Evaluation of long term creep strength of mod. 9cr heat resistant ferritic steel with the aid of system free energy concept

村田 純教*; 渡辺 直也*; 永江 勇二

Advances in Materials Technology for Fossil Power Plants; Proceedings from the 8th International Conference (EPRI 2016), p.487 - 494, 2016/00

In order to evaluate long term creep strength of modified 9Cr ferritic steels, the system free energy of creep ruptured specimens at both 650 and 700C is evaluated as the sum of chemical free energy, strain energy and surface energy, which are obtained by a series of experiments, i.e., chemical analysis using extracted residues, X-ray diffraction, and scanning transmission electron microscopy. Change ratio of the system free energy and creep stress showed the relationship with one master curve irrespective of creep conditions, indicating that the steel ruptures when the applied stress exceeds a limited stress depending on the microstructural state expressed by the change ratio of system free energy. Furthermore, it was found that dominant factor of the change ratio was the chemical free energy change. On the basis of these results, long term creep strength of the steel was evaluated at 700C, for example, 19 MPa at 700C after 10 h. It is concluded that long term creep strength of modified 9Cr ferritic steels can be predicted by the system free energy concept using the ruptured specimens with various creep conditions.