Tony Smith wrote:

> 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

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