Effect of Specimen Size on Intergranular Mode Fracture Toughness of Cr-Mo-V Steel in the Transition Temperature Region

Abstract

A determination procedure of intergranular mode fracture toughness has been studied on the basis of elastic-plastic fracture mechanics and by use of AE technique to detect an onset of microscopic pop-in cracking at the crack front. Experiments were performed on a steam turbine rotor steel (Cr-Mo-V steel) at 150°C, and four different sizes of compact tension specimens having the thickness of 10 mm(0.4TCT), 25 mm(1TCT), 50 mm(2TCT), and 100 mm(4TCT) were used. In combination with the fractographic observation on fractured surfaces, the fracture toughness determined by AE technique, J_iAE, was proposed as the critical J-integral value characterizing an onset of a microscopic intergranular mode pop-in fracture, and its size effects was discussed. Each lower value of K_JAE(=√E·J_iAE/(1-υ²)) obtained from smaller specimens such as 0.4TCT and 1TCT specimens was coincident with that from the large specimens. Furthermore, the toughness values of 2TCT and 4TCT specimens, K_IC(AE), met the size requirement for the plane-strain fracture toughness, and were about 76 percent of the valid K_IC. This evidence suggests that the fracture toughness by AE technique proposed here is a suitable parameter to the toughness value of the brittle fracture initiation more conservative than the plane-strain fracture toughness value.