Creep strength evaluation of a long-term used boiler weldment part on 2.25Cr-1Mo steel by using miniature specimens

Creep damage gradually progress during operation especially at weldment parts of 2.25Cr-1Mo steel boiler pipes. In creep damage assessment of the weldment parts based on stress analysis by a finite element method, creep deformation property of constituent materials in the welded joint has to be obtained. Additionally, it is desired to develop a creep damage assessment method by using a miniature specimen. In this study, in order to clarify influence of a long-term operation on creep strength property, creep tests were performed in inert gas condition using miniature specimens with 1mm diameter taken from each portion of constituent materials of a welded joint in a long-term used boiler header. Although minimum creep strain rates of a base metal and a fine grain are similar to those in a new welded joint, those of a weld metal and a coarse grain are much higher than those in the new welded joint indicating significant reduction of creep strength in the weld metal and the coarse grain occurred during the long-term operation. Creep rupture times of the miniature welded joint specimens are almost equal to those of the standard welded joint specimens. Therefor creep rupture time of the standard welded joint specimen may be predicted by a creep test result of the miniature welded joint specimen. From creep test results of the miniature welded joint specimen taken from creep damaged standard welded joint specimen, it was demonstrated that creep damage greater than medium level can be evaluated by using the miniature welded joint creep specimen. © 2020 The Society of Materials Science, Japan