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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 98 (2012), No. 9

  • Descending Behavior of Particles in Vertical Type Coke Oven by Cold Model and DEM Calculation

    pp. 459-468

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    DOI:10.2355/tetsutohagane.98.459

    For the sake of reduction of reducing agent in blast furnace, the use of ferro coke containing metallic iron as a catalyst was proposed to control the thermal reserve zone temperature. To produce ferro coke, the vertical type of coke oven is used to realize the optimum carbonizing process. In the vertical type coke oven, it is estimated that each particle has a specific residence time derived from the friction effect to wall and the interaction of particles. These phenomena have a great influence on the properties of ferro coke. In this study, the observation of descending behavior in the packed bed was carried out with cold model. The movements of descending particles were analyzed with DEM calculation for cold model and pilot plant. It was found that the friction effect between wall and particles caused the delay of descending speed on the corner of the furnace. Moreover, the mixing effect of particles during descending was studied with the application of diffusion model, using DEM calculation. The residence time distribution model for the vertical type coke oven was newly developed. On the basis of these results, the descending behavior in the pilot plant was clarified.
  • Observation of Microstructure in Thick Steel with High Voltage TEM

    pp. 469-475

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    DOI:10.2355/tetsutohagane.98.469

    HVEM make possible to conduct experiments with relatively thicker specimen than that of used in a conventional medium voltage TEM. Bulk specimen is of big advantage for studying internal microstructures because true strained state of materials is to be easily relaxed in thin specimen and then various micro structural factors that control mechanical properties are subjected to change.
    In this study, HVEM, 1250kV acceleration voltage, was applied to study microstructure in high strength steels using specimens over several 100nm thicknesses. In addition to improving transparency of incident electron, reducing chromatic and stigmatic aberration using an energy filtering technique was useful to conduct observation of such thick and magnetic materials. Density of dislocation and fine carbides from direct observation were consistent to the values estimated from XRD and quantitative chemical analysis. In-situ loading experiment using a tensile specimen holder was also successfully performed to track mutual interaction between nano-sized carbide and dislocations. This in-situ observation proved that dispersed fine carbides in studied steel was to be strong obstacle against to mobile dislocations.
  • Simplified Carburizing Process for Stainless Steel

    pp. 476-481

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    DOI:10.2355/tetsutohagane.98.476

    A ferritic stainless steel was embedded in mixtures of steel and activated carbon powders, and heated at 1273 K for 3.6 ks in flowing N2. The specimen was quenched into the water after heating. This simplified treatment enabled the diffusion of carbon into the stainless steel, which is known to have difficulty in carburizing. The microstructural change was not recognized in the vicinity of the surface in the specimen heated in only activated carbon. However, the carburized layer was formed in the stainless steel when the steel powder was added to activated carbon. The thickness of the carburized layer was more than 200 μm in the case of the mixed powder consisting of 10 vol% steel powder and 90 vol% activated carbon, and increased with increasing steel powder. The surface hardness of the carburized steel also increased with increasing steel powder in the mixed powder. In addition, the carburized layer was observed in the stainless steel heated in mixtures of steel and graphite powders. The details of the carburization phenomenon are discussed on the basis of the measurement of carbon monoxide gas during heating and of the heat treatment without directly contacting with the mixed powder.
  • Kinetic Model of γ to α Phase Transformation at Grain Boundaries in B-Bearing Low Alloy Steel

    pp. 482-490

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    DOI:10.2355/tetsutohagane.98.482

    The effect of Boron (B) on nucleation and growth of ferrite from austenite grain boundaries is examined theoretically assuming the junction of 4-austenite grain boundaries as dominant nucleation sites of ferrite. B segregates to the austenite grain boundaries and reduces the grain boundary energy and thereby retards the ferrite nucleation at the grain boundary. The retardation is expressed as a decrease of nucleation frequency due to an increase of activation energy for nucleation and the calculated value of the fraction of active nucleation sites is in satisfactory agreed with experimental results. The reduction of the austenite grain boundary energy, obtained by applying the Gibbs isotherm for adsorption to the B segregation, is of the same order of as the one which is deduced from results of calculation for decrease in the nucleation frequency based on experimental result. The growth of ferrite is calculated using DICTRA yielding both the volume fraction and the grain size of transformed ferrite as a function of time, which agreed with the experimental results. This suggests the slight influence of B on growth rate. However, the increase of the diffusion cell size due to B addition is considered to be the main reason for the subtle larger grain size of ferrite as compared with B-free steel, which is in good accordance with the experimental fact.
  • Mechanical Properties of Ti-6Al-4V Alloy Using Off Grade Sponge Titanium as Raw Material

    pp. 491-496

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    DOI:10.2355/tetsutohagane.98.491

    The effect of impurity elements on the mechanical properties of Ti-6Al-4V alloy is presented in this paper. Impurity levels of oxygen and iron were varied between 0.1 and 0.7%. Tensile and proof strength increased and elongation, cross-sectional area reduction, and Charpy impact strength decreased as the oxygen content increased. Furthermore, it was found that Japanese Industrial Standards requirements were maintained up to impurity levels of 0.3% O and 0.7% Fe by mass. Finally, the fatigue strength and seizure characteristics of Ti-6Al-4V fabricated from off-grade sponge titanium as the raw material was clarified by a series of experiments. The results showed that mechanical properties of Fe-bearing Ti-6Al-4V alloy are comparable to those of conventional Ti-6Al-4V alloy.
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  • Effect of Hydrogen Pre-Charging on Fatigue Life on Medium Carbon Steels of Different Purity

    pp. 497-502

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    DOI:10.2355/tetsutohagane.98.497

    Fatigue tests were conducted on two kinds of normalized S45C commercial steel bars that are of different purity. Amount and state of hydrogen are determined by thermal desorption spectroscopy (TDS) after cathodic charging. Effects of impurity elements and non-metallic inclusions have been discussed.
    The fatigue life is less damaged in high purity type S45C (S45C/hpt) with same amount of hydrogen pre-charging, although the non-charged S45C steels exhibits almost same fatigue lives.
    The TDS main peak around 335K is lower in S45C/hpt and hydrogen pick-up is smaller. This could be explained by the smaller amount of MnS. Though, it is only the subsidiary reason for the susceptibility.
    The S45C/hpt being sufficiently de-oxidized, the Al2O3 inclusions are reduced in size and quantity. Furthermore, they are wrapped by MnS. The comparative S45C contains silicate inclusions. Both are elongated and cohesive to ferrite matrix; not resulting in the internal failure with hydrogen pre-charging.
    Phosphorus would enhance hydrogen trapping along grain boundary and promote the fracture along it. Grain boundary facet being not found for both steels, it would not deal so much with the present hydrogen susceptibility.
    With S45C/hpt, soluble nitrogen atoms are reduced by sufficient Al addition. This may lead to the late dislocation cell formation and responsible to the cyclic softening that lasts long at the beginning of strain cycling.
    This cyclic softening acts to reduce the dislocation density in S45C/hpt. It would make the steel insensitive to hydrogen substantially through decreased hydrogen concentration.
  • Effects of Alloying Elements on the Hardenability, Toughness and the Resistance of Stress Corrosion Cracking in 1∼3 mass%Cr Low Alloy Steel

    pp. 503-510

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    DOI:10.2355/tetsutohagane.98.503

    The effects of alloying elements on the hardenability, toughness and the resistance of stress corrosion cracking have been evaluated in the 1∼3mass%Cr low alloy steels for 1600mmφ level large-sized turbine rotor forging for geothermal power generation. The chemical composition suitable for the rotor forging which was satisfied with the required material properties and which reduced the C-segregation in the center portion of the ingot was chosen under the restrictive condition on the upper limit of Ni and Cr contents in turbine rotor under the geothermal corrosive environment. The 8 ton sand mold ingot was manufactured and the reduction of the C-segregation was verified in the selected chemical composition. Finally selected 2.25Cr-0.9Ni-0.6Mo-0.25V-0.15Si-1.0Mn-0.25C steel (mass%) is hopeful as the chemical composition for the large-sized turbine rotor forging for geothermal power generation.
  • Effects of Carbon Content and Chromium-Molybdenum Addition on Grain Coarsening Behavior in Hot Working by Small Strain of Steels for Machine Structural Use

    pp. 511-516

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    DOI:10.2355/tetsutohagane.98.511

    The purpose of this research is to clarify the effects of carbon content and chromium-molybdenum addition on grain coarsening behavior in hot working by small strain of steels for machine structural use. The tendency of grain coarsening is as remarkable as high carbon steel. The influence of carbon content is slight to grain coarsening behavior of the austenite grain in hot working with small strain. In low carbon steels, ferrite structure occupies the great portion of microstructure. In these steels, two or more ferrite grains generate from one austenite grain, and those divide an austenite grain. Therefore, the coarse grain or mixed grain of austenite is not directly taken over to a transformed microstructure. It is based on the above reason that the tendency which a coarse grain pearlite structure generates is reduced in low carbon steels. In SCM435 steel, ferrite-bainite structure occupies the great portion of transformed microstructure and includes the coarse bainite structure. This is because coarse grain or mixed grain of austenite structure is directly taken over to a bainite structure.

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