450C is not really in creep range. Process plants have quite regularly expansion joints in those temperatures. Temperature is not the problem. Movement specification is wrong. There is zero change that expansion joint will expand as much as it compresses. Giving such specification on an expansion joint is making the line far more unsafe than giving correct spec.

Only place with very rare exception where pipe engineer must specify equal 2 way movement is movement caused by mechanical movement or for instance wind.

I use only Celsius for the calc. Expansion joints are delivered for ambient temperature, e.g. about 20 C. Temperature increase is 450 - 20 = 430. For the expansion joint to expand the same as it compresses pipe has to cool down to -410 C. In our dimension there is no such temperature. Designing expansion joint for equal movement makes it longer and prone to instability.

Pipe designers spend a lot of time looking into spring rates (and sometimes forgetting pressure thrust). My problem is that most of that time is wasted. High number of expansion joints pass a yield point during their movement. That means that when the pipe cools down the expansion joint has to be pulled to its original length. I do not know any pipe designer who considers that force.

Old times pipes failed because they were too rigid. Today pipes fail because they are too flexible and often also have insufficient axial stops and guides. Stress analysis may say all is perfect and line fails during operation, sometimes during the pressure test.