An improved shock heating test was conducted to investigate the effect of moisture condensed layer on scabbing, and the motion of moisture condensed layer and the time of fracture of sand specimens were measured.
  It was found that when the mold surface is rapidly heated in contact with molten metal or by radiant heat, the moisture condensed layer is formed benearth the mold surface ; simultaneously, expansion force is generated within the dried surface layer and imposes shear stress on the moisture condensed layer. Because of the distinctly weakend condition, fracture occurs on the moisture condensed layer and causes scabs.

View full abstract

  It seems that the ductile-brittle transition of ferritic nodular cast iron depends not only on chemical compositions but also on many microstructural variables. In this study, ferritic grain diameter and mean free path between graphite nodules were adopted as the microstructural variables and the effects of these variables on the toughness were examined mainly by carrying out instrumented Charpy impact test.
  Both finer ferrite grains and smaller mean free pathes showed the tendency to lower the transition temperature. On the other hand, ferrite grain refinement and larger graphite nodules tended to improve the upper shelf energy level. Equations to predict lower yield load and the maximum load transition temperature in the ferritic nodular cast iron were derived by applying the well-known relation in the case of the steel with dispersed spheroidal carbides.

View full abstract

  This investigation was undertaken to clarify the influence of sulfur and manganese on the structure of hypo-eutectic white cast iron and the behaviour of the elements during solidification. The specimens containing 1.9∼3.8%C, 0.008∼0.3%S, 0∼1.0%Mn and 1.0%Cr were solidified unidirectionally in the molds used in previous experiments.
  The size of the eutectic colony became larger with increase in sulfur content, because sulfur extended the temperature range of eutectic reaction. The addition of manganese dissipated this effect of sulfur since sulfur dissolving in the eutectic melt decreased by the crystallization of manganese sulfide.

View full abstract

  Microstructures in quenched specimens with Fe-Ni-C and Fe-C eutectic compositions were analyzed quantitatively in terms of fractional solidification, fraction and number of graphite nodules and average radius of austenite spheres.
  The order of magnitude and the temperature dependence of the growth rate constants of austenite spheres were consistent with the consideration that the growth is controlled by carbon diffusion inward through the austenite shell and there were almost no difference between Fe-Ni-C and Fe-C specimens. The computed number of graphite nodules in unit volume remained almost unchanged during solidification, and the fractional solidification measured was well expressed by the fraction of austenite spheres growing around the graphite nodules. Fractional solidification vs. time curves obtained by the analyses of cooling curves also showed good accordance with those determined in microstructures of quenched specimens. The pronounced tendency for undercooling and its gradual increase in later stages of solidification proved to be attributable to the diffusion controlled growth of austenite spheres and the number of spheres not being enough to compensate their slow growth.

View full abstract

  Nonmetallic inclusions in metals lower fatigue strength, and generally larger inclusions decrease fatigue strength much more than smaller ones. It has been shown in the previous work that the inclusions in spheroidal graphite cast iron, such as Mg and TiC, also lower fatigue strength. But these inclusions are much smaller in size than the graphite nodules which are regarded as a kind of internal defects. The author considered that this characteristic effect may be caused by aggregation of the inclusions in the vicinity of the eutectic cell boundaries. From this point of view, the fatigue strength of spheroidal graphite cast iron was studied as a function of the distribution of the inclusions. Fatigue tests were carried out on a number of ferritic cast irons which contained various amounts of inclusions. Distribution of the inclusions was numerically expressed as “degree of aggregation”, which indicates the deviation of the actual distribution from random distribution (Poisson distribution).
  Fatigue strength sharply decreased with increase in degree of aggregation. That is, though the size of the inclusion is not large, the inclusions have an adverse effect on the fatigue strength by aggregating in the vicinity of the eutectic cell boundaries. The size of the region wherein the inclusions were aggregated was obviously larger than that of the graphite nodules. Consequently, the characteristic effect of the inclusion may be explained by regarding this region as one large defect.

View full abstract

  The changes of α-and β-Si₃N₄ on heating the liquid nitrided Fe-Si alloys (Si1∼3%) were studied by X-ray diffraction, electron microscopy and hardness test.
  In Fe-1 % Si alloy, α-Si₃N₄ was only obtained by nitriding. The amount of Si₃N₄ decreased by heating at 800°C∼1,000°C owing to Si₃N₄ dissolving in γ solid solution. The Fe-Si-N martensite structure was obtained in the sample quenched at 1,000°C. In Fe-2%Si alloy, α-and β-Si₃N₄ were obtained by nitriding. Those Si₃N₄ decreased by heating at 800°C∼1,000°C because it dissolved in the γ solid solution. In Fe-3%Si alloy, α-and β-Si₃N₄ were also obtained and Si₃N₄ decreased by heating at 900°C∼1,000°C. As there is no A₃ transformation in the Fe-Si binary alloy, α solid solution (silico-ferrite) remains up to high temperature. Decrease of two nitrides is not due to dissolution in γ solid solution but to dissolution in α solid solution.
  The amount of α-and β-Si₃N₄ by the nitriding changed with the ability to form iron and silicon nitrides and diffusion rate of nitrogen in Fe-Si alloys. A large amount of Si₃N₄ was found in Fe-2%Si alloy up to 800°C.

View full abstract