Welding
Non-Ferrous
Metals
Treating
Welding
Cast Iron
Welding
Ferrous
Metals
2
Continued
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of the hot section. When welding has been completed, and the metal is cooling,
the cooler section acts as
a restraint against uniform contraction of the metal in, or close to, the weld
itself. Let’s explore some of the practical
aspects of this problem. Expansion
and Contraction in Sheet Metal If
we take a piece of sheet metal and rapidly heat it with a torch along one edge,
that edge will get ”wavy”, as illustrated,
with exaggeration, in Fig. 11-1. Why? Because the cooler metal away from the edge
will not expand as much
as the edge itself, which therefore can increase in length only by ”buckling”
a bit. If we then allow the piece to cool
down, most of the ”buckling” will disappear. However, if the edge is
carefully measured before heating and after
cooling, it will be found that it has shortened a trifle. During the period of
expansion, there was some thickening,
or ”upsetting” of the metal in the edge, in addition to the buckling.
As a practical matter, the upsetting
of the metal at the very edge, and the slight decrease in edge length, are of
little significance. The buckling,
however, can create major problems, with respect to both the welding operation
and to the appearance or utility of
the finished product. In production operations, when it is essential that the
finished weld be perfectly flat, the
usual procedure (regardless of the welding process used) is to clamp the metal
so firmly, and so close to the actual
weld zone, that only ”upsetting” can take place, with all other movement
of the pieces
completely restrained. We’ll be talking more about that subject in Chapter
12. For another
example of the effects of restrained expansion and contraction in sheet metal,
look at Fig. 11-2. Here a piece
of sheet has been cut nearly in two by a slit. Then (as in A) spots along the
edges of the slit are heated
