The API 5L X-grades are typically used in B31.4 and B31.8 applications only. They have poor fatigue properties because of the cold working required to establish their high yield strengths.

Because B31.4 and B31.8 are used mainly for buried piping, thermal cycling usually does not become an issue. Also because they are buried, they are typically below the frost line, so it is unlikely that they will experience a temperature of 0 deg C (let alone 0 deg F). Also note that many of the API 5L X-grades require the use of ASTM A694 (I think) flanges, which are effectively unlisted components in B31.1 and B31.3.

All in all, this is likely to be a poor material choice for a B31.3 application, although as Dave correctly notes, it is not forbidden. There will just be a lot of extra work required in order to satisfy the Code material requirements.