Patent GB-715918-A (GB)
Title: Improvements in welding thin-walled metal tubing
IPC: B23K11/00
Filed: 1952-03-21
Abstract: 715,918. Supply systems for resistance welders; transformers. GENERAL MOTORS CORPORATION. March 21, 1952 [March 31, 1951], No. 7303/52. Classes 38(2) and 38(4) [Also in Group XXII] A method of electrically seam-welding thin-walled metal tubing formed by bending flat strip into tubular shape comprises feeding the shaped strip past a pair of grooved roller electrodes separated by an air gap and supplied with alternating current of a frequency between 600 and 3000 cycles per second, said electrodes forming part of a rotatable transformer comprising a pair of concentric drums forming the secondary inductive element, the primary winding being contained in the annular space between the drums. A pair of roller electrodes 10 and 11 separated by an air gap 9 are connected across a rotatable, single loop of a secondary transformer comprising concentric drums 12 and 13 between which a primary coil 14 is wound on a magnetizable core 15 said coil being connected through slip rings 16 with an A.C. generator 17 whose field winding 18 is supplied with direct current by a D.C. generator 20. A motor 21 drives the generators 20 and 17 and a rheostat 19 regulates the output voltage of the A.C. generator 17. For welding steel tubing having a wall thickness of 0.026 inches to 0.35 inches a 960-cycle generator is used. For a wall thickness of 0.010 inches a higher frequency, e.g. 3000 cycles, is desirable. A steel tube T is placed between the electrodes 10 and 11 and a pressure roll 25 so that the seam S is located in the middle of the gap g between the electrodes. The area hp in Fig. 2 represents the heat pattern adjacent the seam S in which the material of the tube becomes plastic. The heat pattern is controlled by varying the gap g and adjusting the current voltage according to thickness of the tube wall and the speed at which the tube is moved between the elctrodes. For example, the gap g in Fig. 2 is approximatedly twice the wall thickness w which is 0.030 inches when a 960-cycle generator 17 is used and for such wall thickness the voltage range is preferably 1.1 volts to 2.4 volts for a speed range of 40 to 134 feet per minute. The tube T may have a copper, nickel-alloy or stainless steel lining I. applied initially to the flat strip before bending into tube form and the seam S' of the lining is welded simultaneously with the seam S of the tube. When using a low current frequency, e.g. below 180 cycles per second the material in the heat pattern zone is heated by electrical resistance heating due partly to direct conduction across the seam and partly to induced eddy currents and, if the material is magnetic, to hysteresis. When the current frequency is raised above 180 cycles per second the proportion of total heating effect due to hysteresis and eddy currents increases rapidly and the current flowing across the seam decreases proportionally so that a lesser cross-sectional area of metal is required per unit length of tubing to carry this current without causing excessive melting of the metal at the seam. Since the welding current is alternating there is a sinusoidal variation of energy input along the seam as the tube is fed between the electrodes. When a frequency of 960 cycles per second is used, the distance between energy peaks at a tube feed of 120 feet per minute is 0.0125 inches and the zones of maximum energy input are nearly continuous which enables the temperature in the heat pattern to be more easily controlled. Thus a large bead at the seam need not be formed as, due to the controlled plasticity of the metal, a lower pressure can be applied by the backup roll 25 which in turn enables a reduction in power input from the generator 17 to be made. For example, a power input of 135.5 watts per foot run of tubing is sufficient to produce a satisfactory weld in a tube of given material and wall thickness travelling at 400 feet per minute when the generator 17 operates at 960 cycles per second. The welded tube is subjected to a finishing rolling process for truing its shape and smoothing the weld surface.