Life Assessment of Superheater Tubes Fabricated from 2.25CR-1MO Steel

Abstract

In recent years, most of plant owners have tried to extend the continuous run length and shorten a turnaround period in order to enhance the competitiveness. In the case of high temperature components, in which creep is a major damage mechanism, creep resistance decays depending upon the localized condition to which a component is exposed. Thus, condition assessment should be performed with a suitable interval to evaluate the remaining operability. However, conventional techniques to predict the remnant life requires long hours especially for low stressed components like superheater tubes in a boiler. Choosing the most damaged tube among numerous ones is not necessarily easy. Since the distribution of tube skin temperatures is rather erratic, a spot inspection cannot be recommended. Therefore, a non-destructive screening technique would be useful for a timely judgement. Due to carbides coarsening associated with high temperature exposure, hardness decreases continuously with operating hours. In the present work, the effectiveness of hardness measurement to find the weakest tube fabricated from 2.25Cr-1Mo steel was confirmed. The mean rupture life against hardness compensated stress, defined as /H_v, for virgin and service-exposed materials was correlated with Manson-Haferd Parameter by the following equation.

$$ MHP = (\log t_r {\text{ - }}16.053)/(T{\text{ - }}380) = {\text{ - }}0.02975 - 0.01828 \times log{\text{ }}(/H_v ){\text{ - }}0.01146 \times log^2 (/H_v ), $$

where t_r is the mean rupture life, T is the temperature in K and H _v is the hardness prior to a test.