The electroslag process has been developed to weld seams of thick section or joints in a single pass, thus eliminating the need for multiple passes and preparation of bevel, V, U, or J groves and edge preparation.
In this process, two plates, (1 and 2) to be joined together are arranged vertically as shown in Fig. 9.49. One to three electrodes with copper shoes (moulds) on each phase of the plates are used as shown in Fig. 9.50. Powdered granular flux is poured from a hopper. As the weld is made the moulds move up with the welding head.
There is no arc as resistance to electrical flow through the flux creates the heat necessary to melt the metal. The interior temperature of the slag bath for steel is about 1925°C and the surface temperature is approximately 1650°C.
The electrodes used may be either solid wire or flux cored. The extreme heat produced by the molten slag and metal in the weld arc causes the base metal to melt away from the original joint gap and high deposition rates allow welding of very thick sections in one pass.
Several electrodes are used for long welds so that the heat is more uniformity spread. Water cooled shoe or dam-plate fastened to the sides of the workpiece prevents the pool of melted metal from escaping.
These plates also assist the solidification process by removing heat, and are moved up as the weld progresses. Combinations of oxides of silicon, manganese, titanium, calcium, magnesium, aluminium calcium fluoride (flour spar) are used as flux in this process.
In the process of electrogas welding (which is similar to the electroslag welding), an inert gas is used for shielding. Flux cored wire is automatically fed to the molten weld pocket and an electric arc is continuously maintained between the electrode and the weld puddle. Welding speeds of about 1.5 m/hr are possible.
ADVERTISEMENTS:
Process is usually performed in a substantially upward direction. However, the seams of boiler drum can be welded by rotating the drum with a suitable manipulator. Plates in the range of 13 to 200 mm thickness can be welded by this process. Deposition rate is 10-20 kg/ hr/electrode.
Diameter of electrode is 3 mm and metal transfer efficiency is almost 100%. Flux consumption is around 1 kg/100 kg of deposited metal. Constant voltage transformer rectifier rated at 1000 amps at 100% duty cycle is used. For thickness more than 70 mm, oscillation of electrode is desirable.