> 
> > That's a fairly contrived case,
> 
> I don't think so. I think this is a rather normal case when 
> heating up a localized area in a thick metal sheet or a 
> block. Few people work on large sheets or blocks that are at 
> a uniform temperature when gas welding.
> 
> > and is quite possible if you were aggressive enough in 
> heating the hole. 
> > Don't forget, steel conducts heat very well and is quite 
> elastic, two 
> > factors woking against the shrinking hole.
> 
> How is that? Elastic isn't the same as compressible. It's 
> pretty common to have steel almost melting around the welding 
> area, and it still being not even red or even only a bit warm 
> far enough away from the welding area. 
> 
> > Cast iron would be the better choice. 
> 
> For reasons you already mentioned and I agree with, I don't 
> think so. It doesn't have the same elasticity. You may 
> consider cast iron, I don't. 
> 
> > I'm sure people making the main guns on battleships would 
> have noticed 
> > this... or even the people firing them...
> 
> I'm not sure I made myself clear enough. A gun barrel does 
> not fulfill my criteria; it is too thin and probably would 
> heat up fairly uniformly. It probably expands (inside and 
> out). They probably have found a way to deal with that.
> 
> > I couldn't say with 100% certainty (I no longer work with a 
> > metallurgist), but if it did happen, it wouldn't remain 
> that way for 
> > long.
> 
> Well... depends of course. I'm fairly certain that if you 
> take a normal sheet, 1 cm thick, maybe 1 m by 1 m or maybe 
> bigger, and heat it with a welding flame in the middle, the 
> outside remains quite a bit colder than the middle, even 
> after a long time. I'm not sure it would not bend with time, though.
> 
> Gerhard


Unless you can come up with a good reason to rapidly heat the inside of a
20mm x 20mm cylinder with a 1cm hole drilled in it, it's a contrived case.
Getting the inside of a large gun barrel to size is about the closest case
you'll get.  Regardless, it doesn't matter.

Heat spreads out pretty quickly in metal when you heat up one end of it.  If
it didn't, I could throw away my welding gloves.

The metal outside the heating zone doesn't get compressed, it stretches.
Blow up a balloon, same thing.  Heating up the hole is the same as adding
more air to the balloon.  Everything gets bigger.  Elastic.  Cast iron isn't
as elastic as steel, so it is more likely to crack.

If you are trying to get a bearing into a casting, and you hit the hole with
too much heat, the casting will crack.  The hole may be red hot, the edge
may be ice cold, and it will still crack.  Heating the hole stresses the
metal.  The stress isn't relieved by material moving into the hole, it
propagates outwards.  It either expands or cracks.  Note that expanding
includes buckling.  It cracks because the outside can't expand fast enough.

If the material wasn't elastic, nor prone to cracking, then the hole would
get smaller as there is nowhere else to go.  (So cast iron is your best
bet.)  The universe wasn't built that way.  To get the hole to shrink, the
area surrounding the heating zone needs to be non-elastic.  If it's metal,
then it won't be.

Read that paragraph again  :)

You could get the metal hot enough to melt or become vapor, but that's
cheating.

When welding, you need to compensate for the fact that a small area will be
overheated.  The metal will distort.  Every time.  Sheets buckle, tubing
twists.  (A 1cm plate is quite thick, you'll be there a while with that
torch.)  You don't point a gas torch as a point on the metal & leave it
there until it gets to red hot.  You need to heat up a wide area to reduce
the stress caused by expansion.  That's why you see welders waving the torch
about.  Jewellers too, but since their bits of metal are fairly small, they
have the luxury of heating up the whole piece.  Spot welders especially suck
for distorting metal.

I've bashed enough pieces of metal in my life (fitting & turning) and the
hole doesn't get smaller.  Your blacksmith friend was mistaken.  Don't
forget the numbers we are talking about are very small, I didn't realise
blacksmiths use micrometers to measure their work!  And theirs are better
than mine!

Tony

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