Austempered spheroidal graphite cast iron (ADI) is a cast material which accompanies a well balanced combination of superior tensile strength and good ductility. Therefore, its application to the structural components subjected to high strength is increasing recently and is expected to expand more in future. However, the authors recognized that ADI has a characteristic that exposure to water or covering its surface with some kinds of adhesive agents leads to considerable reduction of its tensile properties. This embrittlement behavior and its preventing methods are investigated from several points of view. In this paper, the authors studied about the embrittlement behavior of the fracture toughness characteristics of ADI caused by exposure to water. An ADI sample whose tensile strength and elongation are some 1100 MPa and some 11 % respectively was applied to fracture toughness testing at room temperature following to JSME-S001 (Standard method of test for elastic-plastic fracture toughness J1c) using compact tension (CT) specimens of 15 mm in thickness. In case of a dry CT specimen without any history of exposure to water, the Load (P) vs. Load line displacement (LLD) trace exhibited a relatively ductile fracture state whereby a rapid fracture occurred after decreasing the load somewhat upon passing the maximum load (Pmax) point. However, in case of a wet CT specimen which was tested soon after having been exposed to tap water for an hour, the P vs. LLD trace exhibited a extremely brittle state and a rapid fracture occurred at a point where the P and LLD values are less than half of those of a dry CT specimen. The fracture toughness value at the Pmax point (J(Pmax)) became less than 15% of that of a dry specimen. Fracture toughness testing was carried out with several CT specimens that had been exposed to water for various length of time between 5 minutes and 20 days, and all these exhibited a similarly remarkable embrittlement behavior in fracture toughness characteristics, and length of exposure time to water was recognized to have little influence on the embrittlement behavior. However, in case of a CT specimen which was dried well by slight healing after having been exposed to water for a long lime, its fracture toughness characteristics showed to have recovered its original toughness similar to those of a virgin dry CT specimen. This embrittlement behavior of fracture toughness in ADI caused by exposure to water is considered to result from the hydrogen that is produced by chemical reaction between water and the matrix of the crack tip area during plastic deformation.
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