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A study on sintering between 730∼940℃ for 1∼250 hours in hydrogen was conducted by using spherical silver particles with an accuracy diameter 0.522㎜ and standard deviation 0.006㎜. In case published material data and sintering equations were used, it is shown that surface diffusion dominates the neck-growth stage of sintering for silver particles. The empirical suface diffusion sintering equation has been determined by measuring neck-growth rate, resulting in (x/a)^7 = [36 b_s γ_s ΩD_s/(kTa⁴)]t.

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The effects of substituting W for Mo in a high strength structural alloy steel (US Steel "T-1" alloy) on tensile and impact properties have been investigated. The tungsten content (w/o) varied from 0.25 to 1.00 for the 0.5 w/o Mo, white the remaining alloy compositions were kept constant. It was found that both yield strength and ultimate tensile strength increased with the increasing tungsten content, reaching a plateau strength above 0.75 W. The substitution of W for Mo in the alloy greatly enhanced the Charpy impact energy. At the test temperature of -20℃, the impact energies of 1.00 W alloys were about 40% higher than that of 0.5 Mo alloy. It was also observed that the addition of W was as potent as Mo in preventing temper embrittlement in the alloy.

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The effects of varying carbon content (0.004∼0.098 w/o) on the secondary recrystallization in 3% Si-Fe were studied. It was found that carbon had no great influence on the cold rolled texture behavior. However, increased carbon content enhanced the development of (110) texture during primary recrystallization. When carbon content was low (0.004 w/o), poor secondary recrystallization occurred. Complete secondary recrystallization developed when carbon content was high (0.074 and 0.098 w/o).

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It was investigated, whether high speed steel part could be directly fabricated to near net shape by normal P/M process, ie. powder mixing, compacting and sintering procedure. Fe-, W-, Cr₃C₂and VC powders, whose composition is equivalent to the commercial T1 grade high speed steel, were thoroughly mixed in ball mill, and compacted into a disc-type specimens of 10㎜ diameter and about 5㎜ height at a hydraulic pressure of 5.5 ton/㎠. This compact was sintered in alumina tube furnace at temperature between 1220℃ and 1340℃, both in H₂ atmosphere and in vacuum for various sintering time. The fully densified structure could be obtained only by the sintering at the temperature above 1340℃, or by prolonging the sintering time at 1320℃. The heat treatment behavior of a directly sintered P/M HSS was similar to that of a wrought HSS, so far as the alloying components were fully homogenized during sintering process. The microstructure of the sintered, quenched or tempered high speed steel was also observed in order to elucidate the relationship between the structure and properties of the sintered high speed steel. It could be cautiously concluded that the directly sintered HSS. prepared by normal sintering process could be used supplementary for the production of near net shape HSS part, so far as the more finer Fe-powder would be used and the sintering might be carried out at the optimal temperature.

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High strength low carbon 3.5Ni-1.5Cr-0.38Mo steel containing 0.006% S was melted in the vacuum induction furnace. REM and/or CaSi were added into the melt to control the shape of sulfide inclusions. The specimens were taken in the longitudinal and transverse to the hot rolling direction and then heat treated. The effect of the sulfide inclusion shape control on the property anisotropy such as the impact absorption energy and the ductile-to-brittle fracture transition temperature were studied. Deoxydizing by a aluminum prior to the REM and/or Ca treatment refined the grain, and besides the sulfide or the oxide, complex oxysulfide inclusons are detected among the non-metallic inclusions after the shape control that diminished the inclusion shape factor, I_c (L) below 0.1 from 0.6-0.8. If the shape of sulfide inclusion is controlled by the REM and/or Ca treatment, the tensile property anisotropy is diminished since the ratio in the longitudinal and transverse to the rolling direction, T/L value are improved and T/L values of the impact absorption energy increased from 62.2% to 78.5-89.0%, and could decrease the ductile-to-brittle fracture transition temperature by about 10˚-30℃. The shape control of sulfide inclusion did not show any difference, regardless of the REM treatment or Ca treatment.

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Thermal cycling effect on the mechanical properties of the high temperature austenitized AISI 4340 steel was studied. Specimens austenitized at 1200℃ were thermal cycled at 840℃ and tempered in the range of 200-400℃, after which the tensile properties and impact strengths of the specimens were examined. Thermal cycled specimens tempered in the range of 300-400℃ showed higher tensile and yield strength than that of untreated specimens. Impact strength was also increased by the thermal cycling treatment in all the tempering range of this study. However, after the initial increase in impact strength with increasing tempering temperature, there was an anomalous loss in impact strength when tempering temperature was 300℃. This loss in impact strength was attributed to the tempered martensite embrittlement. High T.S., Y.S. and impact strength were obtained by three cycling treatment and subsequent tempering at 200℃, and their values were 195 ㎏/㎟, 165 ㎏/㎟ and 4.3 ㎏·m respectively. The results indicated that the mechanical properties of AISI 4340 steel was affected by the austenite grain size substantially and the increase of mechanical properties was attributed to the grain refinement by thermal cycling.

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The presence of hydrogen and/or impurity segregation can significantly affect grain boundary strength of iron. Few studies, however, have been attempted to correlate grain boundary strength to impurity concentration along grain boundaries. In this study, grain boundary strength will be quantitatively correlated to impurity segregation in iron with or without the presence of hydrogen. The impurity concentration along grain boundaries was controlled by using iron specimens of different initial bulk impurity concentrations, doping and heat treatments. Quantitative analysis of segregated impurities was carried out using Auger electron spectroscopy with in-situ fracture under ultra high vacuum. The segregation of sulphur clearly decreased the grain boundary strength in both hydrogen-precharged and uncharged iron specimens. On the other hand, the presence of a small amount of carbon can increase grain boundary strength substantially. In the light of present observations, the mechanism of hydrogen-induced inter-granular fracture and the effects of sulphur, carbon and other impurities will be examined.

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