발간논문

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For the purpose of investigating the behaviour of residue Mg in the spheroidal graphite cast iron, the effect of additional amount of Fe-Si-Mg and cooling velocity on the graphite shape were studied by photomicrograph and EPMA. The principal results are as follows. (1) As the additional amount of Fe-Si-Mg and cooling velocity decreased, the degree of graphite spheroidization increased partly. (2) In the case of different cooling velocity, in spite of the same degree of graphite spheroidization the shape of graphite distribution was different. (3) There were little residue Mg and Si in the graphite, but they are much in a boundary of graphite and matrix. In case of spheroidal graphite, the degree was more extreme than that of flake graphite. (4) Silicon content was greater where the residual Mg remained more.

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Reduction kinetics of mixtures of synthetic ilmenite(FeTiO₃) and graphite powder were investigated in the temperature range of 980℃ to 1100℃. Experiments were carried out with various graphite contents under N₂ and CO₂ atmosphere. DTA of the reaction were obtained simultanecusly with the measurement of the weight less. The unique initial rate of reduction of ilmenite was explained in terms of the solid-solid reduction between ilmenite-graphite and the later stage of the reduction was well applied to the Rao`s physicochemical model for reactions between solids through gaseous intermediates. The activation energy of reduction process being occurred mainly by gaseous intermediates was found to be 70.0±2 ㎉/㏖ and the temperature dependence of the specific rate constant, I₁, of Boudouard reaction which occurred simultaneously with the reduction of ilmenite was found to be given by I₁=0.640 exp(-12,360/RT) ㏖/atm-sec-gr. of graphite. Whereas, under atmosphere of CO₂ little reduction of ilmenite was found due to the infiltration of CO₂ into pellet from atmosphere.

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In Al-Si alloy, the variation of mechanical properties with silicon contents was investigated the silicon content being varied from 5% to 25%, and the effects of additives and refining elements were also studied. The results obtained are as follows: 1) Sodium treatment made the primary silicon crystals refined and spheroidized, and made the matrix structure intensified. The effect of P treatment on refining primary silicon crystals was greater than that of Na. 2) Tensile strength showed the maximum value at near the eutectic composition and was improved considerably by addition of Mg and treatment with Na. 3) The variation of matrix hardness with silicon contents was not perceptible and the hardness was improved by addition of Mg and treatment with Na.

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The liquid boronizing of stainless steel to improve its wear resistance has been studied by means of the measurement of the boronized depth and the microhardness, the observation of the microstructure and the test of wear resistance. Liquid boronizing was carried out in molten borax containing various proportion of SiC. The boronized depth and the microhardness at the surface layer were increased with increasing the SiC content, boronizing time and temperature, but boronized depth increased little due to the rapid deterioration of fluidity of molten bath when the amount of SiC in the molten borax exceeded 40%. The boronized depth of the stainless steel was about half that of low carbon steel which was boronized at the same condition, and the boronized depth of 430 stainless steel was larger by 20-30% than that of 304 stainless steel. The wear resistance of the boronized specimen was increased remarkably (about 100 times) than that of the non-boronized specimen, and the wear resistance was improved when tempered the pre-boronized specimen and tested the boronized specimen at elevated temperature. The optimum condition for the best wear resistance was found to be as follows: 1) Bath composition was 25-35% SiC/Na₂B₄O_7. 2) Boronizing temperature was in the vicinity of 950℃. 3) Boronizing time was about 9 hrs.

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The effect of iron (1-6%) addition in titanium on hydrogen diffusion was studied in the temperature range of 890-1020℃ at 200 torr hydrogen pressure. Diffusion coefficient of hydrogen and activation energy for diffusion was determined from the increment of weight during the reaction by thermogravimetric method. According to the relationship between weight gain and reaction time, hydrogen diffusion is the rate controlling step in the reaction with Ti-Fe alloy. The result also indicated that added iron alloying element has almost linearly increased the activation energy for hydrogen diffusion in Ti-Fe system. Temperature dependence of hydrogen diffusion coefficient in Ti-6% Fe alloy is as follows: D(㎠/sec)=4.238×10^(-2) exp(­17714±988/RT)

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