The characteristics of embrittlement in aquatic environments for austempered ductile cast iron (ADI) and as received ductile iron were investigated. Embrittlement of specimen was tested in water-based solution by a cantilever type apparatus by dropping 0.365 mass%HCl solution or ion-exchanged water of 0.013 ml/s intermittently. ADI fractured even in the ion-exchanged water when subject to high bending stress over a long period of time. Fracture strength decreased markedly in the specimen tested in the 0.365%HCl solution. These ADI fractured suddenly without creep deflection. The fracture strength of the as received ductile iron decreased slightly in the 0.365%HCl solution. In the cast iron, hydrogen generated from the galvanic reaction between the graphite and matrix in acid solution. The results suggest that the reduction of strength and ductility of ADI may be caused by hydrogen embrittlement. On the other hand, for as received ductile iron, sensitivity to hydrogen embrittlement was comparatively low because of its ferrite matrix.

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  The major part of decarburization in spheroidal graphite cast iron at high temperature is caused by the diffusion control1ed growth of the decarburized layer. The parabolic growth rate of the decarburized layer in one dimension is formulated as a function of the relative solubility and diffusion coefficient of carbon in austenite. At lower temperature, the growths of ferritic and austenitic layers promote decarburization, the rates of which are calculated. The calculated growth rates at varied temperature are in fair coincidence with the experimental data of prior investigators. Computer simulation shows that the holding time of heating to annihilate al1of the graphite nodules is reduced by increasing the solubility and diffusion coefficient of carbon, and is proportional to the dimension square and the ratio of volume to the surface area of the casting.

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  Molds for precision casting are made by repeating the process of dipping in slurry which consists of fire refractory materials and stuccoing and drying in several times. As rapid drying may cause the mold to crack and peel, the drying time is determined in consideration of the reduction rate of the evaporation components. Although it is considered that the mechanical and thermal properties of green sand mold depend on its moisture content, their relation has not been c1arified quantitatively. Experiments on mechanical strength and thermogravimetry and thermodilatometry were carried out, under the condition in which evaporation components, such as moisture, were contained in the green sand mold. The bending strength of green sand mold containing moisture increases with the decrease of the moisture content ratio. The bending strength of mold is largely influenced by the crystallization of colloidal silica, and rapidly increases more than the crystal melting temperature of silica. The average thermal expansion coefficient of green sand mold containing moisture is negative from room temperature to about 330K, and decreases with increasing moisture content ratio.

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  P-B cast irons are widely applied to the cylinder liner for marine diesel engines, since they are composed of flaky graphite, hard phases (γ-Fe₃C-Fe₃P eutectic) and pearlite matrix which offer higher wear resistance. Recently, the strengthening of the cylinder liner is required to raise engine output. However, large graphite flakes develop in thick castings and reduce the strength. Therefore, the control of graphite structure is essential to attain higher tensile strength. Application of chill blocks to the sand mold, for raising the cooling rate of casting, should refine the solidification structure and provide higher strength. However, experiments on the effects of 20 to 90 mm thick steel chill blocks on the 180mm thick rectangular casting showed that the chill blocks lowered tensile strength on the contrary. Thermal and metallographic analysis of castings revealed that the chill blocks raised both the cooling rate and temperature gradient near the eutectic freezing front. Though a higher cooling rate resulted in finer graphite flakes, a higher temperature gradient made the graphite structure thinner and distributed along heat flow direction in the interdendrite region. These densely distributed thinner graphite flakes lowered the tensile strength.

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