13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Initiation and Propagation Behavior of a Fatigue Crack of Maraging Steel in High Humidity Qiang Chen1,*, Takanori Nagano2, Yuzo Nakamura3 Yoshikazu Maeda3, Norio Kawagoishi4 1 Center for Globalization, Kumamoto University, Kumamoto 860-8555, Japan 2 Department of Mechanical Engineering, Miyakonojo National College of Technology, Miyakonojo 885-8567, Japan 3 Department of Mechanical Engineering, Kagoshima University, Kagoshima 890-0065, Japan 4 Department of Mechanical System Engineering, Daiichi Institute of Technology, Kirishima 899-4395, Japan * Corresponding author: qchen@kumamoto-u.ac.jp Abstract The effect of humidity on the fatigue properties of 18% Ni maraging steels with different hardness and aging structure was investigated under rotating bending fatigue by varying relatively humidity from 25% to 95%. It was found that the initiation and the early propagation of a fatigue crack were accelerated in high humidity, which caused to a large decrease in fatigue strength of the alloys. In fact, the fatigue strength at 107 cycles obtained in RH 85% was less than a half of that in RH25%. Meanwhile, successive observation on surface fatigue progress and the fractographic analysis of fractured specimens elucidated that anodic dissolution was the main reason for promoting crack initiation whilst hydrogen embrittlement due to cathode reaction was responsible for the acceleration of crack propagation. However, the acceleration of crack propagation was suppressed by the formation of reverted austenite. Keywords Fatigue, Maraging steel, Humidity, Ageing condition, Fatigue mechanism 1. Introduction Maraging steel is a kind of ultrahigh strength steel that involves many strengthening mechanisms such as precipitation hardening, solid solution hardening, grain refinement and so on [1]. However, fatigue strength of maraging steel was much lower than expected on basis of its static strength, especially when compared to other steels of similar high static strength. One reason for the low resistance of the alloy to fatigue is due to its high susceptibility to humidity [2]. It is known that the effect of humidity on fatigue strength varies largely with hardness, microstructure and many other factors. However, information on the effect of humidity on the fatigue properties of maraging steel was very limited in comparison with those on its resistance to stress corrosion cracking [3]. In the present study, rotating bending fatigue tests were carried out for 18%Ni maraging steels with different hardness and aging structures in various relative humidity in order to investigate the effect of humidity on fatigue strength as well as fatigue crack initiation and crack propagation behavior. 2. Experimental procedures The materials used were a 300-grade and 350-grade 18%Ni maraging steels. The chemical compositions in mass% of the alloys are shown in Table 1. The steels were solution treated for 5.4ks at 1123K in vacuum followed by air cooling and age hardening in a salt bath. The mean grain size of prior austenite was about 20μm in the both alloys. Table 2 shows mechanical properties of the alloys subjected to various age hardening treatments.
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