Figure 3-31.Effects of expansion and contraction.
contraction are shown in figure 3-31. View A shows a
bar that is not restricted in any way. When the bar is
heated, it is free to expand in all directions. If the bar is
allowed to cool without restraint, it contracts to its
When the bar is clamped in a vise (view B) and
heated, expansion is limited to the unrestricted sides of
the bar. As the bar begins to cool, it still contracts
uniformly in all directions. As a result, the bar is now
deformed. It has become narrower and thicker, as shown
in view C.
These same expansion and contraction forces act on
the weld metal and base metal of a welded joint; how-
ever, when two pieces of metal are welded together,
expansion and contraction may not be uniform through-
out all parts of the metal. This is due to the difference in
the temperature from the actual weld joint out to the
edges of the joint. This difference in temperature leads
to internal stresses, distortion, and warpage. Figure 3-32
shows some of the most common difficulties that you
are likely to encounter.
When you are welding a single-V butt joint (fig.
3-32, view A), the highest temperature is at the surface
of the molten puddle. The temperature decreases as you
move toward the root of the weld and away from the
weld. Because of the high temperature of the molten
metal, this is where expansion and contraction are great-
est. When the weld begins to cool, the surface of the
weld joint contracts (or shrinks) the most, thus causing
warpage or distortion. View B shows how the same
principles apply to a tee joint. Views C and D show the
distortions caused by welding a bead on one side of a
plate and welding two plates together without proper
All metals, when exposed to heat buildup during
welding, expand in the direction of least resistance.
Conversely, when the metal cools, it contracts by the
same amount; therefore, if you want to prevent or reduce
the distortion of the weldment, you have to use some
method to overcome the effects of heating and cooling.
You can control the distortion caused by expansion
and contraction during welding by following the simple
procedures listed below.
Figure 3-32.Distortion caused by welding.