Young-Whan Cho

Non-metallic inclusion and intragranular nucleation of ferrite in Ti-killed C–Mn steel

Abstract The influence of Ti addition on the development of acicular ferrite microstructure during the γ/α transformation in C–Mn steels has been studied. The optical microstructures of the heat-treated specimens with different Ti concentrations were characterized. Transmission electron microscopy analysis with thin foil specimens was carried out to investigate the phase composition of non-metallic inclusions and the local variation of chemical composition around the inclusions. It has been found that an acicular ferrite dominant microstructure could be produced when the Ti concentration is higher than about 50 ppm. The transition from the conventional bainitic microstructure to the interlocking acicular ferrite microstructure occurs in response to the change in the main-component of the non-metallic inclusions from Mn–Si oxide to Ti 2 O 3 . The Mn depleted zones around Ti 2 O 3 particles were detected, which could explain how the intragranular nucleation is facilitated on Ti 2 O 3 particles.

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.

Core shell structure for solid gas synthesis of LiBD4

The formation of LiBD(4) by the reaction of LiD in a diborane/hydrogen atmosphere was analysed by in situ neutron diffraction and subsequent microstructural and chemical analysis of the final product. The neutron diffraction shows that nucleation of LiBD(4) already starts at temperatures of 100 degrees C, i.e. in its low temperature phase (orthorhombic structure). However, even at higher temperatures the reaction is incomplete. We observe a yield of approximately 50% at a temperature of 185 degrees C. A core shell structure of the grains, in which LiBD(4) forms a passivation layer on the surface of the LiD grains, was found in the subsequent microstructural (electron microscopy) and chemical (electron energy loss spectrometry) analysis.

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.

Towards room temperature, direct, solvent free synthesis of tetraborohydrides

Due to their high hydrogen content, tetraborohydrides are discussed as potential synthetic energy carriers. On the example of lithium borohydride LiBH4, we discuss current approaches of direct, solvent free synthesis based on gas solid reactions of the elements or binary hydrides and/or borides with gaseous H2 or B2H6. The direct synthesis from the elements requires high temperature and high pressure (700?C, 150bar D2). Using LiB or AlB2 as boron source reduces the required temperature by more than 300 K. Reactive milling of LiD with B2H6 leads to the formation of LiBD4 already at room temperature. The reactive milling technique can also be applied to synthesize other borohydrides from their respective metal hydrides.

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.