This paper presents the experimental results of an investigation on the fatigue strength of a series of cast irons treated with various quantities of magnesium alloy (Fe-45%Si-20%Mg) ranging from 0.5% to 2.0%. The hardness of the cast irons was modified to high or low levels by annealing or hardening and tempering. The chemical composition of irons was about 3.6%C, 2.6%Si, 0.2%Mn, 0.05%P and 0.02%8. The magnesium treatment changed the ratio of spheroidal graphite to worm-like graphite in the cast irons : the irons treated with less amount of magnesium alloy contained more worm-like graphite.
  Rotating beam fatigue tests indicated that the fatigue limit is independent of the proportion of worm-iike graphite. A further examination of the microstructures revealed that the quantity of inclusions in these irons, such as MgO and TiC of 2-10μ, increased wih increasing amount of residual magnesium. It was considered that the fatigue limit did not increase with increasing spheroidal graphite as such inclusions remarkably lowered the fatigue limit.

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  In the series of authors' works on the characteristics of induction furnace-molten iron, it bas been suggested that nitrogen in iron plays an important role in giving character to the melt. The present work was under taken in order to clarify the above assumption. Inoculated or uninoculated irons with carbon equivalent of 3.8 to 4.5 were obtained by varying carbon and silicon contents. Nitrogen content was varied from about 40 to 150 ppm by adding Fe-Mn-N alloy. The strengthening effect of nitrogen on the inoculated iron was determined by statistical analysis of data obtained for tensile strength, Brinell hardness and transverse strength.
  In the multiple regression equations containing carbon, silicon and nitrogen as independent variables, it became clear that 10 ppm nitrogen increases tensile strength by about 0.7 kg/mm², Brinell hardness by about 3.5 and transverse strength by about 0.8 kg/mm² for irons with carbon equivalent of 3.8 to 4.4. In the iron having carbon equivalent of 4.5, nitrogen showed a peculiar behavior in which the strength dropped to the minimum at about 100 ppm nitrogen. In uninoculated iron, strengthening of nitrogen was slightly smaller than in inoculated iron.
  The mechanism of strengthening of nitrogen was examined in terms of graphite and matrix structures. As a result, some improvement in graphite structure was assumed because supercooling was observed at solidification temperature. But this has not been confirmed by examination on the microstructures. It was shown that the hardness of pearlite matrix increases with nitrogen content, which corresponds to the observed decrease in eutpectoid temperature by nitrogen. This led us to conclude that strengthened pearlite matrix contributes greatly to the strengthening of iron by nitrogen. In addition, it was suggested that 40% of the difference in the strength of iron melted in the induction furnace and cupola was due to nitrogen.

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  For investigating the possibility of metal casting by the plaster mold, various metals were contacted with alpha-gypsum and placed in alumina crucible and were heated at the heating rate of 20°C/min by a differential thermogravimetric analysis equipment in an argon atmosphere. This atmosphere was introduced to KlO₃ solution to observe the thermal decomposition temperature by decolorization due to the SO₂ generating from gypsum.
  Alpha-gypsum which did not contact with metal was fairly stable from 400 to 1,200°C, and SO₂ was generated at 1,330 to 1,360°C by thermal decomposition. But metals which were contacted the thermal decomposition temperature. The reactions between gypsum and metals were classified lowered into three groups : (1) metal contacting little affected the thermal decomposition of gypsum (e.g., Ag and Au), (2) gypsum was deoxidized by contacted metals to some degree and lowered the thermal decomposition temperature and formed CaO and metal oxides (e.g., Cu, Fe and their alloys), (3) deoxidation proceeded further and formed CaS and metal oxidse (e.g., Mg, Zn, Al and their alloys), and some metals belonging to this group reacted exothermically, or did not generate SO₂.
  By the temperature differences between these reactions and melting points of contacted metals, as much as 200°C, Sn, Pb, Zn, Al and it's alloys, some of Cu-base alloys (e.g., brass, aluminum bronze, manganese bronze and beryllium-copper), Ag and Au were considered to be castable by the plaster mold. Ferrous metal was considered to be not possible as it decomposed the gypsum at as low as 900 to 1,030°C. In the case of magnesium and it's alloys, vigorous exothermic reactions were observed at melting points and it was impossible to consider the possibility by this method.

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  With a view to investigating the influence of silicon on the graphitization of gray iron during solidification, various couples of molten iron with different silicon content but with fixed carbon content were bound by molten state joining in both sand mold and graphite crucible using shearcell method. Joining consisted of the following : 1) five couples consisting of standard gray iron with 3.2%C and 1.9%Si and high silicon cast iron with various contents of silicon with the same joining temperature, and 2) a couple consisting of standard gray iron and 5%Si cast iron with varying joining temperature or cooling rate after joining.
  It was clarified that in the combination of standard gray iron and high siilconcast irons the area of pearlitic matrix decreased gradually with the distance from the original gray iron to higher silicon cast iron turning to a ferritic matrix substantially. On the other hand, graphite structure showed a gradual increase in size at first, and then changed to extremely fine graphite in the vicinity of border line of the two irons, and after that increased in size gradually to the normal size of graphite in each high silicon content cast iron. About the same structural change of graphite occurred also in the series of combination of standard gray iron and 5% silicon cast iron by varying the joining temperature of molten gray iron over the eutectic temperature while constantly holding the high silicon content iron at 1,200°C. However, decreasing the joining temperature below the eutectic temperature did not result in any more fine graphite zones, since silicon diffusion had no effect on the occurrence of graphite nucleus. The reason for the occurrence of abnormal zones with extremely fine graphite is assumed to be that any optimum condition for simultaneous generation of numerous graphite nucleus was produced by silicon diffusion from the side with higher silicon in a very short time with no further resolution of existing graphite nodule into the retained molten metal.

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  The characteristics and the mechanism of the formation of the hardened layers generated on the sliding surface of spheroidal graphite cast irons were studied as a function of the ferrite content, which represents the wear loss of the irons, under lubricated conditions using a pin-and-disk type wear machine. The ferrite content of irons was controlled by both heat treatment and amount of Si addition.
  Only in the case of severe sliding conditions, at which the wear became extremely serious, the hardened layer, which were stable for 2% Nital etching reagent and very hard more than Hv 800, generated. Also, the amount of hardened layers were increased in proportion to the decrease of the ferrite content, i.e. wear. It was shown by the X-ray diffraction that the hardened layers consist of α-Fe, α'-Fe, γ-Fe and Fe₃C. There is a linear correlation between the amount of the hardened layers and the integral width of α-Fe(110) and α'-Fe(110).
  Accordingly, it was concluded that the hardened layers were the result of rapid solidification of melting regions on real contacting. It was also found that the retained austenite and the martensite increased with a decrease in the amount of the hardened layers.

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  Shrinkage cavities of gray cast iron are mainly the result of chemical compositions, gases contained, solidification processes, mold conditions, etc. These factors are classified into two groups : the volume changes of molten iron and the movement of mold during solidification. The objective of this investigation was to measure the precise volume changes of molten iron during solidification by eliminating the effect of mold. Several specimens were used, varying the carbon equivalent from 3.6 to 4.3 and the surface levels of molten iron in a 10mm diameter silica tube were continuously measured.
  The volume decreased directly in a linear mode from the pouring temperature to the liquidus in the ratio of 10×10⁻³%/°C, and the volume decreased in a concave curve from liquidus to eutectic temperatures. It was noted that the volume increased remarkably during eutectic solidification. The amount of contraction between liquidus and eutectic temperature increased proportionally with the decrease in carbon equivalent value. That of expansion, in the range of eutectic solidification, decreased as the carbon equivalent value lowered. The total volume change of both contraction and expansion decreased with the lowering of carbon equivalent value and went to zero at carbon equivalent of 3.6, that is, the contraction and expansion canceled out each other.

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