Structural changes of steel 25Kh1M1F in long-term exposure to elevated temperatures and stresses

Conclusions

• 1. In steel 25Kh1M1F subjected to the effect of heat for a time characterized by the temperature-time parameter P (up to 20.5) we have to expect four periods of structural transformations and change of mechanical properties.

• 2. The greatest strengthening effect in this steel is exerted by particles 100–300 nm in size. When their density is greater than 150·10⁸ mm⁻³, there is a linear dependence of the microhardness of the structural components on the density N; with N=(50–130)·10⁸ mm⁻³ microhardness is H 120–130.

• 3. The loss of strength of steel in the first period of operation is connected with the reduced density of particles 100–300 nm in size, in the third and fourth periods with the transformation of the dislocation structure.

• 4. Increased cyclic and static loads applied to rotors in the initial state intensify the course of the process characteristic of the first period, and in addition, they increase the dispersity of particles of the strengthening phase. When loads were applied to metal that had passed through the metastable period, no special features in the change of structure were discovered.

• 5. The state of the metal of rotors after long-term operation is modeled with sufficient accuracy by lengthy holding at elevated temperature without applying a load.