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Transformed ferrite growth was studied using a dual-phase steel cooled at a rate of 0.1℃/sec after intercritical annealing at 830℃ for 30min. A local equilibrium model assuming a diffusion-controlled reaction has been developed to estimate the growth extent of transformed ferrite as a function of time during cooling. The extent of transformed ferrite grown during cooling to temperature T₂X_(T2), is estimated by the following equation: (수식) However, when cooled to temperature T₃ below T₂ at which diffusion fields of separate transformed ferrite begin to overlap, X_(T3) is given as fellows: (수식) The predicted extent of transformed ferrite agreed well with the experimentally measured data.

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Constant-stress creep tests at temperatures of 913,953 and 993 K over a stress range of 140 to 230 MPa were made on AISI 316 stainless steel to investigate the effect of recrystallization occurring during creep. The best creep properties were obtained in the specimens which had been pre-stained 207% arid tested at 953 K under a stress in the range of 147-170 MPa. Above 30% pre-strain the creep properties gradually deteriorated, which was attributed to partial recrystallization. The carbides in the partially recrystallized specimens,0.1-0.3㎛ large, did not coalesce during creep but remained dispersed in the grains and grain-boundaries. In the 30% pre-strained condition, the best creep properties were obtained with the creep stress of 200-230 MPa at 953k. The creep resistance was considerably reduced by 40% pre-strain. It was responsible for the coarsening of carbides and the formation of subgrains.

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Liquid-grain mixture flow for the elimination of isolated pores during liquid phase sintering has been studied. Isolated pores have been created in various Mo-Ni alloys by embedding 3 wt % of spherical Ni particles of 100㎛ diameter in fine Mo-Ni powder compacts. The total Ni content has been varied from 4 to 10 wt %. In the specimens with lower liquid content (=8 vol %), the skeleton of 9 grains around pores remains intact and the pores are mainly filled by liquid, producing liquid pockets at the pore sites. With increasing the liquid content, the dominant mechanism operating for the elimination of large pores becomes liquid-grain mixture (low into pores, forming liquid-grain mixture pockets. Effect of initial Mo powder size on the pore elimination processes is also discussed.

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The effects of high pressure hydrogen environment on the mechanical properties, expecially on the low-cycle-fatigue behavior at room temperature in quenched and tempered SCM4 (AISI 4140) steel, have been investigated. The softening phenomenon and the strength-differential effects were almost the same in air and hydrogen environment. However, the fatigue life in hydrogen environment was reduced significantly. Also the parameters describing the fatigue properties, such as ε＾t_f, Nt, etc. were shown to be very sensitive to the environment. As the hydrogen pressure increases the critical fatigue life, Nc was reduced to a certain limit value. It has been shown that absorption theory among mechanisms of H.E. (hydrogen embrittlement) can not be directly applied. The modes of crack-propagation and faillure revealed that the fatigue crack propagated according to the “Laird’s Model”in air, whilst to the micro-void coalescence, tongue formation and secondary cleavage in hydrogen environment The results of this study may be extremely useful for the prediction of the fatigue life in high pressure hydrogen environment and may verify that the phenomenon of hydrogen embrittlement could be explained by a combination of two effects, the decohesion theory and the planar pressure theory.

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The effect of Nb content on the crystallization temperature, hardness and magnetic properties, and the effect of annealing on the magnetic properties of (Co_(0.94-x)Fe_(0.06)Nb_x)_(78)Si₄B_(18) amorphous alloy ribbons was investigated. With increasing Nb content, the crystallization temperature (T_x) and the hardness increased with approximately linear trend but the Curie temperature (T_c) and the saturation magnetization decreased linearly. The coercive force showed a minimum value at x = 6 (at%). The room temperature magnetic moment was decreased at a rate of about 5.6μ_B per Nb atom. In Nb containing alloys of T_x$gt;T_c, the magnetic properties were improved by conventional annealing as well as by magnetic field annealing. The optimum annealing temperature was 450℃. For the alloy of T_c$gt;T_x without Nb, the magnetic properties were improved only by magnetic field annealing. The best magnetic properties, the coercive force of 7 mOe and the maximum permeability of 540,000 were obtained by magnetic field annealing at 350℃ in the alloy containing Nb of x=4 (at %).

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In order to imporve the wear-and heat-resistance of high carbon steel, a surface alloyed layer was created on a SK-3 steel with tungsten carbide by irradiation of Co₂ laser. The object of this work was to investigate the effects of powder size and coating thickness of tungsten carbide, and the heat treatment on the mechanical properties of the laser surface alloyed layer, The results showed that tungsten carbide was distributed homogeneously throughout the alloyed layer. The amount of tungsten and the hardness in alloyed layer were dependent upon the powder size and the thickness of coated layer. When alloyed layer was formed by overlapping laser tracks, the microhardness of this region decreased slightly. When the specimem was heat treated, the microhardness tended to increase depending upon the amount of tungsten in the alloyed layer.

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