13th International Conference on Fracture June 16–21, 2013, Beijing, China -3reduction of area and the Vickers hardness (HV) obtained for age-hardened 300G and 350G maraging steels. The aging conditions are also listed in this table. One can see from this table that under-aged 300G steel (SA-U0) has the lowest strength and hardness. It also exhibits the smallest reduction of area which represents the ductility. The increase of keeping time in one-step-aging heightens the static strength of 350G steel to the peak strength obtained at 150 ks. However, when the aging-time is longer than this, softening appears. This result suggests that over-aging is induced by the coarsening of precipitates. Fig. 2 shows the relation of the yield strength and tensile strength with the Vickers hardness. It is well known that the strength measured by tensile tests has a linear relationship with the Vickers hardness, i.e., σ=CHV, where Cis a numerical constant and generally about 1/3 [9, 10]. As can be seen from solid lines depicted in Fig. 2, this relation is satisfied in the one-step-aged specimens and the value of Cbecomes 0.334 for yield strength and 0.346 for tensile strength. However the two-step-aged specimens do not exhibit such linear relations in both of the yield and tensile strengths. It has not been clarified why this phenomenon occurs, but the change in hardness seems to be more sensitive to microstructural change than the macroscopic properties obtained by tensile tests. Table 2 shows the tensile properties and Vickers hardness of 300- and 350G steels Grade Aging condition (specification) Yield strength, σy (GPa) Tensile strength, σmax (GPa) Reduction of area (%) Vickers hardness, HV (GPa) 300 753 K, 48 ks (SA-U0) 2.07 2.16 47 6.21 350 753 K, 11 ks (SA-U1) 2.19 2.23 54 6.52 753 K, 100 ks (SA-U2) 2.27 2.31 54 6.76 753 K, 150 ks (SA-P) 2.30 2.44 54 6.91 SA-P + 673 K, 72 ks (DA-U) 2.33 2.42 52 7.35 SA-P + 673 K, 400 ks (DA-P) 2.42 2.55 51 7.69 SA-P + 673 K, 673 ks (DA-O) 2.34 2.44 55 7.35 Fig. 2. The relation of yield and tensile strengths with hardness. 3.2 Change in hardness during second-step-aging Fig. 3 shows the change in hardness with the time of second-step-aging at 473 K and 673 K, which is obtained for SA-P and SA-U0 specimens. The hardness increases very slowly at short time and then rapidly at longer time. SA-P specimen aged at 673 K, however, shows the saturation of hardness increase at 400 ks and the softening due to over-aging at longer time. Instead, the increase 2.0 2.2 2.4 2.6 2.8 6.0 6.5 7.0 7.5 8.0 σy for one-step-aging σ max for one-step-aging σy for two-step-aging σ max for two-step-aging σ y, σ max (GPa) HV (GPa)
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