Read the seamless tube spec, B166, you will see that the final mechanical properties depend on the specific annealing temp.
If there has been any welding or cold work then this temp is clearly too low.
I don't know why you would bother with this temp, you will not even get good stress relief until you heat to 850C or so.
If your service temp is below 550C then don't bother with a heat treatment unless you can go hotter.
= = = = = = = = = = = = = = = = = = = =
Plymouth Tube