ISIJ International
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ONLINE ISSN: 1347-5460
PRINT ISSN: 0915-1559

ISIJ International Vol. 36 (1996), No. 2

  • Thermodynamic Properties of the MGO-BO1.5 Binary System at 1723K

    pp. 133-137

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    DOI:10.2355/isijinternational.36.133

    To determine the thermodynamic properties of the MgO-BO1.5 binary system, the following reaction was used:

    MOn(in slag)+nC(gr.)=M(in liquid copper)+nCO (g)

    where MOn represents MgO or BO1.5.
    The activity coefficients of boron and magnesium in liquid copper were first determined by equilibrating pure liquid B2O3 or pure solid MgO with liquid copper under controlled CO partial pressure in a graphite crucible at 1723K:

    ln γB=0.82+2.81xB, ln γMg=-1.22 (xB≤0.106, xMg≤0.0013)

    The themodynamic properties of MgO and BO1.5 in MgO-BO1.5 binary slag at 1723K were determined as a function of composition by equilibrating the MgO-BO1.5 slag with liquid copper under CO atmosphere. In addition, the interaction parameter (εMgB) in the Cu-B-Mg alloy was determined to be -100 at 1723K.
  • Liquid Phase Boundaries at 1873K in the Ternary CaO-Al2O3-MOx(MOx: MgO, ZrO2) and CaO-SiO2-MOx(MOx: TiO2, MgO, Al2O3) Systems

    pp. 138-142

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    DOI:10.2355/isijinternational.36.138

    The liquid phase boundaries in the CaO-Al2O3-MgO, CaO-Al2O3-ZrO2, CaO-SiO2-TiO2, CaO-SiO2-MgO and CaO-SiO2-Al2O3 ternary slags were determined at 1873K by using a slag-crucible (CaO, MgO, Al2O3 and ZrO2-11mol%CaO) equilibration technique. The equilibrium phase at crucible-slag interface was determined by a microprobe analysis.
  • Nitride Capacities in the CaO-base Ternary Slags at 1873K

    pp. 143-149

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    DOI:10.2355/isijinternational.36.143

    Nitride capacities (CN=(mass%N)·PO23/4/PN21/2) in the CaO-Al2O3-MgO(ZrO2) and CaO-SiO2-MgO(TiOx) ternary slags were measured at 1873K by a gas-slag equilibration technique, using an Mo, Al2O3 or MgO crucible under controlled partial pressures of oxygen (log(PO2/atm)=-11--14) and nitrogen (PN2=0.9 atm). The CN values increased with increasing the contents of nitride former oxides. The CN values in the CaO-AlO1.5-MOx (MOx=MgO, SiO2, ZrO2, TiOx) slags at XCaO/XAlO1.5=0.4-0.7 increased in the order of MgO<SiO2<ZrO2<TiOx and those in the CaO-SiO2-MOx (MOx=MgO, AlO1.5, TiOx) slags at XCaO/XSiO2=1.0-1.5 increased in the order of MgO<AlO1.5<TiOx at a given MOx content.
  • Estimaton of Effective Thermal Diffusivity of Porous Solid Using Data for Image Processing

    pp. 150-155

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    DOI:10.2355/isijinternational.36.150

    Many equations for the estimation of effective thermal diffusivity of porous solid were proposed previously. However, those equations do not always represent the correct effective thermal diffusivity of the porous solid, because of the complexity of the pore structure of the porous solid. Therefore, in this work, effective thermal diffusivity of porous iron was estimated by computer simulation of heat transfer in the porous iron using data of microstructure of the sample. The measurement of the effective thermal diffusivity of the sample by Laser Flash method was also carried out. The estimated results agreed well with the observed data.
  • Simultaneous Behavior of Pulverized Coal Char Combustion and Fine Iron Oxide Reduction by Injecting the Mixture of Coal Char and Iron Oxide

    pp. 156-163

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    DOI:10.2355/isijinternational.36.156

    The present study has been carried out to investigate the simultaneous behavior of combustion of 'pulverized coal char' (described as 'char' afterwards in this paper), and of reduction of fine iron ore using a laboratory scale tube furnace.
    Experimental results show that apparent combustion rate of char is not affected by the ore/char mole ratio simultaneously injected. The reaction rate constant of char was not changed with changing wustite/char injection ratio. On the other hand, the rate constant of char increased proportionally with an increase of hematite/char injection ratio. This implies that hematite behaves as an effective oxygen source for char combustion.
    The main reduction mechanism of fine wustite injected is found to be the direct reduction between molten wustite and unburnt carbon. Similarly, for the case of fine hematites, the direct reduction between molten magnetite thermally decomposed from hematite and solid carbon entrained into the droplet is the main reduction mechanism.
    It was observed that the reduction degree of fine iron oxides mainly depends on the specific combustion heat (kJ/g-ore) which is defined as the ratio of combustion heat of char to the feeding rate of fine iron oxide.
    x

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  • Carbon Monoxide Reduction and Accompanying Swelling of Iron Oxide Compacts

    pp. 164-171

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    DOI:10.2355/isijinternational.36.164

    Pure Fe2O3 compacts were fired at 1373K for one hour then reduced with CO at 973-1373K. Reduction was followed up by means of weight-loss technique whereas volume change was measured by displacement method. The structure of the fired and reduced compacts was examined by reflected light microscope while the different phases were identified by X-ray diffraction technique. These measurements together with kinetic data were correlated for better understanding the gas-solid reaction mechanisms and the accompanying swelling phenomenon. A highest swelling value of 176% was obtained for compacts completely reduced at 1173K whereas a maximum swelling of 224% was obtained for compacts 90% reduced at the same temperature. At all reduction temperatures swelling increased with the increase in reduction extent up to 90% where a maximum swelling value was obtained followed by a small decrease in swelling when compacts were completely reduced. Excessive swelling was attributed to the carbon in metallic iron and/or iron carbide formed and their reactions with oxygen forming CO and/or CO2. A mechanism of disintegration of iron grains and swelling of the compacts has been proposed and correlated with reduction conditions.
    x

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  • Microstructure and Corrosion Behaviour of Ni-P Laser Surface Alloys

    pp. 172-178

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    DOI:10.2355/isijinternational.36.172

    Two different coat thickness electroless Ni-P deposits on mild steel were laser treated at different laser scanning rates. To generate a coated surface of various square centimeters, five laser tracks were realized overlapping 50% of each other. The Ni-P coated samples treated in this way were microstructurally characterized and its corrosion behaviour on 0.1M Na2SO4 solution was determined. As a consequence of the laser interaction, a dendritic solidification microstructure was obtained for all the Ni-P coatings–initially amorphous–tested independently of its coat thickness or the laser treatment condition. The low corrosion resistance of laser treated Ni-P coatings of 45 μm coat thickness makes them not suitable for technological applications. However, laser treatment of Ni-P coated samples with the maximum coat thickness tested (180 μm) enhances its corrosion resistance in the medium studied.
  • Effect of Microstructure of Base Steel on Fe-Zn Alloy Growth during Galvanizing of an Interstitial Free Steel

    pp. 179-186

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    DOI:10.2355/isijinternational.36.179

    The effect of the microstructure of the base steel on the formation and growth behavior of intermetallics on interstitial free steel during galvanizing under various processing conditions (substrate entry temperature, Al content in galvanizing bath) was investigated from the viewpoint of the surface morphology of the intermetallics. In addition, the surface morphology of galvannealed coating was also examined.
    The dependency of Fe-Zn intermetallics growth behavior on the crystal orientation of the base steel was observed clearly in galvanizing in 0.10 mass% or less Al bath, especially at low entry temperatures. In general, ζ crystals are precipitated orderly on (111)α, whereas those are precipitated disorderly on (001)α and (101)α at the outset of the formation of Fe-Zn intermetallics, and the growth rate of Fe-Zn intermetallics on (001)α and (101)α is larger than that on (111)α.
    As an exception, the formation of Fe-Zn intermetallics on (001)α and (101)α at the outset of Fe-Zn interaction is suppressed at the entry temperature of 673K in 0.10 mass%. Al bath, although the clump of orderly precipitated ζ is formed on (111)α.
    The growth of Fe-Zn intermetallics during galvannealing also depends on crystallographic orientation of the αFe grain of the base steel, being retarded on (111)α. A large αFe grain with (111)α on the surface is thought to cause a concave portion on the galvannealed surface.
    Consequently, it is thought that the formation and growth behavior of Fe-Zn intermetallics on IF steel is affected not only by the αFe grain boundary but also by the crystallographic orientation of the αFe grain of the base steel.
  • Effects of Lattice Defect Behaviors on Pre-precipitation Stages of γ' and γ'' Phases in a Ni-base Superalloy

    pp. 187-193

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    DOI:10.2355/isijinternational.36.187

    The pre-precipitation processes of γ' and γ'' intermetallic phases and their effects on age hardening processes in an alloy 718 have been examined in relation to both solution treatment and aging temperature. The results were analyzed mainly in terms of the behaviors of quenched-in excess vacancies as well as constituent atoms of γ' and γ'' precipitates, Ti, Al and/or Nb, around as-grown dislocations. Dislocation dipoles linked to each other formed remarkably during the early stage of aging in the specimen solution-treated at higher temperature. The dislocation dipoles disappeared almost simultaneously with the initiation of γ' and/or γ'' precipitations, and eventually age hardening started. In the specimen solution-treated at the lowest temperature the most rapid age hardening occurred due to the γ' and γ'' intragranular precipitations under the adequate migration of the constituent atoms without the formation of the dislocation dipoles. The initiation of precipitation hardening in the early stage of aging was confirmed to depend greatly on the behavior of the constituent atoms associated with the excess vacancy migration around as-grown dislocations. It is concluded that the formation of dislocation dipoles suppresses the γ' and or γ'' precipitations due to the segregation of the constituent atoms to the dislocation dipoles, and the mechanisms of the segregation and dissipation of the constituent atoms around the dislocation dipoles are discussed.
  • Grain Growth Predictions in Microalloyed Steels

    pp. 194-200

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    DOI:10.2355/isijinternational.36.194

    Empirical models for grain growth predictions are briefly reviewed. It is demonstrated that such models are inadequate for quantitative prediction of austenite grain growth during reheating of as-cast microstructures in microalloyed steels. This inadequacy is mainly attributable to the inability of empirical models to account for abnormal grain growth. Basic principles of grain growth are therefore revisited in an attempt to develop a mathematical model which can account for abnormal grain growth. Such a model has been developed for Ti : N balanced microalloyed steels and is presented here. It is shown that quantitative predictions of austenite grain growth generated from this model fit well with experimental grain growth data obtained during reheating of as-cast slabs of Ti, Ti+Nb as well as Ti+Nb+Mo containing microalloyed steels.
  • Modeling of Transformation Behavior and Compositional Partitioning in TRIP Steel

    pp. 201-207

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    DOI:10.2355/isijinternational.36.201

    Transformation behavior and compositional partitioning in TRIP (Transformation Induced Plasticity) steel was investigated by means of microstructural observation and computer modeling. Studies were made on each of three stages of the continuous annealing process applied to TRIP steel. Ortho-equilibrium partitioning of alloying elements of Si and Mn was attained even in short intercritical annealing time. A transformation model, in which transformation is controlled by carbon diffusion, well described the volume fractional change of ferrite and pearlite during the cooling to austempering temperature. Slower cooling rates significantly increased carbon concentration enriched in untransformed austenite and caused pearlite transformation. Ultimate bainite volume fraction obtained by austempering increased with austempering temperature. Analysis with computer modeling revealed that transformation kinetics above 350°C followed a model based on the diffusional mechanism, while it complied with a model based on the displacive mechanism below 350°C.
  • Precipitation and Growth of γ' Phase in an Fe-38Ni-13Co-4.7Nb Superalloy

    pp. 208-214

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    DOI:10.2355/isijinternational.36.208

    An Fe-38Ni-13Co-4.7Nb base superalloy (alloy 909) is the latest low thermal expansion chromium-free superalloy with a good resistance to SAGBO (Stress Accelerated Grain Boundary Oxidation) embrittlement at elevated temperatures. This investigation is carried out to elucidate the relation between the age-hardening and the nucleation and growth behavior of γ' precipitates in alloy 909 by micro-Vickers hardness test and transmission electron microscopy. The hardness of alloy 909 measured at aging temperatures is ca. HV100 lower than that measured at room temperature. The hardness of specimen aged at 893-1033 K for durations up to 720 ks closely relates to the mean size of γ' precipitates. The growth kinetics of the γ' precipitates in ε phase free region is explained by Lifshitz-Slyozov-Wagner's theory of volume diffusion controlled growth at 943-1033 K. The activation energy for the growth of γ' precipitates is estimated to be 254 kJ/mol which is nearly equal to those of diffusion of Ti or Fe atoms in γ-iron or nickel.
  • Plastic Zones Formation under Different Types of Loading Conditions

    pp. 215-221

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    DOI:10.2355/isijinternational.36.215

    The plastic zone formation under the fracture surface in materials posessing a Body-Centered Cubic Lattice (BCC) structure (steel 20, 40, 45, St3, 15×2MΦA) and materials posessing a Fece-Centered Cubic Lattice (FCC) structure (aluminium alloy D16 and austenitic steels 40Γ18Φ, 40×4Γ18Φ, 03×13AΓ19, 07×13H4AΓ20) was studied by the X-Ray Diffraction method. Specimens were tested under a static, impact, high-speed impulse, cyclic (fatigue) and impact-cyclic loading conditions at the wide temperature range. It was shown that two plastic zones were formed under all above types of loading conditions if the plane stress state was attained at the crack tip. Only one micro-plastic zone is formed under the plane strain deformation state.
    A scheme of two plastic zones formation at the crack tip of specimens tested under a static, impact and impulse types of loading conditions is presented.
    Besides that, it was found that both impulse and impact-cyclic loadings resulted in not only plastic zones formation but work hardening of the material of specimens.
  • X-ray Diffraction Technique for Analysing Failed Components

    pp. 222-228

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    DOI:10.2355/isijinternational.36.222

    General criterions for determination of the local stress state at the crack tip of the materials under different types of loading conditions such as: i) static, ii) impact, iii) high-speed impulse, iv) impact-cyclic, v) cyclic are proposed. They are the ratio of the maximum depth plastic zone to the specimen thickness (hmax/t) and the ratio of the diffraction line width obtained from the fracture surface to the diffraction line width obtained from the unstrained metal ( β/β0). The plane strain deformation condition under static and cyclic loading conditions may be defined more precisely using by above criterions.
    X-ray techniques for determination: i) material fracture toughness of the failed construction, ii) critical fatigue crack length ls and lf, iii) stress of the loading cycle, iv) load ratio R, v) track growth speed are presented. If the fracture surface was completely damaged by corrosion or mechanical affect, X-ray techniques may be used for identification of the type of the fracture surface.
  • Comparison between Full Scale Tests and Small Scale Tests in Evaluating the Cracking Susceptibility of Line Pipe in Sour Environment

    pp. 229-234

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    DOI:10.2355/isijinternational.36.229

    Line pipes have possibility to encounter three types of crack morphology: SSC, HIBC/SWC, SOHIC (Stress Oriented HIC) in the sour environment. For modern line pipes, SOHIC is concerned under applied stress, particularly in softened heat-affected zone. In this study, an X65 UOE line pipe was evaluated as a CAPCIS type full ring test and a full pipe test were compared with tensile tests conforming to NACE standard TM0177-90 method A on the susceptibility to SOHIC in weld.
    A rectangular tensile specimen 6.4 mm in thickness provided the same result as the round bar specimen specified in the NACE standard in terms of threshold stress. Ruptured specimens showed SOHIC preferably induced in the softened HAZ. In contrast, no defect indication was obtained after the completion of the full ring test performed at the applied stress of 90% SMYS above the threshold stress. With regard to the full pipe test, no defect was observed when loaded with 90% SMYS with an exception of bead toe cracking caused by rather high applied stress due to stress concentration. The rectangular tensile test in which a single wide side was exposed to the environment was also conducted. No internal SOHIC was observed at the applied stress of 100% SMYS. And the single side exposure test with weld reinforcement exposed to the sour environment also reproduced the bead toe cracking.
    This study concludes that the NACE standard is more severe than CAPCIS type full ring test which can be substituted with the one-side exposed rectangular specimen and more severe than the full pipe test from the viewpoint of evaluating the susceptibility to SOHIC. This study proposes the single side exposure rectangular test as a realistic substitute for full scale tests.
  • Thermo-mechanical Control Process as a Tool to Grain-refine the Low Manganese Containing Steel for Sour Service Line Pipe

    pp. 235-240

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    DOI:10.2355/isijinternational.36.235

    The study confirms the effect of lowering manganese (Mn) content of sour service line pipe steels on improving the resistance to hydrogen induced blister cracking (HIBC), following which thermo-mechanical control process (TMCP) is considered as an effective tool to strengthen and toughen the low manganese steels. The HIBC is preventable when the Mn content is less than 0.4 mass% even for the steels containing an order higher sulfur than latest ordinary steels for sour service. After confirming the effect of lowering Mn, the Mn content is estimated to obtain the required strength for line pipe steels. Presupposing ordinary rolling facilities available for the TMCP, 0.5 mass% of Mn is necessary to strengthen a steel microalloyed with Nb and Ti. Applying heavy reduction (60%) and rapid cooling (80°C/s) to the 0.5Mn-Nb-Ti steel refines the microstructure and consequently improves simultaneously both yield strength and Charpy transition temperature. As a result of studying the TMCP variables to achieve such grain-refinement, necessary TMCP conditions to obtain the better strength-transition temperature balance are the rolling finish temperature higher than the Ar3 transformation temperature of the steel, the heavy reduction which accumulates equivalent strain of more than 0.9 at 850°C, and the rapid cooling rate of more than 60°C/s.
  • Microstructural Refinement by Cu Addition and Its Effect on Strengthening and Toughening of Sour Service Line Pipe Steels

    pp. 241-245

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    DOI:10.2355/isijinternational.36.241

    The Mn content of sour service line pipe steels is desirable to be low because Mn forms the elongated MnS which acts as nucleation sites of hydrogen induced blister craking, and the segregation streak which provides the propagation path of the cracking. On the other hand, Mn is one of the most effective and economical alloying elements to strengthen and toughen. This study investigates the effect of Cu as an alternative to Mn to strengthen and toughen the line pipe steels containing low Mn. The Cu addition up to 1.7 mass% simultaneously increases yield strength and decreases Charpy transition temuperature, which effect of Cu is attributable to grain-refinement. However, the Cu addition over 2.0 mass% substantially raises the transition temperature with increasing the yield strength, which adverse effect is attributable to the precipitation of during cooling after hot rolling. Analyzing the results of the isothermal holding after hot deformation and the continuous cooling after hot deformation provides the mechanisms of the grain-refinement by solute Cu as follows. The solute Cu in austenite retards the recrystallization of hot deformed austenite, which retains more non-recrystallized austenite enhancing ferrite nucleation. Moreover, the solute Cu in austenite suppresses the austenite-ferrite transformation, which decreases the transformation temperature and thereby prevents formed ferrite grains from coarsening.
  • Comparison of the Mechanical Properties and Wear Resistance between Two Oil-hardening Tool Steels

    pp. 246-254

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    DOI:10.2355/isijinternational.36.246

    The mechanical properties and wear rates of JIS SKS3 and BS BO1 oil-hardening tool steels after quenching from 790-940°C and tempering at 200°C for two hours have been investigated. At the same quenching temperature, the impact toughness and bending fracture stress of BS BO1 tool steel were superior to that of JIS SKS3 tool steel. However, besides quenching from 880°C, the tempered hardness of two oil-hardening tool steels were almost the same. The wear rate of BS BO1 tool steel was a little more faster than that of JIS SKS3 tool steel. Moreover, because of grain growth inhibition by fine vanadium carbides, the prior austenite grain size of BS BO1 tool steel was smaller than that of JIS SKS3 tool steel. The fracture surfaces of both tool steels were similar with ductile dimple fracture occurred at low quenching temperature and brittle intergranular fracture occurred at high quenching temperature. The differences of mechanical properties and wear rates between two oil-hardening tool steels were explained by their prior austenite grain sizes, microstructures, and amounts of retained austenite.

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