EFFECT OF CREEP FRACTURE TOUGHNESS ON CRACK INITIATION AND GROWTH

An investigation was made of the effect of creep fracture toughness on the crack initiation and growth of two low-alloy heat resistant steels 25Cr2Mo1V and 12Cr1MoV. It was found that the resistance to creep crack initiation and growth of steel 25Cr2Mo1V, with a higher creep rupture strength, is much lower than that of 12Cr1MoV. This resistance is increased with the increase of the creep fracture toughness. For steel 25Cr2Mo1V, the creep rupture strength decreases slightly, and the creep fracture toughness and the resistance to crack initiation and growth increase along with the raise of the tempering temperature. As the steels are in tough or brittle state, the initiation and growth of creep crack are varied. In the tough state, cracking is both transgranular and intergranular around the notch tip. The transgranular crack may be either occurred in carbide inside the grains or nucleated at grain boundary and then propagated into grain. However, the intergranular crack may be formed by nucleation, growth and interlinkage of voids. Cracks may be propagated along grain boundaries and penetrated through the grains yet they are discontinuous. In the brittle state, cracks occur along grain boundaries only. They originate from the interlinkage of voids and cause eventually to brittle fracture.