> > Please forgive me for jumping into this thread late. I assumed that 
> > you were talking more or less theoretically about round holes in 
> > uniform, isotropic materials, and applying heat in the 
> general region of the hole.
> 
> We started out with the quiz question "what happens to a hole 
> in metal when you heat the metal?" and there was the obvious 
> answer "it grows, of course". 
> 
> I brought up the idea that the "it grows, of course" requires 
> a number of assumptions to be fulfilled, and claimed (at this 
> point pretty much shooting almost in the dark) that there are 
> situations where the hole may become smaller. 
> 
> At that point this wasn't yet theoretical, and the material 
> was assumed to be steel or iron or similar. As we proceeded 
> to discuss, we elaborated some conditions and for lack of 
> proper tools for executing enlightening experiments, the 
> discussion became more and more theoretical.
> 
> > Sure, if any of those things apply, some dimensions of the 
> hole will 
> > get smaller. What's your point?
> 
> More or less this is my point. Maybe this is obvious to you, 
> but it isn't to me, and it doesn't seem to be to many others. 
> So far I seem to have been unable to convince Tony Smith of 
> that possibility, but this may well be because I don't really 
> understand that much about the details of the effects involved. 
> 
> > Ah, yes, hypothesizing a container that has no elasticity 
> at all kind 
> > of changes the universe of discussion.
> 
> I didn't mean to change the universe with that; it's just one 
> aid for explaining things. I think it doesn't have to change 
> things. If you simplify the temperature gradient to a cold 
> cylinder outside and a hot cylinder inside, there will be 
> stress on both (on the outside cylinder towards increasing 
> size, on the inside cylinder towards decreasing size) and the 
> inside cylinder will likely grow less than it would if both 
> were heated uniformly. What the maximum of that reduction is 
> I can't tell... I'm sure I could calculate it if I spent the time.
> 
> > In that case, shrinkage of the hole requires a reduction of 
> modulus, 
> > as I said before;
> 
> Which happens (see the posted link). One question is whether 
> the reduction is enough.
> 
> > I'll jump back out of this thread now. Please carry on.
> 
> No matter whether you jump out or stay in, thanks a lot for 
> your contribution. It inspired me to research which cleared a 
> few things up in my head.
> 
> Gerhard


I remain unconvinced  :)

The only time I can see the hole getting smaller is if you heat it to around
550C or more.  At this point it loses it's elasticity, and is very
malleable.  It's also lost a lot of its strength (half?  40%?) and goes
downhill rapidly after that.

You could argue that the metal is no longer homogenous, as a phase change
has occurred in the area around the hole.  This why I said heat the hole to
500C.  (Strength starts dropping at 220C.)  

I used to know these numbers, but at least I kinda remember the tempering
colours.  (There's always Google.  These are close enough.)

The steel will deform, and thus the hole may get smaller.  But it won't
shrink evenly, it'll collapse, and remain that way once the heat is removed.
Sheet metal warps, meaning the hole circumference has gotten bigger.

The only material I can think of that behaves the way you describe (shrinks
then expands when heated) is water.  Ice heated up shrinks, and then starts
expanding after 4C degrees.

Making the steel plate with a hole in it to drop a ball bearing thru is
possible, rapid heating of the hole make be troublesome.  Lead or tin (or
solder) might prove to be good test subject, as it has a high COTE (30-40 vs
steels 10-12), but offset by a fairly low melting point.  I can see a lot of
solder sticks from here...

The main reason I remain unconvinced is no-one (save that guy on the
blacksmiths site, but he said a couple of other odd things too) has actually
seen it happen.  There are lots of bits of steel with holes drilled in them,
and surely someone would have gotten a shaft wedged (temporarily) in one by
now, or had a bush/bearing get squished.  Scientists also love the weird
exceptions to rules, and they seem to be fairly quiet.

Tony

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