Evaluation of Creep Residual Life for Modified 9Cr-1Mo Steel Based on Omega Method

Abstract

In order to study the accuracy of creep residual life prediction by the Omega method which is based on creep deformation, a series of creep tests on modified 9Cr-1Mo steel were conducted at 500°C, 550°C and 600°C, and the Omega method was applied to the residual life estimation.
The main results obtained are as follows:
(1) There was a obvious linear portion, which corresponded to the tertiary creep, in the relationship between logarithm of strain and srain. So it was easy to define the Omaga value as a gradient of linear portion.
(2) It was proved that the Omega value depended on stress and temperature in such a way as it was the larger, the lower the stress and the lower the temperature.
(3) By using the Omega value and strain rate which were determined experimentally, the residual life could be predicted within a factor of 1.5 at the stage of 50% and 80% of actual life. It was confirmed that the accuracy of this method was higher than that of the former method based on rupure time.
(4) To apply this method to the residual life evaluation of operating plant materials, the Omega value has to be determined in the lower stress condition. So it is important to develop the extrapolation method of the Omega value based on the laboratory acceralation test to the longer service life.