Dear TJ,
ASME Sec VIII,DIV-1, UCS-56 recommended PWHT cycle for SA203GrE(1 3/16"thk) would be typically @ 595-620 degC, around 1.30 hrs. UCS-66 allows, PWHT at lower temp, with larger soaking time. It's not clear from your email, if this is an external or internal weld , but there may be a scenario, the circ weld and the adjoining BM have to undergo PWHT at the above temperature.
Your best choice for such DMW's to use Inconel-82(GTAW/SAW) and 182(SMAW). The joint could be welded liks an usual pressure vessel C-Seam and locally PWHT or completely furnace PWHT.
Alternatively if possible 203 GrE weld egdes may be butterd with Inconel , PWHT then joined to 304L S.S using Inconel.
203 GrE is weldable with matching ferritic welding consumable,same applies to 304L S.S. However the DMW's would call for Inconel-82 /182 as typically common choice e.g. E-309L would not meet -150 deg F impact strength.
I was fortunate to handle this material SA 203GrE in India, for a number of large Ethylene projects,around 1984-85, soon after finishing my Masters studies.
Welding Nickel steel for such low temperature is quite challenging.We had successfully used Thyssen and Oerlikon welding products.
I have attached some literature for your reference.Feel free to drop a line, if you have queries.
Thanks
Pradip Goswami
Welding Engineer/Specialist