Cr-Mo鋼HAZの低延性クリープ破壊の防止における応力除去焼なましの効果

Low-ductility creep-fracture (LDCF) occured occasionally in heat affected zone (HAZ) of Cr-Mo steels under long-time service. Effect of SR treatment for LDCF of 2 1/4Cr-1Mo and 1 1/4Cr-1/2Mo steel was investigated from the view point of improving ductility of specimen. LDCF was reproduced in laboratory by using synthetic HAZ specimen and creep-rupture test. Two types of intergranular fracture (Type I and Type Ⅱ) were observed in fracture surfaces of specimens. SR treatment was carried out at 975K and 925K for 2 1/4Cr-1Mo and for 1 1/4Cr-1/2Mo steels, respectively. The time for SR treatment ranges from 2 to 200hr. Duvtility of HAZ specimen was improved very much by SR treatment producing the ductile fracture of type Ⅲ when the treating time was long enough. When the treating time was short, the ductility in short time range was large but it decreased in long time range producing the fracture of type Ⅱ. The optimum condition of SR treatment is 20hr at 975K abd 200hr at 925K for 2 1/4Cr-1Mo steel and for 1 1/4Cr-1/2Mo steel, respectively