A few suggestions for welding thin wall chromoly. It's prone to contamination from being exposed to air while it's still hot enough to be reactive. Use a gas lens with a least a #8 cup and a longer post flow. Don't try to weld all the way around the joint. It will build up too much heat. Weld small segments and the move on to another joint to allow it to cool. ER 70S2 is fine, ER312 makes a nicer looking weld.