GB1449125A - Electric arc fusion welding apparatus - Google Patents

Abstract

1449125 Welding by fusion BABCOCK & WILCOX Ltd 15 Nov 1973 [15 Nov 1972] 52799/72 Heading B3R [Also in Division H2] In arc welding, particularly rotary TIG welding of tubes end-to-end and tubes to plates, wherein a welding electrode is traversed over a workpiece by a programme controlled motor and arc initiation is effected by a transitory (e.g. H.F.) voltage, a secondary arc gap device is arranged in parallel with the welding arc gap so that should the welding arc fail to strike upon application of said transitory voltage the latter discharges across the secondary arc gap and is attenuated to reduce the risk of damage to the apparatus. Welding current may be steady D.C. from a rectifier, or pulsed D.C. As shown, in end-to-end tube welding, a TIG head 3, comprising an electrode 7 (e.g. of thoriated tungsten and usually negative), in a ceramic gas nozzle (e.g. feeding argon), is energized from a rectifier 5 and orbited around the workpiece by a motor 4 which is started, stopped and speed-adjusted by a controller 10. A spark gap oscillator 23 delivers arc initiation H.F. voltage to the head 3 on closure of normally open contacts 26. A programmer 6 controls the controller 10 and the rectifier 5 so as to adjust motor speed and/or welding voltage and/or welding current to suit different welding conditions and requirements at different rotational positions of the head in response to signals from a voltmeter 13, an ammeter 14 and, in respect of head position, the motor 4, and may provide for the head to rotate slowly before and after the main welding respectively for arc initiation and preheating and, with low arc current, for reduction of residual weld crater. The controller 10 and meters 13, 14 may be provided in the programmer. A second arc gap device 27 comprises a fuse 33, a cathode 28 (e.g. a tungsten welding electrode) and an anode 29 (e.g. a copper plate) all connected in parallel with the welding electrode and workpiece. The cathode and anode 28, 29 are arranged in a refractory glass tube 30 which is disposed in the head inert gas feed stream, or, as shown, in a tapping therefrom. A control valve 32 is disposed at the glass tube outlet. A test circuit for the fuse comprises a further fuse 34, battery 35, switch 36 and lamp 37. The programmer controls the contacts 26 such that on pressing a start button (not shown), the contacts 26 are closed only until the rectifier output voltage falls below a predetermined value due to welding arc establishment or a predetermined short time elapses, which ever takes place sooner. The arc gap of the device 27 is set, e.g. at 25% greater than the welding arc gap. In operation, before welding, the fuse 33 is checked by closure of switch 36 (or, to prevent operation with a blown fuse, automatically on pressing the start button), and the valve 32 is opened to flood the device 27 with inert gas. On pressing the start button, D.C. voltage from the rectifier is applied across the welding arc gap and arc initiation H.F. high amplitude voltage is superimposed on the D.C. voltage. If, e.g. the welding arc gap is incorrectly set, the workpiece/rectifier connection is defective, or the head is defective and allows air entrainment in the inert gas, and the main arc is not struck, the H. F. voltage discharges across the secondary arc gap to attenuate the H.F. oscillations which otherwise may damage, e.g. the welding head and, by "tracking" across the insulation of the electrode 7, motor 4 and controller 10, the programmer and rectifier, and may provide low resistance paths for D.C. from the rectifier. Should H.F. ionization in the device 27 seek to promote D.C. arc current therein, after the programmer has reopened the contact 26, the fuse 33 will blow.