To much heat in your part. I'd stitch weld it and let it cool a little between runs. Ex 2in weld on the left side, then the same on the right side, then the middle. After that I'd let it cool then repeat the process until you got it all welded. Or if you have a metal table tack it to the table then ...

Hey guys. I'm another new ish guy joining to learn and share my knowledge along the way.
Graduate of Georgia trade school, mig, tig, stick welder. Currently self employed.