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: