Hi sbruno,
There are several approaches that can be taken to help control shrinkage forces. Joint design, design of the assembly, and proper welding sequence are important factors. Here are a few suggestions: 1. Do not overweld. The more metal placed in a joint, the greater the shrinkage forces. 2. Use intermittent welding as opposed to continuous where possible. 3. Use as few welds passes as possible. Fewer passes with larger diameter electrodes are preferable to a greater number of passes with smaller diameter electrodes. Shrinkage caused by each pass tends to be cumulative, thereby increasing total shrinkage when many passes are used. 4. Place welds near the neutral axis. Distortion is minimized by providing a smaller leverage for the shrinkage forces to pull the plates out of alignment. 5. Balance the welds around the neutral axis. This offsets one shrinkage force with another. 6. Use backstep welding. 7. Anticipate shrinkage forces by placing parts out of position before welding, so that they are drawn into the correct position afterwards. 8. Plan the welding sequence. Place weld metal at different points so that as the assembly shrinks in one place, it counteracts the shrinkage forces of the welds already made. 9. Remove shrinkage forces after welding by peening the weld bead. (Engineer approval should be obtained prior to using this method). 10. Minimize welding time. It is preferable to finish the welds quickly, before a large volume of surrounding metal heats and expands. The welding process used, electrode type and size, welding current, and travel speed affect the degree of shrinkage and distortion. There is also a very good book on the market that may be of interest to you, "Distortion Control" authored by John Stewart. It contains a lot of useful information. Check it out.
You haven't provided much info on the configuration of the plates and joint types used. If you are butt welding plates together in the same plane, you will need to use double bevel grooves to balance the weld thickness on each side, and alternate welding layers on each side to counteract shrinkage. If you are making tee (fillet or groove) joints, the welding should also be alternated between sides to equalize the shrinkage as it is welded. If you are welding attachments on one side of a large flat base plate and the opposite side must remain within some flatness tolerance, you will need to restrain the flat plate with temporary stiffeners on the opposite side, then remove them and grind flush when all the attachment welds are finished on the other side. The other option is to use thicker plate and mill it flat when the welding is finished. There may be a bit of trial and error before you get good at estimating how much shrinkage/distortion will occur and how to best counteract it. Mr. Norris posted some good general guidance that should be followed. Use the largest electrode size possible to minimize the number of weld layers. Some folks believe that many small beads with low current creates less shrinkage than a few large beads with large electrodes. Don't believe it, weld some samples yourself and measure the distortion. Higher preheat also helps. If you would normally preheat this thickness to 250 F, increase the preheat to 350-450 F.
Good luck.
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