When cast iron is repeatedly heated and cooled, there occurs a crack caused due to so-called thermal fatigue. Such thermal fatigue comes into problem in the case of cylinder covers for large marine diesel engines which are subjected to severe thermal stresses during service condition. As to this, ductile cast iron showed a good resistivity to the thermal fatigue. As a result of experiments, the requirements of the ductile cast iron for the thermal fatigue are given as follows:
  1) To make the size of spheroidal graphites as small as possible, so as to increase the thermal fatigue strength;
  2) To be free from flake graphite, since it remarkedly lowers the thermal fatigue strength.
  3) The Coffin's equation N^½Δε_p = constant is available for ductile cast iron. The thermal fatigue strength of ductile cast iron can be estimated if the size of spheroidal graphites and their distribution are known, in consideration of the graphite area ratio and the stress concentration coefficint.

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  The authors describe the tests which were carried out in order to explain the relation between the process of nucleation and growth and the crystal structucture of graphites in various cast irons. Flake graphite, spheroidal one and temper carbon were prepared from gray iron, OZ-ductile iron and malleable cast iron by electrolytic extraction. The crystal structures of graphites and carbon were determined by X-ray diffraction. The results obtained are as follows:
  1) The values of the spacing, the lattice constant and the C-C distance determined illustrated that all the specimens had a crystal structure for the typical grapite.
  2) The diffraction lines of the flake and the spheroidal graphites were very sharp, but the lines of the temper carbon were broad compared with the other two lines. Also, the latter (100) and (101) line profiles were more asymmetrical than the former line profiles.
  3) The values of Lc (001) of the flake graphite were all above 1,000 A only at the lower scattering angle side but at the higher scattering angle they became smaller. This tendency of the spheroidal graphite and the temper carbon would be caused by the strain of crystal lattices. The La (110) values of each specimens were above 1,000 A.
  4) The strain of crystal lattice is given by
    β_(obs)•cos θ=η •sin θ+K • λ/D, ············(1)
where β_(obs) is the observed half maximum line breadth, θ the scattering angle, η the parameter of lattice strain, K the shape factor, λ the wavelength and D the crystallite size.
  The results are shown in Fig. 1.
  As shown in Fig. 1, the strain of crystal lattices was the largest in temper carbon, being slightly smaller in spheroidal graphite and almost nothing in flake graphite.
  5) The difference in graphite crystal structures may possibly depend not only upon the nucleation, growth condition and shape of crystals, but the internal stresses of the matrix caused during the solidification.

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  The corrosion fatigue properties of cast irons with various graphite structures were investigated by using a rotary-bending fatigue machine. The graphite structures of the specimens were spheroidal, quasi-nodular and flaky. Distilled water, artificial sea water and 1% aquous solution of sulphuric acid were used as the corrosive solutions. These solutions were conducted to the surface of rotating specimen to make a liquid film on its surface.
  From these experiments it was observed that the fatigue life in the corrosive circumstances was longer than that in the air at any stress levels higher than the endurance limit of the specimen. On the contrary, the former was shorter than the latter at stress levels lower than that limit. Consequently, any defined endurance limit could not be determined in the corrosive circumstances.
  The fatigue lives in the air were measured at various periods of corrosion fatigue tests. A large prolongation of life was observed with specimens subjected to corrosion for a short time, for example about one hour. The favourable effect of corrosion might be attributed to the reduction of the notch effect of graphite tips and fatigue cracks made by corroding the matrix around them. Reversely, the fatigue life decreased after a long period of corrosion because of promoting fatigue cracks by the corrosion.
  The change in fatigue limit occurred in the sequence of fatigue test at stress levels higher than the endurance limit of the specimen was measured. These values of cycles detemined at various stress levels were connected to each other to give a fatigue failure curve of the specimen.
  The formation of fatigue cracks and their propagation were microscopically observed on the surface of specimens during fatigue test. The fatigue cracks were formed at the peripheries of graphite particles and slowly propagated into the matrix and neighbouring graphites until a fatigue damage was developed. The rate of the crack propagation rapidly increased after the fatigue damage had once appeared in the specimen. The length of the cracks on the specimen surface was about two millimeters at the initial stage of fatigue damage. These cracks before the occurence of the damage were called ”not propagating crack” and had no effect upon the fatigue limit of the specimens. Here, the static tensile strength also was not weakened until the fatigue life was nearly consumed. From this fact it was considered that the depth of the fatigue cracks to be initiated was limited only to a shallow range under the surface of specimen.

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  In casting cast iron, the use of metal mold shows a remarkable difference from the ordinary sand technique. The thermal conductivity of the mold metal is very much higher than that of refractories used for sand mold. Accordingly, problems on the use of metal mold may possibly be related to heat transfer in molten metal, mold coating, and metal mold.
  The properties of mold coating in metal mold casting of cast iron may be classified to the three groups which are represented by graphite, zircon, and diatomaceous earth. The respective characteristics of these mold coating materials are :
  1) Graphite type : Heat flux density at the coating layer is remarkably high.
  The differece in surface temperatures between the mold coating and metal mold being very small. The temperature attainable is the highest among the three coating materials.
  2) Diatomaceous earth type : Heat transfer through the diatomaceous earth coating is much suppresed as compared to the case of graphite coating. The temperature difference obtained between the mold coating and the metal mold was more than 200°C, when the surface temperature of mold coating was maintained at higher temperatures than 700°C for a long duration.
  3) zircon type : The properties of zircon type are intermediate, but its surface temperature obtained was the lowest among those materials in question.
The boundary air gap formation and its duration which affect the heat transfer between molten metal and mold coating depended on the kind of mold coating materials and the thickness of metal mold. The air gap formation was most delayed and the duration of the gap was the longest when the diatomaceous earth was used and the thickness of metal mold was 10 to 20mm. When the metal mold thickness was 5 or 60mm, air gaps were repeatedly formed and vanished.
In conclusion, diatomaceous earth was most effective as a mold coating material in question on metal mold casting for cast iron.

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