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Reduction of molten iron oxide with Ar-H₂ plasma was studied using a DC 15KW power sourse. 50g of sample was melted by Ar plasma and reduced in a stream of Ar-H₂ gas, the flow rate of which was 17ℓ/min. Degree of reduction (R) was reached as much as 95% within 12 min by Ar-H₂(14%) plasma, and it was linearly propotional to both the reduction time and the partial pressure of hydrogen. Additives such as CaO, MaO, SiO₂ and A1₂O₃charged in the range of 1-20% showed only trivial effects, however in case of charging serpentine which contained same ratio of acid and basic components, there was a tendence to increase in the reduction degree Kch. Reduction rate constant was calculated, as a function of hydrogen concentration in the Ar-H₂Plasma, the 0.512. Reduction rate and gas utilization % were increased with the increase in plasma temperature, dissociation of H₂and gas ionization such as H→H＾+ + e.

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The purpose of this paper was observation of and kinetic analysis of the precipitation process in heat-resistant alloys to improve the mechanical properties and the thermal properties. Al-Cr alloys were rapidly solidified into a ribbon shape by a melt spinner. The decomposition process of rapidly solidified Al-Cr alloys were examined by means of TEM, X-ray diffraction and micro-Vickers hardness. As a result of an isothermal aging of massively transformed Al-3.43wt.%Cr alloy at 450℃, subgrains formed by polygonization process. With more aging, precipitation occurred on the subgrain boundaries. The decomposition process of massively transformed Al-3.43wt%Cr alloy was analyzed by the cylindrical precipitation model. In the initial stave, n in the Johnson-Mehl-Avrami equation is 2.98 corresponding to the characteristics of the interface-controlled growth of a fixed number of particles. In the middle and the final stage, n is 1.85 corresponding to the characteristics of the diffusion-controlled growth of a fixed number of particles.

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The effect of volume fraction and morphology of bainite on the microstructure and mechanical properties of Fe-0.1C-1.6Mn-0.8Si tri-phase (ferrite-bainite-martensite) steels was investigated. The volume fraction and morphology of bainite were controlled by isothermal holding for different periods at two levels of bainite transformation temperature before quenching in water. The size of martensite particles became finer with increasing volume fraction of bainite in the tri-phase steel. Lower bainite and upper bainite were observed in the tri-phase steel by transmission electron microscopy. The improvement in ductility (elongation and reduction of area) with increasing volume fraction of bainite in the tai-phase steel was attributed to dispersion of fine martensite particles and less hard bainite, that resulted in higher work hardening and relaxation of stress concentration.

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This study was carried out to investigate the effects of densification, formation of Nd-rich phase, and oxidation of powder and addition of Al₂O₃ on the magnetic properties of Nd-Fe-Co-B-Zr system. The sintered density increased with the growth of matrix grains and the formation of Nd-rich phase. The sintering reaction below 1100℃ was retarded when the powder surface was oxidized, and the sintered density was decreased by addition of Al₂O₃. The residual induction (Br) increased with the sintered density and the formation of Nd₂(Fe. Co)_(14)B phase. The coercivity (iHc) improved with Al₂O₃ addition, and reached at a maximum value after sintering at 1040℃. The oxidation of powder, however, decreased the coercivity value remarkably.

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In this paper was investigated the effect of thermal cycling on dimensional change and microstructural change of the interfacial region in W/stainless steel composites, which was hot pressed at 1100℃ and then exposed up to 200 h at 1200℃, with HfN coated and uncoated fibers. After thermal cycling, it was found that axial dilatation of the matrix in W/stainless steel composites with HfN coated fiber occurred conspicuously only in the early stage of cycling. However, the axial dilatation became nearly zero after several tens of cycles, which indicated that mechanical interactions exist between the matrix and the fiber did not exist any more. On the other hand, the matrix side of the interface in W/stainless steel composites with uncoated fiber became rough with increase of thermal cycling, and several voids were observed to be formed in the matrix.

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The effect of transition metals oil the morphology and the yield of β-SiC whiskers has been investigated by vapor phase reaction in SiCl₄-Cl₂C = CH₂-H₂ system. The β-SiC whiskers formed on graphite substrate are mainly of straight shape with blunted tip at their top. Depending on the transition metals added, β-SiC whiskers are in various shapes such as straight, needle-like, gnarled, concentric double-layered, or hollow. The length and the yield of H-SiC whiskers increased by metal addition in the order of Ni$lt;Co$lt;Fe. The solubility of carbon in the metal-Si-C alloy droplet at the tip of the whiskers is associated with the length and the yield of the whiskers.

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The hot embrittlement of KS SPS 5 spring steel in austenite region was studied in order to clarify the cause and mechanism of surface cracking during preheating, hot forming and cooling operations in spring making with rolled steel. The loss of hot ductility was attributed to precipitation of nitrides which were identified as AIN by TEM. Sulfur did not influence the hot embrittlement in this steel because of its decreased solubility due to high Mn content. Through the TEM observation of the size and distribution of the precipitated particles the mechanism of hot embrittlement in spring steel was thought to be relative weakening of the grain boundaries caused by the matrix which is strengthened by AIN precipitates. The effects of the strain rate and the grain size on the hot ductility were discussed.

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The effect of strain induced martensite on the fatigue behavior of AISI 304 austenitic stainless steel was investigated. During low cycle fatigue, the austenitic stainless steel showed a continuous cyclic hardening until fracture. The extent of cyclic hardening increased with decreasing austenite stability. The austenite stability was controlled by different aging time and temperature, which resulted in different carbide morphologies. The fatigue crack propagation rate near △K_(th) varied also with the austenite stability inside the plastic zone at the crack tip. Especially, the near-threshold fatigue crack propagation rate of the grain boundary carbide precipitated condition was the lowest. This was considered to be due to the roughnessinduced closure caused by intergranular facet. A new model for the intergranular facet formation and the fatigue crack propagation of grain boundary carbide precipitated condition was proposed.

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