Non-pressure welding or fusion
welding
‘Non-pressure welding’ is
defined as the intimate joining of metal by the application of heat without the
use of pressure. Local fusion of the surfaces to be joined and the filler metal
takes place by the application of intense heat. The method of welding depends
upon how the heat is produced, namely, electric arc welding, gas welding,
electric resistance welding, etc. Filler metal used in the process must have
the same composition as that of the parent metals to be joined. On cooling and
solidification, a fine grained cast structure is developed having properties
nearly the same as that of the mother metal.
Electric arc welding
Electric arc welding is further
subdivided into two main categories namely.
- Metal arc welding
- Carbon arc welding
Fig.
Electric Arc Welding
In both processes either the
arc is unshielded or shielded by means of a flux or an inert gas environment.
Out of these two processes
‘Metal arc welding’ with flux coated consumable electrode is by far the most
common and cheapest process of welding.
Metal arc welding (Unshielded)
In this process, heat is
obtained from an arc formed between the work and a metal electrode coated with
flux. The metal electrode also supplies molten filler metal to the joint.
Electric energy is converted into intense heat in the arc, which attains a
temperature around 5,5000C. This intense heat in the arc forms a
molten pool in the metal being welded and at the same time, melts the tip of
the electrode. As the arc is maintained, molten filler metal from the electrode
tip is transferred across the arc, where it fuses with the molten base metal.
In actual operation, an arc is
started by striking the work with an electrode and quickly withdrawing it a
short distance. As the electrode end is melted by the intense heat, most of it
is transferred across the arc in the form of small globules to a molten pool. A
small amount is lost by being vapourised
and some globules are deposited alongside the weld as spatter. The arc
is maintained by uniformly moving the electrode towards the work at a rate
equal to the amount of electrode metal transferred to the weld. At the same
time, the electrode is gradually moved along the joint being welded.
Types of electrodes
There are three types of metal
electrodes or rods in use, namely
- Bare
- Fluxed
- Heavy flux coated
Bare electrodes have a limited
use for the welding of wrought iron and mild steel. Because the deposited metal
is affected by the oxygen and nitrogen in the air forming undesirable oxides
and nitrides in the weld metal, it introduces weakness in the joint.
However, 95 % of the total
manual welding production today is with coated electrodes. Heavy flux coating
on the electrode removes the oxides and nitrides in the form of slag which
floats on the surface of the weld. It also prevents oxidation of the surface
metal during cooling. Secondly, it also provides a protective gas shield.
Besides these duties, coating produces ionized vapours to stabilize the arc and
governs the melting rate and the amount of penetration into the parts being
welded.
Heavy coating also permits the
use of larger welding rods, higher current and greater welding speeds.
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