The object in preheating the metal around the weld is to cause it to expand
sufficiently to open the crack a distance equal to the contraction when
cooling from the melting point. In the case of a crack running from the
edge of a piece into the body or of a crack wholly within the body, it is
usually satisfactory to heat the metal at each end of the opening. This
will cause the whole length of the crack to open sufficiently to receive
the molten material from the rod.

The judgment of the operator will be called upon to decide just where a
piece of metal should be heated to open the weld properly. It is often
possible to apply the preheating flame to a point some distance from the
point of work if the parts are so connected that the expansion of the
heated part will serve to draw the edges of the weld apart. Whatever part
of the work is heated to cause expansion and separation, this part must
remain hot during the entire time of welding and must then cool slowly at
the same time as the metal in the weld cools.

[Illustration: Figure 25.--Preheating at _A_ While Welding at
_B_. _C_ also May Be Heated.]

An example of heating points away from the crack might be found in welding
a lattice work with one of the bars cracked through (Figure 25). If the
strips parallel and near to the broken bar are heated gradually, the work
will be so expanded that the edges of the break are drawn apart and the
weld can be successfully made. In this case, the parallel bars next to the
broken one would be heated highest, the next row not quite so hot and so on
for some distance away. If only the one row were heated, the strains set up
in the next ones would be sufficient to cause a new break to appear.