Abstract
Delayed fracture experiments were performed in tap water by using the compact tension specimens of SNCM8 (AISI 4340 steel) tempered at 200°C, 400°C and 600°C. 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-K1 curves. The thicker the plate, the slower da/dt becomes.
(2) The feature of delayed fracture surface is 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 (Kdc)eq on the basis of equivalent crack length, shows no dependence on the thickness, i. e. there exists no effect of hydrogen on (Kdc)eq. The value of (Kdc)eq is larger than the plane strain fracture toughness KIC. This difference may be explained by the existence of the effect of stress history on stress intensity.
(4) The material tempered at 400°C shows a larger value of da/dt than the one tempered at 200°C, and is more sensitive to the delayed fracture. In the material tempered at 600°C, no delayed fracture occurred.
