Jae-Dong Shim

Nucleation of intragranular ferrite at Ti2O3 particle in low carbon steel

Abstract Systematic experiments have been performed to study the intragranular nucleation phenomena of ferrite in Ti-bearing low carbon steels. The fine intragranular acicular ferrite structure in Ti-bearing low carbon steels is induced by heterogeneous nucleation of ferrite plates at fine dispersed Ti2O3 particles, which are thermodynamically stable in the steels. From a steel–Ti2O3 diffusion bonding experiment, it is clearly observed that a local Mn-depleted zone (MDZ) has developed in the vicinity of the steel–Ti2O3 interface. The MDZ formation around Ti2O3 particles is believed to be a dominant driving force for the heterogeneous nucleation of intragranular ferrite. It is also confirmed that the development of the MDZ is associated with the absorption of Mn into the Ti2O3 phase at high temperatures. The width of the MDZ decreases with decreasing bonding (austenitizing) temperature and influences greatly the intragranular nucleation ability of acicular ferrite at Ti2O3 particles.

Ferrite nucleation potency of non-metallic inclusions in medium carbon steels

Abstract In recent years, there has been renewed interest in the role of non-metallic inclusions in controlling the microstructure of steels. The potency of various inclusions and precipitates such as SiO 2 , MnO·SiO 2 , MnS, Al 2 O 3 , Ti 2 O 3 and VN for the nucleation of intragranular ferrite has been examined in the present study. Among them, single SiO 2 , MnO·SiO 2 , Al 2 O 3 , TiN and MnS particles seem to be inert to the nucleation of intragranular ferrite under the present experimental condition. Ti 2 O 3 particles in a Mn-containing steel are very effective for the nucleation of intragranular ferrite, being (Ti,Mn) 2 O 3 particles by absorbing Mn atoms from a steel matrix. On the other hand, Ti 2 O 3 particles in a Mn-free steel are not effective. MnS and Al 2 O 3 particles in high nitrogen steels containing vanadium also appear to be potent for the nucleation of intragranular ferrite. The decrease in transformation temperature causes a change in the morphology of intragranular ferrite from idiomorphic ferrite to acicular ferrite.

Thermoelectric properties of (Bi0.25Sb0.75)2Te3 alloys fabricated by hot-pressing method

Thermoelectric properties of the hot-pressed p-type (Bi0.25Sb0.75)2Te3 alloy were characterized with variation of the hot-pressing temperature and the starting powder size. The roles of the factors which affect the Seebeck coefficient of the hot-pressed (Bi0.25Sb0.75)2Te3 alloy has been elucidated in this study. The donor-like behavior of oxygen could be one of the possible explanations for the higher Seebeck coefficient of the hot-pressed (Bi0.25Sb0.75)2Te3 alloy. Te vacancies formed by mechanical deformation during the powdering process significantly promote the diffusion of second phase Te atoms into their lattice sites so that the matrix Te solubility approaches its equilibrium value at a given temperature in a relatively short length of time. Using the Seebeck coefficient at various hot-pressing temperatures, the micro-phase diagram near the stoichiometric composition of (Bi0.25Sb0.75)2Te3 was evaluated.

Inoculated acicular ferrite microstructure and mechanical properties

Abstract Low carbon–manganese wrought steels with three different microstructures were prepared by different thermomechanical treatments without changing the chemical composition and their mechanical properties were compared. The microstructure of fine interlocking acicular ferrite nucleated intragranularly at Ti 2 O 3 inclusions shows higher strength and good toughness at low temperatures than those of coarser polygonal ferrite/pearlite and aligned ferrite with second phase.

Numerical analysis of molten metal shape in cold crucibles by 3D FEM

A three-dimensional finite-element method with the edge finite element and the H - Φ formulation technique has been adopted to analyze the electromagnetic field distribution in cold crucibles of different designs. The calculated magnetic flux density distribution and the meniscus shape of the molten metal agreed well with the measured values. The influences of the slit width, segment number of the cold crucible, frequency and position of the induction coil on the magnetic flux density distribution in a continuous casting type cold crucible have been calculated. The effects of density and electrical conductivity of liquid metals as well as the frequency of the induction coil on the meniscus shape of the melts have also been studied. Using the information obtained from the results of the calculation, four different designs of bottom support type cold crucibles were compared and an optimum design of cold crucible for melting reactive metals has been proposed.

Fluid flow and heat transfer in molten metal stirred by a circular inductor

Abstract A special electromagnetic stirrer which can produce local pulsating flows as well as an overall vortex flow in molten metals has been devised and experimentally tested using a simplified model system to evaluate its flow control characteristics and mixing efficiency. The influences of frequency, current and the waveform of current on the flow structure and heat transfer in the liquid In–Ga–Sn metal pool have been investigated. It has been found that the double frequency mode resulted in more effective heat transfer process compared to the single frequency mode, even at lower total input power, without significantly incurring a decrease in the averaged flow velocity level.

Thermodynamic criterion of separated eutectic phenomena

About 26 kinds of simple and complex binary eutectics were frozen directionally under the condition of electromagnetic stirring. It was found that anomalous eutectics including most metals/non-metals and some metal/intermetallic eutectics, can be separated on macroscopic scale by flow caused by electromagnetic stirring. Moreover, the eutectics in which one of their component phases possess an entropy of solution over 23 J mol-1K-1 showed notable separation. In addition, process factors also affect the formation of a separated eutectic.

리튬계 수소저장재료의 연구개발 동향

【Hydrogen is in the spotlight as an alternative next generation energy source for the replacement of fossil fuels because it has high specific energy density and emits almost no pollution, with zero

Zone inversion in the Bi/sub 2/Se/sub 3/ rich side of Bi/sub 2/Te/sub 3/-Bi/sub 2/Se/sub 3/ solid solution

CO_2

Evaluation of the micro-phase diagram near the stoichiometric composition of the Bi/sub 2/Te/sub 3/-Sb/sub 2/Te/sub 3/ alloys

emission. In order to use hydrogen safely, reliable storage and transportation methods are required. Recently, solid hydrogen storage systems using metal hydrides have been under extensive development for application to fuel cell vehicles and fuel cells of MCFC and SOFC. For the practical use of hydrogen on a commercial basis, hydrogen storage materials should satisfy several requirements such as 1) hydrogen storage capacity of more than 6.5wt.%