surface of the part; 5 - holes of the cumulative grid.

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

- Fig. Fig. 3. Area for point chemical analysis of the part surface after chromium plating with the addition of a mixture of scheelite (CaWO ₄ ) and serpentine (Mg ₃ Si ₂ O ₇ ) powders to the saturating powder mixture of emitters of electrons and anions O ^2- using a cumulative nickel grid;

- Table 2. The results of point X-ray microchemical analysis (at the points indicated in figure 3 after chromium plating with the addition of a saturating powder mixture of emitters of electrons and anions O ^2- - a mixture of powders of scheelite (CaWO ₄ ) and serpentine (Mg ₃ Si ₂ O ₇ ) using a cumulative nickel grating;

- Fig. Fig. 4. Area for point chemical analysis of the part surface after chromium plating with the addition of a mixture of scheelite (CaWO ₄ ) and serpentine (Mg ₃ Si ₂ O ₇ ) powders to the saturating powder mixture of the emitter of electrons and anions O ^2- using a molybdenum cumulative lattice;

- Table 3. The results of a point micro-X-ray spectral chemical analysis (at the points indicated in figure 4 after chromium plating with the addition of a saturating powder mixture of emitters of electrons and anions O ^2- - a mixture of powders of scheelite (CaWO ₄ ) and serpentine (Mg ₃ Si ₂ O ₇ ) using a cumulative lattice of molybdenum;

- Fig. 5. Area for point chemical analysis of the part surface after chromium plating with the addition of tungsten powder as an electron emitter to the saturating powder mixture;

- Table 4. The results of a point micro-X-ray spectral chemical analysis (at the points indicated in figure 5 parts after chromium plating with the addition of tungsten powder as an electron emitter to the saturating powder mixture;

- Fig. 6. Scheme explaining the operation of the perforated hole of the cumulative grid.

The method is carried out as follows.

The sequence of operations of the proposed method includes: loading degreased parts into a container with a cumulative grid(s) installed near their surface(s); filling them with a saturating powder mixture, sealing the container and heating the container to a temperature of 900-1000°C for 24 hours. 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:

- the use of additional technological equipment - a cumulative grid made of nickel or molybdenum;

- the composition of the saturating powder mixture: in its composition, the saturating mixture, as well as in the prototype, includes chromium, ammonium chloride, but an essential distinguishing feature is the presence of a mixture of serpentine and scheelite powders with a fraction of not more than 100 microns in an amount of 32 wt.% each of the total mass of the saturating powder mixture, which are a source of electrons and oxygen anions.

Cumulative gratings 3 (figure 2) made of nickel or molybdenum have the form of a rectangular plate, are made from sheets of industrial production with a thickness of 0.3 mm by punching holes 5 of a conical shape with a core with an angle at the top of 60 ° until holes with a diameter of 0.2 mm are formed at the top . It has been experimentally proven that the density of holes 5 on the grating should be at least 10 pieces/cm ² . To maintain a constant distance between the cumulative grating 3 and the saturating surface of the part 1, ceramic tubes 2 with a diameter of 2 mm are fixed to the part 1 with a nichrome thread along the perimeter of the sides of the saturation of the surface of the part (Fig. 2a, b) . Then, cumulative gratings are installed on top of the part with fixed tubes and covered with a saturating powder mixture over the gratings.

Saturating the powder mixture, unlike the prototype, has the following composition: 32 wt.% metal powder electrolytic refined chromium (fraction not more than 140 microns); emitters of electrons and oxygen anions: 32 wt.% serpentine powder (fraction not more than 100 microns) and 32 wt.% powder scheelite (fraction not more than 100 microns), and 4 wt.% ammonium chloride (fraction 2-3 mm).

Scheelite powder (oxide CaWO ₄ ) is publicly available, can be obtained both from the natural mineral scheelite (mineral composition: 19.48 wt.% - CaO; 80.52 wt.% - WO ₃ , additionally may contain up to 10 wt.% impurities MoO ₃ ), the deposits of which are also found in Russia (Middle Urals, Chukotka, Eastern Transbaikalia, North Caucasus), and from the products of processing waste from machine-building and electrical production.

Serpentine powder (Mg₃Si₂O₇) is a waste from the enrichment of chromium ore or a mineral of a natural serpentine deposit in N. Ufaley. Mineral composition: 44.1 wt.% - SiO₂; 43.0 wt% MgO; 12.9 wt% CaO; 12.9 wt% - H₂Oh

Ammonium chloride powder is an inorganic compound, an ammonium salt with the chemical formula NH ₄ Cl. In industry and laboratory conditions, it is obtained by the interaction of hydrochloric acid with ammonia or by passing carbon dioxide through a solution of ammonia and sodium chloride. A feature of ammonium chloride is its decomposition into ammonia and hydrogen chloride at temperatures above 338°C. For experiments, a laboratory reagent was used in the form of granules with a fraction of 2-3 mm according to GOST 2210-73.

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 [4]. It is believed that the driving force of such a process of surface saturation is diffusion, described by the well-known two Fick's laws [4–8].

In the theory of transport in solids, two fundamental postulates are formulated, from which it follows [5-7]:

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

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, to accelerate the saturation diffusion process (to increase the depth of chromium diffusion), it is necessary to intensify the operation of the electric field.

When additional sources of electrons and oxygen anions are introduced into the saturating mixture (for example, scheelite CaWO₄and serpentine mg₃Si₂O₇) in addition to the concentration field, an electric field arises during the heating of the mixture. Heating of emitters of electrons and anions to a temperature above the lower threshold of the beginning of emission (450-800°C) and further to saturation temperatures of 900-1000°C determines the appearance of thermoelectrons and oxygen anions in the working space. Iron and iron-based alloys in a heated state are a positive electrode [9]. Consequently, the appearance of thermoelectrons and oxygen anions generates an electric field directed towards the positive electrode, which is the workpiece.

The results of measuring the current in electrical circuits of a saturating powder mixture of serpentine and scheelite when heated to 1000°C shown in FIG. 1. The fixed current indicates the appearance of a directed flow of electrons and oxygen anions to the surface of the saturable part and characterizes the operation of scheelite and serpentine as amorphous solid electrolytes. This directed flow is an additional (to the diffusion flow) carrier of atomic chromium to the surface of the saturated product and an intensifier of the chromium diffusion process.

A cumulative grating 3 (Fig. 2) made of a sheet 0.3 mm thick is installed in front of the saturating surface of the part 1 at a distance of 2 mm (the distance is fixed by a ceramic tube 2 with a diameter of 2 mm, fixed to the surface of the sample with a nichrome thread 4, and the base of the perforated holes of the conical shape 5 of the lattice 3 is located at the saturating surface of the part 1 (Fig. 2a). Each such perforated hole 5 allows enhance the flow of saturating elements in gaseous or molecular form passing through the perforated holes of the cumulative grid.The main diffusion-emission flow (Fig. 6), consisting of saturating metal atoms, chlorides, electrons and oxygen anions, enters the perforated cone-shaped hole through its top. The inner surface of the conical surface of the perforated hole emits electrons, which form additional flows directed towards the optical axis of the cone of the perforated hole, i.e. into the main diffusion-emission flow.As a result of the merging of the main and additional emission flows, a total emission-diffusion flow of greater energy is formed pointing towards the part. Thus, with a constant composition of the saturating powder mixture, it is possible to activate the saturation process in the desired areas of the saturating part. The operation of the perforated holes of the conical shape of the cumulative grid is illustrated by the diagram in Fig. 6.

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

Method execution examples

Example 1. In front of the saturated surface of a degreased part made of steel 35X2H3 with a size of 24 × 10 × 10 mm, a cumulative grid made of a rectangular nickel sheet with a size of 30x15 mm and a thickness of 0.3 mm is fixed with a wire at a distance of 2 mm with perforation with holes with a diameter at the top of 0.2 mm arranged with a density of 10 pieces/cm ² . Then the prepared parts are placed in a sealed container, covered with a saturating powder mixture of the composition: metal powder based on chromium 32 wt.% (fraction not more than 140 microns), and 4 wt.% ammonium chloride (fraction 2-3 mm), electron and anion emitters oxygen: 32 wt.% serpentine powder (fraction not more than 100 μm) and 32 wt.% powder scheelite (fraction not more than 100 μm); heated for 24 hours at a temperature of 1000°C. The result is a depth of the diffusion layer of chromium 53-60 microns (table. 1).

Example 2. The same sequence of actions was performed as in example 1, but a cumulative lattice of molybdenum was used. The result is a depth of the diffusion layer of chromium 60-72 microns (table. 1).

Example 3 (prototype). A degreased part made of steel 35X2H3 with a size of 24×10×10 mm is placed in a sealed container, covered with a saturating powder mixture of the following composition: metal powder based on chromium 35 wt.%, fraction not more than 150 μm; aluminum oxide 50 wt.%, fraction 125-140 microns; ammonium chloride, granulated 3 wt.%, fraction 2-3 mm; as electron emitters, the saturating powder mixture included tungsten powder with a purity of 99.95%, in an amount of 10 wt.%, a fraction of less than 100 μm, heated for 24 hours at a temperature of 1000°C. The result is a depth of the diffusion layer of chromium 47-52 microns (table. 1).

It has been established that in the process of saturation of the surface of parts in containers with the addition of a mixture of powders of electron emitters and oxygen anions (serpentine powder and scheelite powder) and the use of cumulative gratings made of nickel or molybdenum, the depth of the diffusion layer increases compared to the sample saturated using tungsten powder added into a saturating powder mixture as an electron emitter without the use of cumulative gratings (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 2-4 of the results of a point chemical analysis of the composition of the surfaces of parts, with the organized introduction of various emitters of electrons and anions O ^2- and the use of cumulative gratings.

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 3-5 show photographs of cross sections of surface layers of parts saturated in saturating powder mixtures with additives of electron emitters and oxygen anions (a mixture of serpentine and scheelite) with and without cumulative gratings. The results of point X-ray microanalysis of the points indicated in Figs. 3-5 are given in table. 2-4. Each spectrum number in Fig. 3-5 corresponds to the composition of the spectrum with the same number in the corresponding table.

Analysis of the technical results (Fig. 3, 4, 5, table. 2-4) shows that the proposed method of thermal diffusion chromium plating of steels and alloys based on iron using cumulative gratings allows you to get the depth of the diffusion layer more 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 2778388, publ. 08/18/2022 "Method of the contact process of chemical-thermal treatment of steels and iron-based alloys".

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

5. 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.

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

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

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

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

Claims (1)

A method for thermal diffusion chromium plating of a part made of steel or an iron-based alloy using a cumulative grate, which includes loading the said defatted part and a saturating powder mixture into a container, sealing the container and heating to a temperature of 900-1000°C for 24 hours, characterized in that ceramic tubes with a diameter of 2 mm are fixed to the perimeter of the saturation sides of the surface of the specified part using a nichrome thread, on top of the tubes establish a cumulative grating in the form of a rectangular plate made of nickel or molybdenum sheet 0.3 mm thick with perforated conical holes 0.2 mm in diameter at the top, directed by the base to the saturating surface and located with a density of 10 pcs/cm², which, together with the specified part, is covered with a saturating powder mixture, containing the following components: metal powder based on chromium 32 wt.% with a fraction of not more than 150 microns, granulated ammonium chloride 4 wt.% with a fraction of 2-3 mm, and as emitters of electrons and anions O^2- 32 wt.% of serpentine powder with a fraction of not more than 100 μm and 32 wt.% of scheelite powder with a fraction of not more than 100 μm are used.

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