FIELD OF THE INVENTION

The present invention relates to a method for heating metal elements to be joined, especially by forge welding, in which the elements are provided with surfaces to be welded and are positioned so that said surfaces constitute a nar¬ row gap, and in which the heating takes place by means of a high frequency current supplied from a power source of alternating current.

PRIOR ART

From for example Norwegian Patent Application No. 83.3729 there is known a method for joining parts of metal by forge welding, in which the parts prior to the forge welding are heated be means of an induction coil.

In order to provide an efficient joining of said parts, it is of great importance that the heating is concentrated as much as possible in the area of the joint. If the heat is allowed to spred too far from the joint, the heating time will increase, the bending force of the parts will be reduced during the forge welding, the favourable tri-axial stress conditions will be worse and the necessity of heat treatment after the completed welding will increase.

Heating of pipe parts by means of induction heating prior to the joining of said parts, has been suggested and tested by many, but the practical application has been very limi¬ ted, the heating zone having a tendency to be too long. The heat from the coil must be transfered to the metal parts closest to the coil and must spread therefrom and into the joint surfaces, a fact which entils that improper zones of the" metal will be heated.

If the cross section of the material is reduced in the area of the joint for achieving higher tri-axtial tensions

2 during the forging operation, it will be difficult to position the induction coil as close to the joint that heating will only take place in the area of the joint.

From DE Auslegeschrift 15 65 360 there is known a device for continuous electric welding of profiled parts. The device includes connections for two or more V-shaped gaps which are simultaneously and continuously welded in the longitudinal direction of the profiled parts by means of the same high frequency power source. However no instruc¬ tion is -given therein about a heating process relating to the area of a narrow gap which is kept substantially con¬ stant during the heating process.

From DE Offenlegungsschrif 30 07 153 there is known a method for continuous seam welding of pipes by means of high frequency welding, in which method the current is utilized in a better way by including an additional coil for the regulation of the primary high frequency current. However, the method includes the use of a sliding contact system for achieving the continuous welding of longitudin¬ ally extending seams, but this technique cannot be transfe- red to a butt welding of pipes and bolts, which is the case with the present invention.

From US Patent Specification 2 892 914 there is known a method and an apparatus for butt welding based on induction heating, but this known technique has found no practicle application due to the fact that no consentrated heating zone can be achieved in the area of the joint. Further, no gap between the two metal parts to be joined is suggested in US Patent Specification 2 892 914, let alone that the known technique suggests inclined or sloped areas bordering the gap. Due to the lack of a gap between the parts to be joined, the prior art does not allow for flushing the sur¬ faces of the end faces to be joined.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method for heating metal elements to be joined, which more rapidly and efficiently establishes a concentrated heating zone in the area contstiting the gap surfaces of the elements to be welded.

Suprisingly, it has been found that a high frequency cur¬ rent source can be utilized in a manner rendering a quicker and more consentrated heating zone than by induction heating.

The above entioned object is achieved in connection with a method as stated in the preamble, which method is caracte- rised in that the two poles of the power source are connec¬ ted galvanically to each element, respectively, close to the gap surfaces, and that said elements are connected gal¬ vanically across said narrow gap preferably at the opposite side of the power source connections, so as to establish a high frequency resistance paths in the area of said gap surfaces for heating said elements in a narrow zone including said gap sufaces.

Thus, there is established a high frequency resistance heating of said elements right out to the gap surfaces. The reason why the current follows the gap surfaces is meant to relate to the fact that the high frequency current does not choose the path of less ohmic resistance, but the path giv¬ en the lowest total impedance. Consequently it is important that the gap surfaces are positioned close to each other, but without being in physical contact with each other.

By welding a pipe having a wall thickness of 2.-5 cm or more, the method according to the present invention is four to five times more effective than a method based on high frequency induction welding. It is presumed that welding can be completed for very large pipes within approxemately 1 minute without normalizing. Further, the method according

to the present invention includes the advantages of being applicable for all types of metals.

According to a preferd embodiment of the invention two contacts are attached, one on each of the elements, close to the gap and close to each other, and the two elements are connected by means of further contacts and a conductor at the opposite side of the elements, whereafter aternating current is supplied to the two first mentioned contacts.

Preferably, the contacts are clamped onto the respective elements, but can also be attached in another suitable manner. The contacts and possibly the conductor can appropriately be cooled, preferably be means of water.

For a better understanding of the present invention this will be further discussed with reference to the embodiment which is diagra maticly illustated on the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a section through two pipe elements to be welded according to the present invention.

Fig. 2 is a section through two bolt elements to be welded according to the present invention.

DETAILED DESCRIPTION

Fig. 1 depicts a section through two pipe parts 1, 2 which are so positioned relativ to each other that a gap 3 is established therebetween. The gap is defined by gap surfaces 4 and 5 on the individual pipe parts 1 and 2 respectively. The gap surfaces are positioned very close to each other but have no physical contact.

A contact 6 is in a suitable manner attached to the pipe part 1, whereas a contact 7 is attached to the pipe part 2.

These contacts can for exampel be clamped onto the pipe parts. On the diametrally opposite side of the contacts 6 and 7, there are to the pipe parts attached contacts 8 and 9, for example by soldering. The contacts 8 and 9 are connected by means of a conductor 10. The contacts 6 and 7 are in turn connected to a high frequency alternating current source 11.

If the path of the current is followed during a half cycle, the current will for example flow through the contact 6 into the pipe part 1. From here it will pass down to the gap surface 4 and therealong to the diametrally opposite side of the pipe part 1. From there it will flow to the contact 8, down through the conductor 10 and in through the contact 9 in the pipe part 2. From there it will in a corresponding manner flow to the gap surface 5 and there along to the diametrally opposite side, whereupon it flows out through the contact 7 and back to the current source 11. The fact that the current follows the gap surfaces 4 and 5 very accurately, results in that the resistance _* heating takes place just at the desired locations. Besides, the heating takes place so rapidly that the heat only to a smaller degree will have time to spred outwardly in the pipe parts before the temperature in the joint area has become so high that forge welding can be effected.

Because the gap surfaces 4 and 5 do not touch each other, it is to be understood that the gap 3 can be flushed with a reducing gas during the heating.

It is to be understood for the artisan that the invention can find a favourable application beyond the welding of coarse pipes. Thus, the invention is not limited to the illustrated embodiment, but can be varied and modified in several manners within the scope of the attached claims. The gap surfaces can for example be shaped so that they diverge somewhat relativ to each other in an inwardly radial direction. This is especially favourable in connectoin with welding of solid parts, such as rods or

bolts. Thus if the gap surfaces of the rods or bolts to be joined are given a somewhat curved shape, which involves that the current will not follow the shortest path across the gap surface between the contacts, but rather spred out over the gap surfaces for heating said surfaces all over.

It appears that the higher the frequency of the alternating current is, the less the gap surfaces have to deverge to secure a sufficient distribution of the current for heating purposes .

In fig. 2 it is illustrated how the gap surfaces 4a and 5a defining the gap 3a between two solid bolt elements la and 2b to be,joined, have been given a slightly curved skope. The curvature of the gap surfaces allows for a larger dis- tance between the gap surfaces in the area of the common longitudinal axis C of the two elements. Whereas the dis¬ tance therebetween becomes smaller closer to the peripheral areas of said surfaces.

This variation in distance between the surfaces gives a favourable current distribution across the two gap sur¬ faces, so as to allow for a uniform and consentrated heating thereof.

Because the surfaces 4a and 5a are curved in opposite directions they allow for a favourable guidance of the elements when they are pressed together to form a welding joint.

it is to be understood that the Figures illustrate the two parts or elements to be joined immidiately before welding, and that after sufficient heating and possible flushing of the joint or gap 3 or 3a, the elements are pressed together for closing the joint 3 or 3a so as to form a pocket free final welding joint.

As it is seen from the Figure the elements 1 and 2 have been given a reduced cross section in the area of the joint. This is done in order to more easily obtain a tri-

axial stress condition in the joint area when the material, during the pressing together of said elements, is subjected to a floating condition due to the consentrated heating of the gap surfaces. The floating conditions of the material during the forge welding operation can be _. regarded as subjecting the material in the joint area to a load which is 2 to 3 times larger than the plastic yelding of the material.