By the addition of ferrosilicon to molten cast iron, not only is there the inoculating effect but also under certain conditions spheoidal graphite can be formed.
  In order to investigate the role of silicon as an additive, an experiment was carried out using the potentiostatic technique as a means of etching the primary structure. When the melt was treated with large amounts of refined ferrosilicon, microregions in which silicon was highly concentrated were observed in the cast state and spheroidal graphites were mostly formed in such regions. This suggests that the formation of spheroidal graphite is related to the local super-saturation of carbon caused by adding silicon to the melt.

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    In studying the influence of casting conditions on pinholing, it seems to be necessary to first find out in what parts of castings pinhole defects are formed and develop test pieces best suited to study this, and then establish the evaluation method of pinholes formed in the test castings. This report presents the results of the investigation of this relation among the casting thickness, the gating system and the pinholing tendency, and describes the evalution method of pinhole defects in test castings.
    The results obtained were summarized as follows.
  1) No definite tendency to pinhole formation was observed between casting thicknesses of 5 and 20mm.
  2) A gating system decreasing the casting speed was more prone to give rise to pinhole defects.
  3) There was a correlation between the location of pinhole defects formed and the gating system employed. The pinhole defects were apt to be concentrated on the part of castings where the first cold metal entering the mold or the slag formed by the metal-mold reaction finally stayed and solidified.
  4) It was found that the number of pinholes which appeared on the internal surface of the test castings machined by 0.5mm and 1.0mm, respectively, can be regarded as a representative number and volume of pinholes in the castings.

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    Fatigue strength of S.G. cast iron is considered to be influenced by matrix structure and graphite. The influence of pearlite content in the matrix structure was elucidated in the previous report and therefore the influence of graphite size is examined in this report.
    The chemical composition of the S.G. cast iron in this experiment was : C : 3.60%, Si : 2.75%, Mn : 0.24%, P··0.012%, Cr : 0.02%, Mg : 0.040%. Various graphite size was obtained by changing the solidification rate of the casting block.
    The fatigue strength and other mechanical properties are shown on table 1.
From the results of the experiment the author abtained the following conclusions.
  1) The tensile strength is hardly affected by graphite size.
  2) The hardness is greatly affected by graphite size and the minimum value is found where the graphite diameter is 70μ for the ferrite matrix and 40μ for the pearlite matrix.
  3) The elongation and impact value measured at room temperature increased with the increase of graphite size in the ferrite matrix but no change was observed in the pearlite matrix.
  4) Rotation bending fatigue limit was obtained for fatigue strength. Two kinds of specimens were used: the unnotched specimen (with a diameter of 8mm) and the notched specimen (with a noch radius of 1mm). The fatigue strength decreased in proportion to the increase of graphite size, but became constant where the graphite diameter was over 40μ in the case of the unnotched specimen regardless of the matrix structure. This can be explained by regarding the graphites on the surface as notches. However, because the grain size in matrix structure changed in proportion to graphite size the influence of grain size must be also taken into consideration.

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    Formerly one of the present authors introduced a new growth theory of cast iron from experiments on some S.G. irons. By his theory not only the peculiar changes of graphite structure and dilatation-curve of the iron but many complicated growth characteristcs of cast iron of the general type were explained very well. In this paper we again deal with the growth of S.G. iron, because as in the previous work the heating atmosphere was air and low vacuum (10^-1 mmHg) the growth of iron could not be accurately examined until their final stages. In the present work an S.G. iron was heated up to 300 cycles through 600°→950°→600°C in the vacuum of 3×10^-2mmHg. Growth and the metallographic changes of the iron ceased completely after about 200 cycles of heating. Changes of specific volume of the grown iron were also tested. The structure of graphite nodules was observed in detail from the early to final stages by both an optical microscope and electronmicroscope. A considerably difficult problem encountered in the observation of the inner structure of dense graphite nodules in porous iron was solved by using a scanning electronmicroscope developed recently.
    The results obtained were summarized as follows:
  1) The maximum linear growth was 12% after 200 heating cycles, and the rate of growth divided the period of growth into early and later stages. After the dividing point of about 7% of linear growth the rate of growth decreased considerably.
  2) On the growth curve the growing period was followed by a small contraction and then by a level line. In the final stages the iron showed thermal expansion and contraction withou tirreversible change by cyclic heating.
  3) The early growth up to 7% can be explained by the graphite migration mechanism proposed by one of the present authors. The redistribution of graphite and diffusion porosity due to graphite migration make the iron grow during the heatings.
  4) When the growth of the iron reached 7%, its structure changed itself to become completely porous and the cavity of graphite came in contact with the outer atmosphere. This may be confirmed by some calculation on cavity formed by growth.
  5) As grown iron was a completely porous structure, it was easily affected by atmosphere and its rate of growth decreased when its length was measured in air and then heated in high vacuum.
  6) Contraction of grown iron is similar to that of sintered metal.
  7) Metallographic structure of matrix in grown iron changed to a ferritic type by a small amount of carbon in the matrix migrating in the final stages of growth.

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  The effect of hydrogen content of iron melt on pinholing in calcium-treated iron castings cast in green sand moulds has been investigated.
  The melt from a cupola was transferred into a ladle, where the melt was desulfurized by calcium carbide and sodium flouride. After inoculation and calcium treatment in a hand shark, the melt was poured into a test mould.
  In the mould, a disk and a riser were connected to a symmetrical gating system. The disk was 100mm in diameter and 6mm thick and was used as a pinhole test specimen. Samples for hydrogen determination were taken from the melt in the riser. Three types of test moulds were used. Each had a different shape of gating system and were designated as mould No.1, mould No.2, and mould No.3, respectively. The design of mould No.2 and mould No.3 were modeled after mould No.1. The gating system of mould No. 2 was designed to restrain the turbulence of the stream of the melt as much as possible. On the other hand, the gating system of mould No. 3 was designed to bring out the turbulence. When either mould No.1 or mould No.2 was used, the number of pinholes increased with the increase in the hydrogen content of the melt as shown in Fig. 1. In particular, an almost linear relation between the number of pinholes and the hydrogen content was obtained when mould No.2 was used. A large number of pinholes and blowholes were observed regardless of the hydrogen content when mould No.3 was used.
  The change in the hydrogen content of the melt during the previously mentioned treatment was examined, and the hydrogen content was found to increase by the addition of the desulferizer almost up to the same values as those in the riser.

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