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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, youll
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
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
Lets 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 cant get shorter as it
cools. Let it cool down.
Then measure it.
Youll find that its 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