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EXPANSION AND CONTRACTION
All metals expand when heated, contract when cooled. Put a steel bar into a furnace and heat it up to a
temperature of 5000C. It will get longer. Take it out and cool it. It will return to its original length. Further, if you can
find a way to precisely measure the width of the bar while it is hot, and again while it is cold, you’ll find that width
also increased during the heating process. To put it another way, expansion and contraction are ”three-
dimensional”.  If the length of the bar increased one per cent, both dimensions of the cross-section increased one
per cent.
Now suppose that this steel bar, instead of being placed in a furnace so that it could expand in all directions, is mounted between two immovable objects, such as two five-ton blocks of granite, and then heated with a torch until the center of the bar reaches a temperature of 5000C. The granite blocks will effectively keep it from getting longer as it heats up. As a result, it will get ”fatter” than it would had it been free to increase in length. In fact, if the bar is now allowed to cool down to its original temperature, it may wind up a bit shorter than it was at the start, and also a bit ”fatter”.
Let’s try one more example to show what can happen if normal three-dimensional expansion or contraction is
restrained. Take a steel rod exactly five feet long, and heat it up to 5000C or so with no restraint on its movement.
Then, while it is hot, clamp the ends in some way so that the bar can’t get shorter as it cools. Let it cool down.
Then measure it. You’ll find that it’s a bit longer than five feet, and, somewhere along its length, a bit ”skinnier”.
These are key points to remember about expansion and contraction: First, that changes in dimension, if there is no
restraint, will be of the same proportion in all directions. Second, that if restraining forces prevent a change in one
dimension, changes in other dimensions will be greater, and often permanent.
In welding operations, the ”three-dimensional” forces of expansion and contraction are seldom unrestrained.
Heating and cooling are usually more-or-less localized. You generally apply heat to an edge, not to the entire piece
of metal. While the conductivity of the metal will carry heat away from the edge and back into the body of the part,
the edge will reach a temperature of well over 10000C (in the case of steel) while the metal only a few inches away
from the edge may be heated only to 3000C. The cooler metal acts as a restraint to prevent uniform increase in
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