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Up Topic Welding Industry / General Welding Discussion / Shrinkage
- - By overthehill (*) Date 08-04-2002 23:54
Help me out here fellas. When metal is heated it expands, when it cools it contracts, and it contracts more than it expands. Exactly why does metal shrink when it cools? I have a basic understanding of the grain structure change from the heating and cooling process, but don't quite understand what is "lost" for the metal to shrink.
Parent - - By billvanderhoof (****) Date 08-05-2002 04:16
Imagine a bar of steel fitted between two immovable objects. If you begin to heat it it will try to expand- first it will get tight- then it will deflect elastically (like a spring). If you let it cool from this state it would return to the original condition of "just fits". If you heat it further the stress will build further untill it exceeds the yield point at whatever temperature it is at. The bar will then yield becoming effectively shorter. Now if allowed to cool it will be found to be loose. When welding or burning the cooler metal away from the HAZ constrains the heated metal and forces it to yield (cool metal being stronger than hot). Thus after it cools it winds up curved toward the heat.

Hope this helps

Bill
Parent - By CHGuilford (****) Date 08-05-2002 08:47
You explained it very well. Nothing is "lost" it is only relocated like squeezing modeling clay.
CHGuilford
Parent - - By overthehill (*) Date 08-05-2002 15:41
I'm almost embarrassed that I don't get this, except I'm an old, uneducated, dumb welder and I've looked way more stupid than this on many occasions. I've used the "draw" to straighten things for years and the other day I was showing a college kid helper in my shop how to straighten a piece of metal warped from a weld. He ask me why that worked and I couldn't tell him, at least the truth (although that generally doesn't stop me). OK, so the heated metal is expanding. The cooler metal is restricting it. At the "yield point", the heated metal is still expanding. If nothing is lost, does it thicken?
Parent - By kam (**) Date 08-05-2002 19:00
The main reason parts bend after you weld them is primarily due to reduction of stress or stress relief in the parts. That is why they warp. Nothing is lost. Matter cannot be created or destroyed.

What you are doing is relieving the stresses that were put into the metal when it was made at the steel mill. When the steel is processed you can have residule tensile stresses praimary at the surface, or compressive, or neutral. If you have a part which has tensile stresses on the surface and you weld one side you put the stresses on that side you weld to neutral. The stresses on the other side of the part are still there and that is what causes the part to warp.

Hope this helps

kam

Parent - - By BP Maas (**) Date 08-05-2002 21:43
It does thicken. You can confirm this with a caliper by measuring the thickness of a piece of carbon steel before performing your "flame straightening" technique, and measuring after. One of the best examples of this the Seattle Space Needle, those long curved structural shapes were not rolled, they used "flame straightening" to put the curve in.

Regards, Brad
Parent - By NDTIII (***) Date 08-06-2002 03:30
I think an esy way to explain it is that "all" materials expand when heat is applied and contract when it is taken away. Not only metals. This goes down way further than the grain structure. It occurs at the molecular, and even down to the subatomic level or the atoms themselves. The molecules expand when heated and contract when cooled. This accounts for the shrinkage. The metal doesn't go anywhere. The molecules simply become tighter. That gives the metal its refined grain structure.
Parent - By overthehill (*) Date 08-06-2002 04:52
Thanks Brad, and all of you. I've got it.
Up Topic Welding Industry / General Welding Discussion / Shrinkage

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