I am trying to weld a 12" circular .5" thick A48 Class 40 Cast iron plate into a A48 Class 30 cast iron part. The part has a recess machined into it for the circular plate and both the part and insert are chamfered for the weld to penetrate.
I have done this a few times where both parts are Ductile Iron and my method results in an 80-90% chance of success. But this cast iron is kicking my butt.
I preheat both parts with a rosebud to as close as I can get to 450F. I wish I had an oven or something because past 350 is difficult with the size of the part. Anyways once its hot, I weld using a United Alloys Supercast 1/8" rod at 120A DCEP. I run one rod and spin the part 180 and run another rod on the other side, and then back, the 2nd pass is from a fresh area into the start of the previous weld, not starting where the last pass ended. I continue like this until its welded fully around. At this point the whole assembly is still 350F or so. I post heat back up to 450F and cover with thermal blankets folded over, so maybe 16-32 layers. It stays warm so long I leave it till the next morning. Then I check it with Radnor crack check kit and both times now it cracked around the full circumference of the weld. On the outside between the weld and the main part.
What am I doing wrong? Is the solution something I don't have like an oven? Too much amps?
In this first photo the center plate is slightly taller than the face of the outer part, and the entire surface is getting faced on a machine after my weld so the look of my weld doesn't matter so much in that regard.
