スモールパンチ試験法による高Crフェライト系耐熱鋼溶接継手の局所クリープ特性計測

抄録

In high Cr ferritic steels, premature failures at the welded joint have recently been a worldwide issue. This failure occurs at fine-grained heat-affected zone (FGHAZ) and it is known as “Type IV creep damage”. Therefore, it is very important to understand the creep properties of local regions of HAZ such as FGHAZ correctly to clarify the mechanism of Type IV creep damage and enhance the high-temperature strength of welded joints. In this study, a TEM disk-type specimen was removed from the coarse-grained HAZ (CGHAZ), FGHAZ, inter-critical HAZ (ICHAZ) and base metal (BM) of Gr.92 welded joint, and then they were subjected to the small punch (SP) creep test. The FGHAZ simulated specimen was also submitted to the SP creep test for comparing results of those with that of actual FGHAZ. In addition, the finite element analysis was conducted to convert the SP creep rupture data into the uniaxial ones. The experimental results revealed that the creep rupture strengths of FGHAZ and FGHAZ simulated specimen were inferior to those of BM, ICHAZ and CGHAZ. The SP creep rupture data of BM and FGHAZ specimen could be successfully converted into the uniaxial creep rupture ones using the load/stress conversion equation “F = 0.47σ_s”. At the low stress levels, the SP creep rupture data of FGHAZ was in good agreement with that of the welded joint obtained from the uniaxial creep test. This result indicated that the creep strength of FGHAZ itself was one of the important controlling factors of creep strength of welded joint.