Hi reichert
As our company is located in the frozen waste lands of Canada we are confronted with this problem constantly. A few items to consider
1. you can not simple base your MDMT on material spec alone and must also address the design thickness while establishing your allowable design...
You can try the nickel development institue. (www.nidi.org).They provide a free service for answer to questions by material and welding experts involved in stainless steel and nickel alloys. I have used them in the past and they are usually pretty prompt and helpful in their responses.
QAScott
Contact Bruce McWhirter at the Alberta Boiler and Pressure Vessel Safety Association (WWW.ABSA.ca) Bruce is the head of design survey and a wealth of information. I have contacted him in the pass on similiar issues . ABSA has a library of old Codes and standards