qt 400

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?? Randy

Once you get your mill certs, you can use this to calculate preheat to british standards.