발간논문

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To investigate whether tungsten carbide (WC)-hard metal of the higher density and hardness than conventional one could be obtained, tungsten was substituted for cobalt as binder metal, and then this WC-W system was activatedly sintered with adding small amount of Co or Ni. The influences of activators as well as tungsten binder on the sintering kinetics, and on the microstructure of this sintered alloy and its mechanical properties have been studied. The highest fractional volume shrinkage could be obtained in the 10% W contained specimen independently of the sintering temperature and a kind of activators under conditions of the present experiment. The observed activation energy for the process of activated sintering was about 35㎉/mole for the specimen of 10% W. Contrary to the activated sintering of pure tengsten compact, Co was more effective in promoting shrinkage than Ni in the sintering of the present WC-W system. The hardness of the W-cemented WC-hard metal was relatively higher than that of the conventional Co-cemented one. However it showed brittleness and poor strength.

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The effects of silicon and cooling velocity on the matrix structure of spheroidal graphite cast iron have been studied. In the preparation of spheroidal graphite cast iron, Fe-Si-Mg and Fe-Si alloys were used as additive and inoculant respectively. The important results obtained in this study are as follows. (1) As the silicon content is increased by 0.1% in samples with diameter of 10, 20 and 30mm, the amount of pearlite decreases by 4% in hypo-eutectic region but by 1.8% in hyper-eutectic region. (2) However, the amount of ferrite is increased by 4.3% in hypo-eutectic but 1.6% in hyper-eutectic region per 0.1%Si addition in the same samples having various diameters such as 10, 20 and 30mm. (3) The effects of silicion content are larger than those of cooling velocity on the matrix structure.

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It is proposed in the present paper to develop a domestic resin coated sand, and to study the effects of curing temperature and time on mechanical properties of shell molds such as tensile strength, scratch hardness and permeability. The important results obtained are as follows 1. Tensile strength of resin coated sand which have been cured on 90 sec. at 225℃ was maximum, and as curing temperature and time increased above it, the tensile strength decreased. 2. Scratch hardness of resin coated sand which have been cured on 30 sec. at 200℃ was maximum 92S. With increase of curing temperature and time, scratch hardness decreased. 3. With increase of curing temperature and time permeability of resin coated sand increased, and maximum was 220㏄/min. which have been cured on 180 sec. at 300℃.

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Kinetics associated with the formation of chromium diffusion coated layer and chromium carbide onto pure irons and carbon steels in molten lead was studied. Chromium diffusion process was carried out by dipping the specimen in molten lead bath containing dissolved chromium at various temperature and time. The effect of dipping time, bath temperature, and carbon content in the specimen on the diffusion process was described. The result indicated that for the pure iron, the rate controlling step was the chromium diffusion into the metal and the relationship between the diffusivity of chromium, D_a and absolute temperature T was D_a=(1.505) exp[-52.8±7.7(㎉/mole)/RT] The concentration of chromium in surface layer was increased almost linearly with carbon content in the specimen. The result also showed that the hardness of diffusion coated surface in carbon steal was greatly increased due to the formation of carbide.

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It has investigated the magnetic and mechanical properties of sintered cores made of Fe-Si alloy powder containing high silicon content with the goal of making them practical in electric devices. Fe-6.5% Si powders with different particle sizes were compacted under pressures of 8∼9ton/㎠ and sintered at 1100-1250℃ in hydrogen atmosphere. The magnetic properties of sintered cores were improved with increasing the green density of core and sintering temperature. Core loss was superior to those of compressed iron powder cores and electrical steel sheets. In conclusion, the higher the density the better the properties of compacted core and high temperature sintering provides further improvement to them. The Fe-6.5% Si powder cores may be sufficiently good for electric parts such as cores and inductors if their compacted density and sintering temperature are well controlled.

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