Tony Smith wrote:

> 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. =


This may be what the blacksmith is talking about -- that's what they do :)
This is also what I have imagined: the steel around the hole starting to
get red. =


> 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.) =


Hm... steel isn't really a "simple" material :)

Look at this:
http://www.mace.manchester.ac.uk/project/research/structures/strucfire/mate=
rialInFire/Steel/HotRolledCarbonSteel/thermalProperties.htm

All throughout this discussion, we assumed a rather constant thermal
expansion coefficient (I think you call it COTE). But according to this,
around room temperature it seems to be close to 0, and grows with
temperature. =


There is a phase change around 700=B0C, but even before and after that it
doesn't behave that linearly as we assumed. ("Linearly" would be a constant
COTE.)

> 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. =


This is what I've been trying to say. The shrinking of the hole -- if it
happens -- would probably be a plastic effect, not an elastic effect.

Gerhard

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