13th International Conference on Fracture June 16–21, 2013, Beijing, China -1Effect of Second-Stage-Aging on the Fatigue Properties of Maraging Steel Norio Kawagoishi1, Kohji Kariya1, Takanori Nagano2, Yuzo Nakamura 3 1 Department of Mechanical System Engineering, Daiichi Institute of Technology Kirishima 899-4395, Japan 2 Department of Mechanical Engineering, Miyakonojo National College of Technology Miyakonojo 885-8567, Japan 3 Department of Mechanical Engineering, Graduate School of Science and Engineering Kagoshima University, Kagoshima 890-0065, Japan * Corresponding author: nakamura@mech.kagoshima-u.ac.jp Abstract Maraging steels are known to have not only ultrahigh strength but also large toughness so that they are promised to be as used as components or parts with structural integrity. It is also known that maraging steels are conventionally aged in two steps to obtain such good mechanical properties. Some of the present authors showed that one-step-aged maraging steels exhibited the increase in fatigue strength at intermediately high temperatures of 473 K and 673 K, compared with the strength obtained at room temperature. In addition, when low-temperature aging at 473 K and 673 K was given to one-step-aged maraging steels by varying the aging time, this second-step-aging increased the hardness of the steel. It also increased fatigue strength in an environmental situation where high humidity markedly reduced the fatigue life of one-step-aged steels. These results strongly suggest that the second-step-aging at low temperatures produces some additional microstructures which impede plastic deformation and increases resistance to fatigue cracking, even in the environmental conditions which accelerate the deformation and fracture. In the present report, a microstructural model will be proposed to interpret the effect of second-step-aging on the mechanical properties of maraging steel, which will explain the improvement of fatigue strength at intermediately high temperature and in high humidity condition. Keywords Maraging steel, Fatigue, Two-step-aging, Microstructure, Environmental effect 1. Introduction The high strength of maraging steels is principally based on the strengthening due to the martensitic transformation of Fe-18mass%Ni alloy, which also accompanies the introduction of dislocations with high density [1, 2]. According to a recent study of Nakashima et al. [3], the dislocation density introduced by martensitic transformation becomes 7 × 1015 /m2 in ultra-low carbon Fe-18mass%Ni (0.002mass%C), leading to the yield strength of 0.66 GPa. The addition of alloying element such as Mo, Ti and Co increases the strength of maraging steels substantially by the precipitation hardening obtained by aging [1, 2]. As a result, the maraging steels exhibit the ultrahigh strength up to more than 2 GPa, without losing ductility and fracture toughness significantly. These mechanical properties are beneficial from the viewpoints of the structural integrity and the reduction in weight in practical applications. The high strength and large ductility generally improve the fatigue strength. However it is well known that the fatigue properties of steels are very sensitive to environment [4]. Exposure to humid air or to aqueous solutions leads to the prominent deterioration of fatigue strength in high strength steels like maraging steel. Our previous studies [5−7] showed that the fatigue strength of 18Ni maraging steels aged at 753 K was lowered markedly by exposing the steel to humid air with a relative humidity (RH) of 85%. The susceptibility of fatigue strength to humidity increases with aging time in one-step-aged maraging steel, which leads to the lowest fatigue strength at peak-aging [6, 7]. The application of the second-step-aging at 673 K to the one-step-aged steel, however,
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