It is well-known that the strength of flake graphite cast iron is controlled by the morphology of the graphite. Studies on the strength of flake graphite cast iron have mainly been discussed in relation to the shape of graphite as a notch effect so far. However, the observation of the graphite shape with a microscope to evaluate only by the notch defect raises certain doubts. We therefore investigated the effect of the graphite tip radius on the mechanical properies of cast iron (Experiment-1). In Experiment-2, using pure Fe-C-Si cast iron (3.4 mass % Cand 1.8 mass % Si) as the parent material, we evaluated the influence of minor elements such as P and S on the microstructure, and tensile properties of as-cast and ferritized samples.

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  Fe-1.8 % C-Cr-Mo-W-V-Co alloys varying in 4 % and 7 % of vanadium content, 0 % and 5 % of cobalt content and 5 % of each alloying element (Cr, Mo and W) were produced by casting. The characteristics of heat treatment with these alloys quenched and tempered were investigated and their rolling contact fatigue strength was studied. The quenching temperature at which the highest matrix hardness was obtained for quenched alloys became higher as the vanadium content was increased. Tempering at about 773 K resulted in the secondary hardening of the alloys. The high vanadium alloy, which contained less plate-like or rod-like M₂C carbides and more granular MC carbides, showed excellent rolling contact fatigue characteristics. Since the interface between the carbides and matrix had many defects, binding force between the carbides and matrix was small and cracks propagated along the defective interface. Cracks also passed through plate-like or rod-like brittle M₂C carbides which were highly sensitive to notch effects when they intersected the carbides.

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  Structure formation of a ferrite-rimmed spheroidal graphite cast iron with a ferrite structure at the surface layer and a pearlite structure at the inner part has been investigated. The ferrite-rimmed structure hardly formed in small specimens. On the other hand, a large cylindrical specimen with a diameter of φ20 mm or more exhibited a distinct ferrite-rimmed structure. It was found from experiments using large cylindrical specimens with a free surface at the inner part that ferrite forms predominantly at the surfaces free from normal stress. Furthermore, the distribution of pearlite fraction on the cross section of the cylindrical specimen was shown to be related to the distribution of isotropic stress in untransformed austenite induced by ferrite formation from the surface. It is considered that elastic strain energy induced by the ferrite formation at the surface layer raises the eutectoid temperature and promotes pearlite formation at the inner part of the specimen.

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  Ultrasonic vibration is applied to the solidification process of Al-Si alloys, with a horn inserted in the molten and semi-solid alloy. It is found that the primary crystals in the alloys can be refined to different extents depending on the temperature where the ultrasonic vibration is applied. In the Al-Si alloy with a hypo-eutectic composition, the dendrite primary crystals are apparently segmented by applying the ultrasonic vibration in the temperature range from 973 K to 883 K. On the other hand, the segments are found only in the local regions near the horn when the ultrasonic vibration is applied in the semi-solid temperatures, i. e. from 883 K to 850 K. When the primary crystals are angular Si in the Al-Si alloys of hyper-eutectic composition, the vibration can even refine the primary Si grains in the semi-solid area. Modeling the movement of molten alloy in mold with the application of ultrasonic vibration, an acoustic stream is feasibly caused about multiple nucleator in the molten alloy. It is therefore concluded that the primary crystal segments are dispersely distributed all over the molten alloys by such an acoustic stream, which play a role of prompting the nucleation of primary crystals.

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  In order to evaluate a shrinkage tendency of molten spheroidal graphite cast iron, test pieces having a variety of widths with a constant thickness were cast and the shrinkage areas were measured. Also, cooling curves of the surface and the inside of test pieces were prepared to find a relationship between the shrinkage area and the inside solidification ratio when the surface solidified (temporarily referred as S/I). The S/I value at the start of shrinkage occurrence is determined as a critical S/I to shrinkage, with which the shrinkage tendency for the molten iron has been evaluated. The critical S/I to shrinkage are determined for different contents of C, Mo, Sb, Cr and P and inoculation methods and Bi addition. Resultantly the higher amount of C makes the shrinkage lower and minimum at the C content approx. 3.7 %, and it becomes higher with the increase of the amount of alloy element and Bi addition and lower by inoculation. Also, it is found that the shrinkage tendency by adding an alloy element can be converted to C content. Other causes of shrinkage e.g. the lack of graphitization, worse fluidity due to primary graphite, or the difference of solidification time between the surface and the inside are also explained.

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  It is the slag pinhole defects that is occurred frequently within gas defects. There is 3 main factors. : (a) Fe-Mn-Si slag, (b) MnS and its slag, (c) Al of inoculant and Mg of spheroidizer. (b) can be differentiated by Mn and S. (a) and (c) is difficult to be differentiated because the slag component in defects is very similar. To differentiate (a) and (c), I have tested about slag pinhole reappearance, then analyzed by SEM and EDX. In the test of using 3 different inoculant, there were a lot of C. Si and Al were also found. Mn was increased slightly. In the test of using 3 different slag, Mn and Si were found at the same place. C was increased slightly and Al was decreased. It is showed 6 examples of SEM and EDX analysis for slag pinhole defects occurred on products. To differentiate (a), (b) and (c), each element was overlapped by analyzing of EDX phase.

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