Embrittlement of austempered spheroidal graphite cast iron by contact with water and resulting preventive method

Abstract

Austempered spheroidal graphite cast iron (ADI) shows an extremely brittle behaviour in wet conditions when the parallel portion of a tensile specimen is covered with wet tissue paper during testing. The ADI specimens treated in the process window have an excellent tensile strength and elongation in the dry condition, however the former decreased to 70–80% and the latter decreased to 10–15% in the wet condition at room temperature. In the fracture surface of a specimen tested in the wet condition, a white portion consisting of brittle fracture, which is the origin of fracture, was observed in the vicinity of the outer surface of the specimen. The tensile properties in the wet condition decreased as the strain rate decreased, and this brittle behaviour was considered to be controlled by the diffusion process of gas. This water embrittlement was concluded to result from the embrittlement effect promoted by plastic deformation. The water embrittlement was decreased by forming a ferrite surface layer which is not embrittled by water or by avoiding direct contact with water by painting.

Keywords:

• tensile properties
• impact value
• brittle behaviour
• water embrittlement
• hydrogen embrittlement
• ADI
• pealitic SG iron