Crack growth resistance of 18 Ni maraging steel in rotary bending fatigue

In 18 Ni maraging steel, very high static strength and toughness can be obtained by simple heat treatment, therefore it is used for the structures of rockets and aircrafts, tools and various mechanical strength components. Many researches have been performed on the relation between its fracture toughness and heat treatment, but the research on its fatigue strength is relatively few. This research aimed at the detailed study of its fatigue characteristics, and the experimental study was carried out on the resistance to the development of fatigue cracks. It was clarified that the development of fatigue cracks in 18 Ni maraging steel was hardly affected by the aging treatment, but the optimum heat treatment is not known. The materials put to the test and their heat treatment, the experimental method and the results are reported. The resistance to the development of fatigue cracks of 18 Ni maraging steel was very low, and almost same as that of S 35C. This resistance was not affected by the standard heat treatment. The resistance increased largely when about 10 % of austenite was formed by over-aging and furnace cooling from high temperature. (Kako, I.)