The tensile strength and the toughness of austempered ductile cast iron with or without subzero treatment and quench tempered ductile cast iron were investigated. The following results were obtained : (1) The quantity of retained austenite of the cast iron austempered at 623∼773 K are 30∼40 %, and are decreased to 15∼25 % by subzero treatment after austempering. (2) The elongation and Charpy impact values of the cast iron austempered at 573∼773 K without subzero treatment after austempering are higher than that of the other heat treated specimens with the same tensile strength. (3) The retained austenite decreases after the tensile test remarkably. On the other hand, the change of the quantity of retained austenite is little after the bending and the impact test. (4) In the subzero treated cast iron after austempering, the retained austenite is divided into small sections by deformation induced martensitic transformation.

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  Thermal spray enhancing was applied in the cast-in insertion of steel and titanium in aluminum castings. An Al-12 % Si alloy was used for spraying. In a high temperature holding experiment, a fitted cylinder specimen, steel or Ti pipe inside and Al cylinder outside, was held in a furnace at 1073 K for various length of time up to 1000 s. The state of bonding was evaluated by microscopic observation and shear strength test on the cooled specimens. Experimental results showed that liquid phase contact longer than about 400 s yields good metallurgical bonding both in steel and Ti inserts with Al-Si alloy spray coatings as well as Ag and Au plated inserts. No bonding was achieved without the coating. In the cast-in experiments, good bonding was obtained by preheating the mold and inserts with sprayed or plated coatings. Solidification analysis reveald that about 400 s of liquid state contact before solidification of the melt is required for good bonding. Formation of alloyed or intermetallic compound layers, without harmful oxide films, was confirmed at the bonding interface by microstructure observation and EPMA analysis.

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  The effects of microstructure and porosity were investigated on the tensile properties of AC4B type Al-Si-Cu cast alloys containing 7 mass % Si, 3 mass % Cu, 0.17 mass % Mg, 0 to 0.72 mass % Fe and 0 to 0.4 mass % Sr. The specimens were solidified at a cooling velocity of 2.7 K/s or 14.4 K/s in metallic molds. T6 heat treatment was applied to a group of specimens. Since the crack generates from porosity, iron-rich compound and eutectic Si particle, the relation of the tensile properties and the size and morphology of the defects was investigated. Porosities larger than each crystallized phase appeared in the specimens cooled at 14.4 K/s, though the melt was degassed by bubbling a dry Ar gas through it. The crack initiated from larger porosities in the tensile test. In particular, the elongation decreased with the increase in the maximum size of the porosity. On the other hand, larger acicular or Chinese script type iron-rich compounds developed in specimens cooled at 2.7 K/s. The compounds lowered both the tensile strength and elongation, though they tended to raise the 0.2 % proof strength. Higher cooling velocities and proper amounts of Sr addition refined the microstructure and porosities, hence improving the tensile properties of both the as cast and T6 specimens.

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  Successive austempering process, in which upper bainite transformation was immediately followed by lower bainite transformation, remarkably improved the machinability of ADI without impairing the tensile properties. In this paper, the rolling contact fatigue strength (F^∗) of successive austempered ductile iron (SADI) was investigated by using a Nishihara-type wear test machine with a gear oil lubricant. The ductile iron containing 3.52 % C, 2.07% Si. 0.24 % Mn and 0.04 % Mg was austempered at 648 K for 1.8ks and then austempered at 588 K for 0.9 to 18 ks. The amount of γ-pool decreased from 6.52 % to 0.19% with increasing the holding time at 588 K. The F^∗ value decreased with the progress of lower bainite transformation until 9 ks, however that of the iron held for 18 ks exhibited a higher F^∗. Although this F^∗ value is still 30 MPa smaller than that of ordinary ADI austempered at 648 K for 7.2 ks, the machinability of the SADI was considerably higher. This indicates that, the SADI can be provided with a good combination of superior machinability and higher rolling contact fatigue strength.

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  Effects of silicon content are investigated on tensile properties and crack propagation behavior of Al-Si-Cu alloy die castings. The specimens are made by the low velocity shot die casting method. Silicon content of alloy is varied from 8.6 mass % to 15.4 mass %. The tensile strength and the elongation of hypo-eutectic Al-Si alloys are higher than those of hyper-eutectic Al-Si alloys. In the case of hypo-eutectic Al-Si-Cu alloys, a crack propagates through the eutectic phase in inter-dendrite region or α-Al dendrite. In the case of hyper-eutectic silicon alloy, the primary silicon acts as the origin of fracture because primary silicon crystallizes with facet planes. The crack mainly propagates through primary silicon grains or along eutectic silicon. Primary silicon particles tend to segregate in case of low velocity injection casting. This macro-segregation of primary silicon particles deteriorates the mechanical properties. Especially tensile strength decreases with increase of segregation.

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  The effects of the strain rate and contents of phosphorus and magnesium on the intergranular embrittlement (IE) at about 673K in ferritic spheroidal graphite cast iron were investigated by tensile and bending tests. Bending test, which is very useful for evaluating the occurrence of IE, confirmed that IE occurred when the Mg/P ratio exceeded more than about 1.5, even if the P content was as high as 0.03 %. From the electron probe microanalysis on the intergranular micro-cracks observed in the specimen loaded just prior to bending fracture, it was found that IE was caused by the embrittlement of ferrite grain-boundaries consistent with the austenite grain-boundaries formed in the interdendritic region of the eutectic-cell, and that in such grain-boundaries, the magnesium segregated. The spheroidal graphite cast iron with high manganese content, which was also embrittled at 673K when Mg/P was more than 1.5, fractured in intergranular mode even at room temperature. Auger analysis on its intergranular surface fractured at room-temperature in AES chamber revealed that there exists numerous fine metallic magnesium particles, indicating that the intergranular embrittlemen must have been caused by the segregation of the metallic magnesium to the austenite grain-boundary during solidification.

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