It is very important to understand that you do not cool the red area. If
you have spray mist or wet rags you cool the area surrounding the red
section keeping as large a temperature gradient as possible. The larger the
temperature difference the larger the forces are on the plastic red area.
You chase the perimeter of the red until it disappears. At that point there
is no more plastic movement .
In our local area a huge pipe bender is being set up that will heat by
induction using massive amounts of electrical power and at the same time
water spray mist is applied. This is for large pipe over two feet diameter
from what I understand.
Or.... you can build a 3000 ton horizontal press that pushes the pipe. We
built one for a customer over a year ago,.
Randy

"Eric R Snow" wrote in message
...

Greetings Jerry,
What happens is that the heated metal expands. Since the hot metal is
plastic it will move as far as it can, which is where the metal is too
cool to deform. Then when the hot area is cooled the space between the
molecules shrinks and the metal contracts. If left to cool slowly the
displaced metal will move back into it's original shape. But rapid
cooling prevents this so instead the metal shrinks. So rapid cooling
is important. The metal you are shrinking must be able to go through
these hot and cold cycles without damage. So mild steel is a good
candidate for this process. It takes practice but not that much and
once mastered it can be real useful. Another place this method is used
is with removing dents from sheet metal, as in auto bodies. Big dents
in an auto body stretch the metal and to avoid using bondo to fill the
depression heat shrinking the area is sometimes used to fix these
dents. I've done it with a torch and a wet rag. Sheet metal is so thin
that a wet rag can cool the area fast enough.
ERS