旧オーステナイト粒径の異なる18%Niマルエージング鋼の破壊靱性およびストレッチ・ゾーン

抄録

The effect of prior austenite (γ) grain or martensite (α') packet size on the plane strain fracture toughness (K_IC) and the correlation between the stretched zone width and K_IC of an 18% Ni maraging steel were studied by means of fracture toughness testing and fractography.
K_IC was not affected by γ grain size in the fine (6.7-15.7μ) or coarse (118μ) region, while it increased in the intermediate grain size range from 28.3 to 90.4μ. In the latter condition, the plastic zone spread over about one to tow α' packets ahead of the fatigue pre-crack before the onset of unstable fracture.
Stretched zone was observed at the tip of the fatigue pre-crack, and the zone width related linearly to the critical crack opening displacement. The width of the stretched zone was increased by the enhanced relaxation of the stress concentration by the micro-cleavage nucleation after the spreading of the plastic zone over about one to two α' packets.
It seemed that the enhanced blunting process of the fatigue crack led to the increment in K_IC. On the other hand, K_IC was independent of such a structural unit as γ grain or α' packet when the enhanced blunting effect was not introduced.