In the present study, fatigue crack propagation behavior of the developed austempered ductile cast iron (Toughened ADI; TADI) was investigated, and was compared with that of conventional austempered ductile cast Iron.
  The threshold value (ΔK_th) of toughened austempered ductile cast iron is higher than that of conventional austempered ductile cast iron. This result is caused by the excellent crack initiation resistance toughness of the toughened austempered ductile cast iron. Moreover, toughened austempered ductile cast iron shows slower rate of fatigue crack propagation comparing with coventional austempered ductile cast iron. This phenomenon is caused by the crack closure with the strain induced transformation of retained austenite in the toughened austempered ductile cast iron.

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  The mass effect and the hardenability of ADI, using a specially designed apparatus for end-quench test in austempering, were studied. Two types of cone shape specimens were applied for austempering treatment. The changes of Vickers hardness, microstructure, morphology of bainite and retained austenite with distance from quench end at given temperature were investigated with first type of cone shape specimen. The shape of bainite changes and hardness of bainite lowers on the border of pearlite because of increase of transformation temperature.
  Hardenability of iron alloyed with Mo and/or Cu was evaluated with second type of cone shape specimen. A multiple linear regression analysis using the pearlite free half-cooling time data was performed to quantify the effects of Cu and/or Mo on suppression of the pearlite transformation. Half-cooling time is defined as the time required to cool from the austenitizing temperature to the temperature which is midway between the austenitizing temperature and austempering temperature.
  Moreover predicting the micro-structure and hardness of ADI was tried by the heat flow analysis with personal computer. Basing on predetermined end-quench data, the prediction in the way of simulating temperature distribution at pearlite free or 50 % pearlite plus ferrite half-cooling time after dipping into austempering bath by SOLDIA-EX was well coincident with practical ADI.

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  In the previous papers, it has been confirmed that the width of the graphite-free layer of gray cast iron was larger in the case of O₂ atmosphere than that of H₂, and so the oxygen might have a strong effect on the formation of these layers. This effect of oxygen were recognized in this experiment using air atmosphere, and also the graphite free layer was not formed in the case of atmosphere of air deoxydized by aluminum.
  Further, by the flowing-out of not yet solidified liquid method, the formation process of graphite free layer accompanied by the proceeding of solidification of gray cast iron was inquired.

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  As the grain refining process of aluminum alloy, the Ti and B additions have been used widely in aluminum industry. It is recognized that the addition of Ti alone also yields the grain refining effect. Now, it is investigated, how the supercooling for nucleation of solidification changes, and how the grain structure changes according to the contents of Ti and B in aluminum. So, it is clarified that both the TiB₂ compound and the solved Ti are necessary to transition from columnar to equiaxed grain structure, and the equiaxed grain is formed by the heterogeneous nucleation on the TiB₂ particle in the constitutionally supercooled aluminum liquid due to the Ti in solution.

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  Various properties of grown nodular graphite cast iron around wear characteristic were investigated and furthermore the iron was compared with sintered iron for structural parts (SMF4020) whose inner structure was similar to grown iron to promote the probability of utilization as new industrial materials. The resutls obtained were summarized as follows :
1) Density of the iron decreased and damping capacity increased with the growth, but higher mechanical properties were kept even after 10% linear growth (tensile strength; 70kgf/mm², BHN; 180).
2) Wear resistance increased with the growth and melting wear vanished in the range of high friction speed and furthermore adhesive wear vanished in 10% grown iron. Those depended on the results that the iron grew due to the mechanism of irreversible graphite migration and fine graphite nodules redistributed in the matrix and the area of lubrication increased.
3) Wear resistance of all grown irons was superior to that of sintered iron in all friction speed and amount of wear was about from 1/2 to 1/3 that of sintered iron on the average.
4) It was thought that grown nodular graphite iron might be new industrial materials which could be partially replaced with sintered iron. Especially, 2% grown iron was thought to have a bright prospect as it had high tensile strength (80kgf/mm²), high damping capacity (5 times that of as-cast iron), high wear resistance (3 times that of sintered iron at the maximum) and cheaper cost for manufacture.

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  An acceleration of bainitic transformation was observed with the specimens of two stage austempered spheroidal graphite cast iron by the analysis of dilatometric measurement and the microstructural observation. The result of dilatometric analysis indicates that the nucleation of upper bainitic ferrite is activated in the beginning of the transformation. Microstructural observation shows two types of upper bainitic ferrite α_U and α_S growing during the second isothermal holding; α_U nucleates close to the lower bainitic ferrite, and α_S nucleates in pre-transformed austenite matrix independently. And both dilatometric and microstructural studies shows that the acceleration of transformation is caused by the increasing α_U with decreasing of α_S simultaneously.

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