NACE RP 0403 (SP0403) is for Avoiding Caustic Stress Corrosion Cracking of Carbon Steel Refinery Equipment and Piping
Does SP0403 apply to fittings?
If the fitting is going to be used in Caustic service and if the contract that you entered says that NACE RP/SP 0403 is applicable for a given item, then it is applicable.
or Does it apply to the construction of the project?
If the fitting is going to be used in piping construction and that piping is in Caustic service and if the contract that you entered says that NACE RP/SP 0403 is applicable for a given item, then it is applicable.
May a fitting meeting the requiremetn of NACE MR 0175/ISO 15156 be classified as complying to SP0403
Material selection is not in the scope of fitting manufacturer. That is, it is the responsibility of the purchaser to know where the fitting is going to be used. If it is going to be used in region "C" he should not order carbon steel fitting.
Assuming the purchaser orders CS fitting then one has to infer that the purchaser is fully aware where the fitting is going to be used, the obligation of the supplier is as following
For Carbon steel fitting seamless:: it may be as per ASTM
If the item is Welded fitting, then the fitting shall be heat treated after welding and such heat treatment shall be such that the minimum soaking temperature shall be minimun 635 deg c. Normalized and Q & tempered fitting automatically qualifies
The fitting should be given WFMT (SP0403 is not clear whether this is required for weld only or for material, hence it is better to perform for the full fitting)
Also it is recommended that UST of the fitting body be conducted (again SP0403 is not clear whether this is required for weld only or for material, hence it is better to perform for the full fitting)
The above are based on old RP0403-2003 version of NACE, i do not know what the current version says