Accoording to what I have reviewed, QT400 is a Chinese material with the closest Grade being specification being EN-GJS-400-18. Apparently, this is spheroidal graphite cast iron (or nodular iron).
This grade is weldable but an odd combination with structural tubing. What the heck are you doing with this combination?
The actual material is G40.21-350W (Canadian Standard) and the ASTM spec using 50 grade was the closest I could come up with that most people could relate to.
Thanks, this now makes sense to me. The QT400 under this specification is a quenched and tempered plate of high strength low alloy steel.
It is formable and weldable. You are going to have to carefully watch your interpass temperature for welding to this material to avoid altering bulk mechanical properties. I would suggest keeping the interpass temperature below 350 deg F and watch the heat input.
