[TheForge] Shrinking Stainless Sheet metal

Fri Jan 19 22:22:54 EST 2007

Dan:
Good clear explanation, thanks.
By "shrink", in this context, I was thinking in terms of thinner 
stock where both faces would heated pretty equally by conduction, 
but only a small area would be heated. This ought to cause 
thickening as the heated area tries to expand against the 
surrounding material. I understand that the only volume change 
would be from scaling.
I was speculating that it ought to work well with stainless steel.
I once had a go at straightening 1" plate with a rosebud and 
water spray...it was wildly frustrating...felt really dumb at the 
end.
A last Q, if i may....If SS alloys conduct heat slower, wouldn't 
such stuff be good for hydraulic forging press dies, where heat 
loss to the dies is  a big problem?...Pete f

Dan Hayes wrote:
> 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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