On Mon, 12 Mar 2018 11:27:42 -0400, Frank Krygowski
wrote:

On 3/12/2018 9:14 AM, Emanuel Berg wrote:
John B. wrote:

Sure. When you heat, or melt a metal it
expands. If you bond two pieces of metal when
they are hot, or molten, i.e. in it's
expanded state when it cools and shrinks
there has to distortion or if the parts are
not free to move extreme stress will be
imposed. See: https://tinyurl.com/y7feu33f

Still, the area affected is local if we assume
two long tubes being put into one so what is
distorted is the intersection area (plus
change) and by implication the angle or
relative positions of the two tubes/tube parts?

The very first welding project I ever did was back in the 1970s. I
designed and built a little 4' wide, 6'long, 1' deep utility trailer to
tow behind our car. The frame is steel angle. Visualize a two horizontal
4' x 6' rectangles of welded steel, with the upper one resting above the
lower one on 1' tall vertical steel struts.

So after a couple preliminary welding lessons, I welded up the two 4x6
rectangles flat on the workshop floor, then clamped them and the
vertical struts together and, working late at night, hurried to weld the
verticals. I rushed through the job, then left for home.

When I returned the next day, I was very disappointed to see that the
trailer "box" was no longer nicely square. The distortion from welding
had sort of pulled a rear corner upward an inch or more.

I didn't try to correct it, because the trailer was a rush job, and it's
plenty strong. There's not much chance of dangerous residual stresses.
The trailer's done lots of heavy hauling, and I still use it
occasionally. But its not as pretty as I would have liked.


We were making an aircraft work stand that required a 8' x 8'
platform. we got a couple of 4' x 8' x 1/4" plate. Laid them out on
the shop floor all nicely aligned, gaped one rod diameter and all and
weighted them down with some scrap metal and told the newest
apprentice to tack weld these together.... all the way. Do a 1 inch
tack then move up about 6 inches and do another one...
We went off to have a cuppa, or something, and when we came back the
apprentice had decided to make a continuous weld.... much stronger he
figured.

He had gotten about a foot, maybe 18 inches, and the flat 4' X 8'
plats had warped horizontally so that the far end of the joint was
nearly 6 inches wide.

--
Cheers,

John B.