GB2055061A - Improvements concerning copper or copper-alloy tubes for pipes used in sanitary, condenser and other installations - Google Patents

Abstract

A process of making tubes of copper or copper-alloy for resistance to pin-hole corrosion caused by water or water carrying chemically aggressive constituents. The process comprises successive steps of an oxidation heat treatment to a strip of copper or copper-alley to achieve a carbon content reduction to a predetermined value, followed by surface cleaning treatment to remove loose, non-adhering deposits and finally welding the strip to make a tube with the cleaned surface forming the interior of the tube. The tubes have particular application for use in pipework installations of condensers and heat exchangers.

Description

SPECIFICATION Improvements concerning copper or copperalloy tubes for pipes used in sanitary, condenser and other installations This invention relates to an improved process of making copper or copper-alloy tubes for use in pipework installations for liquids such as hot or cold water or water containing chemically aggressive substances e.g. in sanitary engineering, central heating installations, condenser and heat exchanger circuits etc.

The invention also relates to the pipes made in accordance with said process and to their use in condensers and heat exchangers. In this Specification the term "water" is intended to include water containing chemically aggressive substances.

Copper and copper-alloy tubes are currently used on a wide scale for pipework in sanitary engineering as well as in condensers and heat exchangers fitted in electrical, thermal or nuclear plants, in petrol refineries, on board ships or in the chemical industries as well as in seawater purifying plant etc.

In all of these applications the generally good corrosion-resistance of copper and its alloys and their excellent thermal conductivity are of primary importance.

By different additions of alloying elements and, to a lesser degree, also by different mechanical or physical treatments it has generally been possibie to avoid certain types of corrosion which might attack pipes of this kind. A general study on this subject is contained in the article by C. BRECKON and J.R.T. BAINES, in l.Mar.E.1955 pages 1 to 10.

One particular type of corrosion takes the form of localised pinholes which may very quickly allow a refrigerant liquid to leak out and thus put a pipe out of service. This type of pinhole corrosion would seem to be attributable to the use of lubricating oils in the tube-drawing process which decompose and release carbon particles during subsequent heat treatment.

Various methods have therefore been proposed with a view to eliminating the drawing oil residues, to preventing the formation of carbon deposits from said oils, or to removing such deposits by mechanical treatment.

Thus, in order to keep the carbon level at or below 2mg/dm2 it is known to inject a degreasing substance such as trichlorethylene into the tubes, notably in coiled form, prior to heat treatment thereby to reduce the amount of residual oil which could cause carbon deposits. Following this injection of degreasing fluid a stream of water vapour is blown through the tubes to remove any residual traces of the solvent.

It was also proposed to lower the residual carbon content in the tubes after heat treatment by injecting at high speed a mixture of air and abrasive particles such as alumina or silicon carbide. However, this method cannot be used with coiled tubing and its application is strictly confined to straight lengths of tube.

In actual fact the velocity loss in the abrasive particle stream on impact against the curved walls of a coiled tube is too great for the particles to be able to remove carbon or other deposits evenly from the inside of coiled tubing.

Lastly, a residual carbon content of as little as 0.3mg/dm2 has been achieved by injecting the tubes, prior to heat treatment, with a gaseous mixture containing oxygen and inert gases.

During heat treatment in a furnace, generally in a reducing atmosphere, the oxygen reacts with the hydrocarbon residues remaining inside the tubes from the tube drawing process to form volatile gases such as carbon monoxide or carbon dioxide which are then easily expelled from the tube. In this process the oxygen content of the atmosphere which is injected into the tube is carefully controlled in order to avoid the formation or powdrous oxide deposits during subsequent heat treatment. It will be appreciated that this requires operative conditions which are very difficult to control in large scale industrial production.

It is obvious in fact that all these processes are particularly difficult to put into practical operation on an industrial scale for the production of large numbers of tubes, and that it is particularly difficult to accommodate such processes within a continuous copper or copper-alloy tube-making process.

Accordingly, the object of the present invention resides in avoiding the drawbacks appertaining to the known processes according to the state of the art, and more specifi cally in providing a simple and economical method of making copper or copper-alloy tubes for the above mentioned uses and applications.

This invention provides a process characterised by comprising at least the following successively applied stages: treating a strip of copper or copper-alloy in an oxidising atmosphere at such temperature and for such a period of time as to obtain, after heat treatment, a reduction in the carbon content to a predetermined level in order to avoid pinhole corrosion by water in contact with the tube in service; cleaning the surface to remove non-adhering deposits at least on one side of the strip and the edges thereof; and welding the strip by conventional methods to form a tube, with the previously cleaned surface forming the inside of the tube.

The carbon content is generally reduced to a value which is at least less than 2mg/dm2; in the majority of cases it is preferable to reduce the value to below 0.3mg/dm2 and even as low as 0.05mg/dm2.

The process according to this invention enables the direct achievement of carbon contents as low as 0.02mg/dm2 without the slightest problem due to the total accessibility of a strip material to an oxidising treatment, a result which cannot be obtained in the treatment of long tubes and particularly not of coiled tubes.

The skilled man will easily ascertain the right operative conditions, particularly the right choice of temperature and length of time for the oxidation treatment as well as the oxygen content in the gaseous mixture used in the treatment having regard to the technological parameters of the production process in order to arrive at least at the above indicated predetermined carbon level.

The advantage of proceeding in this fashion, as compared with interior oxidation of a tube, resides in the considerably higher degree of regularity and uniformity of the oxidation process. Actually, in the application of oxidation treatment to tubes one cannot achieve completely constant conditions for the compositions of gases and other conditions prevailing inside the tube because the combustion of residual drawing oil results in the formation of carbon monoxide and dioxide, the oxygen-containing gaseous mixture being introduced at one end of the tube and the combustion gases, mixed with the surplus amount of injection gas, being ejected at the other end of the tube. This conventional method must of necessity entail treatment irregularities due to the variable and uncontrollable oil concentration in the tubes and to irregular oxide deposits.

It will also be noted that the majority of all known processes in the art fail to achieve such low carbon contents as can be obtained by the process according to this invention, or to achieve a surface finish and adequate composition of the surface film which is comparable to what can be realised with the aid of the present invention. It may suffice to mention only the enormous difficulties and problems connected with achieving an effective interior sweeping or brushing action in a straight tube, or, still worse, in a bent or coiled tube.

For economical running of such plant it is particularly advantageous to produce the tubes in a continuous process and, for preference, the oxidation treatment and the preceding cleaning treatment would then also be continuously applied.

The oxidation treatment necessarily produces a layer of oxides on the strip surface which is not totally removed by brushing.

However, it has been found, in contrast with the previously held general opinion, that this layer of oxides is not necessarily harmful as long as it is not porous, crumbly and/or very loosely adhering to the metal therebeneath. In a most surprising manner it has been found that a treatment permitting the elimination of deposits of this nature and applied in succession with an oxidation treatment as described suffices to make a product of excellent quality wherein the problems of pinhole corrosion can be avoided even in the presence of highly aggressive water.

By a suitable choice of method for eliminating such deposits it is possible, depending on the desired end result, either to eliminate only the loose, porous and crumbly deposits which may have formed, or to achieve a proper scaling or scouring effect by eliminating loose deposits as well as virtually all adhering oxide deposits.

The said surface cleaning treatment may be applied by brushing, grit-blasting or scouring depending on the required surface finish of the end product.

Here again there is the advantage of treating a freely accessible surface over any width thereof whilst this kind of treatment is very difficult to apply to tubes of great length because of the problems connected with achieving proper circulation of the cleaning medium such as brush, sand or scouring liquid, within a tube. More particularly, it is virtually impossible to achieve effective gritblasting in tubes having a diameter larger than 22 mm or in tubes which have been annealed and heat treated in coiled form.

The above described surface cleaning treatment according to this invention can also be completely automated and applied continuously in succession with the oxidation treatment.

The process has the further advantage that the edges of the strip may be treated simultaneously with the flat strip surface which ensures clean edges, an indispensible prerequisite for the success of the ultimate welding operation.

The tube welding operation is carried out in conventional manner and with the aid of conventional, preferably continuously working equipment. A suitable welding process is notably described in the work "HERSTELLUNG VON ROHREN" edited by Verlag Stahleisen mbH Dusseldorf.

This process is notably applicable for the making of tubes corresponding to the characteristic parameters described in Patent Application No. 80.03284 of 10th September, 1980 of the United Kingdom proceeding in the name of the same Applicants.

The technological progress achieved by the present invention, as compared with the conventional methods of treating tubes after they have been made, resides in that it affords total control in respect of the conditions of the carbon content reducing treatment and the total or partial elimination of oxidic deposits.

This enables a product to be obtained which throughout the production consistently presents an adequate surface condition in that part of the tube which comes into contact with corrosive liquids. Moreover, the edges of the strip are cleaned perfectly at the same time and this improves the quality of the welded joints.

Finally, owing to the fact that the oxidation and cleaning treatments are applied to a plane and accessible surface, these treatments are not only improved in quality but also permit ready inspection and quality control which cannot be easily achieved, especially not visually, when applied to the interior of a tube.

Lastly it will be appreciated that the gases resulting from the oxidation treatment can be easily and continuously removed when the oxidation treatment is applied according to the present invention to a plane strip material, whereas previously oxidation treatment of a tube involves a considerable risk of condensation.

It will also be understood that a tube made in accordance with this invention may be subjected, after its forming following oxidation and cleaning treatment as hereinbefore described, to further treatment of any kind or nature which may be desirable to produce a tube which is suitable for the specific application for which it is designed.

Such further treatment may be of a purely mechanical nature, such as scraping or trimming off the internal welding bead, calibration etc., or of a physico-chemical nature such as annealing, or a conditioning process such as coiling after annealing etc.

Suitable alloys for treatment in accordance with this invention are: arsenical copper (99.40% Cu-As 0.40%;P 0.025%); phosphorous copper (Cu 99.9%; P 0.25%); brass 70/90 (70% Cu, 30% Zn); "Admiralty brass" 70/29/1 (70% Cu-29% Zn; 1 % Sn); brass 76/22/2 (76% Cu-22% An; 2% Al); cupronickel 90/10 (90% Cu-10% Ni, 1.4% Fe and 0.75% Mn); cupronickel 70/30 (70% Cu-30% Ni, 0.6% Fe and 0.75% Mn); cupronickel 70/30 with iron (30% Ni-1.5% Fe, 1.5% Mn); and aluminium bronze (95% Cu-6% Al).

This list comprises the principal alloys envisaged but is by no means exhaustive, the principle of this invention being generally applicable to the principal commercial alloys.

Tubes made in accordance with this invention are characterised by their constant quality. For equal quality their production costs are lower than those of conventionally made products. Moreover, they do not have the eccentricity typical for tubes made by a conventional extrusion process.

The duration of the oxidation treatment applied in accordance with this invention depends on the feed rate of the strip through the furnace; generally the time taken to pass through the reactive furnace zone is between 1 and 3 minutes, the temperature in the furnace ranging between 660 and 810"C.

These products may be used for any of the conventional applications of tubes of this type.

The invention is hereinafter more particularly described with reference to embodiments thereof given solely by way of illustrative examples without limitative character which illustrate in Example 1 the process according to this invention, and in Example 2 a continuous copper tube production process.

Example 1 The copper, or copper-alloy strip is delivered from the coil at the rate of approximately 21 metres per minute and travels continuously through a trichlorethylene bath which removes the main part (about 95%) of oil residues from the strip, The strip then travels continuously through a tunnel furnace wherein the central heat zone has a temperature between 660 and 810"C, heated by natural gas with an excess of air. At the furnace exit cutting tools remove the oxide over a depth of 0.25 mm along the lateral edges of the strip in order to provide a chemically clean metallic surface on the strip sides. The strip is then fed into nine cages comprising forming rolls whereby the strip is bent to tubular form.A copper coil having the dimensions 10 x 10 x 2 mm (square section) through which water flows at the rate of 3 litres per second and through which a high frequency alternating current (350,000 cycles per second) is conducted continuously applies heat to the lateral edges of the formed strip. At the same time, forming rolls of which the throat radius is the outside diameter of the tube being formed, roll the metal to give an axial displacement of 0.35 mm. The welded tube is finally cut to predetermined length by flying shears.

For obtaining a tube with a clean metallic, oxide-free internal surface, the above described oxidation treatment carried out in the tunnel furnace is followed by a brushing operation using bronze bristle (0.3 mm diameter) brushes driven at 1 20 revolutions per minute and travelling in an alternating movement in the axial direction over 1 2 mm.

For obtaining a tube with a clean metallic, oxide-free interior surface the above described oxidation treatment carried out in the tunnel furnace may also be accompanied by gritblasting with alumina particles of a grain size between 1 2 and 44 microns projected at a velocity of 4,5 m/sec.

For obtaining a tube with a clean metallic, oxide-free interior surface the above described oxidation treatment may be accompanied by a scouring or pickling treatment by continuously advancing the strip through an aqueous solution containing 25% sulphuric acid. Example 2 The copper strip is continuously cast with the metal being melted in a furnace of circular section lined with refractory and induction heated in order to achieve perfect homogenity in the metal. At the base of the furnace there is an opening of rectangular section which is 30 mm thick and the width of which equals the circumference of the tube to be welded plus 5 mm for each lateral edge. This rectangular section is surrounded by a double walled copper ring with cold water circulating between the two walls.The copper ring is lined with graphite pieces to provide adequate lubrication for the metal during its solidification. A mechanical system applies a traction force to the strip as it solidifies. The production speed of this strip is approximately 20 metres per minute. On leaving the above described plant the strip has a temperature of about 650"C and is introduced into a preheating furnace at 850"C, which is also induction heated. In the course of four passes the thickness of the strip is reduced from 30 mm to 4 mm by hot rolling and then further reduced to the final thickness (of the tube to be welded), for example 1 mm, by cold rolling. The lateral edges are then cut off by revolving blades, the width of trim being about 4 mm, and the strip is continuously fed to an annealing furnace. The strip is then subjected to the surface treatment as in Example 1, bent and finally welded to form a tube.

Claims (10)

1. A process of making tubes resistant to pinhole corrosion, characterised by comprising at least the following successively applied stages: treating a strip of copper or copper-alloy in an oxidising atmosphere at such a temperature and for a sufficient length of time to obtain, after heat treatment, a reduction of the carbon content to a predetermined level in order to avoid pinhole corrosion caused by water in contact with the tube in service; cleaning the surface to remove non-adhering deposits from at least one side of the strip and the edges thereof; and welding the strip by conventional methods to form a tube, with the previously cleaned surface forming the inside of the tube.

2. A process according to claim 1, characterised in that the carbon content is reduced to a value below 2mg/dm2.

3. A process according to claim 1, characterised in that the carbon content is reduced to a value below 0.3mg/dm2.

4. A process according to claim 1, characterised in that the carbon content is reduced to 0.02mg/dm2 or less.

5. A process according to any one of claims 1 to 4, characterised in that the oxidation treatment is applied for a period of time from one to three minutes at a temperature within the range 660"C to 810"C.

6. A process according to any of claims 1 to 5, characterised in that the surface cleaning treatment is brushing.

7. A process according to any of claims 1 to 5, characterised in that the surface cleaning treatment is grit-blasting.

8. A process according to any of claims 1 to 5, characterised in that the surface cleansing, treatment is scouring or pickling.

9. The process of making tubes according to claim 1 and substantially as hereinbefore described with reference to Examples 1 and/ or 2.

10. A welded tube resistant to pinhole corrosion made by the process according to any of claims 1 to 9.