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Old 12-24-2005, 12:44 PM
Brian Martin,Freelance adviser
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Sandblast warping heat MYTH.

A fellow forum visiter was nice enough to send me this email a long time ago after I posted about heat causing warpage while sandblasting. Very interesting stuff. If you feel the metal while sandblasting, it actually gets cooler from the air being blown on it!!

I deleted his name not knowing if he would want me to post it.


Sandblast warping
XXX XXXXX here - I read your post / recommendations against sandblasting body parts and I am in complete agreement. However, the mechanism that creates warpage is not the heat associated with friction of the abrasives hitting the metal. Rather it is the compressive forces induced within the sheet of metal by the heavy abrasive particles hitting the metal surface. You will not see this same warping when light abrasives are used because they lack the kinetic energy required to compress the molecules in the steel sheet thus leaving the sheet in the same stress state as it was in when formed by the dies at the factory.
This is a common misunderstanding, yet this behavior is well understood by some industries and is exploited by those who understand it to improve fatigue life of certain parts (like connecting rods for instance - they are often shot peened to redistribute stresses and place the surface of the rods in compression to reduce fatigue crack propagation).
When you blast one side of sheet metal, that side of the sheet metal surface will build compressive stresses and will physically grow - this places the opposite side of the sheet into tension - this will result in the side being blasted to bow up toward the blasting source.
In order for warping to occur due to thermal conditioning - the metal actually has to be heated up to a point where the molecules reach a transition state where they go from one packing arrangement to another (close cubic packed to close hex packed) - picture a six pack of beer held together with the plastic rings - that is stress relieved / annealed steel. Now - take the cans out and stack them so that each row of cans is offset by half a can diameter - this is a tighter pack - although less friendly from a packaging standpoint at the grocery store. This new denser orientation is the orientation that steel molecules take on when raised to a sufficient temperature (depends on carbon content and alloys) . . . if you raise steel to this temperature and then quench it - you lock the close packing orientation into this dense packing structure and this results in shrinkage (and increased hardness / brittleness).
Using this technique it is possible to take a piece of 1013 mild steel, whack a piece off, heat the piece up and then drop it in a bucket of water, use a water cooled grinder to sharpen it . . . and then you can cut the parent piece of metal with it as it is harder and denser in this condition.
Soooooo . . . . that is the difference between warping / shrinkage of metal due to sandblasting (compressive mechanism - localized molecular packing) or due to thermal influences (gross - macroscopic transition of molecules to a higher density due to re-orientation of molecular packing).
With all this knowledge in hand (or mind) . . . you now also know that thermal conditioning is typically used to shrink metal on a localized basis where sandblasting will always cause metal to grow on a localized basis.
Hope this makes sense . . . thanks for the great writeup!
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