Thin-walled steel tubes, hollow sections, flange and other assemblies may be joined by an arc process which closely resembles flash welding in the type of apparatus employed. The workpieces are held in clamps, one of which can be moved on the axis of the work. Between the clamps and the joint line two solenoids are placed around the work, one on each side of the joint. These solenoids are energized by a direct current in a manner to produce the same pole on each side of the joint and, to allow them to be placed over the work and removed after welding, they must be split. With the workpieces initially in contact a. d.c. welding source with a range of 28-48 V is connected across the gap. On withdrawing the workpieces from contact an arc is struck across the gap which is then opened to 1-2 mm. The magnetic field created by the solenoids is radial with respect to the axis of the work and this causes the arc to motor around the outer edges of the workpieces (see figure below) which in a few seconds become molten. The gap is then closed rapidly by the moving platen to squeeze out the molten metal and consolidate the weld. A normal machined end is all that is required at the joint and no special treatment of the surfaces of the workpieces is necessary. Welds can be made without any shielding but, if desired, to improve the appearance and quality of the upset metal a shield of argon, nitrogen or other reducing gas may be provided.

The similarities with flash welding are obvious but there are important differences. With flash welding the source of heat is form both resistance heating of molten bridges and short-lives arcs when the bridges are broken. Molten metal is expelled from the joint in the process and there are comparatively long periods of inactivity when no current is passing and there is therefore no heating. With the magnetically impelled arc, however, heating is continu­ous, little metal is expelled and the process is therefore more efficient and the heating cycle considerably more rapid. As the arc tends to adhere to the periphery of the joint this limits the process to welding relatively thin hollow sections of up to 5 mm wall thickness and makes it generally unsuitable for solid sections. Upset forces tend to be less than for flash welding but, because of the rapid heating and smaller heat-affected zone, the rate of upset must be higher. The flash of expelled metal is smaller, smoother and more uniform than with flash welding.