Describe how electric welding is carried out. Where can electric welding be done on a
boiler? What metals can be welded?
In electric welding an electric arc is struck between the electrode, which serves as a filler metal,
and the metal to be welded. The heat which is generated causes the electrode to melt and the
molten metal is transferred from the electrode to the plate. AC or DC current can be used for
welding. When welding a generator is used, two leads are attached to the generator, one is the
electric current and on the other lead a holder is attached which is clamped or grounded to the
material to be welded.
The arc is formed by touching the material with the point of the electrode. The current continue
to pass when the work and the electrode are separated. The heat of the arc melts the metal on
the electrode so that the two fuse together. The melting metal of the electrode for the filler
metal. The electrode rod are usually flux coated. This coating melts at a higher temperature
then the electrode metal, and therefore the coating protrudes beyond the case during welding.
This gives better stability, contact, and concentration of the arc. The coating shields the arc from
the atmosphere by means of inert gases given off. Silicates from the coating forms a slag on the
surface of the hot metal which protects it from the atmosphere as it cools. Also due to the larger
concentration of the slag than the metal as cooling is taking place, the slag is easily removed.
AC welding is more popular than DC welding because
1) More compact plant.
2) Less plant maintenance
3) Higher efficiency than DC
4) Initial cost is less for similar capacity plants
Disadvantage
1. Higher voltage is used therefore high stock risk
2. More difficult to weld cast iron and non-ferrous metals
Circuit is about 15-45 volts and about 80-360 amps. Metals that can be welded, are steel and
ferrous metal, aluminum, and magnesium, copper, and ferrous metals such as stainless steel
without a flux, oxygen arc welding or tag welding.
Electric welding can be done on boiler parts, but it must be carried out by a qualified welder and
under strict codes pertaining to welding of pressure vessel. The welding must also be subjected
to various tests.
