METHOD OF MANUFACTURING TUBES

Abstract: In the manufacture of tubes provided with elevations or recesses, an extrusion billet is preformed (1) and compacted (2) by isostatic pressing whereupon the extrusion billet is extruded (3) into a tube. The extrusion billet consists of at least two materials, forming material (21) and tubular material (22). The forming material is applied to or inserted into a preformed body of tubular material, or inversely, before the body is compacted into an extrusion billet. Alternatively, tubular material and forming material are supplied in powdered state to a mould and are jointly compacted into an extrusion billet. During the extrusion, the forming material forms elevations or recesses in the tubular material. After the extrusion, the forming material is removed (4), preferably by pickling.

Description: Method of manufacturing tubes TECHNICAL FIELD The invention relates to the manufacture of tubes, especially tubes having elevations or recesses. More specifically, the invention relates to the manufacture of tubes in which an extrusion billet, consisting of a tubular material and a forming material, is preformed using powder- metallurgical methods. BACKGROUND ART In the manufacture of tubes by extrusion, the tubes are normally provided with elevations or recesses in the form of channels, grooves, fins, flanges or openigns by mechanical treatment and/or welding in a step following the extrusion. Certain types of elevations or recesses may also be achieved directly during the extrusion. This is true, for example, with grooves or fins which are substantially oriented in the direction of extrusion, that is, above all axially directed and slightly helical grooves or fins . On the other hand, the prior art does not provide for the possibility of extruding tubes with elevations or recesses which are substantially directed in the direction of extrusion. SUMMARY OF THE INVENTION According to the invention, an extrusion billet consisting of at least two materials, tubular material and forming material, is preformed by inserting a forming material into or applying a forming material onto the tubular material and jointly compacting the materials into an extrusion billet so that, during the extrusion, the desired elevations or recesses are formed in the tubular material. After the extrusion, the forming material is removed. The method is especially applicable to the manufacture of corrosion- resistant tubes in which material with a low corrosion resistance may be inserted as forming material in a corrosion-resistant tubular material and be easily removed by pickling after extrusion. The extrusion billet with insertions of the forming material is preformed, according to one embodiment of the invention, by filling the powdered forming material into slots on a starting billet of the tubular material, which has been pro¬ vided with relevant slots, and compacting the powdered forming material and binding it to the starting billet by isostatic pressing, preferably hot-isostatic pressing. Inversely, it is, of course, possible for the forming material to be preformed and for the tubular material to be filled in powdered state around the forming material . The starting billet may be produced in conventional manner by mechanical treatment or by powder-metallurgical pressing, preferably isostatic pressing, or combinations thereof. According to an alternative embodiment, the extrusion billet is manufactured directly from at least two powdered mate¬ rials, one constituting the forming material and the other the tubular material, by compacting the powdered materials into a compound body, with insertions of the forming material in a matrix of the tubular material. The materials may be placed adjacent to each other or be separated by a partition, for example in the form of a sheet. The billet is then compacted by means of conventional powder- metallurgical methods, preferably by hot-isostatic pressing. After the extrusion billet with insertions has been formed, it is extruded in conventional manner, whereupon the inserted forming material is removed, preferably by pickling. With extrusion billets manufactured according to the invention, the possibility is obtained of manufacturing tubes with elevations or recesses in the form of channels, grooves, fins, flanges or openings, regardless of the orientation thereof, without any subsequent mechanical treatment or welding of the extruded tubes being required. By applying the invention, tubes containing elevations and recesses may be manufactured essentially without expensive mechanical after-treatment. At the same time, the material yield is enhanced, which is valuable in the manufacture of high-alloy tubes or tubes of other expensive materials. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in greater detail with reference to figures and examples. Figure 1 illustrates feasible process steps in the manufacture of tubes according to the invention, and Figure 2 exemplifies a number of feasible forms of elevations and recesses. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows how, according to the invention, elevations and recesses in the form of channels, grooves, fins, flanges or openings, regardless of the orientation thereof, are achieved when manufacturing tubes by extrusion. According to the invention, a body is preformed (at 1) of at least two materials, a forming material being inserted into or applied to a tubular material. This body of tubular material with insertions/applications of a forming material is compacted (at 2) into an extrusion billet. The extrusion billet is extruded (at 3) in conventional manner into a tube, whereupon the forming material is removed (at 4) , preferably by pickling, thus obtaining a tube with the desired recesses or elevations. During and after the described process for the manufacture of billets and the tube extrusion, conventional measures (not specified) are, of course, taken, for example to secure and check the quality of the produced tube or to obtain the desired material properties. For example, measures such as surface treatment, heat treatment, adjustment of dimensions and shapes by mechanical treatment may be taken both between the process steps described above and after these. In the manufacture of the extrusion billet used for the extrusion, according to one embodiment of the invention the forming material is inserted into or applied to a preformed starting billet of tubular material, the powdered material being filled into recesses provided in the starting billet, or applied to even out elevations provided on the starting billet so that, after compaction at 2, an extrusion billet is obtained. Inversely, the tubular material may be inserted into or applied to a preformed starting billet of tubular material. Preferably, the insertion or application of tubular material or forming material takes place by placing the starting billet in a cylindrical capsule, whereupon the capsule and any recesses provided in the starting billet are filled with the powdered forming material . Thereafter, the capsule is evacuated and sealed essentially gas-tightly before the capsule, containing the starting billet and the powdered forming material or tubular material, is compacted at 2. Before the starting billet is placed in the capsule, it has been preformed in an arbitrary manner, it then being provided with elevations or recesses, for example by conventional mechanical treatment but preferably by being preformed by powder-metallurgical pressing, suitably isostatic or semi-isostatic pressing. The preformed body with insertions/applications of forming material is then compacted at 2, preferably by hot-isostatic pressing, but also cold-isostatic or semi-isostatic pressing with sintering provides useful extrusion billets. The extrusion billet is then extruded at 3 into a tube, whereupon the forming material is removed at 4. Alternatively, according to a preferred embodiment, at least two powder materials may be supplied to a mould and be directly formed, at 3, into a green body with inser¬ tions/applications of a forming material in a matrix of a tubular material. This green body is compacted at 2 in the manner described above, preferably by hot-isostatic press¬ ing, into an extrusion billet which is then extruded at 3. After the extrusion, the forming material is removed at . Figure 2 shows a number of possible extrusion billets in which a forming material 21 has been inserted into a matrix of a tubular material 22 in order to form, during the extrusion, the tubular material 22 with elevations and/or recesses in the form of channels, grooves, fins, flanges or openings . EXAMPLE Tubes included in heat- or power-generating plants as well as in chemical reactors are required to effectively transfer heat between the media on the different sides of the tube walls . To improve the heat transfer these tubes are often provided with grooves or fins. These fins or grooves are often oriented in a direction deviating from the axial direction, for example spirally oriented. The technique according to the invention is advantageously applied to the manufacture of these tubes, the tubular material normally consisting of a corrosion-resistant material which is shape- resistant to high temperatures, such as, for example, an alloyed steel containing 12 - 30% Cr, and the forming material of an ordinary low-alloy steel.

Claims: CLAIMS 1. A method of manufacturing tubes having elevations or recesses, an extrusion billet being preformed by isostatic pressing and the extrusion billet then being extruded into a tube, characterized in that the extrusion billet is preformed (1) from at least two materials, tubular material (22) and forming material (21), that the forming material and the tubular material are jointly compacted (2) into an extrusion billet, the forming material, during the extrusion (3) , then forming elevations or recesses in the tubular material, and that the forming material is removed (4) after the extrusion. 2. A method according to claim 1, characterized in that the inserted forming material (21) , after the extrusion, is removed (4) by pickling. 3. A method according to claim 1 or 2, characterized in that a powdered material is applied to or inserted into a preformed body, the preformed body consisting of tubular material (22) and the powder of forming material (21) , or inversely, and that the preformed body with applied/inserted material is compacted (2) , preferably by hot-isostatic pressing, into an extrusion billet. 4. A method according to claim 1 or 2, characterized in that tubular material (22) and forming material (21) are supplied in powdered form to a mould and are jointly compacted (2) , preferably by hot-isostatic pressing, into an extrusion billet.

Inventors: SALWEN ANDERS, SALWEN, ANDERS
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1991
Main classification: B21C23
IPC codes: B21C23, B21C37, B22F5, F28F1, F28F21