I had to weld some patches into my back plates to save them. I preheated, welded, post heated and used a needle gun to relieve stress. Then used my welding blanket on top of that to slow down the cooling. I did it in short welds. I also clamped the plates down to a small welding table where I was putting the patches in. Very time consuming. It took a few try’s as some of the welds would crack as it cooled. I preheated more and that finally helped. I welded a lot of cracks in my plates and had no problems. The patches were tough. I am just a hobby welder so that could have been part of the problem. I used a mig with both stainless and mild steel wire. The heat seemed to be what made the difference.
Gary