WO1999016574A1 - An improved method of manufacturing an internally clad tubular product - Google Patents

Abstract

An improved method of manufacturing an internally clad tubular product employs a tubular host and a corrosion resistant tubular cladding member, the external diameter of the cladding member being slightly less than the internal diameter of the tubular host. The cladding member is inserted into the tubular host to form an assembly. A closed environment is formed between the exterior of the cladding member and the interior of the tubular host. Substantially all water and oxygen are extracted from the closed environment by evacuating the closed environment followed by filling with an inert gas, such as argon, the steps of evacuating and backfilling with an inert gas. Thereafter the assembly is heated to about 1850 °F to 2100 °F and the assembly is extruded to reduce the external diameter to the desired diameter, the extrusion serving to reduce the wall thicknesses of the host and cladding member and bond the cladding member to the host. In a most preferred method of practicing the invention, prior to the assembly of the cladding member into the interior of the host, either the exterior surface of the cladding member or the interior surface of the host receives a cladding activator, preferably a nickel phosphorous activator.

Description

AN IMPROVED METHOD OF MANUFACTURING AN INTERNALLY CLAD

TUBULAR PRODUCT

Reference to Pending Applications

This application is not related to any pending applications.

Reference to Microfiche Appendix

This application is not referenced in any microfiche appendix.

Background of the Invention Much of the world's energy involves the production, transportation and refining of crude oil that is inheritantly corrosive - sometimes referred to as "sour crude". Further, in the chemical industry, corrosive liquids and gases are frequently encountered that cannot be safely processed or transported utilizing normally available steel piping. For these reasons there exists a great demand for pipes or similar tubular products that can economically and safely convey corrosive liquids and gases.

To combat the effect of corrosiveness in liquids and gases, pipes can be formed totally of corrosion resistant materials, such as nickel alloys, stainless steel and so forth. However, corrosion resistant alloys are inheritantly expensive and typically may not have tensile strengths required for many applications. For this reason, the need exists for clad tubular products, that is, a tubular product wherein the base external material is of relatively strong but inexpensive carbon steel but wherein the internal surface which is contacted by liquids or gases flowing through the pipe is of a corrosion resistant material. In other words, a great need exists for internally clad tubular products and specifically for such products wherein the host or external portion of the clad product is of a high strength, relatively economical metal, such as steel, and the interior is clad with a corrosion resistant alloy. Such internally clad tubular products thus take advantage of economy and strength of the exterior component of the clad product and simultaneously the corrosion resistant capabilities of the typically more expensive internal cladding material. The concept of cladding tubular products is known. For background material relating to this disclosure, reference should be had to U.S. Patents 4,620,660 entitled "Method of Manufacturing An Internally Clad Tubular Product", issued on November 4, 1986 and 4,744,504 entitled "Method of Manufacturing A Clad Tubular Product By Extrusion", issued May 17, 1988. These two previously issued United States patents teach the basic concept of manufacturing an internally clad tubular product employing a tubular host and a tubular cladding member and are incorporated herein by reference. The present disclosure is directed towards improvements as revealed in these two previously issued United States patents. Further background information can be obtained from the references cited in these two United States patents.

Brief Summary of the Invention

This invention is an improved method of manufacturing an internally clad tubular product employing the basic concepts as taught in United States Patent 4,620,660 entitled "Method of Manufacturing An Internally Clad Tubular Product", and United States Patent 4,744,504 entitled "Method of Manufacturing A Clad Tubular Product By Extrusion", both patents are based on inventions by William C. Turner. The present method is an improvement over the basic concepts of these two previously issued patents which employs a reduced number of steps and, in its more refined aspect, includes more specific and definitive parameters by which superior tubular clad products are obtained.

The first step in the method is the insertion of a tubular cladding member into a tubular host to form an assembly. The tubular cladding member is of corrosion resistant material, such as a nickel alloy or a steel chromium-nickel alloy, that is, stainless steel as examples, it being understood that other corrosion resistant materials can be employed.

The external diameter of the tubular cladding member should be substantially equal to but slightly less than the internal diameter of the tubular host member so that the tubular cladding member can be telescopically positioned within the host member. The host member may typically be a steel or steel alloy tubular member and is of outside diameter that is typically significantly greater than the desired outside diameter of the finished product. Further, both the tubular host element and the tubular cladding alloy element are typically significantly shorter in length than the ultimately desired tubular product.

In a preferred arrangement the length of the tubular cladding member is slightly less than the length of the tubular host member when the host member is essentially of steel and the cladding member either nickel alloy or stainless steel, to compensate for the slightly increased growth in the length of the cladding member during heat up for the extrusion process as will be subsequently described.

After the tubular cladding member is telescopically received in the tubular host member to form an assembly, the next step is to provide a sealed environment between the exterior of the cladding member and the interior of the host member. This is best achieved by welding. A circumferential welding bead is applied at one end of the assembly to close one end of the annular space formed between the members. Communication must be established with the closed annular space and therefore the other end of the assembly is preferably equipped with an enclosure that affords communication, that is, the other end of the assembly is preferably closed by means of an air bag or a manifold, typically formed of relatively thin steel in a semi- toroidal shape providing an inner circumferential edge that is welded to the circumferential end of the tubular cladding member and an outside circumferential edge that is welded to the circumferential end of the tubular host member. To this manifold is attached a small diameter pipe affording communication with the interior of the manifold and thereby to the interior of the annular space between the exterior of the tubular cladding member and the interior of the tubular host.

After the attachment of the manifold, the next step in the manufacturing process is to carefully evacuate the annular space between the exterior of the tubular cladding member and the interior of the tubular host member of substantially all oxygen and water, that is specifically water vapor. This step is critical to the effectiveness of the method. For this purpose vacuum is applied to the manifold and thereby the annular space to extract oxygen and water vapor from the annular space. In order to reduce the oxygen and water level sufficient to achieve the objectives of this invention, it is preferable that a vacuum is applied to about 10^~3 millibars. Further, it is desirable in many instances to reduce the oxygen and water dew points to -60°F or below. To achieve this low level of oxygen and water presence in the annular space, an effective procedure is to first evacuate the annular space followed by refilling the annular space with an inert gas, preferably argon, and again evacuating the annular space. This sequence may be repeated more than once if necessary to achieve the required level of freedom of water and oxygen from the annular space.

Further, the effectiveness of evacuating the annular space of water and oxygen is greatly enhanced by heating the assembly to a temperature exceeding the boiling point of water, that is above 212°F. Most preferably, the evacuation of the annular space is successfully and efficiently accomplished by heating the assembly to at least 250°F to make certain that any water remaining in the annular space is in the form of vapor, that is steam, that is more effectively extracted by the application of vacuum to the annular space. After substantially all water and oxygen are removed from the annular space, it is sealed, that is, the piping connected to the manifold is sealed for the subsequent processing steps. In the preferred usage of this invention, sealing is affected with the space at a 10³ millibar vacuum.

The assembly is then heated to about 1850°F to 2100°F. This relatively narrow temperature range is important to the successful employment of the method and is a more limited critical range than has heretofore been employed in manufacturing internally clad tubular products.

After the assembly has been sealed, evacuated and heated, it is then extruded.

This is accomplished by inserting a mandrel into the interior of the tubular cladding member, which is done after the heating step, the mandrel being substantially the internal diameter of the tubular cladding member, after which the heated assembly is subjected to an extrusion process in the normal method of manufacturing extruded tubular products wherein the external diameter of the assembly is reduced and the length is extended while the internal diameter remains substantially the same. That is, the internal diameter of the initial tubular cladding member is substantially preserved by the mandrel while the external diameter is reduced. The reduction in external diameter results in an increase in the length of the assembly since the volume of the assembly remains the same. The extrusion results in a reduction in the wall thickness of both the internal cladding member and the tubular host member. The force of extrusion of the heated assembly causes the tubular cladding member to intimately bond with the tubular host member so that the yield strength with which the cladding member is secured to the host member approaches or exceeds the yield strength of the cladding member material.

An improved method of this invention includes the steps of first forming an intermediate internally clad tubular product employing the steps as herein above set forth wherein the first extrusion step results in a reduction of the external diameter of the assembly to a diameter that is greater than that of the ultimately desired tubular clad product. The intermediate internally clad tubular product can then be cut into two or more lengths and each length then subsequently subjected to an additional extrusion step to achieve an internally clad product having the ultimately desired outside diameter. In each extrusion step a mandrel is employed so that the internal diameters of the initial assembly, the intermediate assembly and the internally clad tubular product having the ultimately desired exterior diameter remain essentially the same. An intermediate product having an outside diameter exceeding the ultimately desired diameter can be cut into lengths to be again extruded without further concern regarding the annular area that initially existed between the exterior of the cladding member and the interior of the host member since the initial extrusion step fuses or joins these components to eliminate any further need of preserving an environment between the two elements, that is, the two elements no longer exist as independent elements. Stating this another way, after the initial extrusion step the intermediate product can be subsequently extruded without having a manifold secured to it.

The method as heretofore described can be practiced without the use of a liquid interface diffusion bonding material, i.e. a cladding activator. However, in many instances, the use of a cladding activator results in improved shear strength between the tubular cladding member and the host member and further, in most instances, the use of a cladding activator will improve the dependability of the process, that is, there will be fewer failures. Therefore, it is highly desirable in most all instances to apply a cladding activator to either the internal surface of the tubular host or the external surface of the tubular cladding member before these members are telescopically assembled. A highly preferred cladding activator is nickel phosphorous and specifically Ni4P. Another nickel phosphorous that will function, but normally not as effectively as Ni4P, is Ni6P. A combination of these activators may be employed.

The cladding activator is preferably applied utilizing electroless deposition, a plating process that is well known to practitioners in the art. The cladding activator is preferably applied in a very thin layer, usually not exceeding a thickness of about .002". When a cladding activator is employed the other steps as above set forth in practicing the invention remain the same, that is, the use of a cladding activator does not make unnecessary the formation of a closed environment and the extraction of substantially all the water and oxygen from the closed environment prior to the heating and extrusion steps. However, the use of a cladding activator will slightly increase the level of contamination by water or oxygen in the environment that can be tolerated to nevertheless receive an acceptable finished product.

A better understanding of the invention will be obtained from the following description of the preferred embodiments, taken in conjunction with the attached drawings.

Description of the Drawings

Figure 1 is an end view of an assembly for use in practicing the method of this invention for manufacturing an internally clad tubular product. Figure 1 is an elevational view of the end portions of a tubular host having received therein a corrosion resistant tubular cladding member, with a manifold welded to the ends of the tubular host and cladding member. The manifold provides a way of extracting water and oxygen from the annular space between the exterior surface of the cladding member and the interior surface of the tubular host.

Figure 2 is a fragmentary elevational cross-sectional view of the assembly as taken along the line 2-2 of Figure 1. Figure 3 is a fragmentary, highly enlarged view, taken along the line 3-3 of

Figure 1, showing a portion of one end of a tubular host member and one end of a tubular cladding member with a manifold welded in place to form a closed environment with the annular space between the exterior of the cladding member and the interior of the host member. Figure 4 is an enlarged cross-sectional view taken along the line 4-4 of Figure

1 showing a small pipe providing connection with the manifold by which the annular space between the exterior of the tubular cladding member and the interior of the tubular host member are evacuated of water and oxygen.

Detailed Description of the Preferred Embodiments

Referring first to Figures 1 and 2, the basic elements making up an assembly for use in practicing the method of this invention are illustrated. A tubular host member is indicated by the numeral 10 and is typically a carbon steel pipe. The metallurgical composition of host member 10 is preferably selected to provide the major portion of the strength required of the completed clad tubular product, that is, the strength required to resist the tensile force, burst pressure and so forth in the application for which the clad tubular member is intended. The tubular host has a first end 12 and a second end 14 and an internal tubular surface 16. The tubular host member has a wall thickness between internal tubular surface 16 and external surface 18 that is significantly greater than the wall thickness of the ultimately desired internally clad tubular product.

Telescopically positioned within tubular host 10 is a tubular cladding member 20 having a first end 22 and a second end 24. The external cylindrical surface 26 of cladding member 20 is of a diameter slightly less than the diameter of the host member internal cylindrical surface 16 so that the tubular host member is telescopically positionable within the interior of the tubular host member. Between the cladding member external cylindrical surface 26 and the host member internal cylindrical surface 16 is an annular area 28. Cladding member 20 has an internal cylindrical surface 30 that is substantially the internal diameter of the ultimately desired internally cladded tubular member to result from the manufacturing process. After the tubular cladding member 20 is telescopically positioned within tubular host 10, the next important step in the manufacturing process is to form a closed environment that includes annular area 28. This is effectively accomplished by closing the annular area 28 at the circumferential end thereof formed by tubular host second end 14 and tubular cladding member second end 24 which are preferably positioned in a common plane as shown in Figure 2. A weld bead 32 is formed circumferentially around tubular cladding member 20 to thereby close off one end of annular area 28.

The opposite end of the annular area is closed by means of a manifold generally indicated by the numeral 34. Manifold 34 is a toroidal shaped enclosure which, in cross-section, is generally U-shaped with a bite portion 36, a first leg portion 38 and a second leg portion 40 as shown in Figures 3 and 4. Manifold first leg 38 provides a circumferential end surface 42 that is welded by a circumferential weld bead 44 to tubular cladding member first end 22. In like manner, the manifold second leg portion 40 has a circumferential end surface 46 that is welded by a circumferential weld bead 48 to tubular host first end 12. With manifold 34 welded in place, a closed environment between the interior of the manifold and annular area 28 is formed.

To provide communication with annular area 28, a tube 50 is attached to the manifold. One method of accomplishing this is best shown in Figure 4 that shows an opening 52 formed in one wall of manifold 34, the opening receiving a tubular adapter 54 that in turn receives a tube 50.

In order to achieve a high yield strength fusion bond of tubular clad member 20 to host member 10, it is imperative that the annular area 28 separating these tubular members be substantially free of oxygen and water. To achieve this objective a high vacuum is applied to tube 50. While the application of a high level of vacuum for a sufficient length of time through tube 50 and manifold 34 to the annular area

28 may be sufficient in the normal practice of the invention more effort is required to attain the desired complete extraction of water and oxygen from the annular space. Therefore, after a high level of vacuum is applied through tube 50, in the preferred method of practicing the invention, the annular area is then backfilled with an inert gas that is passed through tube 50 and manifold 34 into annular area 28. While other inert gases can function successfully, the preferred inert gas is argon. After the annular area has been filled with argon, vacuum is again applied by way of tube 50 through the assembly to extract the argon and leave annular area 28 with a high level of vacuum therein. This procedure may be repeated as many times as necessary, that is, applying a vacuum, backfilling with an inert gas, and again applying a vacuum until substantially all traces of oxygen and water are removed from the annular space separating the host and cladding tubular elements. For effective bonding of the clad to the interior of the host, annular area 28 is preferably evacuated to about 10^"3 millibars.

In order to increase the efficiency of evacuation of oxygen and water, and particularly water, from annular area 28, the entire assembly as illustrated in Figures 1 and 2, is preferably heated to above the boiling point of water, that is, heated to a temperature exceeding 212°F. A preferred method is to heat the assembly slightly above this temperature to at least about 250°F. With the assembly at this elevated temperature, the steps of evacuating water and oxygen can be more efficiently and successfully completed.

After the evacuation steps have been completed, tube 50 is closed to maintain annular area 28 in a state that is substantially free of oxygen and water. While the method can be successfully practiced as heretofore described, improved yield strength of the bonding force between the cladding member 20 and host member 10 can normally be attained by the use of a cladding activator that is applied prior to the assembly of the cladding member into the host member. The cladding activator can be applied either to the exterior cylindrical surface 26 of cladding member 20 or to the interior cylindrical surfaces 16 of host member 10, or to both surfaces, however, the application of the cladding activator to only one of these surfaces is normally sufficient. In the practice of the method of this invention, the cladding activator is preferably a nickel phosphorous and most preferably Ni4P or Ni6P with Ni4P being the much preferred activator.

The cladding activator can be applied in a variety of ways, however, the preferred method is to employ electroless plating to either the exterior surface 26 of tubular cladding member 20 or the interior surface 16 of host member 10. The electroless plating system works very successfully. A relatively thin layer of cladding activator, indicated by the numeral 56 as seen in Figures 3 and 4, is required. As an example, cladding activator 56 need be applied only to a thickness of about .002".

After the assembly is completely formed as illustrated and as has been described, the next step in the method of manufacturing an internally clad tubular product is to heat the assembly to about 1850°F to 2100°F. It is important that this assembly be heated as uniformly as possible. After or before heating, a mandrel (not shown) is inserted into the assembly and the heated assembly is extruded in an extrusion mill that applies force to reduce the external diameter of the assembly and to thereby to lengthen the assembly. As the assembly is extruded and grows in length the external diameter decreases and the wall thickness of the cladding member 20 and host member 10 decrease proportionally. The extrusion process is carried out to attain the required external diameter of the finished product. In some instances the original external diameter of tubular host member 20 is substantially reduced to attain the finished product and, in other instances, the host member and cladding member as initially employed are more closely dimensioned to the ultimately desired external diameter of the finished product requiring less extrusion. In any event, the extrusion process must supply the necessary pressures between the cladding member and the host member to achieve fusion bonding which requires at least some reduction in the total exterior diameter of the tubular host.

Extrusion which results in bonding of the cladding member to the host member does not require that the ultimately desired external diameter be achieved in a single step. In some instances, it is desirable, after an initial extrusion step, during which the length of the tubular member is elongated in proportion to external diameter reduction, to cut the product resulting from the first extrusion into two or more pieces. These pieces can then be separately again put through an extrusion step to achieve the ultimately desired external diameter. After the initial extrusion step, fusion of the tubular cladding member has been accomplished and further extrusion steps can be employed without the use of a manifold or the evacuation steps as heretofore described since the first extrusion step results in the complete bonding of the cladding member to the host and subsequent extrusion steps function only to convert an already internally clad tubular member to the desired external diameter. The subsequent extrusion steps require heating of the assembly to an extrusion temperature but the temperature in some instances can be slightly less than that required for the initial extrusion step, that is, the subsequent extrusion steps can be carried out at slightly less than the prescribed range of about 1850°F to 2100°F. The tubular cladding member 20 may be seamless or may be a welded tubular product.

Extrusion can be carried out in commonly available steel mills such as by using an extrusion press and the entire method of manufacturing an internally clad tubular product as described herein can be accomplished without any significant or costly changes in tubular product manufacturing procedures except for the necessary additional steps enumerated herein above which do not require substantial or expensive modification of steel mill physical equipment.

The claims and the specification describe the invention presented and the terms that are employed in the claims draw their meaning from the use of such terms in the specification. The same terms employed in the prior art may be broader in meaning than specifically employed herein. Whenever there is a question between the broader definition of such terms used in the prior art and the more specific use of the terms herein, the more specific meaning is meant. While the invention has been described with a certain degree of particularity, it is manifest that many changes may be made in the details of construction and the arrangement of components without departing from the spirit and scope of this disclosure. It is understood that the invention is not limited to the embodiments set forth herein for purposes of exemplification, but is to be limited only by the scope of the attached claim or claims, including the full range of equivalency to which each element thereof is entitled.

Claims

What is claimed: 1. A method of manufacturing an internally clad tubular product employing a tubular host having an internal surface and a corrosion resistant tubular cladding member having an external surface comprising: (a) inserting the tubular cladding member into the tubular host to form an assembly; (b) forming a closed environment between the internal surface of the host and the external surface of the cladding member; (c) extracting substantially all water and oxygen from the closed environment; (d) heating the assembly to about 1850┬░ to 2100┬░F; and (e) extruding the assembly to metallically bond the external surface of the cladding member to the internal surface of the host member.

2. The method of manufacturing an internally clad tubular product according to claim 1 wherein said step of removing from said closed environment substantially all water and oxygen includes evacuating said closed environment followed by charging said closed environment with a dry inert gas followed by again evacuating said closed environment.

3. A method of manufacturing an internally clad tubular product according to claim 2 wherein said dry inert gas is dry argon gas.

4. A method of manufacturing an internally clad tubular product according to claim 3 wherein the steps of charging followed by evacuation are carried out at least two times.

5. A method of manufacturing an internally clad tubular product according to claim 1 including, during the step of removing water and oxygen from the assembly, of heating the assembly to a temperature of at least the boiling point of water.

6. The method of manufacturing an internally clad tubular product according to claim 5 wherein during said step of removing from said closed environment substantially all water and oxygen includes heating the assembly to about 250┬░F.

7. The method of manufacturing an internally clad tubular product according to claim 1 wherein said step of extruding said assembly includes conditions that reduce the wall thickness of said host and said tubular cladding member.

8. The method of manufacturing an internally clad tubular product according to claim 1 wherein said closed environment is evacuated to about 10^"3 millibars.

9. The method of manufacturing an internally clad tubular product according to claim 1 including before step (a) of: applying a cladding activator to either the internal surface of metallic tubular host or the external surface of the tubular cladding member.

10. The method of manufacturing an internally clad tubular product according to claim 9 wherein said nickel phosphorous is Ni4P.

11. The method of manufacturing an internally clad tubular product according to claim 9 wherein said nickel phosphorous is Ni6P.

12. The method of manufacturing an internally clad tubular product according to claim 9 wherein said cladding activator is electrolessly deposited onto either said internal surface of said metallic tubular host or the external surface of said tubular cladding member.

13. The method of manufacturing an internally clad tubular product according to claim 1 wherein said metallic tubular host product is formed of carbon steel.

14. The method of manufacturing an internally clad tubular product according to claim 1 wherein said tubular cladding member is formed of nickel alloy.

15. The method of manufacturing an internally clad tubular product according to claim 1 wherein said tubular cladding member is formed of stainless steel.

16. A method of manufacturing an internally clad tubular product employing a tubular host having a first end and a second end, comprising: (a) inserting the tubular cladding member into the tubular host, the cladding member having a first and second end, the tubular host and the tubular cladding member being of substantially the same length; (b) circumferentially welding the first end of the cladding member to the first end of the host member; (c) welding a circumferential manifold between the second end of said tubular host and the second end of the tubular cladding member to form an assembly having a closed environment between the interior of said tubular host and the exterior of said tubular cladding member; (d) extracting substantially all water and oxygen from said closed environment; (e) heating the assembly to at least about 1850┬░F; and (f) extruding the assembly to metallically bond the external surface of the tubular cladding member to the internal surface of the tubular host member.

17. A method of manufacturing an internally clad tubular product according to claim 14 wherein step (d) is carried out employing communication with said manifold.

18. A method of manufacturing an internally clad tubular product according to claim 14 in which said manifold is formed of carbon steel.

19. A method of manufacturing an internally clad tubular product according to claim 15 in which said manifold is formed of metal material of thickness of about .05 inches.

20. The method of manufacturing an internally clad tubular product according to claim 14 wherein said step of removing from said closed environment substantially all water and oxygen includes evacuating said closed environment followed by charging said closed environment with a dry inert gas followed by again evacuating said closed environment.

21. A method of manufacturing an internally clad tubular product according to claim 18 wherein said dry inert gas is dry argon gas.

22. A method of manufacturing an internally clad tubular product according to claim 19 wherein the steps of charging followed by evacuation are carried out at least two times.

23. A method of manufacturing an internally clad tubular product according to claim 14 including, during the step of removing water and oxygen from the assembly, of heating the assembly to a temperature of at least the boiling point of water.

24. The method of manufacturing an internally clad tubular product according to claim 21 wherein during said step of removing from said closed environment substantially all water and oxygen includes heating the assembly to about 250┬░F.

25. The method of manufacturing an internally clad tubular product according to claim 14 wherein said step of extruding said assembly includes reducing the wall thickness of said tubular host and said tubular cladding member.

26. The method of manufacturing an internally clad tubular product according to claim 14 wherein said closed environment is evacuated to about 10^"3 millibars.

27. The method of manufacturing an internally clad tubular product according to claim 14 including prior to step (a) the step of applying a cladding activator to either the interior surface of said tubular host or the exterior surface of said tubular cladding member.

28. The method of manufacturing an internally clad tubular product according to claim 27 wherein said cladding activator is nickel phosphorous.

29. The method of manufacturing an internally clad tubular product according to claim 29 wherein said nickel phosphorous is selected from one of Ni4P and Ni6P.

30. The method of manufacturing an internally clad tubular product according to claim 27 wherein said cladding activator is applied by electroless deposition.

31. The method of manufacturing an internally clad tubular product according to claim 30 wherein said cladding activator is applied to a thickness of about .002 inches.

32. The method of manufacturing an internally clad tubular product according to claim 16 wherein steps (a) through (f) provide a clad mother pipe of external diameter greater than that of an ultimately desired outside diameter of the internally clad tubular product, including the additional steps of: (g) cutting the internally clad mother pipe into a plurality of intermediate internally clad pipes; and (h) extruding the intermediate internally clad pipes to obtain internally clad tubular products of the ultimately desired outside diameter.

33. The method of manufacturing an internally clad tubular product according to claim 32 wherein the tubular cladding member as employed in step (a), the mother pipe as produced by step (f), the intermediate internally clad pipes obtained by step (g) and the internally clad pipes having the ultimately desired diameter obtained by step (h) each have substantially the same internal diameter.

34. The method of manufacturing an internally clad tubular product according to claim 32 wherein at least one of the externally clad pipes resulting from step (h) is cut into a plurality of lengths that are subjected to an additional extruding step to obtain internally clad tubular products having the ultimately desired outside diameter.