Metallurgical Engineering

High temperature mechanical behaviour of Cu, Cu-Al alloys and Ni was examined by torsion tests with hollow test pieces in conjunction with microstructural observations using both optical and electron microscopies. Dynamic recrystallization occurred in these metals and alloys as the restoration process during the high temperature deformation. Through analyzing the dynamic recrystallization behavior from both mechanical and microstructural points of view, the factor affecting the behavior, such as the deformation condition (Zener-Hollomon parameter), the stacking fault energy, the solute elements and the strains, were discussed in detail.

In steel continuous casting system, nozzles have an important role on the flow rate control of the molten steel and the elimination of the inclusions etc. Based on the fundamental principle of hydrodynamics, the optimal nozzle bore profile was determined for both tundish (TD) upper nozzle and submerged entry nozzle (SEN) with aiming to suppress the turbulence with high kinetic energy generated in the flow of the molten steel. A simulation by flow analysis using computational fluid dynamics (CFD) and a water model experiment were performed and clarified that the turbulence with high kinetic energy could be minimized in the nozzles with newly devised inner bore profile. The actual nozzle devised was manufactured and tested in the steel works with satisfying results. The adhesion of the alumina inclusions to the both nozzles is reduced by lowering the turbulent kinetic energy since the energy loss is minimized at the part. Both stability in the operation and quality of the steels have been brought by the present development.

In steel continuous casting system, tundish upper nozzle has an important role on the flow rate control of the molten steel and the elimination of the inclusions and etc. Based on the basic principle of hydrodynamics, the optimal nozzle bore profile was determined with aiming to suppress the turbulence with high kinetic energy generated in the flow of the molten steel. A simulation by flow analysis and a water model experiment were performed and clarified that the turbulence with high kinetic energy could be minimized in the nozzle with newly devised inner bore profile. The actual nozzle devised was manufactured and tested in the steel works with satisfying results. It can contribute to not only improve the durability of the refractories as materials of the nozzle but also stabilize the casting operation by preventing from clogging of the nozzle.

A new geometrical design of the port in the flow line direction to minimize the maximum port velocity (MPV) has been proposed through the analysis using computational fluid dynamics (CFD) and water model experiments to keep uniform velocity distribution of the molten steel flow throughout the outlet of ports for submerged entry nozzle (SEN) in the continuous casting system. The adhesion of the alumina inclusions to the port part of the SEN is reduced by lowering the turbulent kinetic energy since the energy loss is minimized at the part. Both stability in the operation and quality of the steels have been brought by the present development.

Slip casting behavior of a cylindrical body was analyzed by FEM (finite element method). Based on the assumption that the cake is uniform and incompressible, Laplace equation for liquid pressure can be derived. Cake growth per unit time was calculated by a sequential combination of the Laplace equation, Darcy's law and mass balances. The profile of solid front (the interface between cake and slurry) changed with time from a cylindrical curve to a parabolic one. This three-dimensional profile can be attributed to the interaction of filtration to the bottom and side-wall directions in the cylindrical mold. In the early stage of slip casting, the results of numerical analysis were in reasonable coincidence with the profiles experimentally obtained. It was shown that FEM simulation is valuable for prediction of three-dimensional cake growth which can not be expressed by one-dimensional analytical approaches.

SiC based ceramics have excellent properties in strength, hardness and chemical stability at high temperatures. Such attractive characteristics make them promising candidates for high temperature structural or wear and corrosion-resistant materials. On the other hand, carbides, nitrides or borides of transition metals, such as TiC, NbC, ZrC, TiN, TiB2 and ZrB2 have prominent refractriness with good electric conductivity. Sintered SiC and refractory compounds, however, have low fracture toughness. In order to improve the fracture toughness of ceramic materials, use of refractory compounds as a filler has been attempted.  In this article fabrication and mechano-physical properties of SiC~ (X; carbides, nitrides or borides of transition metals) composites and their microstructure-macro properties relation are reviewed. Refractory compounds particles/SiC matrix composites exhibited improved mechanical properties compared to the monolithic SiC or the compounds.The electric conductivity of the composites can be varied by changing the amount of carbides, nitrides or borides of transition metals. Useof refractory compounds/SiC composites may solve two problems: Iow fracture toughness of ceramic materials and difficulty of machining them by using an electric discharge method.

Ceramics were prepared by hot pressing of chemical mixtures of turbostratic boron nitride with copper. A positive effect of copper on boron nitride crystallization and densification has been found. Preferred orientation of boron nitride grains has been revealed in such ceramics, being opposite to general experience: the c-axis of boron nitride crystallites was arranged preferably within a plane perpendicular to the direction of uniaxial pressure. Such an arrangement is interpreted as a result of the uniaxial compression of the boron nitride grains extensively growing within the c crystallographic axis.

Powders of Si3N4, Al2O3 and SiO2 were mixed with Y2O3 as sintering aid and hot-pressed to form o'-β' sialon. During sintering, the o' phase preferentially precipitated in the temperature region of 1550 to 1600℃ and the β' phase precipitated at temperatures of 1700℃ and above. The resultant microstructure consisted of dual phases of o' and β'. The mechanical properties of o'-β' sialon were improved with increasing amount of the β' phase. When SiO2 required for the formation of the o' phase was all included in the grain boundary phase, o'-β' sialon exhibited a three-point bending strength as much as 1400 MPa. The oxidation resistance of o'-β' sialon is expected to improve due to the presence of the o' phase, but actually was not so good due to the presence of the grain-boundary glassy phase and impurities.

Chemical reactions of synthesis of hexagonal boron nitride and their mechanisms are discussed and classified. Based on their analyses, a ternary B–N–O diagram is suggested, in which virtual compositions of hexagonal boron nitride ceramic and preceding materials, starting with a mixture of raw materials, through oligomolecular semi-organic precursors and turbostratic boron nitride powder are specified. Beside the composition, processes of conversion between particular substances, such as synthesis, decomposition and crystallization of boron nitride powder, and sintering and hydrolysis of ceramic, are characterized by vectors—elements of the ternary diagram. The dynamic model is suggested to be generally applied to systems with prevailingly covalent character, where the polymerization is regarded as a controlling chemical process.

The combustion synthesis of ternary and quaternary carbosulfides of the transition metals was investigated to obtain highpurity materials to be tested as solid lubricants in high-temperature environments. The main carbosulfide formation mechanisms have been clarified in the case of the technologically interesting tantalum carbosulfide, enabling the preparation of reaction products with controlled purity. The formation of molten tantalum sulfide as an intermediate product was found to play a key role for obtaining a homogeneous reaction product, and the presence of excess carbon reduced the oxygen content of the carbosulfide. However, the higher the overall carbon contents, the higher is the amount of carbide impurities in the reaction products. Use of iodine or hexachlorobenzene as a catalyzer minimized the need of excess sulfur in the raw powder mixture, reduced the amount of carbide impurities, and contributed to the homogenization of the reaction products.