RU2607871C1 - Method of producing coatings on articles of low and high alloyed steels, non-ferrous metals or their alloys by thermal diffusion of chromium plating - Google Patents

Abstract

FIELD: metal processing.

SUBSTANCE: invention relates to production of protective metal coating on products of steel, non-ferrous metals and alloys thereof, applied by thermal diffusion method. Method of coating on articles of low or high alloyed steels, or non-ferrous metals or their alloys by coating of metal by thermal diffusion method includes following subsequent steps: loading of processed items in continuously revolving container with simultaneously telescope powder mixture based on chromium powder with admixture of zinc oxide in form of formed over entire volume of container dust cloud from said powder mix, uniform deposition of saturating powder mix on entire surface of product due to tribostatic effect, wherein said loading and deposition are carried out for no more than 15 minutes at speed of rotation of container of 7-10 rpm, heating of container with product in temperature range from 250 to 500 °C at heating rate of 5–10 °C/min and rotational speed of container of 5-8 rpm with duration of heating for no more than 50 minutes, maintenance for no less than 5 minutes at said temperatures and at rate of rotation of container of 2–6 rpm and forced cooling of container with product with increased speed of rotation of container to 7–10 rpm and speed of temperature lowering of 5–10 °C/min and at cooling stage duration of no less than 60 min to allow producing coating with thickness of anti-corrosion layer from 1.5 to 16.0 mcm, which is controlled with duration of implementation of two last stages.

EFFECT: obtaining on surface of treated items uniform along whole surface coating with good operational properties, which are not violated during storage and transportation of products.

7 cl, 2 ex

Description

The invention relates to the field of applying protective metal coatings, in particular coatings on products from low and high alloy steels, from non-ferrous metals or their alloys, from powder mixtures containing chromium powder, the method of thermal diffusion chromium plating, and can be used to increase the corrosion resistance of products, improving their appearance, such as small parts (fasteners), as well as products several meters long (elements of building structures, pipes for various purposes).

In particular, high-strength bolts (GOST 22353-77 and GOST 22356-77) (shear-resistant), which are manufactured by hot stamping or cold heading from alloy steel, are used for the installation of bridge structures. Finished bolts are thermally processed. High-strength bolts are bolts of normal accuracy, they are put into holes of a larger diameter than the bolt. The nuts are tightened with a calibration wrench, which allows you to create, control the tension of the bolts. The large tension force of the bolt tightly tightens the elements to be connected and ensures the solidity of the connection. When a shear forces acts on such a connection, frictional forces arise between the elements to be joined, which prevent the elements from moving relative to each other. Thus, a high-strength bolt, working on axial tension, provides the transfer of shear forces by friction between the connected elements. Therefore, such a connection is often called friction.

To protect the hardware from corrosion (for the period of their transportation and storage prior to the installation of bridge metal structures), it is necessary to take measures to protect them from corrosion (conservation).

There are numerous ways to protect metals from corrosion. The choice of a particular method is determined by the specific working conditions and storage of metal products.

Coatings used to protect metals from corrosion are divided into metallic, non-metallic and formed as a result of chemical or electrochemical treatment of the metal surface.

Metal coatings give the surface layers of metal products the required hardness, wear resistance and corrosion resistance.

Metal coatings are applied in various ways. The most commonly used method is hot dip galvanizing or electro galvanization. In the hot dip galvanizing method, for example, the product is immersed in molten zinc metal, which wetts its surface and covers it with a thick layer up to 100 microns. Then the product is removed from the bath and cooled. In this way, the product is coated with a layer of zinc.

In the galvanic method, metal products are placed in a galvanic bath. Under the influence of electric current, cathodic deposition of a protective metal film occurs on the surface of the product. The thickness of the plating can be adjusted within a fairly narrow range.

The disadvantages of both methods are hydrogen contamination of the metal during the process, poor corrosion protection, etc.

There are a large number of different alloys, compositions and compositions produced by various domestic and foreign manufacturers and used for corrosion protection of hardware, including temporary.

Metallization coatings based on copper-zinc alloys and lead (RF patent No. 2138702 "Protective coating"), as well as on the basis of nickel (RF patent No. 2095469), based on tin (RF patent No. 2177055), manganese (RF patent No. 2258765) possess the lowest value of the coefficient of friction, which prevents thread sticking when twisting hardware. However, they are cathodic coatings with respect to the steel substrate of the hardware. When twisting the hardware, inevitably damage to the coatings on the sharp crests of the thread. Under conditions of moisture from atmospheric precipitation, a galvanic pair occurs between the cathode coating and the steel substrate, which will inevitably lead to rapid corrosion destruction of the steel substrate. Steel corrosion products (rust) have a volume 14 times larger than the monolithic metal, which leads to the rapid detachment of the cathode coatings. In addition, the application of metallization coatings requires special equipment in the form of furnaces for chemical-thermal treatment or magnetron irradiation plants, which complicates and increases the cost of the coating process.

The most widely used method for producing metal coatings on products in order to obtain an anticorrosive layer on their surface has recently been the method of producing coatings on various metal products (steel, non-ferrous metals, their alloys) from metal powders by the thermal diffusion method.

This type of coating is used for products operating in harsh operating conditions, in particular for casing and tubing, assembled into columns using couplings. The pipe is fastened to the coupling in the column by means of a threaded connection. During operation, it is the threaded joint that is most susceptible to wear and various kinds of damage, leading to premature failure of the column. Thermal diffusion zinc coating to the greatest degree than any other provides threaded joints with high wear resistance, tightness and corrosion resistance (TU 1327-001-56591711-03 "Diffusion galvanized tubing and couplings for them", TU 4859-002-50721682- 2008 “Zinc protective thermal diffusion coatings on the elements of threaded joints of tubing”).

A known method of thermal diffusion galvanizing (TDC), in which a zinc source is used a powder with a content of metallic zinc of at least 94% (mass.) And a particle size of less than 75 microns (British standard "Coating of ferrous metals by sherardization" BS 4921: 1988 ) However, it is not technologically advanced due to the adhesion of small particles of powder and sticking to the surface of the products, which occurs due to the proximity of the melting points of zinc and the galvanizing process.

TDC methods are also known in which zinc powder mixtures with inert fillers are used as the zinc source, which increases the refractoriness of the powder mixture and improves the processability (for example, quartz sand). The process is carried out at a temperature of 430-450 ° for 1.5-3 hours, followed by cooling in air (RF Patent No. 2244094). The ratio of iron and zinc in the coating is 0.0638-0.3330, and the microhardness of the resulting coating is 3360-5250 MPa, which corresponds to 6 and G phase. However, these ranges include both all possible values of the ratios of iron and zinc, and all possible values of microhardness for such coatings. This allows you to determine at what ratio the most wear-resistant coating is obtained.

Known methods of TDC using a modified powder with a zinc content of up to 99 mass. % having a surface film of fine-grained oxide particles, wherein the particle size of the powder is from 4 μm to 1 mm, and the particle size of the active zinc oxide in the surface film is from 0.03 to 0.50 μm. [RF patent No. 2170643]. The activity of the modified zinc powder is due to the presence on its surface of a loose layer of zinc oxide particles of a certain size, having a large specific surface on which hydrogen adsorbed during the treatment of the powder with water is adsorbed. This structure contributes to the rapid evaporation of zinc from the surface of the powder particles. The method includes chemical-thermal treatment of products in a rotating retort with the above powder.

A known method of thermal diffusion galvanizing is that steel parts and a saturating mixture consisting of an inert material (sand, aluminum oxide, refractory clay) with a grain size of 60-140 microns and finely dispersed zinc powder with a grain size of 5-60 microns are loaded into a sealed rotating reactor this inert material is loaded into the reactor from 40 to 100 wt. % by weight of parts, heated and maintained at a temperature of 390-430 ° C in an inert atmosphere (RU 2147046).

The disadvantages of this method include, firstly, the low speed of the galvanizing process and, accordingly, low productivity due to the use of zinc powder obtained by air spraying, in which the powder particles are coated with a continuous oxide film, which is a barrier to the sublimation of metallic zinc, which leads the need to increase the duration of thermal diffusion exposure to obtain the required coating thickness at a given temperature, secondly, increased energy costs due to the need for long exposure at a sufficiently high heating temperature, which is required to ensure a sufficient (for practical use) sublimation rate of zinc metal through a continuous oxide film covering zinc powder particles, and thirdly, insufficient coating quality due to narrow grooves overgrowing (for example, threads) with the finest particles of finely dispersed zinc powder, and because of the unattractive presentation having a spotty dark gray color, even fourthly, technological instability due to the possibility of areas without zinc coating appearing on parts when using a saturating mixture with an amount of neutral material of about 40% by weight of the parts, as well as a noticeable slowdown (until complete cessation) of the galvanizing process with an inert material fineness of about 140 microns.

There is another method of applying a zinc coating by thermal diffusion galvanizing, including loading products into a retort of a rotary electric furnace, filling the saturating mixture, sealing the retort, heating it to a predetermined temperature (360-470 ° C), holding at this temperature, depressurizing the retort, unloading the products , washing and passivating them, at the same time, a saturating mixture containing 80-90% zinc is poured into the retort, moreover, to form a zinc coating 1 μm thick, the filling mass of the saturating mixture is 7.8-8.2 g per 1 m ^{2 of} coated surface product characteristics, in addition, during the entire time of the galvanizing process, overpressure in the retort is continuously discharged using the drain pipe (RU 2001131234, 01.20.2004).

The disadvantages of this method include, firstly, the insufficiently high productivity of the process due to the use of zinc powder obtained by air spraying and therefore having a continuous oxide film on the surface of its particles, which reduces the ability to sublimate zinc; secondly, increased energy consumption due to the need to heat the saturating mixture, and with it the processed products, together with the container to a higher temperature in order to provide a sufficiently high ability to sublimate zinc from the surface of particles coated with a continuous oxide film, thirdly, the coating does not have high quality due to the unattractive presentation that has an uneven (spotty) dark gray color.

In addition, the disadvantage of the known methods of TDC is the formation on the treated surface is not uniformly uniform in thickness of the coating, especially on threaded surfaces. This negatively affects the tightness of threaded joints and reduces their operational properties in various application conditions.

The technical task and the technical result achieved is the development of such a method that would provide increased efficiency for obtaining an anti-corrosion coating layer of different thicknesses both on small-sized products and large products, that is, increasing the productivity of the process of thermal diffusion deposition of metal powders.

The stated technical problem and the achieved technical result are provided by the claimed as a invention method of producing a coating on products from low and high alloy steels, non-ferrous metals or their alloys by applying a metal coating to them by the thermal diffusion method, which consists in the following stages being carried out in series:

a) loading the processed product (s) in a constantly rotating container with a simultaneously saturating powder mixture based on chromium powder mixed with zinc oxide, which is formed in the form of a dust cloud throughout the container;

b) uniform deposition of a saturating powder mixture on the entire surface of the product (s), due to the tribostatic effect, with a total duration of stages (a) and (b) of no more than 15 minutes and a container rotation speed of 7 rpm - 10 rpm;

c) heating the container with the product (s) in the temperature range from 250 ° C to 500 ° C at a heating rate of 5 ° C / min - 10 ° C / min and a container rotation speed of 5 rpm - 8 rpm and for a duration heating stage no more than 50 min;

d) holding for at least 5 minutes at the indicated temperature conditions and at a container rotation speed of 2 rpm - 6 rpm and

e) forced cooling of the container with the product (s) with an increase in the rotation speed of the container to 7 rpm - 10 rpm and a temperature reduction rate of 5 ° C / min - 10 ° C / min and with a duration of the cooling stage of at least 60 minutes, which generally provides a coating with a thickness of the anticorrosion layer from 1.5 to 16.0 μm adjustable duration of the implementation of stages (g) and (e).

Moreover, when implementing the method:

a metal powder mixture prepared on the basis of chromium powder is a dispersion of particles of not more than 10 microns with impurities of zinc oxide;

the resulting tribostatic effect is due to the constant rotation of the container, during which the effect of opposite polarity is created, which ensures uniform deposition of metal particles over the entire surface of the processed products;

the metal powder mixture is loaded into the container together with a filler that does not react with the used powder mixtures;

forced cooling is carried out by blowing with cooled air;

forced cooling is carried out by spraying the container with water;

container rotation is carried out both clockwise and counterclockwise.

As a filler, which can be additionally included in the composition of the applied powder mixture based on chromium powder, for example, the following powders are used: silica sand, titanium dioxide, alumina, refractory clay, etc.

The content of metal chromium powders in the used powder mixtures (with impurities of zinc oxide) is 99% wt.

In general terms, the claimed method according to the invention is illustrated by the following non-limiting examples.

The implementation of the invention:

Example 1. The coating process by thermodiffusion saturation with a powder mixture based on chromium powder, the structure of the workpiece can be divided into four main subsequent phases:

1) The phase of creating a chromium-based powder mixture with impurities of zinc oxide and possible additives of the indicated dust cloud fillers throughout the container and uniformly depositing zinc particles on the surface of the processed products due to the tribostatic effect;

2) The phase of heating to a given temperature (250-500 ° C) at a certain speed and duration of heating;

3) The holding phase at a given temperature (for a certain time period of at least 5 minutes (5-10 minutes);

4) The phase of forced cooling for a certain time of at least 60 minutes (60-80 minutes) and a certain rate of temperature decrease;

The phase of creating a dust cloud over the entire volume of the container and the uniform deposition of particles of the powder mixture on the surface of the processed products due to the tribostatic effect. The container is in constant rotation, at which the rotation speed is from 7 to 10 revolutions per minute, and it is preferable to rotate the container for this phase at a rotation speed close to the maximum speed, which gives a better friction effect between the particles of the powder mixture of air and the surface of the parts. The time allotted for this phase is not more than 15 minutes, and heat is not supplied to the container over the entire time period of this phase. Thus, this phase occurs at room temperature.

At the end of the first phase, the phase of heating to the indicated temperature begins by switching on the heating elements or gas burners that supply heat to the outer wall of the container, and through it the heat penetrates into the container, evenly heating the surface of the processed products and metal particles (chrome), which are already due to tribostatics, it is as if glued to the surface of the products with a uniform layer. The container is during the passage of this phase in constant rotation at a minimum rotation speed, but not less than 5 rpm. In this case, the container rotates in reverse mode, changing the direction of rotation every three minutes. Starting from about 250-260 ° C and up to 500 ° C, the diffusion of metal atoms of the powder mixture into the structure of the processed products begins. The time allotted for this phase is up to 50 minutes, and in this case, heating starts from room temperature, and the heating rate is 5-10 ° C / min. When the set temperature is reached, the heating is turned off and the next stage immediately begins, namely the holding phase at the set temperature. The holding phase at a given temperature is necessary to control the thickness of the coating layer, and therefore, its time range is from 5 to 15 minutes and the minimum temperature fluctuations in the range of not more than 4 degrees. In this regard, the total residence time of the container in the heating furnace can be from 45 to 80 minutes, which, of course, is several times shorter than all previously known thermal diffusion methods. At the end of the holding phase, the last phase of the process begins, namely the forced cooling of the container: with the help of, for example, motors, cold air is pumped and fed through special guide hoses directly to the outer walls of the container, which, as in the previous phases, is in constant rotation, and the rotation speed in this phase, as well as in the first phase, increases to maximum revolutions. The objective of this phase is to quickly remove the temperature gradient to 260 ° C and then gradually reduce the temperature gradient to room temperature. The time of the final phase is up to 70 minutes. In this case, the extreme value of time is allotted for complete cooling to room temperature.

The following are specific other examples of the invention - the deposition of a metal coating using powder mixtures, namely chromium powders with an admixture of zinc oxide, as well as with possible additives of fillers, such as titanium dioxide and others, under various specific conditions for the implementation of various stages of the process.

Example 2. The method is carried out analogously to example 1, but as a powder mixture, a mixture of chromium powder with impurities of zinc oxide (chromium content 99%) is used. The parts (threaded parts of the fastening group) are loaded into a constantly rotating container with simultaneous saturation of the powder mixture; a dust cloud is created from the specified powder mixture (particle size 5 μm); carry out uniform deposition of powder on the entire surface of parts (products) with a total duration of this first phase of 15 minutes; at a container rotation speed of 10 rpm Heating is carried out to 500 ° C at a heating rate of 10 ° C / min, container rotation speed of 10 rpm and a heating duration of 20 minutes Exposure is carried out for 5 minutes at a container rotation speed of 4 rpm. Forced cooling is carried out by increasing the speed of rotation of the container to 10 rpm and the speed of decreasing the temperature of 10 ° C / min. Stage duration 50 min. The thickness of the anti-corrosion layer is from 1.5 microns to 5.0 microns. Cooling is carried out by spraying the container with water. Get a coating with uniform thickness over the entire surface.

All coatings obtained according to examples 1, 2 in accordance with the method according to the invention have a uniform thickness of the anti-corrosion layer over the entire surface, which does not deteriorate under the influence of various atmospheric factors during storage and transportation, as well as during their subsequent operation, that is, they have a uniform structure that provides an improvement in their operational properties. The method itself, in comparison with the known ones indicated as the prior art, is more technologically advanced, allows to obtain coatings with a small thickness of the anti-corrosion layer.

Claims (12)

1. A method of obtaining a coating on products of low or high alloy steels, or non-ferrous metals, or their alloys by applying a metal coating to them by the thermal diffusion method, which includes the following stages that are carried out in series: a) loading the processed products into a constantly rotating container with a simultaneously saturating powder mixture based on chromium powder with an admixture of zinc oxide in the form of a dust cloud formed over the entire volume of the container from the specified powder mixture; b) due to the tribostatic effect, uniform deposition of the saturating powder mixture on the entire surface of the product, while the above loading and deposition is carried out for no more than 15 minutes at a container rotation speed of 7-10 rpm; c) heating the container with the product in the temperature range from 250 to 500 ° C at a heating rate of 5-10 ° C / min and the rotation speed of the container 5-8 rpm with the duration of the heating stage not more than 50 min; d) holding for at least 5 minutes at the indicated temperature conditions and at a container rotation speed of 2-6 rpm and d) forced cooling of the container with the product with an increase in the speed of rotation of the container to 7-10 rpm and a rate of temperature decrease of 5-10 ° C / min and with a duration of the cooling stage of at least 60 minutes to provide a coating with a thickness of anticorrosive layer from 1, 5 to 16.0 μm, which is regulated by the duration of the implementation of stages (g) and (d). 2. The method according to claim 1, characterized in that the metal powder mixture based on chromium powder consists of particles with a size of not more than 10 microns with impurities of zinc oxide. 3. The method according to claim 1, characterized in that the tribostatic effect is provided by constant rotation of the container, during which they create the effect of different polarity for uniform deposition of metal particles over the entire surface of the processed products. 4. The method according to claim 1, characterized in that the powder mixture containing metal powder further comprises a filler that does not react with the used powder mixture. 5. The method according to claim 1, characterized in that the forced cooling is carried out by blowing with cooled air. 6. The method according to claim 1, characterized in that the forced cooling is carried out by spraying the container with water. 7. The method according to claim 1, characterized in that the rotation of the container is carried out both clockwise and counterclockwise.