Tetsu-to-Hagané
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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 87 (2001), No. 11

  • Structures and Reactions at Metal/Electrolyte Interfaces at the Atomic Level

    pp. 679-684

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    Until recently, there had been only few in situ methods available for the structural determination of an electrode surface in solution at the atomic level. Now several recent investigations have demonstrated scanning tunneling microscopy (STM) to be a powerful new technique for in situ characterization, with atomic resolution, of surfaces under potential control. The object of this article is to highlight some of the recent progress made, mainly in the authors' laboratory, on in situ STM with atomic resolution. Several selected topics are focused on, including instrumental and experimental aspects of in situ STM, observation of clean platinum electrode in solution, specifically adsorbed anions (halides, cyanide, sulfate) on metal surfaces, and adsorption of organic molecules (benzene and other small molecules) on clean and iodinemodified electrodes.
  • Micro-structural Analysis of Coke Using the Overlaying Mesh Finite Element Method

    pp. 685-691

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    In order to clarify the bulk breakage mechanism of metallurgical coke in blast furnace, the overlaying mesh finite element analysis that can consider both macro and micro-structure simultaneously is applied for complex micro-structural geometry such as coke. We introduced the stress intensity factor, a criterion of stress concentration around the crack tip, which can get from displacements of singular points by the template mesh putting on the crack tip and evaluated characteristics of fracture for high-strength coke.
    The numerical results show that the stress intensity factor (K*I) is dependent on porosity (ε) linearly and then we proposed the following equation considering a shape parameter (C) and the stress intensity factor (B) at ε=0.
    K*I=B( 1-Cε)
    This equation shows that the stress intensity factor decreases with porosity because principal stress around pores in the neighborhood of the crack tip becomes larger and pores prevent stress around the crack tip from concentrating. This negative effect of relaxation becomes larger as porosity increases. Here the parameter C is a specific value, it depends on pore shape and does not depend on crack length.
    We conclude that this numerical analysis is very useful for estimation of coke quality.
  • Proposal of Independent Two Phase Growth during Solidification in Austenitic Stainless Steels

    pp. 692-699

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    Solidification morphologies of austenitic stainless steel weld metals that solidified with primary ferrite were investigated in terms of crystallography. At the fusion boundaries, austenite grows first with plane-front morphology from the base metal austenite in an epitaxial manner. Then, ferrite forms on the growing austenite by keeping the Kurdjumov-Sachs orientation relationship with the austenite. The ferrite grows, as the primary phase, more rapidly with dendritic morphology than the planar austenite. Though the phase diagram indicates that the formation of the austenite results from the eutectic reaction in the primary ferrite solidification mode, no specific orientation relationship was confirmed by crystallographic studies between the primary ferrite and the interdendritic austenite. The austenite is found to grow independently of the primary ferrite, growing along <100> direction, even when the primary ferrite changes its growth direction. Consequently, it is suggested that the austenite in the interdendritic regions is not restricted by the primary ferrite during the growth. The growth manner of the primary ferrite and secondary austenite is named as "independent two-phase growth", and was confirmed not only in weld metals but in cast metals.
  • The Effects of Inclusions in Steel on MnS Precipitation in Fe-Si Alloys

    pp. 700-706

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    "In-situ" observation of MnS precipitation in Ce-added and Zr-added Fe-Si alloys was made on cooling using a confocal scanning laser microscope. In Ce-added and Zr-added sample, the temperature at which MnS precipitation started became higher because of the heterogeneous nucleation of MnS on Ce2S3 and ZrO2. By SEM observation, it was found that many MnS precipitates were neighbored on Ce2S3 and ZrO2. But in case of the sample contained much Ce, the temperature at which MnS precipitation started became lower and the amount of MnS precipitates became less. It was because the sulfur was consumed in Ce2S3 and the solubility product of manganese and sulfur became small. The number of MnS precipitates in δ and γ phase was very different in the sample contained neither Ce nor Zr, but that number became almost equal by the addition of Ce and Zr.
  • Effect of Oxide Composition on Solidification Structure of Ti Added Ferritic Stainless Steel

    pp. 707-712

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    Effect of oxide composition on the solidification structure of 16mass%Cr-0.15mass%Ti-0.009%N ferritic stainless steel was investigated. As for the results, the oxides in steel ingots was observed to be covered by TiN and contained Al and Mg. When the Mg/Al mass ratio of oxides, determined by EDS analysis, ranged from 0.3 to 0.5, an equiaxed fine-grain structure was produced. According to Al2O3-MgO phase diagram, the oxide composition range corresponds with Spinel phase composition at molten steel temperature. And the disregistry between Spinel and TiN is very low. Therefore, it is presumed that the particles of Spinel accelerate the formation of TiN in molten steel and then an equiaxed fine-grain structure is produced. The mechanism of Spinel dispersion in molten steel is discussed on the information of phase diagram for Al2O3-MgO-TiO2 system and the nucleation theory. It is speculated that the spontaneous nucleation of liquid Al-Mg-Ti oxide occurs due to low interfacial energy with molten steel and then the oxide droplets grow into Spinel.
  • Impact Properties of S45C under Tensile Loading

    pp. 713-718

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    The authors developed a weight-dropping impact testing machine which evaluates the toughness of materials under tensile impact loading. The relation between the time t and load P is measured with the developed testing machine. The impact strength σmax, is determined from the maximum load P max; impulse F s from the time t and load P. Also, the "impulsivity Tim"-the ratio of the material impulse F s to the standard impulse F ss calculated by means of the maximum load P max-is defined. A toughness evaluation method based on the relationship between the impact strength σmaxand the "impulsivity Tim" was reported in a previous paper. In this paper, heat treated S45C, well known for its static mechanical properties and Charpy impact value, is used as the specimen in the developed testing machine. It is confirmed that the developed testing machine is useful as a toughness evaluation method under tensile impact loading.
  • Characteristics of Luders Deformation and Their Computer Simulation Based on a Work-softening Model

    pp. 719-726

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    We proposed a new constitutive equation to explain the Portevin-LeChatelier effect. In the present study, we have examined whether the equation is applicable to the yield point phenomena of low carbon steels or not. By using the constitutive equation, some characteristics of the yield point phenomena, i.e., 1. stress difference between upper and lower yield points, 2. yield point elongation, 3. strain rate in Luders band and 4. velocity and width of the band, were predicted. To determine these characteristics experimentally, tensile tests on low carbon steels were carried out at room temperature at various strain rates. Local strains were measured by using strain gage stuck on the specimen surface. The prediction based on the constitutive equation showed a good agreement with the experimental phenomena. Some discussions are also done regarding on the validity of assumptions adopted.
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