I am currently evaluating the design of some large but lightly stressed welded storage frames.  We have many years of experience of similar designs fabricated in mild steel (S275/43A) operating in a -20°C environment.  

My concern is that that the new generation of frames will be in service at -80°C and must have a 25 year design life.

Brittle fracture is obviously the main concern. There is no stress cycling to speak of of fatigue is unlikely to be an issue.  Specifiying 304 stainless steel would solve the problem but adds significant cost. Aluminium is prohibitively expensive because of the welding costs.