13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- Figure 9. TEM micrographs showing microstructure of (a) SA-P and (b) DA-U Figure 9 shows microstructures of SA-P and DA-U steels observed by TEM. Rod-shaped particles of ~10nm or less in length and spherical ones are observed in SA-P steel, which were identified as Ni3(Ti, Mo) and FeMo respectively by their diffraction patterns of TEM and the X-ray diffraction of electrolyte-extracted particles. In DA-U steel, it is difficult to confirm different particles by means of their microstructural features. Tewari et al. reported that fine particles of nano-sized ω-phase precipitated at low aging temperature in DA-U steel [5]. However, any other precipitates were difficult to distinguish, though it can be predicted from the phase diagram of Fe-Mo alloy that Mo atoms are supersaturated during the second aging at 473 K. Since a high density of dislocations are initially introduced by martensitic transformation, the supersaturated Mo atoms may migrate by the pipe diffusion along those dislocations so as to form the moderation of concentration or the clusters around the dislocations, which impede the motion of dislocations and cause the hardening at 473 K. DA-U steel is in such a way strengthened by the second aging at 473k. On the other hand, in high humidity, hydrogen atoms generated in accompany with anodic reaction may diffuse into the matrix and concentrate at grain boundaries, inducing hydrogen embrittlement [9, 10]. It is concerned that the clusters of supersaturated Mo atoms around dislocations and the irregularity of concentration in the second aging may trap some hydrogen so as to suppress hydrogen embrittlement in high humidity. 4. Conclusions The effect of aging condition on the fatigue strength of 18% Ni maraging steel of grade 350 in long life region in high humidity was investigated under rotating bending in relative humidity of 25% and 85%. Fatigue strength of both single aging and double aging hardened steels was markedly decreased in high humidity environment. However, the decrease of fatigue strength was suppressed by the double aging. Although most of fracture surface was characterized with lath boundary cracking regardless of the humidity and aging condition, a few brittle facets of prior austenite comparable to a grain size were observed at the origin of fracture in high humidity. The main reason for the decrease of fatigue strength in high humidity was the acceleration of both crack initiation and its early growth due to hydrogen embrittlement. The decrease in humidity susceptibility by the 100nm (a) (b)
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