> > If you heat the middle (containing a hole) of a piece of metal, it 
> > will expand.  But the rest of the piece isn't getting out 
> of it's way 
> > to make expansion easy.  This introduces stress in the 
> piece of metal.
> 
> Right: "introduces stress". So what does this stress do to the hole? 
> 
> This is a normal situation when (gas) welding, isn't it? I 
> don't think people usually heat the complete sheet up to 
> welding temperature when they weld a piece to a sheet. So 
> there is then a hot area in the middle, surrounded by a 
> cooler area. What happens to a hole in the hot area in the 
> middle? (You probably shouldn't try that with cast iron, but 
> that's a different story.)
> 
> So far even Tony's argument ("'Gadzooks!'") didn't really 
> sound too scientific about what happens in this situation to a hole. 


The hole gets bigger.

For thin metal the metal around the hole will buckle (welding is a pain,
regardless of method).  For thicker stuff (i.e. cast iron) it may crack the
metal outside the heated zone.

That's why you heat up the whole piece.  Some people dip big castings into
heated oil baths.

The answer is either nothing / too small to measure / it gets bigger / it
get bigger and damages the piece, resulting in Gadzooks! being heard.

Note 'gets smaller' isn't on the list.

Tony

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