As to the second reason for preliminary heating. It is understood that the
metal added to the joint is molten at the time it flows into place. All the
metals used in welding contract as they cool and occupy a much smaller
space than when molten. If additional metal is run between two adjoining
surfaces which are parts of a surrounding body of cool metal, this added
metal will cool while the surfaces themselves are held stationary in the
position they originally occupied. The inevitable result is that the metal
added will crack under the strain, or, if the weld is exceptionally strong,
the main body of the work will he broken by the force of contraction. To
overcome these difficulties is the second and most important reason for
preheating and also for slow cooling following the completion of the weld.
There are many ways of securing this preheating. The work may be brought to
a red heat in the forge if it is cast iron or steel; it may he heated in
special ovens built for the purpose; it may be placed in a bed of charcoal
while suitably supported; it may be heated by gas or gasoline preheating
torches, and with very small work the outer flame of the welding torch
automatically provides means to this end.
The temperature of the parts heated should be gradually raised in all
cases, giving the entire mass of metal a chance to expand equally and to
adjust itself to the strains imposed by the preheating.
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