|Welding distortion means more than just changing the shape of the object you're welding. Welding distortion means localized yielding of the material and locking in new stresses that may not have been considered in the original design. The phrase "stress relieving" never took on greater importance.
This is probably one article where the pictures can speak for themselves. The object shown in the picture to the lower left was a simulation of the welding stresses imposed upon a typical lifter installation in a rotary dryer unit. If you've ever wondered why welding is minimized to skip welding and fillet welds, here's why. Just look at this distortion! A frontal view of the full penetration weld can be seen in the lower right picture. Ultrasonic inspection of this sample showed the weld to be a very good full penetration weld with no lack of fusion or poor penetration.
|The imposed welding stresses are phenomenal. Assuming that each weld pulled up on the shell material uniformly, we're talking 3/8" each on 316 stainless steel that is 1/2" thick! This material has a minimum yield strength of 75ksi. The weld pulled straight up on the shell material with absolutely no leverage whatsoever, talk about stress. The picture below illustrates the total deformation of the shell material. Was the distortion caused by excessive heat input, or too many weld passes? Take a look at the pictures to the bottom left and right. There are about three to four weld passes on each side, well within the expected number of passes for the size rod used. Their half-moon shape indicates good heat control.|
|What this all boils down to is that if the material being welded is not allowed to distort in order to relieve the stress, it will tear itself in order to relieve the stress, or worse yet, it won't tear until it is in service for awhile.
In the case above, let's say we used a jig to keep the material from distorting. More than likely the weld would have sucked in the back side of the shell plate. However, let's just say that the shell plate was 2" inches thick. That shell plate isn't going to suck in one bit, and that's when a "lamellar tear" may occur. Such a tear would occur on the bottom side of the weld to the shell, and it would not be detected visually. It would be detected using ultrasonics. However, if the weld was not inspected, it would then go into service with the tear.
Remember this - when you put that material in a jig to keep it from distorting during the welding operation, don't be fooled. The welding stresses are still there.
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