Dear pbrown,
last month we had the same matter for a cryogenic line in a LNG plant, for which we considered whether to use a high Ni alloy or a SS304.
In particular we have taken 9%Ni (ASTM A353 and A553) and 36%Ni (Alloy 36) into consideration:
-pipes are available for both materials (at Eisenbau & Kraemer); just to give an idea the price of Alloy 36 pipe is about four times the price of 9%Ni pipe;
-no fittings availability up to now;
-9% Ni is listed in B31.3;
-Alloy 36 neither is listed nor is considered in a Code Case, so design in accordance with B31.3 is more onerous: in particular, due to the shortage of mechanical properties (even by vendor), allowable stresses at different temperatures can not be determined.
In conclusion we have discarded Alloy 36 and we are still looking for 9%Ni fitting.
Regards.