If your job follow U.S CODE, maybe AWS D1.1 charpter 3 preqaulify preheat tempature also can be followed.
If you want save time and cost to avoid preheat, you have to qualify that:
PQR withourt preheat and thickness equal to the product and the stress also same, then, your hardness test value lower than the requirment at BS EN ISO 15614, your owner should be accept.
So, hardness test on the product also help, normally HAZ hardness value will be highest area.
Such as AH 36 to FH 36, 350 HV10 was required normally, for offshore product hardness lower than 325 HV10 much better.