There are too many material types and uses for steel castings, that you question could not be answered specifically without a lot more info. But a good place to start is the ASTM specification that the casting was made too. The ASTM specifications many times say whethere PWHT is require, optional, or prohibited, etc, and they also may define it by the size of repair. Typically major repairs are 10 sq inches, 1" deep, 20% of the wall, or one that leeks during hydrotest. Often major repairs require PWHT, but there are many differences based on what material you are welding and what the intended service is.