Dear,
You can see the preheat requirements for process piping in ASME 31.3 clause 330 table 330.1.1 and clause 131 in ASME 31.1 for power piping. You can also get the preheat and interpass temp. in AWS D1.1 (structural welding code) table 3.2.
Before applying the preheat consider the chemisty, thickness and material supply conditions etc.
Thanks.