발간논문

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The effect of the intermetallic compound, AlN, on the mechanical property of low car bon steel was investigated. For this study, two different steels with different aluminum and nitrogen content were prepared. The one was Fe-0.1C-0.31Al-0.016N (steel A) which has excess aluminum compared with the stoichiometric ratio of one to one of Al and N, the other was Fe-0.1 C-0.04 Al-0.011 N (steel B) whose Al and N content was very close to one to one atomic ratio. Since the specimen whose Al and N content is close to one to one atomic ratio has greater common solid solubility of aluminum and nitrogen at the lower temperature compare to the steel with large deviation from one to one ratio, finer aluminum nitride precipitants are obtained through solution heat-treatment in steel B to inhibit grain growth during subsequent heating. The fine grain size and the precipitation hardening effect are believed to act for the enhancement of tensile strength and ductility of the low carbon steel.

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The effect of Ni doping on W powder on the sintering behavior, mechanical properties, and electrical contacting characteristics were investigated to improve the performance of electrical contact material. On the basis of these experimental results, the following conclusions were obtained. In the lower Ag content, Ni-doping seems to be considerably more effective on densification of the W-Ag-Co system, than dopent Co, but both dopent, Ni, Co, were less effective in the higher Ag content. In the sintering of Ni doped W-Ag-Co system, the volume expansion appeared as well as in pure W-Ag-Co system. By the model approach and X-ray diffraction analysis, it was identified that the formation rate of intermetallic compound, Co_7W_6, was reduced by Ni doping, but there was no difference between Ni doped W-Ag-Co system and pure W-Ag-Co system in chemical composition and microstructure after sintering for 3hrs, and it was observed that dopent Ni accelated the densification of W-Ag-Co system. Hardness was increased and contact erosion was decreased by Ni doping of W powder in W-Ag-Co base contact material. Therefore Ni doped W-Ag-Co base sintered alloys are recommended as a electrical contact material that requires the frequent make and break like a vibrator.

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An investigation has been performed on the effect of high temperature thermomechanical treatment (HTMT) on the mechanical properties, especially strength and toughness of AISI H13 hot work tool steel. The experimental results showed that HTMT increased the strength and toughness of hot work tool steel. 45% deformation at 980℃ increased the yield strength about 9.9% and the impact energy about 60% compared with conventional heat treatment. The improvement is due to the dispersion of fine carbides precipitates within the martensite lath and the refinement of martensite lath size.

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Planar anisotropy of fatigue crack growth rates in 2024-T4 aluminum alloy and cold worked and recrystallized aluminum strips has been investigated with emphasis on effects of precipitates, inclusions, and preferred orientation. All specimens except recrystallized aluminum specimens showed the rapider rate of region II fatigue crack growth in the direction of load normal to the rolling direction than in the loading direction parallel to the rolling direction. Such an anisotropy was attributed to arrangement of precipitates and inclusions in the rolling direction for 2024-T4 aluminum alloy, and to grains elongated in the rolling direction for cold rolled aluminum strips. The effect of the preferred orientations seemed to be less important to the anisotropy than the mechanical fiberings. The crack propagated along the <110> directions on the {100} planes for the 2023-T4 aluminum specimen. The crack opening displacement at the fatigue rate transition from region II to region III for 2024-T4 aluminum and cold rolled commercial purity aluminum was calculated to be 6-9㎛.

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2024 aluminum alloy was extruded over a temperature range of 300℃∼443℃ and the effect of extrusion temperature on strength and microstructure of the extruded state (F state) and T4 state has been studied. Extrusion at 300℃ showed the lowest strength among the studied extruded specimens, even lower than the homogenized state. The extrusion temperature which resulted the highest strength of the extruded was 350℃. It is concluded that major controlling factor of the strengthening of extruded state is precipitates, and secondary factor of it seems to be the sub-grain size. The strength of the T4-state increased with increasing the extrusion temperature.

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The abrasive wear resistance of medium carbon Cr-Mo steel which was quenched and tempered at various temperatures was studied in relation to its hardness and microstructure. Special emphasis was placed on the effect of the tempering temperatures. A controlled dry wear tester manufactured to simulate the abrasive condition was used to measure the wear weight loss, and wear rate. The wear rate (W_M) was inversely proportional to bulk hardness (H_B) and simply expressed by the following semi-empirical equation. W_M=300×(60)^(-n)×1/H_B, ㎎/㎏-m where n is strain hardening exponent. It is also found that the martensitic structure of the steel tempered approximately at 200∼250℃ shows excellent high stress abrasive wear resistance.

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The phase transformation of amorphous Co_79B_21 alloy prepared by the roller quenching method was studied by DTA, electrical resistivity measurements and X-ray diffraction. The crystallization fraction was deduced from electrical resistivity. The amorphous forming range of Co_(100-x)B_x system was found to be 19≤×≤27 (at％). The first transformation of amorphous Co_79B_21 occured at 420℃ and was exothermic reaction. The transformation from the amorphous state to β-Co (fcc, a=3.54Å) was controlled by diffusion growth. The activation energy for crystallization was 50 Kcal/mole for the isothermal annealing of amorphous Co_(79)B_(21) alloy.

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To produce oriented silicon steel, it is necessary to develop (110) [001] texture by the secondary recrystallization. The inhibition of a normal grain growth is required for the secondary recrystallization. The grain growth inhibition is known to be associated with the presence of dispersed particles or solute elements segregated in grain boundary. The effect of sulfur on the secondary recrystallization in 3% silicon steel with 0.03-0.4 w/o manganese and 10-25 ppm boron were investigated by the various annealings and the analysis of sulfur composition. Secondary recrystallization was observed only in the strip with excess sulfur which remained after the formation of manganese sulfide. Otherwise normal grain growth was observed. It was also observed that secondary recrystallization started from the surface of the strip which contained 0.035 w/o manganese and 0.053 w/o sulfur, and that desulfurization was a diffusion controlled process at 850℃ and 960℃ in hydrogen atmosphere. From these results, it is concluded that secondary recrystallization in our specimens occurred due to the inhibition of primary grain growth by excess sulfur rather than manganese sulfide particle.

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