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

Tetsu-to-Hagané Vol. 78 (1992), No. 6

  • Development of Steel Structure Fabrication Technology from Viewpoint of Structural Mechanics and Welding Dynamics

    pp. 847-853

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  • Applications of Thermal Spraying for High Temperature Corrosion Environments

    pp. 854-859

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    [in Japanese]
  • Hydrogen Embrittlement of Ni-base Superalloys Used for H-II Rocket Engine

    pp. 860-869

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  • Effective Thermal Conductivities of Products in the Stepwise Reduction of Agglomerated Iron Ore

    pp. 870-877

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    Effective thermal conductivities of fired pellets, nonfired pellets and sinter which were reduced in a stepwise manner to magnetite, wustite and metallic iron by CO-CO2 or H2 gas have been measured by the laser flash method in the temperature range from room temperature to 1273 K.
    As the reduction proceeded, porosity of the samples changed from 23% to 62%. Measured effective thermal conductivities of the samples were remarkably smaller than those of dense materials. Thermal conductivities of metallic iron and hematite depended strongly on temperature in comparison with wustite. As the samples were reduced, the effective thermal conductivity decreased and reached a minimum value in wustite, but increased to 510 times in metallic iron. The treatment of reduction in stepwise at 1273 brought a little difference among the effective thermal conductivities of the three kinds of samples.
    The influence of reducing gas, namely ; pure CO or H2, on the temperature dependence of the effective thermal conductivity of metallic iron was observed. This is probably attributed to the generation of different structures of the samples. The data measured were correlated on the basis of the unit cell model. As a result, this model succeeded in explaining the actual bonding condition among neighbor grains and the equations obtained agreed well with the data observed.
  • Effects of Flux Composition and Oxidizing Conditions on Dephosphorization of Chromium-containing Hot Metal by Using CaO-CaF2-based Fluxes

    pp. 878-885

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    Removal of phosphorous from chromium-containing hot metal by using CaO-CaF2-based fluxes was investigated at 1480 ± 20°C. Experiments were carried out using a 300 kg induction furnace in which the fluxes were injected into the metal by using a gas mixture of O2 + Ar. Experimental results showed that the optimum ratios of CaO/CaF2 and O2/(CaO + CaF2) for dephosphorization were approximately 1/1 and 40 Nl/kg, respectively. Under these conditions, about 50 per cent of phosphorous was removed by using 70 kg/ton of the flux from the metal containing 6 mass% of carbon and 28 29 mass% of chromium. The contents of Cr2O3 of the slag after the treatment were as low as 12 mass%. When the ratio of O2/(CaO + CaF2) was increased to more than 40 Nl/kg, the loss of chromium by oxidation was increased without any favorable effect on the degree of dephosphorization. In addition, when chromium contents are more than 17 mass%, the observed value of LP( (mass% P)/[mass% P]) was good agreement with the calculated one on the assumption that the oxygen potential is determined by the reaction of CaO·Cr2O3 formation.
  • Decarburization Reaction in Ultra-low Carbon Iron Melt under Reduced Pressure

    pp. 886-893

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    The kinetics for the decarburization of iron melt in the ultra-low carbon range under reduced pressure were studied, using an induction furnace with a 20 kg melt. The results obtained are as follows:
    1) The decarburization reaction proceeded to less than 5 ppm of carbon, and the decarburization rate constant, K, decreased as the carbon content decreased.
    2) As the pressure in the chamber decreased, the decarburization rate increased with a carbon content of more than 10 ppm. On the contrary, the dependency of decarburization rate on the pressure in the chamber was small with less than 10 ppm of carbon.
    3) As the influence of the sulfur content on the decarburization rate was small with a carbon content of more than 10 ppm and the decarburization rates were also not affected by the mass transfer of CO in the gas phase under the experimental conditions, it is concluded that the decarburization rate was controlled by the mass transfer of carbon in liquid iron.
    4) The fact that the decarburization rate with a carbon content of more than 10 ppm was affected by the total pressure of the chamber was explained by the change in interfacial area of the reaction between the gas and liquid phases resulting from CO boiling in the bulk melt.
  • Analysis of Thermal and Residual Stresses of a Low Alloy Cast Steel Ingot by the Use of Viscoplastic Constitutive Equations Considering Phase Transformation

    pp. 894-901

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    It was found that the applied temperature range of ANAND'S equations based on an internal variable theory, which describe the rate- and temperature-dependent constitutive relations, can be extended to lower temperatures less than a homologous temperature of 0.6 for a high carbon low alloy cast steel. Furthermore, phase transformation was introduced into these constitutive equations, thereby making it possible to represent stress-strain responses for all temperatures by a set of constitutive equations. To verify the above treatment under continuous cooling situation, including pearlitic transformation, a compression test with constant strain rate was performed. The result showed a reasonable agreement with the calculated stress-strain curve and the necessity to take into account 'transformation plasticity'. The main purpose of this paper is to develop a finite element method based on thus extended constitutive equations capable of calculating thermal stresses during an entire ingot casting process. The effects of transformation plasticity were also incorporated. The residual stresses measured at the surface of ingot have shown a reasonable agreement with the calculated values, verifying the method proposed. Although the analysis was done for this particular steel ingot, this method should be useful for analyzing other casting processes and materials.
  • Apparent Viscosity of Semi-solid Metals

    pp. 902-909

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    Cold model and stirring experiments of Al-10 mass% Cu semi-solid alloys on continuous cooling were carried out to investigate factors affecting viscosity and the relationship between apparent viscosity and stirring condition of semi-solid metals.
    In the cold model experiments, the suspension viscosity of sphere particles was lower than that of irregular shape particles, and decreased with widening of particle size distribution. These behaviors were explained well by the equation proposed by MORI-OTOTAKE which is a function of particle shape, the critical fraction solid for fluidity and the fraction solid of suspension. The measured apparent viscosity of Al-10 mass% Cu semi-solid alloys was regressed with this equation. The regression curves were well fitted with the measured values, and the coefficients of regressed equations depended on solidification rate and shear rate. The apparent viscosity increased with increasing solidification rate and decreasing shear rate. The critical fraction solid for fluidity increased with decreasing solidification rate and increasing shear rate. These results depended on the shapes of suspended particles in semi-solid metals. In case of the dendritic structures with many arms or the network structures, the viscosity was increased and the critical fraction solid was decreased. The apparent viscosity equation of Al-10mass% Cu semi-solid alloy is proposed as follows,
    ηaLa{1+2.41×105C1/3γ-4/3/2[1/fs-1/(0.72-8.82C1/3γ-1/3)]}(Pa·s).
  • Practical Application of an Advanced 12Cr Rotor to 593°C-700MW Steam Turbine Plant

    pp. 910-917

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    The largest class IP rotor forging with 1200mm in maximum diameter, for steam turbine of 700MW class to be operated at 538/593°C steam temperature, has successfully been manufactured with an advanced 12Cr rotor material which we call this alloy TMK1, through the electroslag remelting (ESR) process from 55t ingot. The quality and material characteristics of this advanced 12Cr rotor forging were evaluated for various locations of this rotor.
    In spite of a large ingot, neither chemical composition changes nor mechanical properties differences with positions in the rotor could be recognized. This is due to the ESR process and the advantages of the ESR are fully exhibited. Significant improvements in toughness and creep rupture properties were recognized in this rotor forging. Especially, the improvement of creep rupture strength in this advanced 12Cr steel is mainly attributed to dispersion strengthening by V4C3 and NbC in addition to solid-solution strengthening by Mo.
    It was confirmed that this rotor had sufficient characteristics for a large 12Cr steel rotor for 593°C steam temperature.
  • Evaluation of Creep Deformation and Rupture Life of 1.3Mn-0.5Mo-0.5Ni Steel by Modified θ Projection Concept

    pp. 918-925

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    Creep curves of 1.3Mn-0.5Mo-0.5Ni steel were analyzed using the modified θ projection concept based on the following equation:
    ε=ε0+A{1-exp(-αt)}+B{exp(αt)-1}
    where ε is the strain, t is the time, and ε0, A, B and α are the parameters determined by curve fitting. The equation could describe the measured creep curves over the whole creep stages up to 90 percent of the rupture life. The modified θ projection concept could successfully predict long term rupture lives obtained independently on the same material.
    Heat-to-heat variations in creep properties were discussed on the basis of the creep curve analysis. The parameters ε0, A and α were invariable among all the heats investigated. Only the parameter B related to the weakening process was different among the heats, indicating that the difference in the weakening parameter B was primarily responsible for the heat-to-heat variations.
  • Effect of Graphite Nodule Spacing and Temperature on Fracture Toughness in a Thick Walled Ferritic Spheroidal Graphite Cast Iron

    pp. 926-933

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    Elastic-plastic fracture toughness JIC has been determined by the R-curve method and an ultrasonic single specimen method over the range below room temperature. Samples with high nodularity and with three kinds of graphite nodule spacings were taken from a thick walled cylindrical casting. Fracture toughness specimens used were compact tension (CT) specimens with 25mm in thickness.
    Ductile-brittle transition occurred at a low temperature region. In the ductile fracture region, JIC increased with increasing graphite nodule spacing and with decreasing temperature. Estimated plain strain fracture toughness divided by yield stress was constant over the ductile fracture region. This parameter, therefore, can be used as a material constant representing the resistance to ductile fracture in the material used.
    For the behavior of JIC in regard to graphite nodule spacing and temperature, a model of the onset of crack propagation is proposed using void growth and coalescence with the precrack tip.
  • Creep Fracture Modes at High Temperature in SUS321H

    pp. 934-940

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    Creep fracture modes of the SUS321H steel have metallographically been examined over the periods up to about 15×104 h at temperatures between 525 and 900°C in order to draw a creep fracture mechanism map of a SUS321H type steel.
    The creep fracture modes have been divided to one transgranular creep fracture at shorter rupture time and three types of intergranular creep fracture ; the cracking at the surface of the specimen, the creep cavitation and the cracking at sigma/matrix interface.
    The fracture due to proceeding of surface crack was observed in the wide ranges of temperatures and stresses in SUS321H steel. As this fracture mode due to the surface cracking was not observed in other steels, this mode seems to be characteristic this steel. In this steel grain boundary carbides of M23C6 disappears gradually by change of the carbides from M23C6 to TiC within grain during creep. This disappearance of the grain boundary carbides suggests that the acceleration of the grain boundary sliding rate leads to a marked formation of the surface cracking.
    The fracture due to the creep cavitation was observed in the temperature range lower than 550°C. In these lower temperatures, the many carbides are still remains at the grain boundaries and the creep cavities occures at the interface of the grain boundary carbides and austenite matrix.
    The fractures due to cracking at the interface of sigma/matrix were observed at longer rupture times, similar to SUS304H and SUS316H steels.
  • Improvement of Mechanical Properties of Ti-15V-3Cr-3Sn-3Al TIG Weldment by High-low Two-step Aging

    pp. 941-946

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    High-low two-step aging treatment was applied to a Ti-15V-3Cr-3Sn-3Al alloy TIG weldment in order to improve the mechanical properties of it. Strength of a fusion zone and base metal is rather different after aging at a single temperature because of aging enhancement in the fusion zone. In the first high temperature aging of the two-step aging, coarse alpha particles which strengthen little and suppress streng-thening by fine alpha precipitation in following low temperature re-aging, precipitate in the fusion zone more than in base metal because of the enhancement of aging in the fusion zone. Hence, strengthening of the fusion zone in re-aging is less than in the base metal and comparable strength between the fusion zone and the base metal is obtained through the two-step aging. Bi-modal microstructure of coarse and fine alpha precipitates in beta matrix produced by this aging treatment improves the fracture toughness of the fusion zone of the weldment.
  • High Temperature Creep Properties of High Purity Chromium

    pp. 947-954

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    The high temperature creep properties of a pure chromium have been investigated by comparing with that of a gamma single phase Ni-20Cr alloy. The larger creep resistance and creep rupture strength of the pure chromium rather than those of the Ni-20Cr alloy were attributed to the decrease in the ratio of testing temperature to the melting point due to the difference of the melting point between a chromium and a nickel.
    The higher creep rupture ductility of the pure chromium than that of a Ni-20Cr resulted from the evolution of subgrain followed by dynamic recrystallization to inhibit the initiation and propagation of cracks. Above 50MPa, the increase in the stress exponent of minimum creep rate, n value, of the pure chromium with increase in the stress could be explained by the grain refining due to the evolution of subgrains followed by the dynamic recrystallization.

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