Microstructure of decarburized spheroidal graphite cast iron is affected by the temperature and time of decarburization, as well as by cooling rate thereafter. To make clear these decarburizing conditions, experiments and numerical simulation were carried out. The isothermal growth rate of decarburized layer obtained experimentally is in good coincidence with that calculated formerly. Precipitations of ferrite and carbon take place beside decarburization during cooling. On further cooling below eutectoid temperature, ferritization proceeds in hyper-eutectoid zone of decarburized layer as well as non-decarburized part, while ferrite-pearlite mixed structure is formed in hypo-eutectoid zone. The latter striking structure interposed by ferritic matrix zones is called “pearlite band”. Numerical simulation shows that pearlite band is likely to be formed when the product of the initial decarburized depth by the square root of cooling rate is larger than about 0.01cm・(K/s)^1/2 for cooling from 1273K, and 0.006cm・(K/s)^1/2 for cooling from 1173K.

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  In the present work, attempts were made to develop a new type of cast iron, stainless spheroidal carbide cast iron whose chemical composition is x%C-20%Cr-10%Ni-y%V-Fe. This cast iron exhibits excellent characteristics in corrosion and abrasive resistance. In this cast iron, the morphology of vanadium-carbide was observed to change depending on melting conditions. At relatively higher temperature such as 2023K spheroidal carbides crystallized, while flower-like carbides crystallized at relatively lower temperature such as 1723K. Spheroidal carbide particles have a tendency to coagulate with each other when the temperature decreases and appear flower-like. These flower-like particles were found to break down to spheroidal particles again when the temperature was elevated. Thus we can conclude that the morphology of vanadium-carbide is not affected by the thermal history, but depends only on the pouring temperature.

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  In order to clarify the flow of semi-solid metal through the solid network, pressure infiltration experiments were carried out controlling the solid morphologies (globular and dendritic structures), fraction of solid, and applied pressure. Semi-solid AC4CH and ZDC2 alloys were compressed by argon gas at a pressure of 0.50MPa to force liquid components into permeating the ceramic foam filter. The results indicated that filtrated metal weight increased with pressurizing time, simultaneously yielding the consolidated grains of globular structure in the cake layer on the filter. At low fractions of solid, permeability depended strongly upon the solid morphology, namely a dendritic structure produces lower permeability due to the large flow resistance. For semi-solid AC4CH alloys with dendritic structure, cake filtration was observed for most fractions of solid. On the other hand, for semi-solid AC4CH alloy with globular structure, depth filtration was observed for low fractions of solid, f_s＜0.35. Cake filtration was prevailing for high fractions of solid, f_s＞0.35. The flow with the cake filtration mechanism is related to liquid flow through the solid network in semi-solid metals. For semi-solid ZDC2 alloy, dentritic structure is rearranged to globular structure during pressurization due to solid phase movement such as breaking off and rotation of the dendrite arms, and grain growth.

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  Friction welding of pipe shaped spheroidal graphite cast iron was carried out under welding conditions such as peripheral speed (rotational speed), friction pressure, friction time and forge pressure. The dimensions of the test pieces were 42mm in outside diameter and 28mm in inside diameter respectively. As a result, tensile strength increased with increasing peripheral speed, friction pressure, friction time, and forge pressure. High tensile strength specimens were broken on the base metal. On the other hand, tensile strength decreased with increasing layer of deformed spheroidal graphite.

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