13th International Conference on Fracture June 16–21, 2013, Beijing, China -2recovered the susceptibility comparable to the under-aged states. In another previous study [8], the fatigue strength of a 300G maraging steel under-aged at 753 K was shown to increase at test temperatures of 473 K and 673 K. It was also demonstrated that the hardness increases with the number of cycles at these temperatures. The second-stage-aging of this type is, hence, very effective to improve the fatigue properties at moderately elevated temperatures. The results in the previous studies mentioned above suggest that microstructural change is induced by the static second-step-aging to improve the fatigue strength in humid air, and that dynamical aging is also caused during deformation at moderately elevated temperatures. In addition, these effects appear prominently in fatigue properties. In the present study, we try to understand the effects of the second-step-aging on the fatigue properties of maraging steels from the microstructural aspect. For this purpose, we reassess the results of previous studies. In particular, we investigate the dependence of hardness change on aging time and temperature in detail, taking into account the microstructural analyses by means of transmission electron microscopy (TEM). Based on these reassessment and analyses, the strengthening mechanism due to the second-step-aging and its role in suppressing the deterioration of fatigue properties will be discussed. 2. Experimental procedures Chemical compositions of 300G and 350G maraging steels used are shown in Table 1. Both steels were supplied as rods of 13 mm diameter, and experienced the solution treatment conducted at 1123 K for 5.4 ks. The 300G steel was aged at 753 K for 48 ks, which led to an under-aging state. For the 350G steel, two under-aging conditions were obtained at 753 K by using the aging times of 11 ks and 100 ks, while the peak-aging was attained by heating the specimen at 753 K for 150 ks. The 300G steel specimens were subjected to rotating bend test with a frequency of 50 Hz at room temperature, 473 K and 673 K, to examine the effect of test temperature on the fatigue properties. For the 350G steel, the peak-aged specimens were subjected to second-step-aging at 473 K and 673 K by changing the keeping time, prior to tensile and rotating bend tests at room temperature. The fatigue tests of the 350G steel specimens were carried out in air with the RH of 25% and 85% to examine the influence of humidity on the fatigue properties. The details of experimental procedures used for the above fatigue tests had been reported in the previous papers [5−8]. Hereafter we assign S and D to the one-step-aging and two-step-aging, and U, P and O to the under-, peak- and over-aging, respectively, as will be shown in Table 2. The second-stage-aging at 473 K and 673 for various time up to 800 ks was applied to the under-aged 300G steel and the peak-aged 350G steel. The Vickers hardness of the polished surfaces of these specimens was measured by using a load of 9.8 N to examine the change in hardness during the second-step-aging. The microstructures of one-step-aged and two-step-aged 350G specimens were also observed by TEM. In addition, an electrolyte extraction technique was used to identify the precipitates and evaluate their contents. Table 1. Chemical compositions of maraging steels (mass%). Grade Ni Mo Co Ti Al Mn Si S C Fe 300G 18.4 5.14 9.09 0.89 0.11 0.01 0.05 0.002 0.005 bal. 350G 17.89 4.27 12.36 1.3 0.08 0.01 0.01 0.001 bal. 3. Results 3.1 Relation between the tensile properties and hardness Table 2 shows the yield strength ( σy) given by 0.2% proof stress, the tensile strength ( σmax), the
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