Creep Deformation and Fracture Behaviors of High Cr Ferritic Steel Welded Joint by Rectangular SP Specimens

Abstract

Modification of small punch (SP) creep test was investigated to develop a new procedure for evaluating the creep property and damage of welded joint of high Cr ferritic steels by a miniaturized specimen. Disk and square-types specimens are usually clamped between the upper and lower dies in the isotropic manner in the conventional SP creep test, whereas the specimen used in this study was rectangular (10 × 6 × 0.50t mm) and was clamped at both ends of the specimen. This newly proposed SP creep test was applied to the welded joint of P122 steel pipe, which had been actually used in a fossil power plant for a long time. The specimens were removed from the mid section of the thickness of pipe so that the HAZ was located on the center of specimen perpendicular and parallel to the longitudinal direction of specimen respectively. In addition, the rectangular specimen consisting of only base metal and the conventional SP test specimen (10 × 10 × 0.50t mm) with the HAZ were also subjected to the SP creep test for comparisons. Experimental results revealed that, by arranging the HAZ perpendicular to the longitudinal direction, the creep rupture strength decreased significantly reflecting the lower strength of the fine grain HAZ (FGHAZ). This decrease in creep strength was attributable to the acceleration of initiation and growth of cavity and/or microcrack at the FGHAZ. Consequently, it is indicated that the present modified SP creep testing technique has a high potential as a tool for evaluating the susceptibility to Type IV cracking and/or the creep damage of welded joints.