발간논문

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Effects of minute addition of titanium dioxide on sintering behavior and magnetic properties of barium ferrite were investigated. It was found that those additions weakened magnetic properties except coercive force. In case of 1.2ω/0 TiO₂, however, the ferrite exibited (BH)max=0.94×10^6 G·Oe at Br=2,060 gauss. These values are closely compared to those of barium ferrite without TiO₂. Ba0·5.6Fe₂O₃ was selected as barium ferrite and TiO₂ was added up to 1.8ω/0 by increment of 0.3w/o. The optimum sintering temperature way lowered as TiO₃ addition increased. It was 1250℃ for 1.2ω/0 TiO₂ addition, compared to 1280℃ without TiO₂. Sintering time was 1 hour and compacting pressure was 1.5ton/㎤. The density of sinterd pellet was increased with TiO₂ addition. The pellet density was about 4.8g/㎤ at optimun sintering temperature of each TiO₂ addition.

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Effect of electrode covering on the mechanical properties of high tension steel weld has been investigated with domestic covered electrodes and prepared electrodes with various ingredients. Fluxing materials in the heavily covered electrode give the arc considerable stability, help to float out impurities from within the molten puddle, and develop inert gases which keep the outer surfaces of the molten weld from oxidizing. An addition of 1∼2% graphite in the electrode covering increases strength of the weld metal resulting from carburizing effect, whereas a larger amount of graphite (3∼5%C) makes it brittle causing CO gas blow hole formation. Excess amount of manganese in the covering materials may cause harmful effect, forming fish eye. Addition of iron also results in strength increment and gives the arc stability. Molybdenum produces superior strength and appearance of the weld metal and gives better weldability. It is, therefore, suggested that an addition of ferro-molybdenum to the covering materials would help to produce sound weld for high tension steel.

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Cathodic polarization of sintered nickel oxygen electrode in 30% KOH solution can be decreased with the addition of silver electrocatalyst in the nickel powder matrix. Iron wire mesh may be used as a plaque material, while copper or brass showed large polarization. Temperature and oxygen partial pressure have a little effects for the cathodic polarization. The limiting current density for electrochemical oxygen reduction is 3.5mA/㎠ for the electrode with 5% silver activator, and the upper range for discharge current as a secondary cell is not more than 1.3㎃/㎠ at room temperature and atmospheric pressure.

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Usually forged steel billets are used to extrude tubes or rods. In this investigation, cast steel billets were hot-extruded to seamless pipes. By using cast billets, it is possible to elminate hot working operations of the material prior to extrusion. Fine fiber structures and mechanical properties equivalent to extruded products from for ged billets were obtained from the cast billet extrusion. Billet preparation and lubricating processes are described in details. The change of micro-structures and mechanical properties by hot-extrusion of cast steel billets are discussed.

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This research aims to find out the maximum carburization point in 3 tons Herould type basic electric arc furnace by using low carbon steel scrap (0.2∼0.25%) and charcoal for nodular cast iron making. The maximum carburization content was obtained in 40∼50 min after melt-down of charge in the furnace, the most suitable seeding temperature was over 1,460℃ and the sulfur content was decreased to 0.008∼0.0l% by adding carbide powder in the ladle.

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Qualitative agreement exists between theoretical extrusion stresses, based on the upper-bound approach, and experimental values on the cold-extrusion of carbon steel billets. 0.26% and 0.50% carbon steel billets were successfully cold-extruded into round bars, gears, and spline shafts, up to 60% area reduction through conical converging dies. The effect of die geometry on the forming load and the charge of mechanical properties of materials are discussed in details. Fine fiber structures and high surface hardeness were obtained through cold-extrusion of steel.

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