Method and apparatus for heating opposing surfaces of two elements to be joined

#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 narrow gap, and in which the heating takes place by means of a high frequency current supplied from a power source of alternating current.

#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 spread 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.

#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.

#FIG. 1 depicts a section through two pipe parts 1, 2 which are so positioned relative 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 example be clamped onto the pipe parts. On the diametrically 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 diametrically 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 diametrically 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 spread outwardly in the pipe parts before the temperature in the joint area has become so high that forge welding can be effected.