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

Tetsu-to-Hagané Vol. 90 (2004), No. 5

  • High Temperature Oxide Scale Morphology and Secondary Cooling Property of Cr Bearing Low Alloy Steel

    pp. 237-243

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    The high temperature oxidation behavior in air atmosphere and air-mist cooling property of 1 ton ingot are investigated using plain carbon steel (P.C. steel), chromium bearing steel (Cr steel) and chromium and nickel bearing steel (Cr-Ni steel) in order to clarify the effect of surface oxidation on cooling property. The results obtained are summarized as follows.
    At 1573K, Cr enriched outer scale layer and internal oxide consist of Fe2SiO4 eutectic oxide are formed on Cr steel, whereas characteristic subscale of Ni bearing steel is formed on Cr-Ni bearing steel. These characteristics disappear at lower oxidation temperature.
    At mist cooling, cooling rate, and hence heat transfer coefficient of Cr steel is significantly reduced compared with P.C. steel and Cr-Ni steel. Characteristics of scale morphology and cooling property due to chromium bearing disappear by Ni addition. Comparing with other cooling experiments, specimen dimension has significant effects on cooling properties.
    As air-mist cooling intensity is mild in this work, insulation effect of surface scale or subscale layer is insufficient to explain the cooling property. It is considered that the morphology change of cast surface is caused by the formation of chromium oxide layer at the inside of outer scale and hence prevention of FeO or Fe2SiO4 formation which clamp scale layer.
  • Surface Topography Characterization of Electro-galvanized Steel Sheets

    pp. 244-249

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    Surface morphologies of electro-galvanized steel sheets were quantitatively characterized, and the relationship between the surface morphologies and the appearances were studied. First, the surface morphologies were directly connected to the appearances by comparing the laser microscope images with the scanning electron microscope images. Consequently, the relationship between the appearance and the surface morphology was qualitatively explained. Then the surface morphologies were measured using topographic scanning electron microscope and were quantitatively characterized calculating 3D parameters from the topographic data. The surface morphologies of the electrogalvanized steel sheets consisted of the shape of the Zn deposit and the roughness of the substrate. Extracting the shape of the Zn deposit from the surface morphology data by means of high-pass filtering, 3D parameters according with the qualitative explanation were obtained.
  • Effect of Tensile Orientation on Rafting of γ' Phase in Single Crystal Ni-base Superalloy, CMSX-2

    pp. 250-256

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    Change in the shape in coarsening of the cuboidal γ' phase during creep of the single crystal nickel-base superalloy, CMSX-2, with the three different pole orientations, nearly [001], [011] and [111] to stress axes. The volume fraction of γ' precipitates was nearly equal to 70%. Creep tests were conducted at temperatures of 1173, 1273 and 1323K. At higher stresses, there were no significant differences in the time to rupture and the minimum creep rate between the [001] and [011] crystals, while, crystal with [111] orientation shows two times larger minimum creep rate. SEM observations of three planes for each single crystal were conducted as follows, for (001), (100) and (010) planes in [100] crystal, for (011), (100) and (011) in [011] crystal, and for (111), (211) and (011) in [111] crystal. Consequently, it is confirmed that the cuboidal γ' turns its shape to the platelet which is evolved along the (001) plane for three single crystals. TEM observations showed that the connection of cuboidal γ' occurred to [020] and [200] directions, and the γ' plate was formed parallel to (001) plane, irrespective to the crystal orientations. The rafting in the single crystals with [011] and [111] orientations to stress axis has not been reported. So the reason of the formation of the rafting in the single crystals with [011] and [111] orientations was discussed.
  • Effect of Strain Rate and Temperature on Nucleation of Dynamic Recrystallization at Triple Junctions in Fe-32Ni Alloy

    pp. 257-262

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    Effect of strain rate and temperature on nucleation of dynamic recrystallization (DRX) at triple junctions (TJs) in Fe-32Ni alloy was investigated in compression at the temperature from 1123 to 1323K and at strain rate from 2.0 ×10-5 to 2.0× 10-3 s-1. It was revealed that DRX grains nucleated preferentially at TJs at the strain lower than the peak strain. DRX nucleation at TJs became easier with decreasing strain rate and increasing temperature. More than 80% of DRX grains nucleated at TJs were twins. The mechanism of the preferential DRX nucleation was discussed in relation with the effect of grain boundary misorientation composing TJs and angle between loading axis and sliding grain boundary.
  • Effects of Quenching Temperatures and Chemical Compositions on Saturation Magnetization of Fe-Cr-Mn-C and Fe-Cr-Ni-Mn-C Alloys

    pp. 263-270

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    Quenching temperature dependences of saturation magnetization, J(T), were investigated in five Fe-Xmass%Cr-2.0mass%Mn-Ymass%C (X=1.97-9.92, Y=0.89-0.61) alloys and four Fe-6.0mass%Cr-Zmass%Ni-2.0mass%Mn-0.75mass%C (Z=1.04-3.02) alloys in order to elucidate the correlation between magnetic properties and microstructures as well as to develop an alloy exhibiting both ferromagnetism after heat treatment at lower temperatures and feeble magnetism after quenching from higher temperatures. The starting materials, consisting of ferromagnetic α (or α') and carbide phases, were prepared by heating at 1053K and the following slow cooling. They all exhibited high JJ values of 1.57-1.75 T. The J values of the alloys decreased by quenching from higher temperatures between 1073 and 1423K, and decreased with increasing quenching temperature. Especially, the J values began to decrease at lower quenching temperature in Fe-Cr-Mn-C alloys with lower chromium and higher carbon content, and in Fe-Cr-Ni-Mn-C alloys with higher nickel content. These behaviors were closely related to the quenching temperature dependences of the paramagnetic retained austenite content, γR(%). That is to say, the γR began to increase at lower quenching temperature in these alloys. In the lower chromium and higher carbon content alloys, the carbide particles dissolved at relatively low temperatures. Therefore, the decrease in martensite start temperature, Ms, started from the lower quenching temperature. On the other hand, in the higher nickel content alloys, the dissolution degree of the carbide particles was almost independent of the nickel content. However, the Ms decreased because of the high nickel concentration in γ phase.
  • Effect of Ti and Alloying Elements on Microstructure and Toughness of Simulated HAZ of 780 MPa Class Steels

    pp. 271-277

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    The effect of Ti on microstructure and toughness of simulated HAZ of conventional high carbon type 780 MPa class steels and low carbon bainitic steels was investigated. The effect of alloying elements on microstructure and toughness of simulated HAZ of low carbon bainitic steels was also investigated. It was found that toughness is deteriorated by Ti addition in conventional high carbon steels because of the decrease in hardenability, but in low carbon bainitic steels toughness is improved because facet of Charpy impact surface is refined without decreasing hardenability. It was also clarified that toughness of low carbon bainitic steel of which hardenability is increased by non-carbide forming alloy is superior to that increased by carbide forming alloying elements. The difference could be explained by increasing of driving force for ferrite nucleation caused by lowering transformation temperature.
  • Improvement of Discoloration Resistance of Vacuum Annealed Commercially Pure Titanium Sheets in Atmospheric Environments

    pp. 278-285

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    Titanium has been applied to roofs and walls of buildings. It is known that titanium used as architectural material discolors in atmospheric environments. This discoloration is mainly caused by interference color due to the growth of oxide film on the surface of titanium. Authors have proposed a discoloration mechanism, in which TiC begins to dissolve by acid rain, turning into an oxide, and an oxide film grows. In this paper, the formation and reduction mechanism of TiC was examined on the surface of titanium cold rolled sheets finished with vacuum annealing. An atomic bond between Ti and C in cold rolled sheets was detected with XPS, and it was found that C is unremovable by degreasing process. A large amount of TiC is formed on titanium surface vacuum annealed at lower temperature than 600°C. In this case, a discoloration index is as large as 25 after a discoloration accelerating test, in which titanium sheet is immersed in sulfuric acid solution for several days. On the other hand, when TiC is reduced with raising the vacuum annealing temperature to 650°C, C on the surface area diffuses into the inside of the titanium sheet and the surface C concentration decreases. Consequently, the discoloration index also decreases to onethird of that processed with lower annealing temperature. In addition, this modified annealing process was confirmed to be effective uniformly in the commercial large sized cold rolled coils. The discoloration level is equivalent to that of the polished surface titanium.
  • Removal of Heavy Metals in Synthetic Bottom Ash of Burnt Urban Waste by Using of Polyvinyl Chloride

    pp. 286-293

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    Synthetic bottom ash of burnt urban waste containing heavy metal oxides, CdO, Cr2O3, Cu2O, PbO and ZnO, is reacted with polyvinyl chloride, PVC. The synthetic ash is prepared from chemical reagents of heavy metal oxides, Al2O3, CaO and SiO2. It is revealed that the heavy metal oxides except Cr2O3 are removed as metal chloride from the synthetic ash by PVC. It is considered that the heavy metals in bottom ash of burnt urban waste can be removed by use of end of life PVC.

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