高強度鋼SNCM8の遅れ破壊の破壊力学およびフラクトグラフィによる検討

概要

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Delayed fracture experiments were performed in tap water by using the compact tension specimens of SNCM 8 (AISI 4340 steel) tempered at 200℃, 400℃ and 600℃. The effects of tensile strength and thickness on the characteristics of delayed fracture under the plane-strain condition were discussed. The results obtained are summarized as follows: (1) In thick specimens, hydrogen is not fully dissolved, so that the crack front has a curvature. Thus. the specimen. thickness affects the shape of apparent da/dt-K_1 curves. The thicker the plate, the slower da/dt becomes. (2) The feature of delayed fracture surface i~~ predominantly intergranular. The fractographic feature along the crack front varys qualitatively with the change of crack front curvature. (3) The delayed fracture toughness, defined as (K_<dc>)_<eq> on the basis of equivalent crack length, shows no dependence on the thickness, i.e. there exists no effect of hydrogen on (K_<dc>)_<eq>. The value of (K_<dc>)_<eq> is larger than the plane strain fracture toughness K_<IC>. This difference may be explained by the existence of the effect of stress history on stress intensity. (4) The material tempered at 400℃ shows a larger value of da/dt than the one tempered at 200℃, and is more sensitive to the delayed fracture. In the material tempered at 600℃, no delayed fracture occurred.
1978-01-15