©
COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON
II
inch or 7 inch lengths. Torches
may be either air or water-cooled. The air-cooled types actu-
ally are cooled to a degree by the
shielding gas that is fed to the torch head through a compos-
ite cable. The gas actually surrounds
the copper welding cable, affording some degree of cooling.
Water-cooled torches are usually used for applications where the welding
current exceeds
200 amperes. The water inlet hose is connected to the torch head. Circulating
around the torch head, the water leaves
the torch via the current-in hose and cable assembly. Cooling
the welding cable in this manner allows the use of a smaller diameter cable that
is more flexible
and lighter in weight. 2.3.3.1
The gas nozzles are made of ceramic
materials and are available in various sizes and
shapes. In some heavy duty, high current applications, metal water-cooled
nozzles are used.
2.3.3.2 A
switch on the torch is used to energize the electrode with welding current and
start the shielding
gas flow. High frequency current and water flow are also initiated by this
switch if the
power source is so equipped. In many installations, these functions are
initiated by a foot control
that also is capable of controlling the welding current. This method gives
the operator full
control of the arc. The usual welding method is to start the arc at a low
current, gradually increase
the current until a molten pool is achieved, and welding begins. At the
end of the weld,
current is slowly decreases and the arc extinguished, preventing the crater that
forms at the end
of the weld when the arc is broken abruptly. 2.3.4
Shielding
Gases -
Argon and helium are the major shielding gases used in gas
tungsten arc welding. In some
applications, mixtures of the two gases prove advantageous.
To a lesser extent, hydrogen is mixed
with argon or helium for special applications. 2.3.4.1
Argon and helium are colorless, odorless,
tasteless and nontoxic gases. Both are inert
gases, which means that they do not readily combine with other elements. They
will not burn
nor support combustion. Commercial grades used for welding are 99.99% pure.
Argon is
.38% heavier than air and about 10 times heavier than helium. Both gases
ionize when present
in an electric arc. This means that the gas atoms lose some of their electrons
that have a negative
charge. These unbalanced gas atoms, properly called positive ions, now
have a positive charge and are attracted
to the negative pole in the arc. When the arc is positive
and the work is negative, these positive ions impinge upon the work and remove
surface oxides or scale in the weld
area. 2.3.4.2
Argon is most commonly used of the
shielding gases. Excellent arc starting and ease
of use make it most desirable for manual welding. Argon produces a better
cleaning action
when welding aluminum and magnesium with alternating current. The arc produced
is
