발간논문

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A study was made of the effects of alloying elements on microstructural factors of nine high speed steel (HSS) rolls manufactured by centrifugal casting method. Particular emphasis was placed on the role of hard carbides located along solidification cell boundary and the type of the martensite matrix. Microstructural observation, X-ray diffraction analysis, and hardness measurement were conducted on the rolls to identify carbides. Various types of carbides mostly segregated in the intercellular regions were dependent upon alloying elements and their amount, and played an important role in overall roll hardness. The effects of alloying elements were analyzed on the basis of the liquidus surface diagram, suggesting that the proper contents of carbon, tungsten, molybdenum, chromium, and vanadium were 1.9-2.0%, 3-4%, 3-4%, 5-7%, and 5-6%, respectively.

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Hot deformation behavior of CP(Commercially Pure)-Ti has been studied using Thermacmaster at various strain rates (0.001s^(-1)-10s^(-1)) and temperatures (750℃-950℃). Steady state flow behavior was established at strain rates of 0.1s^(-1) and below and flow curves tend to exhibit a small degree of flow softening at comparatively high temperatures. Dynamic material behavior during hot deformation has been studied using a model of Prasad et al. The result shows that a maximum material flow occurs under the condition of the strain rates 0.001s^(-1)-0.001s^(-1) at temperatures 750-800℃. Dynamic recrystallization is observed to be a primary dynamic process occurring at these conditions. A microstructural instability region has been also determined using Prasad et al.`s model and was generally in agreement with the result of experimental observation that such region is associated with the formation of shear bands. These results lead to a determination of an optimum condition for hot deformation of CP-Ti.

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The texture and plastic anisotropy of three different aluminum alloys (Al-Cu, Al-Mg and Al-Mg-Si) for auto body panel materials has been studied. In order to improve the formability of aluminum alloys, it is necessary to optimize the texture which gives rise to plastic anisotropy. The anisotropic properties were characterized by determining the R-values, and then compared to that calculated from quantitative texture data. The formability tests were also performed to obtain forming limit diagram(FLD) by using the stretching machine. Lankford parameter(R) values were predicted by the continuum mechanics of textured polycrystals (CMTP) method, which were compared with experimental R-values. The sheet having a random texture and a high n value showed an improved formability.

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It is known fact that the presence of each element in insoluble system like W-Cu does not influence the mutual reduction process. Therefore, reduction is generally carried out at lower temperature in order to gain smaller particle size. However, in a nano sized mechanically mixed WO₃-CuO system, grain growth of tungsten particle and distinct solid state sintering are observed during reduction process. This implies the possibility of change of powder morphology by diffusion during reduction. This study is focused on scanning electron microscopy and dilatometry in order to investigate the effects of reduction temperature not only on the reduced powder morphology but also on the sintering behavior. It is observed that powder gets smaller and more round upto reduction temperature 980℃ due to breakage of composite powder and solid state sintering. As reduction temperature exceeds 980℃, the powder begins to grow once again. These results are well in agreement with the dilatometry experiments. In W-Cu system, sintered density is almost always determined by the rearrangement stage, which is determined by the powder size and its morphology. This dilatometry experiments show that the densification of reduced W-Cu powder is largely depended on the reduction temperature and shows the best at 980℃.

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A new hot metal pretreatment process has been developed at Pohang Works to produce low phosphorus steel economically. A new process could be characterized by simultaneous dephosphorization and desiliconization with oxygen and lime based flux after desulfurizing hot metal. In this study, the influence of the operational conditions such as hot metal and slag compositions and blowing condition was investigated. Dephosphorization ratio of hot metal was increased with the [Si] and [Mn] oxidation ratio. On the other hand, hot metal dephosphorization ratio was decreased with (%T.Fe) and basicity in slag. By the use of dephosphorized hot metal in a converter, the phosphorus content at blowing end was less than 0.01wt%.

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A mathematical model of the rotary kiln for the calcination of limestone was developed by considering the reaction rate of calcination, mass and energy balance of solid and gas. A system of strongly nonlinear ordinary differential equations and algebraic equations with two point boundary conditions were solved by iterative algorithm. The measured outlet temperature of gas and calculated combustion temperature by K-value method were used as boundary conditions. By applying this developed model, the axial temperature profiles of gas, solid bed and wall and the axial concentration profiles of solid and gas were calculated from operating conditions. The temperature of outer wall and CaCO₃ conversion calculated by model were in good agreement with those measured from running rotary kiln at Kwangyang Steel Works.

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The effects of the duplex treatment on the isothermal oxidation and high temperatures wear resistance of AISI H13 steel were investigated in this work. High temperature wear tests at 500℃ were performed under no lubricant condition in air using a ball-on-disk type tribotester and measurements of oxidation resistance of the duplex treated specimens at 700℃ and 900℃ were obtained by isothermal oxidation test in air. Experimental results showed that oxidation behaviours of duplex treated H13 steel was remarkably dependent upon the type of the intermediate layer between the duplex-treated layer and the substrate. The duplex type A specimen, which has calorized layer as an intermediate layer, did not show any serious oxidation and only partial fracture of the duplex treated layer was observed after 100hours. On the contrary, duplex type B specimen, which has nirided layer as an intermediate layer, showed rapid oxidation of the duplex treated layer even after only 20 hours exposure at 900℃ and the duplex treated layer were completely detached from the base metal after exposure to 900℃ for 50 hours. Among three specimens, only plasma nitrided specimen showed the worst oxidation resistance. From the wear test at 500℃, the wear volume was calculated and the minimum weight loss in duplex type A specimen and the minimum weight loss in the plasma nitrided specimen were measured. The maximum wear depths of wear scars were measured to be approximately 60㎛ for duplex type A, 70㎛ for type B, and 100㎛ for the plasma nitrided specimen.

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NiCrAlY/8wt%Y₂O₃-ZrO₂, two layer TBC was fabricated by plasma spray method and its surface modification was performed by CO₂ laser and then the change of characteristics and failure mechanism during thermal shock test were investigated. The more specific energy of laser beam, the more crack width and thickness of melted layer originated by laser treatment. In the same specific energy, they decreased by reducing laser power. After laser treatment, residual stress was altered tensile stress, adhesion strength reduced due to the horizontal crack at the bottom of interface between melted and unmelted layer, surface corrosion of ceramic layer was suppressed due to densification and the life cycle by thermal shock test were increased. The failure procedure during the thermal shock test was as follows; buckling occurred first, then spalling followed after a few cycles and fracture region was expanded. The failure resulted from buckling is believed to occur because of compressive stress developed by cooling. It is thought that thermal shock lifetime after laser treatment is improved owing to the stress relaxation caused by vertical crack to surface and residual tensile stress.

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The mechanical alloying process has been used to produce intermetallic-dispersion-strengthened Al alloys. While the mixing of elemental powders precedes the milling operation in a conventional mechanical alloying method, the mixing method of pre-fabricated intermetallic powders with AI powders has been suggested in this study. Dispersoid intermetallic powders have been made by pulverizing as-arc-melted tetragonal DO_(22)-, DO_(23)-structured A₃(Ti,V,Zr) and cubic Ll₂-structured Al_(66)Mn_9(Ti,Zr)_(25), monolithic compounds. Intermetallic dispersoids in Al alloys found to be stable after the heat-treatment at 425℃ up to 400 hours and overall, specimens maintained original microstructures with a fine distribution of intermetallic compounds. Due to the fine and stable dispersion, alloys showed high compressive yield stress levels at room and elevated temperatures.

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Titanium carbide, TiC-x mole% Ni composites, and functionally-graded materials (FGMs) of TiC-x mole% Ni were synthesized by an electrothermal combustion (ETC) method. TiC-65 mole% Ni composite was not ignited by indirect tungsten coil heating, but can be synthesized by an electrothermal combustion. The velocity of the combustion wave decreased with increasing addition of Ni and increased with an increase in the applied field. Functionally-graded TiC-Ni materials were made from reactant layers with compositions of Ti + C + x mole% Ni with x ranging from zero to 65 by an electrothermal combustion. In the FGM products a nearly linear change in composition in the graded region was observed in samples with 0 ≤ x ≤ 65 with x being the mole% Ni.

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