why axial shift is maintaining -0.73 mm in our steam turbine?
Because it can!
Seriously, if you want a sensible answer please ask a sensible question with some detail included. With the information in your question we can not even tell if you have a problem. The first step of any systematic approach to trouble shooting is to define the problem. Without this step no solutions are possible.
__________________
If you can't explain it simply, you don't understand it well enough. (A.E.)