It is said that the property of bentonite, especially in its state of mulling with water, is not uniform showing varying values according to the differences in production site, kind and grain size. Hitherto, however, there is no agreement on considerations on the binding effects of bentonite on green sand mold because of using different bentonites produced in different sites, which were all mulled by the same content of water.
  The authors considered that it was very important to decide the water content suited for each of the bentonite with differing properties. So, in this work, six kinds of bentonite were selected, and the relationship between the properties of bonding materials with different water content and the properties of green sand mold were studied.
  The results were as follows:
  (1) In order to estimate the properties of green sand mold bonded with bentonite, it is important to examine the nature of bentonite, however, it is even more important and effective to examine the properties of bonding materials containing bentonite.
  (2) The properties of green sand molds which are produced with bonding materials of equal quality and fixed water content under the same mulling conditions are varied according to the differences in the production site of bentonite.
  (3) The properties of green sand mold vary according to whether the water content of bonding material is over or below water content at the plastic limit of bentonite.
  (4) When the water content of bonding materials is not more than the water content at the plastic limit of bentonite, the properties of green sand molds are influenced by mulling time.

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  The metal mold casting process in cast iron is, recently, being employed on a considerable scale because of its economic and technical advantages. But the process is limited for commercial production because of the short service life of the metal mold. For the development of the process, a metal mold material with a long service life is desired. In the metal mold casting process, cast iron mold is ordinarily used. The metal mold is destroyed by cracks resulting from repeated heating and cooling in actual use.
  The authors attempted to improve the thermal shock resistance of cast iron as a metal mold material by alloying Al, Cr, Mo, Si and Mn. This report investigated the effects of the alloy elements on the thermal shock resistance of cast iron by statistical analysis. High temperature microhardness of the cast iron alloyed with Al, Cr and Mo which showed high thermal shock resistance was measured in the range of room temperature to 800°C. Also, with respect to the thermal shock resistance, cast irons alloyed with Al, Cr and Mo were compared with siliconized cast iron which was packed in Fe-Si-Cr alloy powder containing 20∼30%Si prepared in high purity and treated in H₂ for 6 hrs at 1100°C.
  The results are as follows:
  (1) In the thermal shock test whereby the specimens were heated to 1,000°C for 2 min. and cooled in blasted air for 2 min., cast irons alloyed with 1.0∼3.0%Al, 0.5∼1.0%Cr and Mo each showed good thermal shock resistance. And there were no adverse effects on castability and mechanical properties within these compositions.
  (2) In addition to the high thermal shock resistance, high temperature hardness of the cast irons alloyed with Al, Cr and Mo was improved and was within H_V 85∼125 at 800°C, while cast iron without alloy elements rapidly lost hardness above 600°C and indicated H_V 50 at 800°C.
  (3) By alloying Al, the formation of the surface oxide was prevented almost perfectly. Also, internal oxidation was neutralized by Cr and Mo additions, but the degradation of the mechanical properties due to internal oxidation in the alloys was inevitable. In comparison with the cast irons alloyed with Al, Cr and Mo, the siliconized cast iron which had good thermal stability was substantially superior as a metal mold material.

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  The present study was undertaken to obtain fundamental information regarding crazing (craze-crack) formation process in cast iron. For this purpose, two kinds of flake graphite cast iron with different matrix (ferrite or pearlite) prepared by means of heat treatment were tested using the testing apparatus for crazing developed by the author. The crazing formation was observed by optical and electron microscopes, and discussed in relation to the microstructure of the cast iron.
  The following results were obtained :
  (1) Within 50∼500 thermal cycles, the relation between the numbers of cycle (N) and the amount of crazing (a_N) was obtained as the exponential function (a_N=k₁·N^α, where k₁ and α are constants for material).
  (2) The origin of crazing was considered to be the graphite portion at the heated surface.
  (3) In the matrix portion, the propagation of crazing was observed as being influenced by the three-dimentional configuration of flake graphite structure in gray cast iron.
  (4) Compared within the same thermal cycles, the effect of the difference of the matrix on the characteristics of crazing was not great.
  (5) In the graphite portion, crazing was observed passing through the inner part of the graphite. But in the matrix portion, the direct relation between the path of crazing and metallurgical factors of the matrix (ferrite grain boundary, lamellar structure of pearlite, etc.) was not found.

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  Recently, hardness measurement has gained importance as an inspection to guarantee castings produced in foundries, because it is rapid and usually non-destructive. However, only a few study which deal with the conversible relationship of various hardness measurements for gray cast iron have been published up to date.
  This study is a basic investigation on the conversible relationship of hardnesses in gray cast iron which may be utilized in the hardness inspection and on some problems associated with various hardness measurements in gray cast iron. The test method which was employed for this study, was H_B (Brinell hardness), H_S (Shore hardness) and H_RB (Rockwell hardness; B scale) based on JIS. The specimens used for this study were produced by using 15 kW high frequency induction furnace, and their carbon equivalent range fell between 3.5 and 4.4. The hardness range of the specimens used was from H_R 90 to H_B 250.
  The results of the experiment are summarized as follows :
  (1) It was confirmed that hardness value of gray cast iron is mainly influenced by the dimensions and testing site of the specimens, casting quality, and testing conditions.
  (2) The variation in individual hardness value in gray cast iron is greater in the relatively soft iron, and it decreases with increasing hardness.
  (3) In order to increase the accuracy of hardness measurement, the surface of the specimens should be polished with No. 320 emery polishing paper for H_B and H_S testing, and No. 120 emery paper is recommended for H_RB testing.
  (4) The hardness conversion between gray cast iron and plain carbon steel is very different. Therefore, when the conversion value for plain carbon steel is applied to gray cast iron, considerable conversion error is caused due to the difference in quality.
  (5) The following regression equation was calculated by the computer for hardness correlation between H_B and H_S, as well as B_B and H_RB.
  Correlation between B_B and H_S :
      H_B=−104.7+8.483H_S
  Correlation between H_B and B_RB :
      H_B=−213.8+13.052H_RB−0.18733H_RB²+0.001026H_RB³

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