The present invention relates to a method for heating closely spaced end portions of two pipes to joined by forge welding or the like, wherein two diametrically opposed contacts are attached to each end portion, through which contacts each end portion is supplied with high frequency alternating current for resistance heating of the material in the respective end portion, the contacts for one of the pipes being located in close proximity to and in the same axial plane as the contacts for the other pipe.

A method of this type is disclosed in US-PS 4 736 084, which hereby is included by reference. This prior art method is successfully used for joining solid work pieces and thick-walled pipes, the heating current concentrating itself in the material closest to the end surfaces of the pipes due to their close proximity. However, for thin- walled pipes, e.g. made from C/Nn steel, the proximity effect may not be sufficiently high so that the current may leak away from the joint surfaces of the pipes. Further¬ more, the contact points or junctions between the pipe end portions and the ecetrical contacts give rise to a wider heating zone resulting in a colder portion in the joint between the contacts.

The purpose of the present invention is to improve the method in question so that it may successfully be usee also for thin-walled pipes and for materials having high thermal and/or electrical conductivity.

In accordance with the invention, this is achieved by a method as recited in the introductory paragraph, the method being characterized in that the contacts for one of the pipes at least close to the pipes are arranged to form an angle with the contacts for the other pipe, whereby the

projections of the contacts onto said axial plane appear to cross each other.

According to an advantageous embodiment of the invention the contacts for one of the pipes are run through slots in the contacts for the other pipe.

In areas of the joint remote from the contact points or junctions the heating current may tend to spread somewhat from the joint surfaces and reduce the rate of heating. To concentrate the current close to the joint also in these areas, it is suggested according to the invention to place ferrite bars parallel to the longitudinal axes of the pipes close to the pipe wall in diametrically opposed areas between the contacts. These ferrite bars may be arranged both on the inside and on the outside of the pipes.

The effect of crossing the contacts increases with increa¬ sing angle between the contacts. Concequently, in accor- dance with the invention, the angle is adjusted in order to avoid overheating of the pipe material between the con¬ tacts.

The invention will be described in more detail in the following in conjunction with the exemplifying embodiments shown in the appended drawings, where

Fig. 1 is a perspective view illustrating a prior art method,

Fig. 2 is a view similar to that in Fig. 1 but illustrating schematically an embodiment of the present invention,

Fig. 3 is an elevation view corresponding to the left part of Fig. 2 showing an alternate embodiment, partly in section along the line III - III in Fig. 4,

Fig. 4 is a section along the line IV - IV in Fig. 3, and

Fig. 5 is a section on the line V - V in Fig. 2.

Fig. 1 illustrating a prior art embodiment shows two pipes 1 and 2 having end portions 3 and 4, respectively, to be joined by means of forge welding. Prior to joining the two pipes their end portions 3, 4 are heated by means of a high frequency alternating current supplied by a source 5 through contacts 6, 7 to the upper pipe 1 and 8, 9 to the lower pipe 2.

The current density in the end portions 3, 4 is illustrated by the lines 10. As apparent from these lines, the current is less dense midway between the contacts 6, 7; 8, 9 and also near the junctions between the contacts and the respective pipe end portions. These effects lead to insufficient heating of the middle areas and the material between the contact pairs 6; 8 and 7; 9.

According to the invention, the contacts 6 - 9 are rear¬ ranged as shown in Fig. 2, i.e. the contacts within each pair 6; 8 and 7; 9 appear to cross each other in the plane of the drawing, which corresponds to an axial plane 14 through the center line of the pipes 1, 2 and the contacts (Fig. 5) . As indicated by the current density lines, the problem areas at the contact junctions and the middle areas have greatly improved. At this time it is not fully understood why these beneficial effects occur and therefore no explanation will be attempted here.

Figs. 3 and 4 illustrate how the contacts 6 and 8 may be formed to obtain the crossing effect. The contact 8 has two spaced parallel bars forming a slot 11 therebetween through which the contact 6 extends, an angle α being formed between the crossing parts of the contacts. In order to avoid an inadvertent short circuit between the

contacts in the crossing area, an insulating material like mica may be placed between the contacts 6 and 8 in the slot 11.

Crossing of the contacts 6, 8 has the effect of improving heating of the zone 12 between the contacts. By increasing the angle α the heating will be intensified and therefore it is necessary to der ine the angle α for each application depending on material composition, pipe thickness and diameter, contact distance and electrical parameters in order to avoid overheating of the zone 12.

It is believed that the close spacing of the pipe portions 3 and 4 give rise to a "proximity effect" that keeps the current density strong near the joint between the pipe portions. However, even with crossing contacts there will be some tendency for the current density to be less midway between the diametrically opposed contacts 6, 7; 8, 9, as illustrated in Fig. 2. This effect may be due to the high magnetic permeability in the steel pipe and steel in tools inside the pipe or a screen around the pipe. In order to keep the current density high also in the areas between the contacts, it is suggested according to the invention to place elongate ferrite bars 13 parallel to the central longitudinal axes of the pipes 1, 2 near the pipe wall in those areas where the current density is to be improved, e.g. as shown in Fig. 5. The ferrite bars 13 are arranged symmetrically about a diamtral axial plane 14 through the pipes and contacts 6, 7; 8, 9. Ferrite bars may also be placed on the outside of the pipes 1, 2 instead of, or in addition to, the bars 13 on the inside.

The ferrite bars may consist of ferrite particles sintered in a mass of insulating material. The bars may have a cross-section of about 30 x 30 mm and a length of about

150 mm, for instance when used for welding pipes of 273 mm external diameter and 12,7 mm thickness. For such pipes,

using an electrical effect of 180 k at 100 V and a frequ¬ ency of 15000 Hz, the necessary heating for forge welding the pipes to each other may be obtained in about 30 sec.

It will be understood the changes and modifications may be made in the exemplifying embodiments described in the specification and shown in the drawings without departing from the scope of the invention as defined in the following claims.