logic confounded by wisdom or was it the other way around ???
thermal expansion and negative thermal expansion (contraction) I understand somewhat, being a welder and all. I heat up a pipe and it expands a certain amount, and when cooling it contracts, but some materials contract more than thermal expansion. Then we mix materials with differing thermal expansion and negative expansion rates and we wind up with even more issues. Stupid atoms, chemistry and science.
Quote:Handbook - ESAB Welding & Cutting ‑ North America
EXPANSION AND CONTRACTION
All metals expand when heated, contract when cooled. Put a steel bar into a furnace and heat it up to a temperature of 5000C. It will get longer. Take it out and cool it. It will return to its original length. Further, if you can find a way to precisely measure the width of the bar while it is hot, and again while it is cold, you’ll find that width also increased during the heating process. To put it another way, expansion and contraction are ”three-dimensional”. If the length of the bar increased one per cent, both dimensions of the cross-section increased one per cent.
Now suppose that this steel bar, instead of being placed in a furnace so that it could expand in all directions, is mounted between two immovable objects, such as two five-ton blocks of granite, and then heated with a torch until the center of the bar reaches a temperature of 5000C. The granite blocks will effectively keep it from getting longer as it heats up. As a result, it will get ”fatter” than it would had it been free to increase in length. In fact, if the bar is now allowed to cool down to its original temperature, it may wind up a bit shorter than it was at the start, and also a bit ”fatter”.
Let’s try one more example to show what can happen if normal three-dimensional expansion or contraction is restrained. Take a steel rod exactly five feet long, and heat it up to 5000C or so with no restraint on its movement.Then, while it is hot, clamp the ends in some way so that the bar can’t get shorter as it cools. Let it cool down. Then measure it. You’ll find that it’s a bit longer than five feet, and, somewhere along its length, a bit ”skinnier”.
These are key points to remember about expansion and contraction: First, that changes in dimension, if there is no restraint, will be of the same proportion in all directions. Second, that if restraining forces prevent a change in one dimension, changes in other dimensions will be greater, and often permanent.
In welding operations, the ”three-dimensional” forces of expansion and contraction are seldom unrestrained. Heating and cooling are usually more-or-less localized. You generally apply heat to an edge, not to the entire piece of metal. While the conductivity of the metal will carry heat away from the edge and back into the body of the part,the edge will reach a temperature of well over 10000C (in the case of steel) while the metal only a few inches away from the edge may be heated only to 3000C. The cooler metal acts as a restraint to prevent uniform increase in dimensions of the hot section. When welding has been completed, and the metal is cooling, the cooler section acts as a restraint against uniform contraction of the metal in, or close to, the weld itself. Let’s explore some of the practical aspects of this problem.
Expansion and Contraction in Sheet Metal
If we take a piece of sheet metal and rapidly heat it with a torch along one edge, that edge will get ”wavy”, as illustrated, with exaggeration, in Fig. 11-1. Why? Because the cooler metal away from the edge will not expand as much as the edge itself, which therefore can increase in length only by ”buckling” a bit. If we then allow the piece to cool down, most of the ”buckling” will disappear. However, if the edge is carefully measured before heating and after cooling, it will be found that it has shortened a trifle. During the period of expansion, there was some thickening, or ”upsetting” of the metal in the edge, in addition to the buckling.
As a practical matter, the upsetting of the metal at the very edge, and the slight decrease in edge length, are of little significance. The buckling, however, can create major problems, with respect to both the welding operation and to the appearance or utility of the finished product. In production operations, when it is essential that the finished weld be perfectly flat, the usual procedure (regardless of the welding process used) is to clamp the metal so firmly, and so close to the actual weld zone, that only ”upsetting” can take place, with all other movement of the pieces completely restrained. We’ll be talking more about that subject in Chapter 12.
For another example of the effects of restrained expansion and contraction in sheet metal, look at Fig. 11-2. Here apiece of sheet has been cut nearly in two by a slit. Then (as in A) spots along the edges of the slit are heated rapidly, with a torch. Not too much seems to happen. The cold part of the metal restricts any general expansion, so that the forces tend merely to ”upset”, or thicken the edges, at the spots most strongly heated. However, when the piece is allowed to cool, the slit closes at its open end, and one end may even slide past the other, as indicated inB. The forces of contraction, rather than eliminating the ”upsetting” that took place during the heating, have shortened both sides of the slit, and the metal in the uncut end of the sheet has had sufficient elasticity to allow it to act as a hinge. The point we’d like to make is this: that what happens during the cooling period is seldom the reverse of what happens during the expansion period.
Fig. 11-1. When the edge of a piece of sheet steel is heated, expansion will cause the hot edge to warp or wave. (The effect shown here is much exaggerated). After cooling,however, the heated edge is likely to be a bit shorter than it was originally.
Fig. 11-2. Because of what may be termed the ”hinge effect”, heating and cooling will tend to close the gap between parts unless some method is provided to prevent that from happening.
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