I am not sure about the carbon equivalent of QT 400 but that would be the
place to start. You often can get that from your supplier. Did it come
with a mill certification? The hardness isn't a concern. what is the
amount of carbon in the alloy
I am guessing that if you preheat to 100 C it should be good enough.
3/8 thick will often allow you put minimal preheat input.
To test you might try setting up two pieces of scrap in a Tee and do a
fillet about two inches long. Snap the assembly open and examine the
fracture. If you have the weld tearing out the parent metal you need
preheat. Repeat the test with preheats until you don't have tearout.
I notice that you posted about welding to stainless. I am wondering
which stainless wire alloy you chose. 309??? 312??