1999 年 17 巻 3 号 p. 466-473
Low-ductility creep-fracture (LDCF), which arised occasionally in heat affected zone (HAZ) of heat-resisting Cr-Mo steels, was reproduced in laboratory. Synthetic HAZ specimens (HAZ specimens) were prepared by giving weld-thermal-cycle to round bars of 1 1/4Cr-1/2Mo and 2 1/4Cr-lMo steels, and creep-rupture tests were made on those specimens at the testing temperatures of 775 and 825 K. HAZ specimens showed very small ductility (reduction in area smaller than 10%) at each time to fracture. The specimens produced intergranular fracture along the grain boundaries of prior-austenite.
The heat treatments of stress-relieving (SR) was given to the HAZ specimens ; heating temperatures of 975 K and 925 K, respectively, were selected for 2 1/4Cr-lMo and 1 1/4Cr-1/2Mo steels on the basis of practical SR conditions of those steels. The ductility of HAZ specimens of 2 1/4Cr-lMo steel was improved by SR for 20 h at 975 K. That of 1 1/4Cr-1/2Mo steel was improved as well by SR for 200 h at 925 K, but this treatment reduced significantly its fracture stress. Therefore, it is practically difficult for 1 1/4Cr-1/2Mo steel to remove LDCF by SR treatment. The influence of SR treatment on LDCF was discussed from the metallurgical view point in some details.