발간논문

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Austenite formation and mechanical properties of HSLA steel of different starting structures has been studied by intercritical annealing(IA). The different starting structures are: ferrite＋pearlite (FP1), martensite(M1), cold worked ferrite＋pearlite(FP2) and cold worked martensite(M2). Austenite was nucleated at ferrite grain boundaries in FP1 and FP2 and also at the interior of the grains in case of FP2. By increasing the IA time globular austenite structures were distributed uniformly all over the space enclosing the ferrite phase. Austenite in M1 and M2 was nucleated at the interface between prior martensite laths and the globular strutctures were formed from the former acicular structures. By increasing the IA time M2 has shown well developed globular austenite structure than M1 and the agglomerated continuous austenite networks surrounding ferrite phases were formed. Tensile strength in all the tested samples of different starting structures was increased by increasing the IA time. Any noticeable change in elongation was not found in FP1, FP2 and M2 by the increase of IA time. Exceptionally elongation of M1 was increased by increased IA time. Chapy V-notch impact values of all the different starting sturctures were decreased by increasing the IA time.

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The plasma spray process was used to deposit. coatings of ZrO₂- MgO powders onto SM45C substrate, and the characteristics of as-deposited and heat. treated coatings have been investigated. Particulary, the variations of microstructure, porosity, wear resistance, thermal shock resistance and thermal barrier in ZrO₂- MgO coatings after heat treatment under lower pressure have been investigated. It was found that coatings of ZrO₂- MgO is consist of layered structure with thickness 2-3㎛, and with grain size of 0.O6㎛. The amount of prosity was increased with increased spray distance, and the lowest amount of porosity was obtained at the arc current of 450A, and at the spray distance of 50mm. After heat treatment, the amount of porosity was found to be decreased, and wear resistance, microhardness and thermal shock resistance were improved. However, the thermal barrier were decreased.

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The effects of Sn addition on the core loss of a non-oriented electrical steel containing 3.1wt% Si were investigated in the range of 0.20-0.70㎜ in thickness and 50-400㎐ in frequency. When a small amount of Sn was added to the steel, it was not only segregated on grain boundaries, resulting in smaller microstructure, but also enhanced texture development favorable to magnetic properties. Due to these phenomena, the addition of Sn is found to be effective in reducing the core loss, especially the eddy current loss, at higher frequency and thinner gauge.

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The effects of alloying elements on microstructures and fatigue life in STB2 bearing steel, were investigated through the measurements of particle distribution and alloy content in carbides for different chemical composition and manufacturing processes. Among alloying elements, chromium has a strong influence on the distribution of carbides. Finer spheroidized carbides and finer retained carbide particles were distributed after austenitizing and quenching. The microchemistry of carbides also changes during spheroidizing and austenitizing. The chromium content in carbides was decreased during spheroidizing and increased again during austenitizing. The effect of the chromium addition was found to increase fatigue life markedly, while those of other elements such as manganese and silicon, were found to be not so effective on the fatigue life.

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This paper was perfomed to invetigate the effect of extrusion on the mechanical properties of K₂O·6TiO₂/Al composites in order to optimize hot extrusion condition. K₂O·6TiO₂whisker was mixed with Al powder. The mixture was degassed for 1 hour at 450℃ and then pressed under the pressure of 100 ㎫ in vacuum at 600℃. These composites obtained range from 10 to 30 v/o whisker. Hot pressed composites were hot extruded at 500 and 550℃, respectively. Extrusion ratio was 10:1, 15:1, 25:1. After hot extrusion, microstructures observation showed that whisker clustrs strainghtened narrowly parallel to extrusion direction. It was observed that relative density of composites increased as extrusion ratio increase, showing 99.6 % at 10 v/o whisker. The tensile strength at both room temperature and elevated temperature temded to increace WHS extrusion ratio. In extrusion ratio of 10:1, however, tensile strength of the composites with 30 v/o whisker was observed to be lowered compare with that of as-hot pressed composites. As thermal exposure time increased, tensile strength of the composite with 20v/o whisker decreased. This may be attributable to the matrix weakness and reduction of whisker strength due to exit of K ion at elevated temperature.

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Silver overlaid phosphor bronze and nickel inlaid phosphor bronze strips have been fabricated by cold roll bonding. Effects of surface cleaning methods of component strips, the rolling speed, the initial fraction of thicknesses and the reduction ratio on bonding were investigated. Good bonding required wire brushing of bronze strips and rolling reduction ratios over 50%. Annealing alove 400˚C for 1 hour to reroll overlaid metal for the sake of controlling dimensions caused pore band formation near the interface between the two component metals. However sufficient rerolling reductions could remove the pore band. Modification of the shape of nickel strip could improve bonding between the component strips of nickel inlaid phosphor bronze. (Received February 10, 1993)

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In this study, the effects of matrix microstructure of 2XXX Al-SiCw composites of the mechanical properties were analyzed. The used materials were powder metallurgy processed 2124 and 2009 aluminum composites reinforced with 15 volume percent SiC whiskers. Fracture parameter analyses and microstructural observations revealed that the improved fracture toughness of the 2009 Al-SiCw Composite, compared to the 2124 composite, was due to the increase in the critical microstructural distance ℓ when Mn-containing particles are absent. The 2009 composite was also heat treated in T4 and overaged conditions. The overaged 2009 composite had lower ductility and also much lower value of critical fracture strain compared to that of the T4 condition. Detailed fractographic analyses indicated that the most common form of damage was the cracking of SiC whiskers in the overaged condition and the decohesion of Al/SiCw interface in the T4 condition, respectively. The change of fracture mechanism can be explained by the increased load transfer to the whiskers in the overaged condition resulting in the whisker cracking and early failure during fracture process.

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The crystallization characteristics of the Fe_(80-x)Ni_xB_(20)(20≤x≤40) and (Fe_(0.5)Ni_(0.5))_(80-x)Bx(16≤x≤24) amorphous alloy systems have been investigated in detail using differential scanning calorimetry, X-ray diffraction and transmission electron microscopy. Measurements of the exothermic peak temperature (T_p) and the activation energy during crystallization were obtained by differential scanning calorimetry. In the Fe_(80-x)Ni_xB_(20) amorphous alloys activation energy for crystallization increased with increasing x, and in the (Fe_(0.5)Ni_(0.5))_(100-x)B_x amorphous alloys it decreased with increasing x. Two-step crystallization was observed in specimens with x≤18 of (Fe_(0.5)Ni_(0.5)(100-x)B_x alloys; amorphous→amorphous＋γ-(FeNi)→γ-(FeNi)＋o-(FeNi)₃B. By the X-ray diffraction, the phases of crystallization products of these amorphous alloys are (ⅰ)fcc-(Fe,Ni), t-(Fe,Ni)₃B or fcc-(Fe,Ni), o-(fe,Ni)₃B, t-(Fe,Ni)₃B for high Fe content, (ⅱ)fcc-(Fe,Ni) and o-(Fe,Ni)₃B for equal contents of Fe and Ni, and (ⅲ)o-(Fe,Ni)₃B for high B content.

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The effects of γ` precipitates and Y₂O₃, dispersoids on high temperature tensile and creep properties of mechanically alloyed Ni-20Cr-4W alloy were studied. The zone annealing was performed to increase grain size and grain aspect ratio. The zone annealing method was found very useful to increase high temperature strength and creep resistance. The addition of Y₂O₃into Ni-20Cr-4W alloy not only gave this alloy dispersion hardening, but also improved thermal stability of γ` precipitates at high temperature. Up to 950℃, γ` precipitates contributed to high temperature properties more efficiently than Y₂O₃dispersoids. However, the contribution of Y₂O₃to high temperature strength was greater in creep than in high temperature tension. The mechanically alloyed Ni-20Cr-4W alloy showed the creep threshold stress.

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Variations of hardness, tensile properties and textures were investgated in cold rolled and/or annealed commercial C-grade and T-grade maraging steels. The increase of hardness due to cold reduction was small. During annealing treatment performed in the course of cold rolling, grain refinement occurred due to transformation to austenite and partial recrystallization in austenite, which raised hardness slightly on subsequent aging. T-grade maraging steels reached peak hardness faster than that of C-grade though the peak hardness of both were similar, Deformation texture components developed in maraging steels were (100) $lt;O11$gt; and (111) $lt;O11$gt;, (111) $lt;112$gt; and these were sharpened with cold reduction. During annealing treatments in the course of and/or after cold rolling, (100) $lt;011$gt; component decreased considerably but other components decreased insignificant.

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