The present invention relates to strip cladding, and in particular to the type of strip cladding in which electromagnetic pole pieces are provided for the purpose of causing a magnetic field in the region of the electrode strip.
In Barger, U.S. patent application Ser. No. 596,878, now issued as U.S. Pat. No. 4,027,135, an apparatus was disclosed in which a favorable tie-in between adjacent cladding strips or weld beads was accomplished by the provision of a pulsating magnetic field on the trailing side of the strip electrode. The Barger application is hereby incorporated by reference. The pulsating magnetic field in the Barger application was caused by two magnetic pole pieces, one on either side of the deposited strip, each being slightly to the trailing side of the electrode strip. The electromagnets were pulsed in such a manner as to cause a pulsating magnetic field that opposed the magnetic field created on the trailing side of the strip electrode by the welding current. The changing magnetic field caused forces on the molten welding flux covering the molten clad strip, so as to agitate both the molten welding flux and the molten strip. This agitation of the molten strip prevented the formation of a channel between adjacent strips so that it was not necessary to manually fill inter-strip channels.
Strip cladding is often carried out on the interior of vessels; a carbon-steel vessel, for instance, might be coated with a stainless-steel cladding. The welding head is positioned at a point on the interior of the vessel, the vessel is then rotated, and a circular or spiral strip is deposited on the interior of the vessel by appropriate axial motion of the head. In the past, when the welding head came close to a flange on the vessel, the operation had to be stopped, and the remainder of the clad was applied manually. As can be appreciated, this manual cladding operation is quite time consuming and expensive. It was nonetheless necessary, because the flange constituted an obstruction that prohibited the cladding from being deposited right up to the edge of the flange.
Another limitation on the use of strip cladding has been the difficulty of depositing cladding strips in relatively small-diameter cylinders. It is sometimes necessary to clad the interiors of fluid conduits that are not large enough to accommodate both the welding head and the strip-electrode spool. In such instances, the welding head is inserted into the cylinder, and the electrode strip is fed from a spool located outside the conduit. In order to lead the electrode strip from the plane of the spool to the plane of the welding head, the electrode strip must be spiraled, and the room taken up by the spiral sets a lower limit on the size of the cylinder that may be clad circumferentially.