13Cr鋼の疲労挙動に及ぼすじん性の影響

抄録

Recently, several studies have been performed to clarify the correlation between fatigue crack growth rate and toughness of materials. The results so far indicate that the effect of toughness on fatigue behavior is fairly complicated.
In the present study, constant amplitude fatigue crack growth rate experiments were made on 13Cr steel, which is being used mainly for rotating parts of pumps, turbines and compressors, at the following three temperature levels: -25°C (low toughness), 100°C (ductile-brittle transition temperature), and 200°C (high toughness), to examine the relationship between the crack initiation time, the crack growth rate and the toughness of the material.
The results obtained are as follows:
(1) The material toughness has little effect on the fatigue strength provided the ultimate tensile strength remains constant.
(2) At -25°C the initiation time of the fatigue crack is later than at 200°C, but the stable crack growth rate at -25°C is approximately 50% higher than at 200°C. This difference in crack growth rate is probably related to the fact that for the material with low toughness, the region of fatigue crack growth is apt to contain a number of intergranular, cleavage and quasi-cleavage facets, and they decrease as the toughness increases.
(3) For the material with low toughness, rapid fracture initiates when the maximum value of the stress intensity factor, K_max reaches the fracture toughness. And the rapid fracture occurs in such a brittle manner that the evidence of predominant cleavage or intergranular failure can be seen. On the other hand, when the testing temperature is higher than the ductile-brittle transition temperature, the fatigue fracture toughness tends to exceed the fracture toughness and the rapid fracture region shows ductile dimpled fracture.