발간논문

Home > KJMM 논문 > 발간논문

Thermal spray coating layers were prepared on the carbon steel plates with iron-chrome-boron alloy powders using a detonation gun under various fuel gas contents. The coating layer was composed of Fe rich Fe-Cr solid solution and (Cr. Fe)_xB particles, which had relatively a tow friction coefficient such as 0.05 and showed high wear resistance. The transition from crystalline to amorphous phase occurred during the wear treatment. TEM observation revealed that Fe-Cr solid solution phase with super-saturated B and Si was one of the main contributors for the transition. Oxide inclusions formed at an excess fuel gas condition may impede the crystalline to amorphous transition by lowering the solute content in Fe-Cr solid solution phase.

keyword :

The present study is concerned with the processing and the microstructural analysis of VC/carbon steel surface allayed materials using irradiation of high-energy electron beam. The mixtures of DC powders and flux (50%MgO-50%CaO) were placed on a plain carbon steel substrate, and then electron beam was irradiated on these mixtures using an electron beam accelerator. The microstructures of the irradiated surface layers mere examined by optical microscopy, scanning electron microscopy, and transmission electron microscopy. Residual pores were found in the specimen processed without flux, but hardly found in the specimens processed with a considerable amount of flux. As a result of irradiation, the V content was homogeneously maintained throughout the melted region, and fine vanadium carbides were formed in the melted region. These microstructural modifications including the formation of VC greatly improved hardness, especially high-temperature hardness up to 500℃.

keyword :

Simultaneous oxidation-chloridation behavior of austenitic stainless steels by NaCl in air was investigated at 650℃. Oxide scale formed on the surface of every test alloy consisted of inner spinel layer and outer Fe₂O₃ layer. Condensed and gaseous chlorides were formed at the alloy/scale interfaces by NaCl vapors which penetrated via pores or cracks of the scales. Phase equilibria and the partial pressures of gaseous species at the interfaces between alloys and product phases were calculated by computer-assisted thermodynamic calculations using the Thermo-Calc computer program. A mechanism was proposed for the overall oxidation process including the initial reaction of alloys with NaCl, the rapid formation and growth of oxide scales, and the transport of gaseous species through the oxide scales.

keyword :

Giant magnetoresistance(GMR) of Co_(15)Ni_xCu_(85-x) Melt-spun ribbons is closely correlated with the microstructures produced by the spinodal decompodition. The solid solution range is extended by the replacement of the Cu by Ni in the as-quenched state. The compositional wavelengths obtained by the subsequent isothermal aging in Co-Ni-Cu are shorter than those in Co_(15)Cu_(85) binary ribbons, resulting in the increase of the surface-to-volume ratio. The largest MR ratio of 8.2% in high field has been achieved in the Co_(15)Ni_5Cu_(80) ribbon aged at 723 K for 4h. Further, the field dependence of MR ratio in low fields becomes larger with the Ni content and the largest mean MR ratio between 0 and 300 Oe, 2.6 %/ΔH, was obtained in Co_(15)Ni_(10)Cu_(75) ribbon. This behavior would be correlated with the shorter wavelength in the modulated microstructures.

keyword :

The interfacial reaction behavior between the melt-spun, amorphous 54.8 at% Cu-39.6 at% Ag-5.6 at% Ce alloy and the pure Sn solder was investigated at 240℃ and 180℃ and was compared with that between the crystalline alloy obtained by a crystallization treatment of the amorphous alloy and the Sn solder. The amorphous and crystalline alloys showed no qualitative difference in reaction behavior at 240℃. The reaction zone with a layered structure of Cu_6Sn_5/Ce_5Sn₃/Cu_6Sn_5+Sn(l)/(Cu-Ce)+Sn(l) grew fast as the liquid Sn dissolved preferentially Ag of the alloys. During the annealing of the Sn-wetted alloys at 180℃ for 100 hours, the reaction zones formed in the amorphous and crystalline alloys showed similar layered structures but the growth behavior of the reaction zone was significantly different: a reaction-limited growth in the amorphous alloy and a diffusion-limited growth in the crystalline growth. The difference can be at-tributed to the fact that the bulk diffusivity of Sn in the amorphous alloy is much lower than the grain boundary diffusivity of Sn in the crystalline alloy.

keyword :

Circulation rate of melt, one of the important factors which determine decarburization rates of RH system, was calculated by a 3-dimensional numerical simulation technique. A new model for analytic calculation of the shape and volume of plume zone formed by side blowing of Ar gas was designed to obtain the driving force of melt circulation. Circulation flow rates were measured in the 1/10 scale water model and in an actual RH operation to verify the calculated results. The predicted circulation rates showed a good agreement with both experimental results, Because the new program can calculate the circulation rate with various operating conditions and dimensions of RH system, it could be very helpful to determine the optimum operating conditions and to design new RH systems.

keyword :