Because of the excess of acetylene always present there is no
danger of oxidizing the metal being heated.
The only care required by this torch is to keep the small air passages at
the nozzle clean and free from carbon deposits. The flame should be
extinguished when not in use rather than turned low, because this low flame
rapidly deposits large quantities of soot in the burner.
CHAPTER V
OXY-ACETYLENE WELDING PRACTICE
PREPARATION OF WORK
_Preheating._--The practice of heating the metal around the weld
before applying the torch flame is a desirable one for two reasons. First,
it makes the whole process more economical; second, it avoids the danger of
breakage through expansion and contraction of the work as it is heated and
as it cools.
When it is desired to join two surfaces by welding them, it is, of course,
necessary to raise the metal from the temperature of the surrounding air to
its melting point, involving an increase in temperature of from one
thousand to nearly three thousand degrees. To obtain this entire increase
of temperature with the torch flame is very wasteful of fuel and of the
operator's time. The total amount of heat necessary to put into metal is
increased by the conductivity of that metal because the heat applied at the
weld is carried to other parts of the piece being handled until the whole
mass is considerably raised in temperature. To secure this widely
distributed increase the various methods of preheating are adopted.
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