The effects of graphite morphology on the thermal shock properties of grain cast iron were investigated using a newly developed thermal shock test to reproduce thermal shock cracks similar to those occurring on the roll surfaces of hot strip mills. In iron specimens with spheroidal graphite, the thermal crack started at the surface and then propagated straight and deeply. On the other hand, when the amount of branched graphite was increased, the depth of the crack became shallower than that of spheroidal graphite iron. This is because the crack split into multiple branches, changed its direction, and propagated from there. In addition, the results of a modified compact tension test show that branched graphite specimens have stronger resistance to fractures than spheroidal graphite ones. These results suggest that increase of branched graphite in grain cast iron makes it possible to improve thermal shock properties through the prevention effects of branched graphite against crack propagation.

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  The continuous cooling transformation (CCT) curves of multi-component white cast irons with manganese (Mn) and nickel (Ni) (up to two percent each) were investigated at the austenitizing temperatures of 1273K and 1373K.
  Both pearlite and bainite transformations in the CCT curves shifted to the long time side with an increase in either Mn or Ni content regardless of the austenitizing temperature. In particular, pearlite transformation stopped when two percent Mn or one percent Ni was added. With regard to temperature, the CCT curve of cast iron austenitized at 1373K shifted to the long time side more than that austenitized at 1273K. The Ms temperature decreased when either Mn or Ni content increased.
  Relatively high hardness levels up to 600HV20 were obtained in the cast iron samples with two percent of either Mn or Ni even at the cooling rate of 0.12K/s or half cooling time of 4.5 × 10⁴s, because the pearlite and bainite transformations were delayed by the presence of the alloying elements. Maximum hardness was obtained around the cooling rate at which bainite transformation nose started occurring, and began decreasing with increasing Mn or Ni content.

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  Ultrasonic radiation during solidification is known to be an effective tool for achieving grain refinement in various kinds of alloys. Since the mechanism of grain refinement by ultrasonic vibration is not fully understood yet, it is not widely applied to practical casting processes. Through the observation of non-equilibrium α-Al grains appearing in sono-solidified hypereutectic Al-Si alloys, the authors have proposed the crystallizing mechanism of α-Al grains, in which the nucleation of α-Al grains arises from extremely high pressure generated by the collapse of acoustic cavitation in molten Al-Si alloys. However, the mechanism cannot explain the refinement of primary silicon in hypereutectic Al-Si alloys, because the liquidus temperature of primary silicon becomes lower with increasing pressure. In the present study, the refining mechanism of primary silicon was examined through their morphology observations after applying ultrasonic radiation to molten Al-Si alloy, during solidification and at constant temperatures. Fine primary silicon was recognized in Al-18mass%Si alloys quenched from above the liquidus temperature after ultrasonic radiation. It was also found that acoustic cavitation during the sono-solidification leads to the generation of inhomogeneous nuclei, that is, cavitation bubbles, so that a large number of inhomogeneous nuclei of cavitation bubbles causes the grain refinement of primary silicon even above the liquidus temperature.

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