Doesn't the impact requirement depend on the nominal thickness? I used to have to do room temp or +40 CVNs on WCB (don't try any low temp since it probably won't pass), but I can't remember if it was from section VIII or III. Anyway, since I can't help you there, I do remember that even though it is based on thickness, the thickness used do determine CVN requriements on the valves we welded didn't have anything to do with the thickness of the valve, but on the thickness of the attaching pipe. So watch out for that one on castings.