REMAINING LIFE PREDICTION BY THE ISO-STRESS METHOD OF BOILER TUBES AFTER PROLONGED SERVICE

Abstract

The prediction of remaining creep life by the Iso-stress method was investigated for 153500h service-exposed boiler tubes of 1.25Cr-0.5Mo-Si, 2.25Cr-1Mo and 9Cr-1Mo steels. At first, the internal pressure creep rupture strength of the service-exposed tube specimens of the steels was shown to be the same as the tensile creep rupture strength of the sheet specimens taken from the tubes. Then the temperature-accelerated creep rupture tests under iso-stress were carried out by the tensile creep rupture tests, because of simplicity of the tests. The Iso-stress method was verified for the virgin specimens of the steels by comparing the predicted long-term rupture times as long as 105h with the data in the NRIM Creep Data Sheets. The predicted rupture times agreed with the observed data within a factor of 2 or 3. The temperature-accelerated tests of the sheet specimens taken from the service-exposed tubes showed that the linear extrapolation of the rupture times to the service temperature gave the remaining creep rupture life of about 8×105h for the 1.25Cr-0.5Mo-Si steel and about 1.6×106h for the 2.25Cr-1Mo steel. However, the Iso-stress method could not be applied for the 9Cr-1Mo steel, because the logtr versus T curve was not linear, where tr is the creep rupture time and T is the testing temperature. It is concluded that the Iso-stress method is useful when the significant microstructural change does not take place during accelerated creep rupture tests at high temperatures.