GB2041008A

GB2041008A - Vacuum Metallising Internal and External Surfaces of Metal Pipe

Info

Publication number: GB2041008A
Application number: GB7903593A
Authority: GB
Original assignee: Ukrain Nii Sp Stalei Splav I F
Current assignee: Ukrain Nii Sp Stalei Splav I F
Priority date: 1979-02-01
Filing date: 1979-02-01
Publication date: 1980-09-03
Also published as: GB2041008B

Abstract

Metal pipes 4 are coated on their inside and their outside in a vacuum metallizing chamber 1. To coat the inside surfaces, a metal having a melting point below that of the pipe itself is put inside the pipe. The pipe is then fed past metal evaporators 6 which produce a diffusion coating of metal on the outside surfaces of the pipes. The pipes are rotated about their own axes as they pass through the chamber 1 and are heated to above the melting temperature of the metal inside the pipe. <IMAGE>

Description

SPECIFICATION Method and Apparatus for Applying Protective Coatings to Metal Pipes The present invention relates to a method and apparatus for applying protective coatings to metal pipes.

The invention is especially useful in the production of heat exchangers in which pipe walls are used to separate media having different chemical and physical properties, for example, in heat and power engineering where the external media are combustion products of high-sulphur fuel and the internal media are water and water vapour.

The manufacture of pipes from metal which displays sufficiently high corrosion resistance in two media with different chemical and physical properties often requires the use of special highalloy steels, which are expensive. Moreover, it is sometimes practically impossible to find an alloy with the required properties.

The problem in question can be solved either by producing bimetallic pipes, or pipes provided with a protective coating.

In the first instance considerable difficulties of a technical nature are involved which render the production technique economically unprofitable.

As to the production of located pipes, it should be observed that the protective coatings applied to pipes are often made of cheap low-alloy and low-carbon steels and, therefore, are not sufficiently corrosion-resistant when placed in two different corrosion media.

The known methods for applying protective coatings to metal pipes permit coatings of similar or substantially similar composition to be found on the inner and outer surface of pipes.

Out of many known types of coatings applied to pipes, diffusion coatings possess the highest technological properties.

Zinc coated pipes are most widely applied in industry, then aluminum coated pipes. Pipes with diffusion coatings from refractory metals, such as chromium, manganese and silicon, are produced in an extremely limited number. The reason for this lies in the fact that the metal coating techniques now in use are characterized by low production efficiency, being at the same time labour-consuming and failing to ensure the required quality of the metal being protected. For example, the diffusion metal coating method widely applied now consists in that the pipes are placed in a metallization mixture, including a powder of the metal being applied, an inert additive and a halide or ceramic powder saturated with the halogen compound of the metal being applied.The production process comprises the following stages, namely: preparing the pipes to be metallized, charging the pipes into a container, filling up the interspace between the pipes and the interior thereof with a metallization mixture (filling), sealing the container and heating it in a heat-treatment furnace and holding it at a constant, preset temperature, cooling the container within the furnace, cooling the container outside the furnace, unsealing the container, and removing the particles of the metallization mixture from the surface of the pipes. When carrying out diffusion chromium coating of pipes from low-carbon steel for a depth of up to 0.2 mm, the isothermal hold-down time amounts to 10-1 5 hours, and the overall duration of the metal coating process, heating and cooling periods included, lasts from 30 to 50 hours.With the metal being subjected to high temperatures (1000-1 1500) for such a long period of time, its structure and physical properties undergo changes and are not alway restorable in the course of subsequent heat treatment of the pipes. Chemical composition of the metal also tends to change (it undergoes decarbonization) in the zone adjacent to the protective diffusion layer.

The above-mentioned disadvantages restrict the application of pipes with protective coatings.

Due to the impairment of the strength characteristics of the pipes themselves, protective coatings have found but limited application in the protection of pipes made from high-strength and refractory low-alloy steels.

The process of diffusion metal coating is commonly performed in one-chamber heattreatment apparatus of intermittent action. These types of apparatus are disadvantageous in that they do not lend themselves to any appreciable increase in the production output since increases in the unit power and in the quantity of pipes being charged result in longer heating and cooling periods, as well as in non-uniformity of heating of the pipes.

A technique of chromium diffusion coating of pipes is known, which is performed in a threechamber heat-treatment furnace in which the first chamber is used for heating the pipes and chromium (metallizing agent), in the second chamber the pipes being coated are saturated with chromium, and in the third chamber the pipes and metallizing agent are cooling. The chambers are arranged successively and separated from one another by means of vacuum gates. To perform the process of diffusion chromium coating, pipes are placed in an open container made in the form of a box, and are then covered with chromium orferrochromium powder. Thereafter, the container is positioned on a movable trolley which is brought inside the first chamber. The production capacity of these furnaces remains low.In addition, the metal of the pipes undergoing diffusion chromium coating is prone to vigorous decarbonization.

Vacuum straight-through or other types of apparatus of continuous action for diffusion metal coating of pipes are not known to be in use.

However, similar types of apparatus are used for diffusion coating of steel strip. The strip undergoes metallization in these apparatuses while being passed over a metallizing agent heated to the temperature of active evaporation.

This type of apparatus has a chamber wherein the strip is unwound from a roll, a metallizing chamber with a metal evaporator wherein the strip is metallized, and a chamber wherein the strip is wound into a roll after the metallizing operation.

Two types of transporting means are used to transport the pipes through heating chambers in known tube coating apparatuses, i.e. the roller type and wheel type transporting means. Both types enable simultaneous transportation of a plurality of pipes which are thereby capable of forward motion while concurrently rotating about their axes. Where one-sided heating of pipes is required, the wheel-type transporting means is preferable in that it ensures a greater number of revolutions performed by the pipe within a given section along its travelling path. This leads to more uniform heating of the pipe surface. The use of similar types of pipe transporting means in vacuum metal coating apparatuses fails to ensure uniform heating and metallization of pipes.This is due to the fact that in vacuum furnaces, unlike in hearth heat-treatment furnaces wherein heat is transferred both by heat radiation and convection of a heated gas, the heat transfer from a heating source (which is the source of vapour from the evaporating metal) is effected exclusively by way of heat radiation. Therefore, during simultaneous transportation of a plurality of pipes disposed in a line the pipes found in the centre of the line are heated to a higher temperature than those disposed at its periphery even if equidistant from the heating source.

Due to heat losses the heating source per se has a somewhat lower temperature at the furnace peripheral sections than at its centre.

The present invention provides a method for applying protective coating to metal pipes comprising diffusion metal coating of pipes in a vacuum chamber containing a metal being evaporated, wherein prior to effecting the coating of pipes a metal is fed into the interior of each pipe with the melting temperature thereof being substantially below the melting temperature of the pipe metal, whereas the metal coating of the pipes is carried out at a temperature above the melting temperature of the metal disposed within the pipe which is continuously rotating about its axis while longitudinally advancing within the vacuum chamber above the evaporated metal.

The method according to the invention for applying protective coating to metal pipes permits protective diffusion coatings from various metals to be obtained on the pipe internal and external surfaces. The coatings can be selected to be corrosion-resistant to aggressive media in which the pipes are to be used.

The method of the invention also makes it possible to substantially reduce the time period required for heating, metallizing and cooling of the pipes being treated. This became possible to accomplish through direct heating of a pipe (or a plurality of pipes) in contrast to the known methods in which the pipes being coated are heated through a container and a powdered metallizing agent. In carrying out diffusion coating the temperature of a pipe is selected in accordance with the effect it has on the metal quality, as well as on the rate of coating. Upon completion of the coating operation the cooling of the pipes is likewise effected at a high rate, since after metallization the pipes are transferred to a water-cooled holding chamber.For example, in performing diffusion chromium coating of the external surfaces of pipes a layer of chromium 0.1 5 to 0.20 mm thick is formed on low-carbon steel pipes in a time period of 5 to 6 min as against the time period of 10 to 1 5 hours required to conduct metal coating of pipes according to previously known methods. It should be observed that the duration of the process is only limited by the time period required for the formation of the diffusion layer on the pipes being treated.

It is due to the high speed at which protective diffusion layer is formed that the decarbonization of the surface layers of metal adjacent to the diffusion layer is substantially reduced or prevented.

In addition, owing to the high rate of applying protective coatings to pipes it becomes possible to enhance the production process and lower the cost of the pipes provided with protective coatings.

Pipes whose internal and external surfaces are coated with protective coatings of different composition are most frequently used in various heat-exchanging installations. The selection of coatings for the external and internal surfaces of pipes is based upon the degree of corrosion resistance of these coatings in working media. An important factor to observe here is that the metal applied to the internal surface of a pipe must have a melting temperature below that of the pipe metal and that it should dissolve in the pipe metal as the diffusion coating is being applied. With regard to the metal for the external surface of a pipe, its melting temperature can be variable (higher, equal or below the melting temperature of the pipe metal).The temperature at which the coating process is conducted and its duration are selected in accordance with the desired thickness of the coating to be applied. In those cases when the temperature of the coating process has no deleterious effect on the properties of the pipe metal, the process is run at the highest temperature where it is still possible to transport the coated pipes. When, however, the heating temperature during metallization affects the structure and properties of the pipe metal, and these properties can not be or are not restored in the course of subsequent heat treatment, the process temperature is selected such as to have minimum adverse effect on the properties of the pipe metal.

According to one embodiment of the invention, the melting temperature of the evaporating metal is above the melting temperature of the metal disposed in the interior of each pipe.

This makes it possible to expand the range of selection of protective coatings for use in different aggressive media, enabling coatings of refractory metals to be applied to the external surfaces of pipes; such metals having melting temperatures above the melting temperature of the metal disposed inside the pipe and above that of the pipe metal.

According to another embodiment of the invention, the metallization of pipes is conducted at a temperature above the melting temperature of the evaporating metal.

As a result, it becomes possible to substantially raise the temperature of the diffusion coating process and thereby to step up the production process as a whole.

This, in turn, permits the production efficiency of the metallizing process to be enhanced and the cost for the coated pipes to be reduced.

According to still another embodiment of the invention, the leading end of each pipe is raised during the coating process relative to the trailing end thereof so that its longitudinal axis forms with the horizontal plane an angle of not more than 1 5" in the travelling direction of the pipes.

This embodiment of the invention can be used to advantage where long pipes are being coated.

Using this feature, it is possible to coat the internal surface of a pipe using lumps of metal variable in shape, serving as the metallizing agent.

The lumps are placed in the leading end of the pipe. In the process of coating, the metal disposed inside the pipe melts down under the effect of heating and gradually moves towards the trailing end of the pipe to merge therein as the latter is heated to progressively higher temperature. By altering the angle of inclination of the pipe, it is possible to vary the time period during which the pipe remains in contact with the molten metal, as well as the parameters of the layer being applied (thickness of the layer and concentration of the element being saturated).

The invention also provides an apparatus for applying protective coatings to metal pipes, comprising a vacuum chamber accommodating a metal evaporator, a pipe delivery chamber and a pipe holding chamber, both chambers being provided with vacuum gates and storing devices and arranged one on either side of said vacuum chamber, and means for transporting the pipes to be coated through said chambers said means comprising wheels similar in diameter and fixed in stands spaced apart from one another along the travelling path of the pipes, the axes of rotation of the wheels in each stand being parallel to one another and coinciding with the axes of rotation of the similarly arranged wheels in other stands, the centres of rotation of the wheels being arranged on an arc the highest point of which is on the vertical axis of symmetry of the stand.

The apparatus according to the invention for diffusion metal coating of pipes permits coatings of different chemical composition to be simultaneously applied to the external and internal surfaces of the pipes being treated.

Uniformity in thickness of the layer being applied is assisted by equal temperature conditions maintained in the process of coating. This is helped by the arc-like arrangement of wheels in each stand and, consequently, the arrangement of the pipes on the stands. Since the evaporator temperature is at a maximum in its central portion and some what lower along its periphery, the pipes which pass above the central portion of the evaporator are further away from the evaporator than are other pipes.

According to one embodiment of the invention, each stand is provided with a mechanism adapted to enable it to rotate about the vertical axis of symmetry of the stand.

As a result, it becomes possible to vary the relation between the speed of rotation of the pipes and that of their translatory motion, and also enables the space relationship between the pipes to be varied in accordance with their diameters, thus ensuring effective coating thereof.

The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a general longitudinal sectional view of an apparatus for carrying into effect the method of the invention; Figure 2 is a view of a means for transporting the pipes to be coated (as viewed from the side at which the pipes are fed); Figure 3 is a cross-section taken along the plane Ill-Ill of Figure 2; and Figure 4 is a top view of a means for transporting the pipes to be coated.

Referring now to the drawings in detail, the apparatus shown for applying protective coating to metal pipes comprises chambers 1, 2, 3, (Figure 1 ) intended respectively for metallizing, supplying and holding pipes 4. The chambers 1, 2, 3 are arranged in succession one after another and are partitioned by means of vacuum gates 5.

Mounted within the metallizing chamber 1 are metal evaporators 6. The metal evaporator 6 is basically a closed vessel made from a refractory material and formed with openings for the passage of the pipes 4. Arranged inside the evaporator 6 are heating elements 7 and baffles 8. The chambers 2 and 3 intended respectively for delivery and holding of the pies 4 are similar in construction and incorporate grid-like devices 9 for storing the pipes 4. The devices 9 are movable in a vertical direction with the aid of an actuator 10. To enable the passage of the pipes 4 through the chambers 1, 2, 3, the apparatus is provided with transporting means 11 (see Figure 2-4) which is made in the form of stands 12 spaced apart from each other along the travelling path of the pipes 4 and fitted with wheels 1 3 with diameters similar to those of the pipes.The axes of rotation of the wheels 13 in each stand 12 are parallel to one another and coincide with the axes of rotation of the wheels 13 similarly disposed on the adjacent stands 12. The centres of rotation of the wheels 13 in each stand 12 are arranged on an imaginary arc the highest point of which is on the vertical axis of symmetry of the stand. The wheels 13 are set in rotation with the aid of a drive 14 through bevel gears 1 5 and idle gears 16.

In order to be able to set the apparatus of the invention in an inclined position the chambers 1, 2, 3 are mounted on a frame 17 which bear against a mechanism 1 8 and is pivoted to a foundation at its opposite end.

The metallizing chamber 1 can be constructed so as to accommodate one or a plurality of metal evaporators 6. To vary the speed of rotation of the pipes 4, each stand 12 is provided with a means enabling it to rotate about its vertical axis of symmetry.

The apparatus of the invention operates in the following manner.

The pipes 4 prepared for coating and filled with a metallizing agent are charged into the storing device 9 incorporated in the chamber 2. The lower row of pipes is positioned on the wheels 13. Next, the chambers 1, 2, 3 are sealed and then evacuated by means of vacuum pipes to a residual pressure of from 10-i to 1 0-8 mm.Hg.

The metallizing agent disposed within the evaporators 6 is heated by means of the heating elements 7 to a temperature of active evaporation. When the metallizing agent has reached a required temperature, the vacuum gates 5 being open, the drive 14 of the transporting means 11 is actuated. As this happens the pipes 4 positioned on the wheels 13 are rotated as they are concurrently transferred from the chamber 2 to the chamber 1. While passing over the evaporators 6 the pipes 4 are heated and the external surfaces thereof are coated with the evaporating metallizing agent.

The metallizing agent disposed inside the pipes 4 is heated by the heat conducted through the pipe walls, whereupon it melts down to spread out along the interior surface of the pipe 4. The temperature of the pipes 4 is regulated by means of the baffles 8. After passing through the metallizing chamber 1, the pipes 4 enter the chamber 3 wherein they are removed from the wheels 13 by means of another grid-like storing device 9. To enable coating of the next batch of the pipes 4, the storing device 9 incorporated in the chamber 2 is brought one step down. After all the pipes 4 have been coated, the vacuum gates 5 are closed, and the next quantity of pipes 4 to be coated is fed into the chamber 2, while the coated pipes 4 are discharged from the chamber 3.

To vary the speed of rotation of the pipes 4 while maintaining their speed of forward motion constant, or to vary the interaxial distance between the pipes 4 of variable diameter, the angle a of rotation of the stands 12 can be changed (Figure 4) by rotating the stands about their vertical axes of symmetry. Where metallizing agent for the coating process is used in the form of lumps of metal, or where it becomes necessary to regulate the thickness of the coating layer inside the pipe 4, the entire apparatus is tilted to an angle of up to 150 by means of the mechanism 1 8, so that the leading ends of the pipes 4 are raised in relation to their trailing ends.

The present invention can produce pipes suitable for application in various heatexchanging equipment used in suiphuric-acid production, where sulphuric acid is disposed inside the pipes and running water outside. A diffusion coating of silicon, resistant to sulphuric acid, is applied to the interior surface of the pipe, and a diffusion coating of chromium to the exterior surface. To obtain these types of coatings a 45-percent alloy of silicon with iron is fed into the interior of the pipe, whereupon the pipe undergoes coating above an evaorator containing chromium. The coating process is conducted at a temperature of 13500C.Within a period of five minutes there forms on the exterior surface a layer of chromium 0.15 to 0.20 mm thick, with the concentration of chromium on the surface being of the order of 40 to 47 per cent. A diffusion layer of silicon is formed on the interior surface which is 0.25 to 0.30 mm thick with the concentration of silicon on the surface ranging from 17 to 25 per cent.

The invention also lends itself readily for application in the production of boiler tubes for heat surfaces. In this case a diffusion chromium coating is applied to the exterior surfaces of the tubes The coating process is conducted at a temperature of 1250 to 1 3500C for a time period lasting from 6 to 7 minutes. During this time a layer of chromium 0.15 to 0.20 mm thick is formed on the surface. After undergoing heat treatment, the coated tubes have a structure and mechanical properties similar to those possessed by uncoated tubes.

Claims

1. A method for applying protective coatings to metal pipes, wherein a metal is fed into the interior of each pipe, with the melting temperature of said metal being substantially below the melting temperature of the pipe, and a diffusion metal coating process is carried out on the outside of the pipes in a vacuum chamber with a metal evaporator at a temperature above the melting temperature of the metal in the interior of each pipe, the pipes being continuously rotated about their axes while axially advancing within said vacuum chamber through said metal evaporator.

2. A method as claimed in claim 1, wherein the melting temperature of the metal being evaporated is above the melting temperature of the metal fed into the interior of each said pipe.

3. A method as claimed in claim 1 or claim 2, wherein the metal coating process is carried out at a temperature above the melting temperature of the metal being evaporated.

4. A method as claimed in any preceding claim, wherein in the course of the metal coating process the leading end of each said pipe is raised relative to the trailing end thereof so that its longitudinal axis forms an angle of not more than 1 50 with the horizontal plane in the travelling direction of said pipes.

5. An apparatus for applying protective coatings to metal pipes, comprising: a vacuum chamber with a metal evaporator; chambers for the delivery and holding of pipes, both chambers being provided with vacuum gates and storing devices and arranged one each side of said vacuum chamber; means for transporting said pipe through said chambers, said means comprising wheels similar in diameter and fixed in stands spaced apart from one another along the travelling path of the pipes, the axes of rotation of said wheels in each of said stands being parallel to one another and coinciding with the axes of rotation of the similarly arranged wheels in other stands, the centres of rotation of said wheels in each said stand being arranged on an imaginary arc with the centre thereof being farthest from the horizontal plane in which said metal evaporator is arranged.

6. An apparatus as claimed in claim 5, wherein each of said stands is provided with a mechanism for rotating it about its vertical axis of symmetry.

7. A method for applying protective coatings to metal pipes, substantially as hereinabove described with reference to, and as shown in the accompanying drawings.

8. An apparatus for applying protective coatings to metal pipes substantially as hereinabove described with reference to, and as shown in the accompanying drawings.