Abstract
Crack propagation behavior under creep-fatigue conditions at elevated temparatures was studied with a SUS 304 stainless steel, a quenched 21/4Cr-1Mo steel and a Hastelloy X, and the results were discussed in the light of fractography.
The crack propagation rates of the 304 steel and the Hastelloy were correlated linearly with cyclic J integral range ΔJ. In the region of low propagation rate, the crack propagation rates of the 304 steel, the Cr-Mo steel and the Hastelloy were about the same. In the region of high propagation rate, on the other hand, those of the Cr-Mo steel were higher and those of the Hastelloy were slightly lower than those of the 304 steel. This difference in crack propagation rete is probably associated with the difference in creep ductility of these materials.
In the case of C-P type loading, in all materials, a transition of fracture morphology from the transgranular fatigue type to the intergranular creep type occurred when the ratio of creep component to fatigue component φ exceeded a critical value of about 1. In the C-C type loading, transgranular fatigue fracture appeared also in the case of φ ratio larger than 1, where the fracture morphology in C-P type loading was intergranular. It is suggested that reversible slip along grain boundaries caused by compressive creep possibly recovers forward slip along the boundaries caused by tensile creep.
No appreciable difference was found between crack propagation rates of the 304 steel in air and vacuum, and it was consistent with a small difference observed on fracture surfaces in these environments.
