in the weld zone is displaced by a shielding gas in order to prevent contamination
of the molten weld puddle.
This contamination is caused mainly by nitrogen, oxygen and water vapor present
in the atmosphere.
As an example, nitrogen in solidified
steel reduces the ductility and impact strength of the weld and can
cause cracking. In large amounts, nitrogen
can also cause weld porosity. Excess
oxygen in steel combines with carbon to form carbon monoxide (CO). This gas can
be trapped in the metal,
causing porosity. In addition, excess oxygen can combine with other elements in
steel and form com- pounds
that produce inclusions in the weld metal. When
hydrogen, present in water vapor and oil, combines with either iron or aluminum,
porosity will result and
underbead weld metal cracking may occur. To
avoid these problems associated with contamination of the weld puddle, three main
gases are used for shielding.
These are argon, helium and carbon dioxide. In addition, small amounts of oxygen,
nitrogen and hydrogen
have proven beneficial for some applications. Of these gases, only argon and helium
are inert gases.
Compensation for the oxidizing tendencies of other gases is made by special wire
Argon, helium and carbon dioxide can
be used alone, in combinations or mixed with others to provide defect
free welds in a variety of weld applications
and weld processes.