Cross-weld Creep Behavior and Life Prediction of Low Alloy Ferritic Steels

Abstract

The reliability of elevated temperature components depends upon understanding the creep behavior of welds, which often determine the components' lives. The most likely manner of failures at high temperatures for the serviced welds fabricated from low alloy ferritic steels should be Type IV cracking. In the present work using 1.25Cr-0.5Mo and 2.25Cr-1Mo steel welds, however, the location and the morphology of the ultimate failures were influenced by a couple of factors, namely, the magnitude of stress, temperature, time to rupture, specimen geometry and inherent creep properties owned by each alloy. Thus, the consistent relationship between the feature of creep damage and the life consumption to be utilized for the remnant life assessment of welds has not been derived.As the alternative, the authors have examined the cross-weld creep behavior at the tertiary creep stage. Despite the variation of failure types, which were Type I, Type III, Type IV and ductile rupture at the parent material, the omega method predicted the rupture life with the accuracy of a factor of 2. Furthermore, it was found that the linear relationship between e and e appearing at the tertiary creep stage was also available in the life prediction. The slope of strain rate versus strain derived from the above relationship was correlated with the rupture life independently of failure types, specimen geometry and materials and the same level of accuracy as that of the omega method was obtained. From the usefulness of this technique and observation of grain boundary damage at the Intercritical HAZ (ICZ), the authors has interpreted that grains around creep cavities are still subjected to a significant level of stress, suggesting that apparent feature of grain boundary damage dose not necessarily exhibit the remaining creep strength of welds.KEY WORDS: 1.25Cr-0.5Mo steel; 2.25Cr-1Mo steel; creep, welds; life assessment.