[TheForge] Shrinking Stainless Sheet metal

Sat Jan 20 11:04:10 EST 2007

Peter,

What I'm picturing is a situation equivalent to a sheet metal panel welded
inside a heavy steel frame, where the sheet metal panel ended up warped. If
the idea is to then apply heat to a bulged area to shrink it, similar to
hitting a heat shrink storm window with a hair drier, it's not going to
work. The surrounding material isn't going to be stiff enough to restrain
the heated area (when hot) to effectively "upset" the heated area. All
that's going to happen is the heated area will get hot and grow, and then
cool and shrink back to exactly where it was to begin with.

If welding was limited to the edges, and you were to cut the panel out with
a saw, I'd expect it to lay relatively flat. The only permanent effects of
welding on the panel will be limited to areas where the panel yielded in
tension as a result of being over stretched between two peripheral welds.
The cause of the distortion is limited to the welds. Unfortunately and for
the most part, it's not reversible. It could have been minimized by clever
tacking, weld sequencing, heat control, preheat, etc., but once the welds
are complete, there is little you can do to the sheet metal panel to correct
the situation. There may be something you can do by adding to, or removing
and re-welding the peripheral welds, but not much you can to working on the
panel itself.

Andy is absolutely correct that there are situations where you can change
the volume by heat treatment. However, these situations are limited to those
where the internal structure (crystallographic) is permanently altered by
heating. Tempering martensite (Andy's example) is probably the most common.
If your panel is austenitic stainless steel it's not going to move much.

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 10:23 PM
To: dhayes at dthayes.com; Sponsored by ABANA
Subject: Re: [TheForge] Shrinking Stainless Sheet metal

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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