MEANS OF EXPLOSION WELDING

The present invention relates to a method for mutually attaching at least two metal tubes disposed within each other by means of explosion welding, wherein a casing of explosive material is arranged around the outer tube and this casing is being made to explode to establish a metallurgical connection between the two

5 tubes, wherein the interior of the inner tube is filled with a certain material before the

casing is being made to explode.

WO 2011/053124 A1 discloses a method as described above, wherein, before the explosive material is being made to explode, the inner tube is filled with a

10 granular material, after which the space in the tube left free by the granular material

is filled with a liquid.

The material that the inner tube is filled with, forms an anvil that the inner tube is hit against upon the explosion, by which a connection to the outer tube is obtained. 15 As compared to a method in which the inner tube is only filled with a liquid, the

application of granular material in combination with liquid has the advantage that certain undesired deformations that may occur upon the explosion especially in longer tubes, remain limited.

20 It has appeared that the quality of the inner surface of a tube obtained with the known method leaves things to be desired sometimes. Then, the inner surface is rough to such an extent, that when the tube is stretched to a greater length, a risk of crack formation exists. Roughness of the inner surface may also lead to other

undesired situations, such as an increased risk of ignition of an inner tube layer out of

25 zirconium in case that certain substances are fed through a tube having such an

inner layer.

An objective of the present invention is to solve, or at least alleviate the above mentioned problems. In particular, an objective is to provide a method according to 30 the introductory paragraph, wherein the tube that is eventually obtained, has

relatively good surface quality. At least one of these objectives is reached by a method wherein the material that the inner tube is filled with, comprises relatively small, hard spheres.

The invention is based on the insight that the roughness of the inner surface of the tube obtained with the known method results from a print that is left by the granular material inside the inner tube on the inner surface thereof. By the invention, a roughness that is too high to an extent is avoided by the relatively small, hard spheres leaving as a print at the most a collection of relatively small, smooth dents, instead of a collection of pointed indentations.

An additional advantage of a tube having a relatively high surface quality, such as one that can be produced by a method according to the invention, is a relatively long life. As a preferred material for the spheres, the invention foresees a ceramic.

Ceramic has been found in practice to be excellently suitable for the application described here. This means, that the material per se can form spheres of sufficient hardness, that are small enough to fill to a sufficient extent an inner tube that is to be fused to an outer tube, such that the whole of the spheres can, at least in

combination with a liquid, form an anvil that the inner tube is hit against upon the explosion, so as to establish a sufficient degree of fusion, wherein, at the same time, the particles themselves are big, or at least smooth enough, to prevent the spheres from leaving, upon the explosion, a print on the inner surface of the inner tube, that provides a roughness that is too high to an extent for certain applications. Here, ceramics are meant to comprise in any case oxide ceramics, nitride ceramics, and carbide ceramics.

Preferably, the ceramic is sintered, so that the hardness of the spheres is relatively high. A hardness of the spheres that has been found suitable for the described application is 7 or more on the Mohs scale.

In principle, the size of the spheres may vary, and may be adapted also on the diameter of the inner tube. For larger tube diameters, the use of spheres having a relatively large diameter such as 5 mm may suffice, whereas for smaller tube diameters, the use of spheres having a diameter of 1 mm or smaller may be required.

In practice, at least the use of spheres out of sintered bauxite has lead to good results. These spheres had a mean diameter of about 0,75 mm, a diameter distribution of about 20/40 mesh, and a sphericity and a roundness of about 0.9 on the scales of Krumbein & Schloss.

For a type of sphere that to be applied in a certain situation, the material, the mean diameter, the diameter distribution, the hardness, and the sphericity may in principle be chosen independent from each other in order to optimize certain process steps, or to obtain a certain degree of anvil action or a desired surface quality. That does not take away, however, from also specific combinations of certain values for different parameters being able to provide certain advantages.

The invention is described in more detail with reference to the drawing, wherein Figure 1 shows a longitudinal cross section of two tubes disposed within each other, wherein the interior of the inner tube is filled with a combination of a liquid and a solid material in the shape of relatively small, hard spheres.

Figure 1 schematically shows a longitudinal cross section of a work piece 1 , consisting of two tubes 2 and 3 fitting inside each other. The hollow interior of the inner tube 3 is indicated with 4. An explosive layer 5 is arranged around the work piece 1. The diameters are not critical, but have not been depicted to scale.

As an explosive material, any material may be used that is known for the explosion welding of tubes.

In the method according to the prior art being carried out, the interior 4 of the inner tube 3 is filled first with a granular material, after which the interior 4 of the inner tube 3 is topped up with water or another liquid, before the explosive material 5 is being made to explode.

In fact, the method according to the invention may be carried out as known in the prior art, with the exception of the filling of the interior 4 of the inner tube 3.

According to the present invention, the interior 4 of the inner tube 3 is filled with a material comprising relatively small, hard spheres 6. After providing the spheres 6 inside the inner tube 3, the inner tube 3 may be additionally filled like in the prior art with a liquid 7 such as water, to obtain good anvil action. The spheres then form a network of mutually connected interstitial spaces, wherein the fluid 7 within said network functions as a non-compressible backing when the material 5 is being made to explode. The liquid 7 will not easily be able to move within the labyrinth of interstitial spaces, so that a potentially occurring pressure wave, that may cause undesired deformations, is attenuated or even completely avoided. Compacting the quantity of spheres 6 after providing them, for instance by knocking and/or vibrating and/or pressing appears unnecessary in practice.

It will be clear to a skilled person that the invention is not limited to the exemplary embodiment described above, but that various alternatives and

modifications are possible within the scope of protection of the invention as defined in the appended claims.