The present invention relates in general to explosion welding .

Explosion welding is a known per se technique for

attaching together metal objects, such as plates or pipes. In short, two objects are placed on each other, and on top of that a layer of explosive material, is arranged. When the

explosion material is made to explode, the two objects are pressed together powerfully to such extent that they melt together in the attachment zone. A large advantage with

respect to other welding techniques is that the heat input is relatively low, so that the objects as a whole do not melt and no or only a very thin mixing zone results.

A special application of explosion welding is attaching together two concentric pipes within each other, wherein the outer pipe forms a coating for the inner pipe. In such case, the explosive material is arranged around the outer pipe, and the explosion front moves along the length of the pipes. The explosion can be used to clamp the outer pipe against the inner pipe, but a metallurgical joint will result if the

explosion is controlled well. In a particular embodiment, the inner pipe is made of inexpensive carbon steel and the outer pipe is made of a more expensive stainless type of material such as for instance inconel 625.

In order to have the circular pipes maintain their shape well, it is necessary that the interior of the inner pipe is filled with a medium that can resist the shock of the

explosion well, and a liquid is used for this purpose because of the fact that a liquid is non-compressible. This may be a cheap liquid such as water. This process is well known on a small laboratory scale; the maximum achievable length in this case is about 50 cm. However, there is a need to apply this process in pipes having commercial lengths of 3 m or 6 m, and possibly even larger. In such case, it appears that a problem occurs in practice, of which the precise cause is not entirely clear to us, but which seems to be connected to the propagation of the shock wave in the water. The problem expresses itself in that the work piece exhibits a trumpetlike deformation: the diameter of the work piece increases as the distance to the end of the work piece decreases, i.e. seen in the direction of propagation the explosion front, and it is even possible that the work piece tears open at its end.

The invention aims to provide a solution to this problem.

These and other aspects, features and advantages of the present invention will be further clarified by the following description with reference to the drawings, in which same reference numerals indicate same or similar parts, in which indications "under/above", "higher/lower", "left/right" etc. only indicate the orientation shown in the figures, and in which :

figures 1A and IB schematically show cross sections of a work piece;

figure 2 shows a cross section comparable to figure IB on larger scale, of a work piece according to the present

invention;

figures 3 and 4 illustrate some variations of the present invention .

Figure 1A schematically shows a transverse cross section, and figure IB schematically shows a longitudinal cross

section, of a work piece 1, consisting of two pipes 2 and 3 fitting into each other. The hollow interior of the inner pipe 3 is indicated at reference numeral 4. An explosive layer 5 is arranged around the work piece 1. The pipes have a length of for instance 6 m. The diameters are not critical, but are not shown to scale. For instance, the inner pipe 3 may have an outer diameter of 5 cm and a wall thickness of 5 mm, and the outer pipe 2 may have a wall thickness of 1 mm. Any material known for explosion welding of pipes may be used as explosive material, so that it is not necessary to describe more details of this material.

When implementing the method according to the state of the art, the interior 4 of the inner pipe 3 would be filled with water before the explosive material 5 is made to explode. In fact the method can be implemented as is known in the state of the art, with the exception of the filling of the interior 4 of the inner pipe 3. According to the present invention, the interior 4 of the inner pipe 3 is first filled with a granular material 6, after which the interior 4 of the inner pipe 3 is filled with water (or another liquid) .

The granular material 6 is preferably condensed well, for instance by knocking and/or vibrating and/or pressing.

Preferably, the grains do not have a smooth surface, but have a rugged surface. A suitable material is for instance gravel or coarse sand. Preferably the granular material has been obtained by crumbling stony material. Otherwise it is noted that this material can be reused.

The granular material 6 forms a network of interstices connected to each other, which are filled with the water. When the explosive material 5 is made to explode, in the usual manner, the water (or other liquid) 7 in the interior 4 will still function as non-compressible backing. However, the water will not easily be able to displace itself in the labyrinth of interstices so that the consequences of a pressure wave are reduced or even avoided entirely.

The present invention can be applied with pipes within a large range of diameters, actually without the diameter forming a restriction. Obviously, in the case of larger diameters also the diameter of the hollow space 4 is larger, and that means that a larger amount of the granular material is needed (this amount scales with the square of the radius) . In such case, it might happen that the total package of granular material is still compressed slightly by the

explosion. Therefore, in a further elaboration, the present invention proposes to arrange a solid inner core 8 centrally in the hollow space 4, which may possibly but not necessarily be made of steel. This embodiment variation is illustrated in figure 3. The space 14 filled with the granular material 6 between the inner core 8 and the inner wall of the inner pipe 3 now has an annular contour; the diameter of the inner core 8 is preferably selected in such a way that the thickness of this annular space 14 (indicated at 9 in figure 3) is in the order of about 2,5 cm.

Figure 4 illustrates a variation wherein a hollow pipe 40 is filled in advance with granular material 6 and liquid 7, and then this filled pipe 40 is arranged in the hollow space 4 of the inner pipe 3. The pipe 40 can be made of a plastic. An advantage of this variation is that filling the pipe 40 can be done in advance, so that loading the pipes 2 and 3 on site can go faster.

It will be clear to a person skilled in the art that the invention is not limited to the exemplary embodiments

discussed in the above, but that several variations and modifications are possible within the protective scope of the invention as defined in the attached claims. For instance, it is possible that the outer pipe is made of an inexpensive type of steel while the inner pipe is made of a more expensive stainless type of material, in which case the inner pipe will then typically have a smaller thickness (for instance 1 mm) than the outer pipe (for instance 5 mm) : in this way, the inner surface of the inexpensive steel pipe is then provided with a high quality coating. It is also possible that an inexpensive steel pipe is provided with a high quality coating on both its inner surface and its outer surface by placing three pipes in each other.

In the above it has been described that the pipes 2 and 3 are first placed within each other and then the interior 4 of the inner pipe 3 is filled with granular material. It is also possible that the interior 4 of the inner pipe 3 is first filled with granular material and then the filled inner pipe 3 is arranged within the outer pipe 2.