High-Temperature Crack Tip Deformation and Crack Propagation in Heat-Resistant Steels

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Behavior of crack propagation and local deformation near the crack tip during high-temperature straining (T=923K, =-5x10^<-6>/s) was examined in three types of heat-resistant steel ( 316H austenitic steel, 9Cr-lMo martensitic steel and 2(1/4)Cr-lMo ferritic steel), in order to make clear the mechanism of creep crack propagation. Main results and conclusions are summarized as follows: (1) The rate of crack propagation and frequency of cavitation near the crack tip decreases in the order of austenititic steel, martensitic steel and ferritic steel, though the crack tip opening becomes larger in this order. (2) Even in the austenitic steel which exhibits typical intergranular crack growth with grain-boundary cavitation and is brittle macroscopically, the degree of stress concentration at the crack tip is quite small (1.2-1.3), indicating that a stress concentration at the crack tip is relaxed extensively by local plastic deformation near the crack tip. (3) With increasing degree of biaxial tensile state of stress, the crack propagation rate increases, being accompanied by an increase in the degree of stress concentration and the frequency of cavitation and also by decrease in the size of crack tip area where work-hardening and cavitation are concentrated. (4) The results (1)-(3) indicate that crack propagation depends on the establishment of critical states of both work-hardening and cavitation in crack tip area. Crack propagation rate in a material, which exhibits work-softening during deformation, is very low.