A105 is not an acceptable substitute if the client has specifically called for A105N. The normalizing heat treatment will ensure the forging has superior properties to the "as forged" material.

There is a wide range of technical documents (papers etc.) available which question the acceptability of A105 material within the range of services for which it is typically accepted by established design codes and the client is likely aware of these technical problems and thus specified A105N material.  A few specific examples of issues:

API RP14E:  Para. 4.1 (b) specifies that A105 material should be normalized for service temperatures below 30 F (-1 C).

ASTM A105:  The ASTM specification itself requires all flanges for service greater than ASME Class 300 be normalized as well as all piping components other than flanges, which are NPS 4 or greater, and intended for service in systems rated greater than ASME Class 300, to be normalized.

NACE MR0175/ISO 15156:  In general, NACE/ISO requires carbon steel material used in sour service to be in a heat treated condition (e.g., normalized). However, there is a general exception to this for A105 material (para A.2.1.3) but some end users do not allow the use of this exception.

You would also want to read the document "Recommendations for Pipe Flanges made in Forged Steel complying with ASTM A105" published by the Belgium Chemical Risks Directorate which reinforces (in much more detail) the limitation expressed in API RP 14E.

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