--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.
[Illustration: Figure 26.--Cutting Through the Rim of a Wheel (Cut Shown
at A)]
If welding is to be done near the central portion of a large piece, the
strains will be brought to bear on the parts farthest away from the center.
Should a fly wheel spoke be broken and made ready to weld, the greatest
strain will come on the rim of the wheel. In cases like this it is often
desirable to cut through at the point of greatest strain with a saw or
cutting torch, allowing free movement while the weld is made at the
original break (Figure 26). After the inside weld is completed, the cut may
be welded without danger, for the reason that it will always be at some
point at which severe strains cannot be set up by the contraction of the
cooling metal.
[Illustration: Figure 27.
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