In order to clarify the fatigue crack growth mechanism for austempered ductile iron, crack nucleation and growth characteristics were examined by means of fatigue crack propagation test, X-ray diffraction and TEM observation. The main crack was observed to propagate successively connecting the detached cracks which were nucleated in retained massive austenites near the crack-tip. Volume fraction of the retained austenite on the fatigue fracture surface was measured quantitatively by using an X-ray diffractometer. The retained austenite decreased with increasing effective stress intensity factor range. TEM observations of fatigue-deformed areas near the crack tip clearly revealed plate-like or lenticular martensites in the retained massive austenites. A fatigue crack growth model was proposed for the austempered ductile iron, based on the above experimental results.

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  A series of die casting experiments were performed to clarify melt cooling behavior with early solidification in the sleeve. The study was, in particular, directed towards investigating the effects of melt volume ratio in the sleeve, plunger speed, and sleeve preheat temperature on melt solidification.
  At the sleeve bottom near the gate, melt temperature fell partly under the liquidus point, even at an early stage just after the melt filled into the sleeve. Super-cooling occured as to eutectic reaction, especially near the bottom and gate side. Solidified metal was observed to be stripped off the sleeve wall and transfer to near the gate inlet, giving the product of an abnormal structure if it flowed into the mold cavity. Such scattered structure increases with the decrease of the melt volume ratio in the sleeve and plunger speed, and with lowering of the sleeve preheat temperature. The simulation reasonably explains the cooling characteristics of the melt in the sleeve and the effect of sleeve preheat temperature on melt solidification behavior.

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  In this study, white cast iron powders were made from cast iron melts by means of gas and water atomization methods and the annealing behavior of these powders was studied by SEM observation of the annealed structure. Furthermore, the annealing behavior of cast iron poured into a metal mold (10 mm φ) was also studied. The experimental results are summarized below.
  The graphitization rates of these cast iron powders were extremely higher than those of cast iron poured into a metal mold and graphitization began at the surface of the cast iron powder. The carbide of gas-atomized cast iron powder was completely decomposed in 120 s at 1273 K. On the other hand, carbide of cast irons poured into metal mold completely decomposed in 4.3 ks at the same temperature. However, the graphitization rate of fine powder produced by crushing cast irons poured into metal mold was very high and the graphitization time of these was similer to that of gas-atomized cast iron powder. It was confirmed that the graphitization time of cast iron powder is affected by the modulus (Volume/Area) of powder, the former being directly proportional to the latter. The results suggest that graphite preferentially precipitates at free surfaces of cast iron powder.

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  Cast-in bonding of WC-Co alloy on the surface of cylindrical cast iron was performed, and the thermal conditions for metallurgical bonding are discussed based on experiments and solidification analysis. In the experiment, WC-Co alloy specimens (20mm in diameter 10mm in height) were bonded on the bottom surface of 82mm diameter cylindrical ductile iron at various heights. Good metallurgical bonding with enough bonding strength was possible at a melt/insert volume ratio of 100, while the bonding was only partial with a volume ratio of 50. The border volume ratio for good bonding was between 50 and 100, indicating that bonding on the surface needs larger volume ratio than wholly cast-in insertions. Bonding was improved by protruding WC-Co inserts 1mm into the cast iron melt. Solidification analysis with an axially symmetric two-dimensional model was performed by the finite difference method. The analysis showed that where good bonding was obtained, a solidified layer at the interface remelts and remains in liquid phase contact for more than 30 seconds. The liquid phase contact time necessary for bonding is shorter than that in mild steel insertion in cast iron. Where bonding failed, remelting does not occur and there is no liquid phase contact.

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  One defect which occurs in the aluminum chill casting is blacking scab defect. We investigated the mechanism and prevention of this defect using a liner-type test piece inserted in the corner of the die cavity.
  Results of investigations show that aluminum alloy sticks to the mold wash physically when the product is ejected from the die, creating Al-Si-Fe-O compound. To prevent the occurrence of blacking scab defects, the following are necessary : ①smoothing and closing of mold wash surface, ②packing and closing of interstices in the mold wash layer, ③selection of inactive chemical and stable binder for aluminum alloy melt.
  The developed mold wash consists of three layers (under coat ; conventional mold wash, middle coat ; sodium aluminate binder mold wash, primer coat ; titanium oxide mold wash). This structure was found to increase life by three to five times.

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