Selection of molding binder in CO₂ gas hardening molding process and examination of properties of the mold with new molding binders were carried out. The reactivity among the several binders, Ca(OH)₂ and CO₂ gas was investigated. As the results, alkali-neutralized copolymer of isobutylene and maleic anhydride showed a good property as a binder of the mold. The hardening reaction between CO₂ gas and the mold sand with this binder proceeded more rapidly than the mold sand with PVA aqueous solution or sodium-silicate solution. This property enables the molding process using this binder to decrease the amount of CO₂ gas required for hardening it. Gray iron castings weighing about 20kg per piece were tentatively cast using this mold. This process yields precise casting free from defects and has preferable collapsibility.

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  Tools of a plug mill for seamless pipe rolling are damaged very shortly when used in heavy mechanical and thermal loading. So improving durability of the tools has been attempted by making them by forging of high carbon and chromium cast steel. Forgeability of high chromium cast steel was tested by hot rolling of 50kg short ingots with chemical composition of 1.2 to 2.0% carbon and 22 to 27% chromium. The structure of the ingot heated at 1,000 to 1,150°C consists of eutectic carbide of Cr₂₃C₆ type and austenitic matrix. After forging by hot rolling, test materials were heat-treated and examined in mechanical properties, characteristics of fatigue strength and improvement of wear at elevated temperatures.
  There are good forgeability of high carbon and chromium cast steel with suitable chemical composition selected by empirical equation of Cr(%)=12+7.7×C(%). Structure and mechanical properties of the materials are greatly improved by hot rolling, especially in toughness, fatigue life and anti-wear properties at elevated temperatures.

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  A special probe ‘Multi-Transducer’ which is able to detect near-surface defects of casting was developed by the authors, and echo pattern and echo peak difference caused by size and shape of casting defects were improved. The application of this probe was taken in a housing case of iron casting and the depth detection sensitivity for various size and shape of defects was examined comparing with X-ray photography. The probe is possible to estimate in an accuracy of ±1.5mm in case of blow hole. The development of a portable Multi-Transducer system was also discussed.

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  The change in properties of the molding sands which were prepared by adding 2 to 3% pasted binder consisting of starch phosphate, zircon flour and graphite was investigated under repeated use. The dry and hot strengths of the dry sand molds with this binder were higher than those of the sand mold with starch phosphate alone. It was found that no defect could be obtained in iron castings by replenishing this heat-resistant starch phosphate as small as 1.5% to the molding sand for repeated use. However, ten time repeated use of the molding sands makes the residual starch reduce to 40% of the initial amount, so that the amount of starch replenished should be corrected at an appropriate time.

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  The effects of heat treatment and Si content on the fracture toughness of black heart malleable and spheroidal graphite cast irons with ferritic matrix structure were studied. In spheroidal graphite cast iron, two kinds of heat treatment were carried out, that is, malleablizing and low temperature ferritizing annealings in which graphite nodules were loosely and/or tightly enveloped in the matrix structure. In black heart malleable cast iron containing 1.21-1.81%Si and spheroidal graphite cast iron containing 1.87-3.45%Si, it was difficult to measure effective K_IC values specified by ASTM-E399. The silicon content of both malleable and spheroidal graphite cast irons was clearly confirmed to influence upon their J_IC values. J_IC values of spheroidal graphite cast iron was not influenced by the malleablizing and ferritizing annealings.

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  Toughness of the nickel added spheroidal graphite cast irons with various matrix structures was examined. Ferritic cast iron is most excellent in ductility, however its strength is low, and the addition of nickel in this matrix iron lowers the toughness. Pearlitic iron shows increased proof stress by the addition of nickel, but nickel addition shows no remarkable improvement in impact properties. Martensitic iron is essentially brittle, but addition of nickel can improve the impact transition temperature and absorbed energy. Bainitic iron shows good combination of high strength of about 140kgf/mm² and toughness and had lower transition temperature than that of ferritic iron. Improved mechanical properties in the bainitic iron are obtained with nickel content at least 3 percent.

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