Technical field

[0001 ]The present invention relates generally to a method for manufacturing of seamless metal tubes as well as seamless tubes manufactured with the method. In particular the invention relates to an improvement in the

manufacture of hollow bodies to be used as an intermediate in the manufacture of seamless tubes.

Background

[0002] Generally it is known in the prior art how to manufacture seamless tubes.

[0003]For instance WO 2006/045301 and EP 1814679 disclose a method starting with a thick-walled hollow block which is formed from a massive block. The hollow block is then rolled and forged in several steps to a tube of the desired dimensions. The massive block is pierced to produce a thick-walled hollow block. This piercing step is a problem in the art. It is both complicated and expensive. Especially high alloyed materials as stainless steel and nickel alloys gives difficulties at piercing.

[0004] US 5,540,882 discloses a method for powder metallurgical manufacturing of a body having a through hole, for example a hollowed tool blank or thick-walled tube, providing a tube in an outer capsule, so that there is formed a space between the tube and the capsule filling the space between the tube and the capsule with a metal powder for forming a desired body closing the capsule hermetically compacting the closed capsule and its contents using hot isostatic compaction at a temperature exceeding 1000°C, so that the metal powder is compacted to complete density. The method further comprises the step of hot working the capsule through at least forging or rolling.

[0005]Although it is known how to manufacture for instance tubes from metal powder there is room for improvement, in particular for large objects it is desired to provide a more economical method. [0006]Thus it is a problem how to improve the manufacturing of the seamless tubes from powder and make it more economical.

Summary

[0007] It is an object of the present invention to obviate at least some of the

disadvantages in the prior art and provide an improved method of

manufacturing a seamless metal tube.

[0008] In a first aspect there is provided a method for the manufacture of a

seamless metal tube comprising the steps of: a. providing a capsule having the shape of a hollow block,

b. filling the capsule with a metal powder,

c. subjecting the filled capsule to hot isostatic pressing to obtain a density of 95-98% of TD to obtain a hollow block,

d. working the hollow block into a finished seamless tube in one or more steps, during which step(s) the density is increased to full density, whereby the cross sectional area of the hollow block is decreased at least with a factor 2.5.

[0009] In a second aspect there is provided a seamless tube manufactured with the method described above.

[0010]One advantage is that the seamless tube is simple and inexpensive to

manufacture, not least since the required time in the hot isostatic press is reduced because full density is not aimed for during the hot isostatic pressing.

[001 1 ]Another advantage is that full density can be guaranteed because of the densification after the hot isostatic pressing where the cross sectional area is reduced at least 2.5 times.

[0012]Another advantage is that the starting material for the further shaping has a fine grain structure compared to the traditional cast material with a coarse grain structure which is more sensitive to cracks. [0013]A further advantage is that a capsule protects the powder and material inside the capsule, for instance from oxidation but also from cracking. The capsule serves to protect the material from cracking since the capsule takes up strain from subsequent shaping and forging steps. The practice today is to use ingots as starting feed stock, which usually exhibits a coarse cast structure sensitive to cracks and also show lower hot ductility than a corresponding powder body.

[0014] Using this method it is possible to manufacture tubes of compound

materials, as well as tubes of alloys which are otherwise difficult to make tubes of. Tubes for high temperature applications are also possible to manufacture.

[0015]The invention provides a desired possibility to obtain a starting material for the manufacture of seamless tubes, which material is free or essentially free of cracks and also has such a homogenous composition of the metal structure that it does not lead to cracks later in the operation steps.

Detailed description

[0016]Before the invention is disclosed and described in detail, it is to be

understood that this invention is not limited to particular compounds, configurations, method steps, substrates, and materials disclosed herein as such compounds, configurations, method steps, substrates, and materials may vary somewhat. It is also to be understood that the terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting since the scope of the present invention is limited only by the appended claims and equivalents thereof.

[0017] It must be noted that, as used in this specification and the appended claims, the singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise.

[0018] If nothing else is defined, any terms and scientific terminology used herein are intended to have the meanings commonly understood by those of skill in the art to which this invention pertains. [0019]The term "cross section" as used throughout the description and the claims denotes an intersection of the hollow block and a plane. The plane is perpendicular to the extension of the tube. The cross sectional area is only the intersection where it comprises material, i.e. the middle hollow part of the hollow block is not a part of the cross sectional area.

[0020]There is disclosed a way to prepare a hollow billet for the further shaping steps, such as for instance hot forging steps, which can for example consist of an apparatus which has an inner mandrel and two hammers which are operating towards each other and which are forging out the tube successively against the mandrel to a longer tube with a thinner wall. During the forging operation the hollow billet is turned/rotated so each part of the circumference is subjected to the same deformation/shaping.

[0021 ]This technique requires first of all that the ingoing hollow block from the first part of the operation is free of cracks and also have such a homogenous composition of the metal structure that it does not lead to cracks later in the operation steps. The invention aims at providing such an ingoing hollow block.

[0022]The above technique described is suitably used for production of tubes with big diameters and/or heavy wall thicknesses. For smaller dimensions typically diameters under about 250 mm the normal route is conventional extrusion, or hot rolling according to the so called Mannesmann process. However it cannot be ruled out that for special cases the present method can be used also for these tubes, for instance if it is desired to manufacture the tube of a special material where it is suitable to start with metal powder.

[0023] In a first aspect there is provided a method for the manufacture of a

seamless metal tube comprising the steps of: a. providing a capsule having the shape of a hollow block,

b. filling the capsule with a metal powder,

c. subjecting the filled capsule to hot isostatic pressing to obtain a density of 95-98% of TD to obtain a hollow block, d. working the hollow block into a finished seamless tube in one or more steps, during which step(s) the density is increased to full density, whereby the cross sectional area of the hollow block is decreased at least with a factor 2.5.

[0024]The hole in the hollow block is a through hole. In one embodiment the

hollow block is shaped as a tube with very thick walls.

[0025]The density is not 100 % of Theoretical Density (TD) after the hot isostatic pressing, but instead in the interval 95-98% of TD. This saves time and thereby cost regarding the hot isostatic press. The density is increased in the next at least one densification steps to full density, i.e. close to 100% TD or as close to 100%TD as it is practical to achieve. It has been discovered that by decreasing the cross sectional area of the hollow block by a factor of at least 2.5 times after H IP the desired density, i.e. full density can be reached. The density obtained after such a compaction is close to 100% TD, or as close as it is possible to get in practice.

[0026] In one embodiment the capsule comprises an inner tube, an outer tube and two end caps, wherein the end caps are massive, i.e. not hollow. In one embodiment at least one of the inner tube and the outer tube is a spiral welded tube. In one embodiment the outer tube is a spiral welded tube. This embodiment has the advantage that the probability for crimps is reduced.

[0027] In one embodiment the capsule is adapted to forging the hollow block. The capsule remains after step c) so that the capsule is still on the hollow body when the shaping starts.

[0028] In one embodiment the inner tube comprises at least one bellow shaped section. In certain embodiments, depending on the material used for the capsule itself, the material inside the capsule, their temperature extension coefficients and so on it may happen that the inner part of the capsule is heated slower than the outer part during the HIP. If this is the case it may lead to that the inner part of the capsule is subjected to such a stress that it cracks. A solution to this problem is to use an inner part of the capsule with one or more bellow shaped section which can be subjected to strain and expand so that the capsule does not crack. Such an embodiment can be realized by using an inner tube in the capsule comprising at least one bellows shaped section.

[0029] In one embodiment the metal powder comprises carbon steel. In another embodiment the metal powder comprises stainless steel. An advantage is that the capsule can be filled with any metal powder or mixture of different metal powders and also additional additives. Tubes of stainless steel are considered to be difficult to manufacture using methods according to the state of the art.

[0030] In one embodiment the metal powder comprises at least one oxide which oxide has a melting point higher than the melting point of the elemental metal in the metal powder. In one embodiment the oxide is at least one selected from the group consisting of aluminum oxide and zirconium oxide. Such an addition greatly improves the high temperature properties of the material and provides an excellent tube for high temperature applications. In one embodiment the oxide has a melting point at least 100°C higher than the metal powder, wherein the oxide is stable at the melting point of the metal powder, and wherein the oxide does not react with the metal powder at the melting point of the metal powder. In one embodiment the oxide is a metal oxide. In one embodiment the oxide is the form of a powder with an average grain size smaller than 1 μιη.

[0031 ] In one embodiment the filled capsule is subjected to cold isostatic pressing before step c). This step will make the process more economical by subjecting the material to CIP (cold isostatic pressing) before HIP (hot isostatic pressing). Then the density of the material increases before HIP and the heating time during the HIP process will be shorter, which gives a more economical process.

[0032]The density after step c) is in the interval 95-98 % TD, i.e. the density is less than 100% of TD after step c), the material is not compacted to full density. This makes the process more economical since the time in the H IP (hot isostatic press) can be reduced. In particular for large objects this is important. Examples of large objects include tubes and similar object. The density after step c) should be 95% of TD or greater so that it is gas tight, however it should not be more than 98% of TD after step c). First this will reduce the time for the HIP which will make the process more economical. The shaping using for instance forging in the following step will increase the density to full density, i.e. 100% of TD or close to 100% of TD. The density and quality of the part is achieved by compacting so that the cross sectional area of the hollow block after H IP is reduced by a factor of 2.5. Any small porosity in the hollow body will disappear during subsequent shaping and forging. After the final densification the density will be full density and a skilled person knows how this desired full density can be reached for different materials and different shaping methods. As an additional precaution during shaping and forging there is the capsule which protects both against oxidation and cracking in difficult materials.

[0033] In one embodiment at least one of the ends of the hollow block is cut off. In many embodiments an end part of the tube has to be cut of and discarded, a part can be cut off from at least one of the ends of the finished tube.

[0034] In one embodiment the hollow block is heated before the one or more steps in step d). After the HIP the material in the hollow block is hot, and in one embodiment this heat is utilized so that the hollow block does not have to be heated so much before the shaping steps. This makes the process more economical. It is of course also possible to let the hollow block cool down after HIP and then start the subsequent shaping.

[0035] In one embodiment step d) comprising forging. Forging includes for instance hot forging.

[0036] In one embodiment step d) comprises radial forging in which an internal tool (mandrel) is inserted into the hollow block and at least two forging jaws act on the outer surface of the hollow block, wherein the hollow block is rotated and axially displaced during the idle stroke phase of the at least two forging jaws. In one embodiment the forging jaws are working symmetrically and simultaneously on the hollow block so that the net force exerted on the mandrel is minimized.

[0037] In one embodiment the capsule is removed after step d) with the aid of at least one acid, preferably sulfuric acid. This is particularly suitable for steel with high corrosion resistance such as stainless steel.

[0038] In one embodiment AI2O3 is added between the capsule and the metal

powder before the hot isostatic pressing and wherein the capsule is removed mechanically after step d). In one embodiment AI2O3 is added on the inside of the capsule before it is filled with metal powder. This addition of AI2O3 is different from adding AI2O3 to the entire powder mixture in order to improve the high temperature properties of the finished tube. The addition of a layer of AI2O3 inside the capsule is suitable for instance when manufacturing a tube comprising steel.

[0039] In a second aspect there is provided a seamless tube manufactured with the method described above.

[0040]AII the described alternative embodiments above or parts of an embodiment can be freely combined without departing from the inventive idea as long as the combination is not contradictory.

[0041 ]Other features and uses of the invention and their associated advantages will be evident to a person skilled in the art upon reading the description and the examples.

[0042] It is to be understood that this invention is not limited to the particular

embodiments shown here. The embodiments are provided for illustrative purposes and are not intended to limit the scope of the invention since the scope of the present invention is limited only by the appended claims and equivalents thereof.