Subsurface porosity defects have been identified as hydrogen-nitrogen pores or CO-related pores by many researchers. Based on this finding, efforts have been made at the production site to control a wide range of factors necessary for reducing these defects, however subsurface porosity defects could not always be prevented. To resolve such problems, in this study, films on the internal surface of porosities detected after machining were precisely observed and analyzed by using FE-EPMA, after which the causes of the defects were investigated.

  The inner surface of the defects was basically composed of oxide and lustrous carbon films, and sometimes additional oxide films were observed on the dual films. The lustrous carbon films exhibited clear folds or creases, and some of them were broken through or were deformed by dendrite growth. The formation mechanism of the dual layer and its characteristic features could not be explained by authorized theories. But it can be explained by assuming that the films on these defects are generated on the surface of the melt and are entrained in turbulent flow. In addition, many small pores in size of 100 - 200 μm were detected around porosity defects, of which inner surface were composed of lustrous carbon films exhibiting clear folds or creases.

  From these features, small pores might be entrained in turbulent flow at the gating system, and some of small pores might rise in cavities and remain trapped under the solidified skin. The subsurface porosity might be formed on these small pores if there is enough hydrogen and nitrogen gas in the melt.

  Given that the addition of 1% or less of Cu to spheroidal graphite cast iron significantly affects the matrix structure and mechanical properties, it is thus vital to deepen knowledge on the effects of phenomena occurring in the matrix structure. The purpose of this study was to fabricate seven types of spheroidal graphite cast iron with Cu added up to 0.6%, to investigate the relationship between mechanical properties, phase constitution of spheroidal graphite/ferrite/pearlite, and ferrite phase structure, and to discuss the effect of Cu addition. First, the correlation between the pearlite area ratio and the increase in tensile strength and Brinell hardness with increasing Cu content was clarified. TG-DTA measurements were performed to analyze the matrix structure. It was found that A_r3 decreased with increasing Cu content and only A_r1 was present at 0.6Cu. In addition, using the full width at half maximum (FWHM) of (110) α as an index, the lattice strain increased with the amount of Cu addition. These results suggest that the addition of Cu may promote the slower growth of spheroidal graphite near the A_cm line, while the growth of spheroidal graphite proceeds rapidly in the austenite region in the absence or presence of a small amount of Cu.

  We investigated the reasons for why shell mold strength decreases when green sand is reclaimed by calcination at lower temperatures. When calcination is performed at temperatures above 740℃, the shell mold strength increases. The temperature of 740℃ is close to the OH group dehydration temperature for bentonite, which is present below 740℃, and is thought to reduce shell mold strength. To investigate this relationship, we added bentonite to shell mold sand, and confirmed that it reduces the shell mold strength. Using scanning electron microscopy, we found that bentonite prevents resin adhesion to sand particles and causes embrittlement of the resin coating. Moreover, the results of gel-permeation chromatography and powder X-ray diffraction analysis indicated that intercalation of bentonite in the phenolic resin reduces shell mold strength.

  The objective of this experiment is to increase the heat resistance of high Si spheroidal graphite cast iron (3.3%C-4%Si-0.6%Mo) which is used as turbine housing. It is known that when Al is added to the spheroidal graphite cast iron melt, nodularity and fluidity decreases, but heat resistance increases. This experiment thus aimed to demonstrate the effective utilization of Al. In samples added with a small amount of Al, microstructure was observed and mechanical properties and fluidity were measured. High temperature oxidation tests and high temperature tensile tests were also carried out. The high temperature oxidation results were used as measures of heat resistance.

  The results of the experiment showed that good heat resistant cast iron can be fabricated without decreasing the mechanical properties and fluidity. When the added Al was less than 0.3%, the nodularity was high and elongation of mechanical property did not decrease greatly. Al was distributed across the matrix structure. When the added Al was over 0.3%, the removal of sticking slag inside the ladle posed as a significant factor to resolve the difficulty of mass production. Therefore the optimum Al addition amount was considered as 0.2%. As operations such as melting, casting, sand molding etc. are carried out at the manufacturing site at our company, the experimental results are easy to apply to both the fabrication of the prototype and the practical products.

  Cast iron welding improves the performance of joints using welding rods for cast iron. However, such rods are more than twice as expensive as those for mild steel. The authors found that when spheroidal graphite cast iron is melted and rapidly solidified, graphite nodules aggregate and move to the surface. If the graphite nodules in the fusion zone move to the surface, the carbon concentration may reduce, thereby suppressing the generation of ledeburite. In this study, by irradiating with YAG laser, I-type butt-welded joints were fabricated using L materials (with an average nodule diameter of 52 μm) specimens without groove width. Ferrite and pearlite matrices joints were undermatch joints. Applying PWHT (Post Weld Heat Treatment) restored the joint strength to more than 90 % that of the base metal. The impact value for the as-welded joint dropped to less than 15 % of that for the base metal. However, it could be recovered up to about 40 % of that for the base metal by PWHT. Ferrite matrix joints after PWHT exhibited similar level impact values as the as-welded pearlite matrix joints. Moreover, the surface of spheroidal graphite cast iron with different numbers and size of graphite nodules was irradiated with YAG laser. As a result, at irradiation feed rates above 100 cm/min L, materials specimen exhibited a much lower hardness in the fusion zone than S materials (with an average nodule diameter of 27 μm) specimen. The graphite nodules in the fusion zone moved to the surface during rapid melting by laser irradiation. In the S materials specimen studied, more ledeburite, martensite and retained austenite formed near the fusion boundary than with the L materials specimen.