발간논문

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In multi-pass welding, the effect of 2nd pass on the microstructure and dynamic fracture toughness was discussed. The 2nd pass was found to enhance bond toughness in the range of 2.5 kJ/㎜ and 4.5 kJ/㎜ heat input. The reason seems to be that the size of Martensite -Austenite constituent was refined and its amount was decreased. The size of lower bainite and tempered martensite were also refined. TEM study revaled that the M-A constituent contained lath or twined martensite and it behaved as a crack path.

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Ribbons of rapidly solidified Ni-Mo-B alloy were produced by chill block melt spinning. Powders were manufactured from ribbons by a pulverizing process and then consolidated by extrusion after heating for 2 hrs at 1100℃. In order to improve the mechanical properties, and additional aging treatment was also performed. The microstructure and mechanical properties of the alloy, such as room and elevated temperature hardnesses, wear resistance, and impact strength, were evaluated. Mechanical properties of the alloy were correlated to the microstructure, in view of the application of the alloy to the high temperature wear resistance fields.

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The matrix structure and the morphology of CV graphite are affected by the addition of various kinds of elements, which are contained in the CV graphite cant iron, additives, fading time and cooling rate. The effects of cooling rate on the matrix structure, the shape of CV graphite, mechanical properties and propagation of ultrasonic wave in hypereutectic CV graphite cast iron were studied. When the diameters of the specimens were reduced from 50 ㎜ down to 20 ㎜, the following results were obtained : 1) CV graphite count showed 10.4 % increase and CV graphite length showed 17.3 % decrease. 2) The content of graphite and ferrite showed 10.8 %, 3.7 % decrease and that of pearlite showed 36.2 % increase. 3) Hardness and tensile strength showed 3.1 %, 10.3 % increase and impact strength showed 1.5 % decrease. 4) Ultrasonic wave velocity showed 1.1 % increase.

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The effect of boric acid on microstructural evolution during the consolidation at atmospheric pressure(CAP) process of T-15 highspeed steel has been analysed in comparison with the effect of elemental boron. X-ray diffraction, X-ray microanalysis, optical and scanning electron microscope are utilized to examine the sintered structures. The microstructural and metallurgical features resulting from these two different sources of boron were essentially identical. Therefore the sintering enhancement effect of boric acid proved to be that of elemental boron which is formed through the reduction of boric acid during the CAP process.

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This paper describes the results on the formation of magnetite particles to be used in making magnetic ink from the waste pickling liquor of steel which contains 8.9 wt.% of Fe ion (in which Fe²+ ion occupies 8.58wt.%) under conditions of pH 11, temperature of 80℃, air blowing rate of 100 cc/sec, and Fe^(2-)/Fe^(+3) ㏖-ratio of the waste pickling liquor being 0.5-27.1) Oxidation is easily proceeded in 200 g of waste liquor added with 1% HNO₃from Fe^(+2)+ ion to Fe^(+3) ion at 70℃ with air blowing rate of 100cc/sec. 2) Magnetite particles at Fe^(+2)/Fe^(+3) ㏖-ratio of 0.5-1.0 were of ultra-fine sizes less than 110 angstrom. They have magnetic susceptibility of 48-60 emu/g at 6kOe and showed super-paramagnetic characteristics and thus could be used in making magnetic ink. 3) It was possible to produce magnetic ink by coating the oleic acid on the surface of magnetize particles made from the waste pickling liquor of steel and dispersing them into kerosene oil.

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Consolidation of amorphous Fe_(80.5)Si_(6.5)B_(12)C₁ and Co_(70.3)Fe_(4.3)Si_(16.4)B_9 alloy powders was performed by the powder rolling process. As the basic idea of this process, the decrease of viscosity of amorphous metal with increasing temperature was used. Thus the optimum heat treatment temperature(Top) for consolidation was investigated. The Top(Fe system : 738K, Co system 763 K) coincided with 95% of the crystallization temperature(Tx) measured by differential scanning calorimetry at a heating rate of 20 K/min. The consolidated amorphous alloy maintained the low coercivity of the initial powder. A ball-milling treatment of amorphous powder far consolidation improved the compressive strength of the bulk specimen by 10㎏/㎟.

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Two layer thermal barrier coatings failed by the thermal expansion mismatches beteen metallic substrate and ZrO₂ ceramic coat. Therefore, in this study, NiCr-ZrO₂(Y₂O₃) composite coat layers were introduced between substrate and ZrO₂ceramic coat in order to increase the durability by reducing the thermal expansion mismatches and by improving the bond strength. The NiCr alloy within composite coat oxidized prior to oxidation of NiCrAlY bond coat and the specimen failed within the composite coat layer. Oxidation products in composite coat layer were NiO and Cr₂O₃. The efficiency of the thermal barrier coatings increased with temperature up to 800℃ . The temperature of the substrate decreased by about 110℃ when the surface of ZrO₂ ceramic coat was heated to 800℃.

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The sintering behavior of TiC-Fe alloy composite has been investigated. Milled powder compacts of Fe∼33%TiC(wt.%), Fe-6.7%Cr-33TiC, Fe-6.7 %Mo-33%TiC and Fe-3%Cr-3%Mo∼0.6%C-33%TiC(Ferro-TiC C grade) were sintered between 1200℃ and 1450 C for 0 and 0.5 h. Most of the shrinkage during the sintering was observed to occur be low the liquid please formation temperature indicating that the solid state sintering of Fe alloy matrix plays an important role in the densification of the specimen. After liquid phase sintering at 1450℃ for 30 min., some pores remained. Subsequent HIP treatment, however, eliminated the remaining pores and increased TRS value.

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The form of heat transfer equation which can be used for estimating convective heat transfer coefficient in gas atomization was revised in accordance with the dimensional analysis, and was given as Nu =canst Re^n Using the experimental data of Ranz and Marshall, the heat transfer equations were proposed by the following relationships Nu=2.32 Re＾(0.23) for 2
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A chemical hysteresis and its temperature dependence in the LaNi_5-H system have been investigated by using a Sievert-type apparatus. The initial hydrogenation rate was controlled by the valve opening and the initial hydrogen pressure of a hydrogen gas reservoir. The pressure-composition isothermss in LaNi_5-H measured at 10, 30, and 50℃ varied sensitively with the hydrogenation rate and the temperature, implying an existence of a chemical hysteresis and its temperature dependence in this alloy. The chemical hysteresis decreased with increasing the temperature. By an annealing treatment of the activated LaNi_5 the temperature dependence of the chemical hysteresis decreased remarkably.

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