13th International Conference on Fracture June 16–21, 2013, Beijing, China -6Fig. 7. SN curves of 350G maraging Fig. 8. Crack growth (a) in dry air steels in dry and humid airs. and (b) in humid air. 473 K and 673 K intersect with that of room temperature at 0.4 ks (Nf ≈ 2 x 104), and the dynamic aging becomes prominent at longer time than this. It also predicts that low cycle fatigue region at shorter time than 0.4 ks shows normal temperature-dependence in which the fatigue strength is lowered by increasing temperature. One can see from Fig. 3 that static aging at 673 K already appears at shorter time than 0.4 ks. However there is no increase in hardness within 2 ks when the second-step-aging is conducted at 473 K, while dynamic aging resulting in the same increase of fatigue strength as that of 673 K starts in much shorter time. These results suggest that the microstructural change at 473 K and 673 K is enhanced with the motion of dislocations during fatigue tests. From the analysis of static aging described in the previous section, it is believed that solute atoms segregate at dislocations and impede their motion. Almost the same increase of stress amplitude at 473 K and 673 K suggests that the atmosphere of solute atoms around dislocations is saturated so quickly as to have the same suppression effect on the motion of dislocations. However, the enrichment of solute atoms may lead to the formation of dense or large precipitates at 673 K for longer duration, which results in larger fatigue limit than that of 473 K as the stress level is lowered. 3.4 Effect of two-step-aging on environmentally assisted fatigue cracking Fig. 7 shows the SN curves of one-step-aged and two-step-aged 350G maraging steel specimens which were tested in the air of 25% RH (dry) and 85% RH (humid) at room temperature [7]. In the dry air, the one-step-aged specimens show larger fracture strength in peak-aging than in under-aging, while fatigue strength is not affected by two-step-aging. Fatigue tests in the humid air, however, result in a marked decrease in fatigue strength, especially at high cycle fatigue region. The one-step-aged specimens exhibit the increase in the susceptibility to the humid air as the aging time increases from under-aging (SA-U1) to peak-aging (SA-P). As a result, the peak-aged specimens showed the lowest fatigue strength. The addition of second-step-aging at 673 K to the SA-P specimens increases the fatigue strength significantly compared to the original value of SA-P specimens. Cracks are initiated at very small number of cycles in the humid air compared to in the dry air, as demonstrated by Fig. 8. After the initiation stage, cracks grow steadily in both conditions, but the crack growth is accelerated in the humid air. In particular, abrupt crack propagation seems to occur 0 200 400 600 800 1000 105 106 107 Stress amplitude σ a (MPa) Number of cycles to failure Nf SA-U1 SA-P DA-U DA-P 25%RH 85%RH 0.001 0.01 0.1 1 10 0 1 2 3 4 SA-P(25%RH) DA-P(25%RH) Crack length l (mm) Number of cycles N (x105) 0.001 0.01 0.1 1 10 0 0.2 0.4 0.6 0.8 1 SA-P(85%RH) DA-P(85%RH) Crack length l (mm) Number of cycles N (x105) (a) (b)
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