[TheForge] Shrinking Stainless Sheet metal

[Dan Hayes]

Peter,

You cannot permanently "shrink" a metal object by general heating or
cooling. Of course the object will expand if it is heated and contract if it
is cooled but it will return to its original size and shape when returned to
the original temperature.

You can, however, distort an object by rapid differential heating. For
example, if you heat one side of a plate the heated portion will try to
expand. If the heated area is retrained by the bulk of the material, instead
of expanding, the heated area will experience a compressive stress equal to
that which would result from allowing the material to expand thermally, and
then compressing it back mechanically. If the stress exceeds the yield point
of the material, the heated material yields. If the plate is then allowed to
cool to a uniform temperature, the plate will be bowed with the "shrunken"
spot (the inside surface) being the material than yielded in compression.

The overall volume of the plate will remain unchanged. The heated/yielded
side will be in tension (shrunken). The other side will be in compression
(expanded). The sum total of compressive and tensile stresses will be zero.
The sum total of contraction and expansion will be zero. The plate will be
in equilibrium.

This principal applies to all objects, regardless of the shape, size or
complexity.

The above method of distorting and object is different that what happens
when molten weld metal is deposited in a joint. In this case, there is real
shrinkage of the joint. By that, I mean the joint (i.e. the V-notch) really
does get smaller thereby bending the part. This is because the weld metal is
hotter than the bulk of the adjacent material when it solidifies and
therefore shrinks more as the whole assembly cools to room temperature. The
volume of air is replaced by hot molten weld metal, the molten metal
solidifies and bonds to both sides of the joint, and draws them in as it
cools and contracts.

Both phenomenon occur during welding.

As for weld shrinkage, the primary factors are the coefficient of thermal
expansion (higher = greater shrinkage) and the difference in temperature
between the weld pool at time of solidification and the bulk of the weldment
(this is why preheat reduces distortion).

As for distortion due to differential heating, the worst case is a high
coefficient of thermal expansion, low ratio of yield strength to modulus of
elasticity, and low thermal conductivity. This makes it easy to heat a local
area to the point of yielding. By worst, I mean most difficult to control
accidental distortion. On the other hand, the same piece should be the
easiest to shape intentionally.

I hope this helps.

Dan

-----Original Message-----
From: theforge-bounces at mailman.qth.net
[mailto:theforge-bounces at mailman.qth.net] On Behalf Of Peter Fels And Phoebe
Palmer
Sent: Friday, January 19, 2007 2:11 AM
To: Sponsored by ABANA
Subject: Re: [TheForge] Shrinking Stainless Sheet metal

Sorry for being dense, but isn't the warping a sign of heating 
induced shrinkage?
I ask because i've been having a lot of trouble using heat 
shrinking effectively on compound curves in sheet steel.
  Gotta figure it out before i start quacking like a duck, any 
more often than i do...pf

James Binnion wrote:
> No,the high thermal expansion and the low thermal conductivity is what 
> causes the warping and further heat just makes it worse in my experience.
> 
> Jim
> On Jan 18, 2007, at 8:28 PM, Peter Fels And Phoebe Palmer wrote:
> 
>> Thanks James;
>> Shouldn't those same properties make it easier to heat shrink?...pete f
>>
>> James Binnion wrote:
>>> Stainless has a 25% higher thermal expansion rate than mild steel. 
>>> This is why it is such a pita to work when you are welding  it in 
>>> thinner sections it will warp at the drop of a hat it is also a poor 
>>> thermal conductor, much worse than mild so the heating is localized 
>>> more than with the mild and the thermal expansion is acting on a 
>>> smaller area resulting in greater warping.  If possible keep beads 
>>> short and move to a different area to do next bead and keep working 
>>> different areas around whole perimeter till the complete weld is done 
>>> to keep from getting too much heat in one area. This will help but it 
>>> is still likely to warp.
>>> Jim
>>> On Jan 17, 2007, at 1:38 PM, Ries Niemi wrote:
>>>>
>>>> Theoretically, it should move much like mild steel, but its so damn 
>>>> hard to get to move, ever, that it seems to be harder to heat shrink.
>>>>
>>>> ries
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