13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- 2. Experimental procedure The material used was a commercial 350-grade maraging steel. The chemical composition was shown in Table 1. The steel was solution treated for 5.4ks at 1123K in vacuum followed by air cooling and age hardened in salt bath. The mean grain size of prior austenite was about 20 μm. Figure 1 shows aging curves, in which hardness is increased by the two-step aging as stated in introduction. The reason for the increase in hardness by the two-step aging will be discussed later. Aging conditions investigated were a single peak-aging at the conventional aging temperature of 753K, which is denoted as SA-P, and a two-step or double aging by including an under-aging treatment at 473K after the conventional aging SA-P, which is denoted as DA-U, hereinafter. The two-step aging showed a definite hardening. These conditions were indicated by open marks in Fig.1 and were specified in Table 2. Further study on the variation of mechanical properties with different combination of two-step aging conditions can be found in the related presentation. [6] Figure 2 shows the shape and dimensions of plain specimen. Specimens were machined after the solution treatment, and then age hardened as specified in Fig. 1. Prior to fatigue testing, specimens were sanded by emery paper followed by electro-polishing to remove work affected layer and secure surface state for easier observation. Fatigue tests were carried out using a Ono-type rotating bending fatigue machine with a capacity of 15 N·m, operating at about 50Hz in relative humidity (RH) of 25% and 85%, respectively. The humidity was moderated in a range of RH±5%. Table 1. Chemical composition (mass%) Grade C Si Mn Ni Mo Co Ti Al Fe 350G 0.001 0.01 0.01 17.89 4.27 12.36 1.3 0.08 Bal. Figure 1. Aging curves of maraging steel 1 10 100 1000 10000 500 600 700 800 900 1 10 100 1000 800 900 DA-U SA-P Single aging : 753K Hardness, HV Aging time, t ks Double aging : 473K
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