13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Effect of Aging Condition on Fatigue Strength of Maraging Steel in Long Life Region Qiang Chen1,*, Qingyuan Wang2, Norio Kawagoishi3, Kohji Kariya3, Yuzo Nakamura4, Nu Yan5 1 Center for Globalization, Kumamoto University, Kumamoto 860-8555, Japan 2 Department of Civil Engineering & Mechanics, Sichuan University, Chengdu 610065, China 3 Department of Mechanical System Engineering, Daiichi Institute of Technology, Kirishima 899-4395, Japan 4 Department of Mechanical Engineering, Kagoshima University, Kagoshima 890-0065, Japan 5 Department of Mechanical Engineering, China Three Gorges University, Hubei 443002, China * Corresponding author: qchen@kumamoto-u.ac.jp Abstract A novel aging treatment was proposed to improve the fatigue strength of maraging steel by taking the effects of aging condition and humidity into account. Rotating bending fatigue tests were carried out for a 350 grade of 18% Ni maraging steel in the long life region up to 108 cycles in the relative humidity of 25% and 85%. Aging conditions under investigation included a conventional aging or the so called single aging at 753K, and a two–step aging or double aging by ageing at lower temperature of 473K succeeded to the single aging. Through the double aging, the susceptibility of fatigue strength to humidity was significantly improved without any deleterious effects. The main reasons for the decrease in fatigue strength in high humidity are due to the promotion of crack initiation and the acceleration of small crack propagation. The improvement of fatigue property in high humidity by the double aging was explained from the roles of the additional precipitation of supersaturated Mo atoms during the second lower temperature aging. Keywords Fatigue, Maraging steel, Aging, Humidity, Rotating bending 1. Introduction Maraging steel has the highest tensile strength and high ductility among steels used in practice [1]. However, the fatigue strength of maraging steel is much lower than expected from its high static strength [2]. The reason for the lower fatigue strength is due to the high sensitivity of maraging steel to both notch [3] and humidity [4]. Since maraging steel is mainly hardened by the precipitation of intermetallic compounds such as Ni3(Ti, Mo) and Fe2Mo in martensite structure, the strength of the steel depends largely on the aging conditions. Many studies have been carried out on the precipitations in the steel. For example, it is well known that the in-coherent particle of Ni3(Ti, Mo), which is one of the primary strengthening particles, precipitates at high aging temperature beyond 723K, while the particles of Fe2Mo, ω phase and Mo cluster precipitate at low aging temperature below 723K [5]. Precipitation at high temperature is usually referred to as high temperature phase whilst that at low temperature as low temperature one. Considering the above-stated relation between aging temperature and precipitation, it is expected that additional precipitation and hardening may happen during a two-step aging or the conventional high temperature aging followed by a low temperature ageing, because the concentraion of Mo exceeds its solubility at low temperature. However, aging treatment of maraging steel is usually conducted under a constant high temperature only, and the study of the mechanical properties, especially the fatigue strength, of the two-step aged steel at high and low temperatures has not been reported. In the present study, the effect of aging condition on the fatigue strength of 18%Ni maraging steel of grade 350 in long life region was investigated. A novel aging treatment was proposed for improving fatigue properties of the maraging steel. Fatigue tests up to 108 cycles were performed in relative humidity of 25% and 85% under rotating bending. Aging conditions examined were the conventional aging at 753K or the so called single aging, and a two–step aging or double aging at 473K subsequent to the conventional aging at 753K.
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