What is the service temperature for this alloy?

Hardness degradation is a result of microstructural degradation, that comes with creep damage.

Since creep damage is time and temperature dependent primarily, it is important to look at those conditions. Then comes the role of stress.

Stress causes a strain rate in the material and adds to weakening of the material if the strengthening mechanisms are not in place. Loss of strength of an alloy is again a result of exposure to temperature.

Your question can be answered better if you can mention the service temperature and the full chemical composition (if available).

Dear Sir,

I am really appriciate for your answer and the servse temeratrue is around 450-650 degree C, and the material used here are in the family of 2.25Cr-1Mo steels, such as, P/T22, P/T 23 and P/T 24.

Best regard,

Qihua