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

Tetsu-to-Hagané Vol. 83 (1997), No. 7

  • Formation of the Vertical Slits in the Lower Part of a Sintering Bed and Their Effect on Sintering

    pp. 413-418

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    The conditions for stable formation of vertical slits in the lower part of the sintering bed and their effect on the sintering were investigated. The effect of the slits was evaluated experimentally with a test pot, feeding simulator and commercial plant. The total pressure drop was decreased and the permeability in the bed was improved by the formation of vertical slits. The gas flow rate in the slits was calculated at 1.8 times as large as that of a conventional bed. Moisture condensation, which decreases the void fraction in the sintering bed, is reduced by this increased gas flow rate. To form slits, plates were inserted vertically in the lower part of the raw material on the pallet of sintering machine. The ratio of the height of the plates to the distance between plates should be under 0.5 for stable feeding without stagnation of the raw material.
  • Influence of Sample Temperature and Hygroscopicity of W-Electrode on Carbon Analytical Value in the Analysis of Low Carbon Steel by Atomic Emission Spectrometry

    pp. 419-424

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    In our previous paper concerning the analysis of low carbon steel by atomic emission spectrometry, carbon intensity was found to increase considerably with the rise of sample temperature before discharge and with the time intervals between analyses. However, the mechanism of this intensity rise has not been known. Therefore, we tried to establish a model for this mechanism.
    The results were as follows:
    (1) The analytical value of carbon increased 0.2ppm with 1°C rise in sample temperature before discharge. It is explained from the additional atomic emission; (1)an oxide layer produced on the sample surface was thicker as the sample temperature rose.; (2)carbon monoxide gas (CO gas) was formed through the oxidation of carbon in the sample by the oxide layer (C+O_??_CO).;(3)the formed CO gas was introduced in a discharge column and decomposed to C and O to give rise to the carbon intensity.
    Therefore, to analyze carbon in low carbon steels accurately, the sample temperature before discharge should be adjusted to room temperature. If the sample temperature is higher than room temperature, we must grind the sample again after cooling the sample to lower temperature than room temperature and adjust the sample to room temperature.
    (2) Carbon intensity increased when the time intervals between analyses were prolonged. It is considered that this increase results from water adhesion to W-electrode; (1)in the time intervals, the dust as by-product of discharge adhered to W-electrode.; (2)as being discharged under this condition, CO2 in water adsorbed to the dust was introduced in discharge column and decomposed to C and O to give rise to carbon intensity. Therefore, to analyze carbon in low carbon steels accurately, we needed to remove the dust adhered to W-electrode by discharge of cleaning or heating of Ar gas as an atmosphere.
  • Numerical Analysis of Stress Intensity Factors of Crack in Subsurface Layer of Work Roll for Rolling

    pp. 425-430

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    A simplified model for stress intensity factors of mixed modes was derived in consideration of non-dimensional stress intensity factors obtained by Murakami et al. A numerical analysis model for stress intensity factors of mixed modes in the subsurface layer of work roll of rolling was also obtained by applying this simplified model to the deformation analysis model for the thermal elastic-plastic stress and strain in the subsurface layer of work roll with stress and thermal loads. Then, the stress intensity factors in the subsurface layer of work roll in hot rolling were calculated with this numerical analysis model. It has been found that the stress intensity factors of tensile and shear modes on the side of intermediate roll is larger than those on the side of rolling material and that the crack propagation is estimated to occur on the side of intermediate roll, etc.
  • Rolling of Flat and T-shaped Profiled Wires by the Satellite Mill

    pp. 431-436

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    The authors have developed a new type rolling mill named the satellite mill. The mill comprises one large diameter roll (central roll) and five smaller diameter rolls (satellite rolls) arranged along a circumference of the central roll. Material is passed continuously through the five gaps between the central roll and the satellite rolls and deformed into a profiled cross section. Since all rolls are driven at the same roll speed, longitudinal compressive stress is produced between stands during rolling. The elongation is greatly suppressed. The mill has been applied to the production of round-edged flat wires and two kinds of T-shaped profiled wires from round circular wires. The obtained rolling characteristics and product properties are compared with those by a conventional rolling method. It is shown that the transverse metal flow is enhanced and the filling ability to the roll groove is excellent in satellite-mill rolling. The effect is more apparent in rolling of T-shaped wire having thinner ribs. It is supposed that the satellite mill is favorable for profiled wire production.
  • Siliconizing of Iron by Molten Magnesium-Silicon Alloy Bath

    pp. 437-441

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    A molten magnesium-silicon alloy bath containing 1 to 12 mass% of silicon was used for siliconizing iron by keeping the specimen in the bath for 180-900s at 973-1123K. The composition of siliconized layer, which consisted of fine-grained and columnar-grained sublayers, was identified almost the same as Fe3Si, using the Energy Dispersive Spectrometer and the X-ray Diffractometer. Micro-Vickers hardness number of the layer was about 500. The thickness of the layer varied with the siliconizing temperature and time. Then the diffusion coefficient was calculated as,
    D=1.91×10-4exp[-152.6(kJ/mol)/RT] (m2/s) for Mg-3%Si alloy bath, and
    D=4.54×10-4exp[-163.5(kJ/mol)/RT] (m2/s) for Mg-12%Si alloy bath.
    On the growing way of the columnar-grained sublayer which is thicker than the fine-grained sublayer, insoluble particles deposited on the surface of the specimen can be taken and dispersed into the layer by this siliconizing process.
  • Siliconizing of Iron and Steel by Molten Magnesium Bath with Silicon Carbide Powder

    pp. 442-447

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    A molten magnesium bath was used to siliconize iron and steel specimens embedded in SiC powder for various periods at 973-1123K. The siliconized layer obtained was composed of Fe3Si phase as the matrix and SiC phase as dispersed particles.
    Therefore, a hard composite layer, as high as 820 of Vickers hardness number, could be obtained.
    The thickness of the layer varied depending on the siliconizing temperature and time. Then the thickness was measured as to iron specimens siliconized with 5μm-SiC powder and then the diffusion coefficient was calculated as,
    D=34.22×10-4exp[-186.30(kJ/mol)/RT] (m2/s). The thickness of the layer could also vary with SiC powder size of 2 to 40μm, the smaller in the size, the thicker in the layer. However, SiC powder of 5μm was thought to be suitable practically.
  • Compositional Gradient and Ion Selectivity of Cr-substituted Fine Goethite as the Final Protective Rust Layer on Weathering Steel

    pp. 448-453

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    The compositional gradient and ion selectivity in the final protective rust layer of the Cr-substituted goethite, α-(Fe1-x, Crx) OOH, on a weathering steel have been discussed by examining the spontaneously and artificially formed Cr-substituted goethites. It was found that the Cr content in the spontaneously formed final protective rust layer increases gradually toward the [rust/steel] interface. This increase in the Cr content decreases the crystal size of the goethite and leads to a denser Cr-substituted goethite rust layer which provides higher protectiveness against atmospheric corrosives. It also became evident that the goethite with Cr content higher than approximately 3 mass% possesses cation selective ability. Thus, it is supposed that the spontaneously formed Cr-substituted goethite final protective rust layer has the ability to suppress corrosive anion penetration, at least near the [rust/steel] interface where the Cr content is estimated to be more than approximately 5 mass%. The structure of the [rust/steel] interface was also examined by transmission electron microscopy and the presence of a thin interfacial layer was pointed out. A detailed structure of the final protective rust layer is newly proposed with considering the compositional gradient and the structure of [rust/steel] interface.
  • Evaluation Method of Delayed Fracture Property of High Strength Steels

    pp. 454-459

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    To evaluate delayed fracture properties of martensitic bolt steels containing Cr, Mo, and V, the relationship between critical content for delayed fracture [Hc] and environmentally absorbed content [HE] in diffusible hydrogen have been investigated. The critical value [Hc] and the environmental value [HE] were determined by accelerated delayed fracture tests under constant applied load using specimens with uniformly distributed hydrogen and by cyclic corrosion tests in 5% NaCl aqueous solution as an environmental condition, respectively. The diffusible hydrogen was detected by hydrogen thermal desorption analysis after respective tests.
    On the other hand, the delayed fracture resistance was defined as fracture ratio [F] of bolts exposed in the seaside testing site.
    Based on the result that ( [HC] [HE])/[HC] increases monotonically with a decrease in fracture ratio [F], it is proposed that the parameter ([HC]/[HE] )/[HC] would be a quantitative indication showing delayed fracture susceptibility of high strength steels in service environment.
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  • Delayed Fracture Mechanism in High Strength Steels by Acoustic Emission Source Wave Analysis

    pp. 460-465

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    Acoustic emission(AE) source characterization was carried out to examine the mechanism of delayed fracture in high strength steels. In static load delayed fracture test, incubation time for crack initiation was observed. Crack initiates at the region of maximum triaxial-stress state. After crack initiation, cracks propagated discontinuously. The diffusible hydrogen content demanded for first intergranular crack initiation was more than that demanded for crack propagation. The AE source characterization showed that the inter-granular crack diameter were 40 to 80μm and crack evolution rata were 30 to 40m/s.
  • Microstructural Changes during Creep and Life Assessment of Mod.9Cr-1Mo Steel

    pp. 466-471

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    Several microstructural changes take place in a material during the course of creep. These changes can be a measure of creep life. In this paper, microstructural changes in Mod.9Cr-1Mo steel were studied and it was examined which is a good measure of creep life.
    Microscopic structural changes, such as void growth, lath structure uniformly oriented to the tensile axis and elongation of grains, are evident only in the necked portion of ruptured specimens. These macroscopic structural changes are not useful for creep life assessment.
    Lath width increases and dislocation density within lath decreases with increasing creep duration. These changes in dislocation substructure start in the early stage of creep life, and cause the increase of strain rate in the tertiary creep stage. The lath width and the dislocation density reach a saturated value before rupture. The saturated values are independent of temperature, and uniquely related to creep stress normalized by shear modulus. The extent of these microstructural changes are greater at lower stresses under which the material is practically used. These facts suggest that the lath width and the dislocation density within lath can be a useful measure of creep life.
    Hardness of crept specimens is closely related to the lath width and the dislocation density within lath. The changes of these microstructural features can be evaluated by the measurement of hardness.

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