Creep-fatigue life estimation of circumferential welds in thermal high energy piping systems

To prevent accidents in high energy piping systems of thermal power plants, it may be important to accurately estimate the creep-fatigue life of circumferential piping welds. The main failure mode in the damage cases was Type IV damage which was creep-dominated damage occured at the fine-grained HAZ(FGHAZ). The damege mode is considered to change from longitudinal failure at the base metal to circumferential Type IV failure when the ratio of axial stress to hoop stress exceeds a certain value. The allowable thermal expansion stress in the design, the stress relaxation property of the material, and the creep rupture strength of welded joints are investigated, as factors affecting creep damage of circumferential piping welds. Fundamental creep-fatigue tests were perfomed using cross-weld specimens and their lives were estimated by a linear damage summation rule. In the life estimation, it seems necessary to consider elastic follow-up phenomena where strain transfers to the HAZ.