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  • 1.
    Alexis, Jonas
    et al.
    Swerea-Mefos.
    Jönsson, Pär
    KTH, Superseded Departments, Metallurgy.
    Jonsson, Lage
    Heating and electromagnetic stirring in a ladle furnace – a simulationmodel2000In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 40, no 11, p. 1098-1104Article in journal (Refereed)
    Abstract [en]

    A three-dimensional simulation model coupling heating and induction stirring in an ASEA-SKF ladle furnace was developed. Data of the heat transfer from the area to the steel bath were predicted in a separate model and included as boundary conditions in a ladle model. The are model considers the contributions of heat transferred by of each of the following mechanisms: radiation, convection, condensation and energy transported by electrons. Predictions were made to simulate the change of temperature distribution in the ladle during simultaneous heating with electrodes and stirring by induction. A first attempt was made to compare the predictions with measured temperatures from a 100 t ASEA-SKF ladle. The agreement was found to be fairly good when heat-flux data for a 25 cm are length were used as input to the ladle model. This indicates that the model can be used for more in-depth studies of the effects of heating for ladles that are inductively stirred.

  • 2.
    Aune, Ragnhild E.
    KTH, Superseded Departments, Metallurgy.
    Thermodynamic studies of some high temperature systems1997Doctoral thesis, comprehensive summary (Other scientific)
  • 3.
    Aune, Ragnhild, E
    et al.
    KTH, Superseded Departments, Metallurgy.
    Seetharaman, Seshadri
    KTH.
    Experimental studies of heat capacities of Nd2Fe17, Nd2Fe17N1.49 and Nd2Fe17N1.75 alloys in the temperature range 273-773 K1998In: High Temperature Materials and Processes, ISSN 0334-6455, E-ISSN 2191-0324, Vol. 17, no 4, p. 299-311Article in journal (Refereed)
  • 4.
    Aune, Ragnhild, E
    et al.
    KTH, Superseded Departments, Metallurgy.
    Sichen, Du
    KTH.
    Seetharaman, Seshadri
    Phase Relationships in the System Mn-Co-C1997In: The Japan Society of Calorimetry and Thermal Analysis - Netsu Sokutei, Vol. 24, no 4, p. 171-178Article in journal (Refereed)
  • 5.
    Aune, Ragnhild, E
    et al.
    KTH, Superseded Departments, Metallurgy.
    Sridhar, S
    Sichen, Du
    A Galvanic Cell Study Of the Ni-W-O System in the Temperature Range 1034-1317 K1994In: Journal of Chemical Thermodynamics, ISSN 0021-9614, E-ISSN 1096-3626, Vol. 26, no 5, p. 493-505Article in journal (Refereed)
    Abstract [en]

    Experimental studies of the standard molar Gibbs free energy of formation of NiWO4 have been carried out using a solid-state galvanic cell involving (zirconium oxide + calcium oxide) electrolyte in the temperature range T = 1034 K to 1317 K. The galvanic cell used can be represented as

    -Pt|{(1 - δ)Ni + δW}(f.c.c.), NiWO4(s), WO2.72(s)|(1 - x)ZrO2 + x CaO|NiO(s), Ni(s)|Pt+,

    where δ « 1. The e.m.f. against temperature plot shows a break at T ≈ 1167 K, indicating a possible phase transformation occurring in the ternary mixture. These studies were complemented by X-ray-diffraction studies of some ternary mixtures equilibrated at T = 1073 K and 1273 K. A thermodynamic assessment of (nickel + tungsten + oxygen) was carried out as part of the present work, incorporating the present results along with those available in the literature for the ternary as well as the three binaries involved. The isothermal sections at T = 1073 K and 1273 K are presented.

  • 6.
    Borgenstam, Annika
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Engstrom, A.
    Höglund, Lars
    KTH, Superseded Departments, Materials Science and Engineering.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    DICTRA, a tool for simulation of diffusional transformations in alloys2000In: Journal of phase equilibria (Print), ISSN 1054-9714, E-ISSN 1544-1032, Vol. 21, no 3, p. 269-280Article in journal (Refereed)
    Abstract [en]

    In the present paper, a general survey of the diffusion-controlled transformations (DICTRA) software is given. DICTRA is an engineering tool for diffusion simulations in multicomponent alloys. The simulations are based on multicomponent diffusion and thermodynamic data, both obtained by analyzing and assessing experimental information. This allows for many different cases to be studied as soon as the underlying data are available. DICTRA is not a complete simulation tool because only geometries that can be transformed into one space variable can be treated, but many well posed problems of practical interest may be solved. The program contains several different models, which are discussed in the present paper. Each model has its own applications and several examples from recent simulations are given in order to demonstrate the usage of the particular models.

  • 7. Du, H.
    et al.
    Somers, M. A. J.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Microstructural and compositional evolution of compound layers during gaseous nitrocarburizing2000In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 31, no 1, p. 195-211Article in journal (Refereed)
    Abstract [en]

    Compound layers developed at 848 K during gaseous nitrocarburizing of iron and iron-carbon specimens were investigated for several combinations of N and C activities imposed at the specimen surface by gas mixtures of NH3, N-2, CO2, and CO. The microstructural evolution of the compound layer was studied by light microscopy and by X-raydiffraction analysis. Composition-depth profiles were determined by electron probe (X-ray) microanalysis. Layer growth kinetics was investigated by layer thickness measurements. The influence Of the N and C activities on the microstructural and compositional evolution and the growth kinetics of the compound layers formed is discussed for the iron substrate. The results indicate that the microstructure is governed by a fast C and a slow N absorption at the surface in an early stage of gaseous nitrocarburizing. The influence of carbon in the substrate on the microstructural and compositional evolutions and on the growth kinetics was evaluated from comparing the results obtained for: a: normalized Fe-0.8C alloy with those for iron under identical nitrocarburizing conditions.

  • 8. Ekroth, M.
    et al.
    Frykholm, R.
    Lindholm, M.
    Andren, H. O.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Gradient zones in WC-Ti(C,N)-Co-based cemented carbides: Experimental study and computer simulations2000In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 48, no 9, p. 2177-2185Article in journal (Refereed)
    Abstract [en]

    The formation of surface zones with a composition gradient during sintering of WC-Ti(C,N)-Co cemented carbides has been studied experimentally and by computer simulations. The microstructure has been investigated with SEM and EPMA. The simulations are based on a solution of the multicomponent diffusion equations, coupled with thermodynamic calculations using thermodynamic descriptions of the individual phases. The results from the simulations are in good agreement with the experimental results, indicating that diffusion and the thermodynamic properties are the two major factors that control the gradient structure formation.

  • 9.
    Eriksson, Robert
    et al.
    KTH, Superseded Departments, Metallurgy.
    Jonsson, Lage
    KTH, Superseded Departments, Metallurgy.
    Jönsson, Pär G.
    KTH, Superseded Departments, Metallurgy.
    Effect of entrance nozzle design on the fluid flow in an ingot mold during filling2004In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 44, no 8, p. 1358-1365Article in journal (Refereed)
    Abstract [en]

    Earlier studies have shown that the mold powder reacts with Steel during filling of the mold during ingot casting of ball-bearing and tool steels, which results in formation of inclusions. Since all inclusions are bad for the material properties for these grades, this is a phenomenon that needs to be avoided. In this study, fundamental mathematical modeling of the filling of an ingot has been used to predict the fluid flow characteristics. A special effort has been made to model the effect of a modified inlet allowing for a larger volume flow. Predictions made by the presently used numerical model, indicates that a successive increase in the opening angle of the inlet nozzle leads to a gradual decrease in the disturbance of the free surface, during mold filling. Furthermore, the horizontal velocities are lower, which results in lower values of the Weber number which is an indication of less chances for mold powder entrapment into steel. It is concluded that both these improvements leads to decreased possibilities for the creation of inclusions in the steel due to interaction with the mold powder.

  • 10.
    Eriksson, Robert
    et al.
    KTH, Superseded Departments, Metallurgy.
    Jönsson, Pär
    KTH, Superseded Departments, Materials Science and Engineering.
    Gustafsson, A.
    Determination of inclusion characteristics in 'low-carbon' steel during up-hill teeming2004In: Scandinavian journal of metallurgy, ISSN 0371-0459, E-ISSN 1600-0692, Vol. 33, no 3, p. 160-171Article in journal (Refereed)
    Abstract [en]

    Clean steel production requires good control of the size distribution and the composition of the inclusions at all stages of the steel making process. So far, there exists very little information on the behavior of the inclusion population during ingot casting of steel. Therefore, this study focus on providing information on the inclusion population before and during up-hill teeming of 'low-carbon' steel, based on plant trails carried out at Ovako Steel's facilities in Hofors, Sweden. Liquid steel samples were acquired from the casting ladle just before the start of the filling of the ingot molds and from the rising steel in the molds during the filling of the molds. Thereafter, the inclusion compositions for the different size classes were determined using a scanning electron microscope equipped with energy dispersive X-ray. It was found that the inclusion compositions were more complex in the samples taken in the mold in comparison to the ones taken in the ladle. In some cases large exogenous inclusions most likely formed due to a reaction with the mold flux were found. When a mold was filled without mold flux reoxidation of the steel due to reaction with the atmosphere took place. The inclusion size distributions were also determined using light optical microscopy and classified according to the Swedish standard SS111116 (JK chart II). The number of inclusions found in the sample taken in the mold were higher than in the sample taken in the ladle. The general conclusions are that more in-depth research is necessary to understand the reactions taking place during filling of an ingot.

  • 11.
    Eriksson, Robert
    et al.
    KTH, Superseded Departments, Metallurgy.
    Seetharaman, Seshadri
    KTH, Superseded Departments, Materials Science and Engineering.
    Thermal diffusivity measurements of some synthetic CaO-Al2O3-SiO2 slags2004In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 35, no 3, p. 461-469Article in journal (Refereed)
    Abstract [en]

    In an attempt to systematize the knowledge of the heat conduction of liquid silicates, the effective thermal diffusivities of some synthetic slags containing CaO, Al2O3, and SiO2 have been measured, using the three-layer laser-flash method on a differential scheme in the temperature range of 1625 to 1825 K. The effective thermal diffusivities measured, which are a combination of the phononic and photonic heat-transfer mechanisms, were found to increase with increasing temperature for all the presently investigated slags. The slag compositions were chosen in such a way that the changes in the effective thermal diffusivities would reflect the changes in the structure of the slags. It was observed that, at a CaO/Al2O3 molar ratio of 4.42, an increase of the SiO2 content had very little effect on the effective thermal diffusivity values. On the other hand, addition of SiO2 to a slag with the CaO/Al2O3 molar ratio of 2.59 resulted in a significant increase in the effective thermal diffusivity. The addition of Al2O3 to slags with a constant CaO/SiO2 molar ratio resulted in a marked increase in the effective thermal diffusivity. Both these trends indicate that there might be an influence of the network formation in silicate melts on the effective thermal diffusivity.

  • 12. Erneman, J.
    et al.
    Schwind, Martin
    KTH, Superseded Departments, Materials Science and Engineering.
    Liu, P.
    Nilsson, J. O.
    Andren, H. O.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Precipitation reactions caused by nitrogen uptake during service at high temperatures of a niobium stabilised austenitic steel2004In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 52, no 14, p. 4337-4350Article in journal (Refereed)
    Abstract [en]

    Precipitation phenomena in type 347 austenitic stainless steels have been investigated after long-term heat treatment and creep in air at 700 and 800 degreesC. Nitrogen uptake was observed during long-term creep testing at 800 degreesC. No such effect was observed at 700 degreesC although times up to about 70,000 h were used. The major phases precipitated after long time exposure at 800 degreesC were primary Nb(C,N), Z-phase, Cr2N and M23C6, while primary Nb(C,N), secondary Nb(C,N) and sigma-phase were the major phases at 700 degreesC. Z-phase precipitated in both intragranular and intergranular form at 800 degreesC. Large precipitates exhibiting a core/rim structure showed a rim of Z-phase surrounding undissolved primary Nb(C,N). The microstructural evolution during creep deformation in air at 800 degreesC was modelled thermodynamically. The model satisfactorily predicts nitrogen uptake and the essential features of the evolution of the microstructure with time. The precipitation sequence could be qualitatively described, although it was not possible to model the formation of all precipitates.

  • 13.
    Fakhrai, Reza
    KTH, Superseded Departments, Metallurgy.
    Modelling of carry-over in recovery furnaces1999Licentiate thesis, comprehensive summary (Other scientific)
  • 14.
    Fakhrai, Reza
    et al.
    KTH, Superseded Departments, Metallurgy. KTH, Superseded Departments, Materials Science and Engineering.
    Blasiak, Wlodzimierz
    KTH, Superseded Departments, Materials Science and Engineering.
    Combustion Performance of the Kraft Recovery Boiler Versus Black Liquor PropertiesIn: Energy Conversion and Management, ISSN 0196-8904, E-ISSN 1879-2227Article in journal (Other academic)
  • 15.
    Fakhrai, Reza
    et al.
    KTH, Superseded Departments, Metallurgy.
    Blasiak, Wlodzimierz
    KTH, Superseded Departments, Metallurgy.
    Numerical Simulation of Black Liquor Combustion in Kraft Recovery Boiler1998In: Exposé över Förbränningsforskning i Sverige, 1998Conference paper (Other academic)
  • 16.
    Fakhrai, Reza
    et al.
    KTH, Superseded Departments, Metallurgy.
    Blasiak, Wlodzimierz
    Theoretical Analysis of Interaction Between Fuel Drop and Walls during Black Liquor Combustion in a Kraft Recovery Furnace2001In: Energy Conversion and Management, ISSN 0196-8904, E-ISSN 1879-2227Article in journal (Other academic)
  • 17.
    Fredriksson, Patrik
    KTH, Superseded Departments, Metallurgy.
    Differential thermal analysis of the iron-platinum system2004In: Scandinavian journal of metallurgy, ISSN 0371-0459, E-ISSN 1600-0692, Vol. 33, no 3, p. 183-188Article in journal (Refereed)
    Abstract [en]

    In the present work, a phase diagram investigation of binary Fe-Pt alloys was carried out by means of the differential thermal analysis method. 12 different alloys were investigated with a NETZSCH STA 449C Jupiter((C))DCS unit in the temperature range of 300-1673 K and 3 alloys with a Setaram HTC 1800 K-DSC 2000 K calorimeter in the temperature range of 3001988 K, respectively. The measurements were carried out in Ar as well as H-2 atmospheres. The results obtained show a good agreement with previous investigations and bring new information for order/disorder phase transitions for FePt and FePt3 alloys, respectively, in the temperature range 1420-1610 K. Use of He instead of H-2 in some of the trials showed that the impact of hydrogen on the measurements was insignificant.

  • 18. Frykholm, R.
    et al.
    Ekroth, M.
    Jansson, B.
    Andren, H. O.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Effect of cubic phase composition on gradient zone formation in cemented carbides2001In: International Journal of Refractory Metals & Hard Materials, ISSN 0263-4368, Vol. 19, no 06-apr, p. 527-538Article in journal (Refereed)
    Abstract [en]

    Cemented carbide cutting tool inserts with a surface zone depleted of hard cubic carbides and enriched in ductile binder phase have been studied experimentally using scanning electron microscopy (SEM), electron probe micro analysis (EPMA) and transmission electron microscopy (TEM). The results are compared with simulations based on a solution of the multicomponent diffusion equations, coupled with calculations using thermodynamic descriptions of the individual phases. The materials in the study are based on WC-Ti(C,N)-Co, WC-Ti(C,N)-NbC-Co and WC-Ti(C,N)-TaC-Co. The surface zone is formed by creating a gradient in nitrogen activity in the material, leading to an outward diffusion of N. Due to thermodynamical coupling between N and Ti, the outward diffusion of N will lead to an inward diffusion of Ti, and a surface zone depleted in cubic carbides is formed. Additions of elements like Ta or Nb will affect the width of the surface zone. A material with a Ta-containing cubic phase will have a narrower surface zone than a material with a Nb-containing cubic phase. Ta or Nb additions also affect the distribution of the different phases in and adjacent to the surface zone.

  • 19. Frykholm, R.
    et al.
    Ekroth, M.
    Jansson, B.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Andren, H. O.
    A new labyrinth factor for modelling the effect of binder volume fraction on gradient sintering of cemented carbides2003In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 51, no 4, p. 1115-1121Article in journal (Refereed)
    Abstract [en]

    To increase the cutting performance of WC-MC-Co cemented carbide tools, it is common to use a high temperature CVD process to coat them with thin wear resistant layers. During the process cracks are unavoidably introduced in the coating. To prevent crack propagation it is of interest to create a tough surface zone in the substrate, enriched in WC and binder phase. A way to create such a zone is to sinter a nitrogen-containing cemented carbide in a nitrogen free atmosphere. This formation of gradient structures has been extensively studied using microscopy and simulations, and it has been shown that the process is driven by diffusion in the binder phase. However, the diffusion paths are partly blocked by the dispersed particles. This effect can be formally handled by considering effective diffusivities by introducing a so-called labyrinth factor, lambda. In prior work it has been assumed that lambda =f(2) where f is the volume fraction of the binder. The validity of this assumption has been studied by simulations and experimental analysis of gradient sintered WC-Ti(C,N)-Co cemented carbides containing 5.0, 6.7, 10.0 and 20.0 vol% binder phase. It was found that by using the labyrinth factor lambda = f(2) instead of a better correspondence between experiments and simulations can be achieved.

  • 20. Gustafson, A.
    et al.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Possible effect of Co on coarsening of M23C6 carbide and Orowan stress in a 9% Cr steel2001In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 41, no 4, p. 356-360Article in journal (Refereed)
    Abstract [en]

    it has long been known that a cobalt addition increases the resistance to tempering in steels. This may be due to the fact that Co raises the Curie-temperature which retards diffusion. In the present work the effect of Co on coarsening of M23C6 in the 9% Cr steel P92 is studied by computer simulations. The results show that a final average radius of the carbides after 30 000 h at 600 degreesC decreases with 30 % with a Co addition of 10 mass %. This raises the Orowan stress with 30 %. Moreover, ii is assumed that slower particle coarsening also leads to a retarded coarsening of the martensite lath structure.

  • 21.
    Göthelid, Mats
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Haglund, S.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Influence of O and Co on the early stages of sintering of Wc-Co: A surface study by AES and STM2000In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 48, no 17, p. 4357-4362Article in journal (Refereed)
    Abstract [en]

    The influence of oxygen on the sintering behavior of WC-Co has been investigated by Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). Deposition of Co on the WC(0001) surface and subsequent annealing at 650 degreesC results in a 2x2 reconstructed pre-cursor layer on top of which Co grows in weakly bound islands which can be moved on the surface by the STM rip. Annealing at 850 degreesC removes excess Co and leaves only the 2x2 surface. Oxygen exposure of the 2x2 surface results in a clustered cobalt oxide overlayer which on annealing at 750 degreesC breaks up and restores the 2x2 structure as the metallic Co wets the surface.

  • 22.
    Hallberg, Malin
    et al.
    KTH, Superseded Departments, Metallurgy.
    Jonsson, Lage T. I.
    KTH, Superseded Departments, Metallurgy.
    Jönsson, Pär G.
    KTH, Superseded Departments, Metallurgy.
    A new approach to using modelling for on-line prediction of sulphur and hydrogen removal duving ladle refining2004In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 44, no 8, p. 1318-1327Article in journal (Refereed)
    Abstract [en]

    A simplified model has been developed for on-line determination of sulphur and hydrogen contents in the steel during vacuum degassing in an ASEA-SKF ladle furnace at Ovako Steel in Hofors, Sweden. The simplified model was developed based on results from fundamental mathematical model simulations of hydrogen and sulphur refining for a number of cases representing normal production situations. More specifically, mass-transfer coefficients were determined from the simulations and thereafter used to develop separate simplified models for sulphur and hydrogen refining. Predictions from using the simplified process models agreed well with sulphur and hydrogen data from full-scale plant trials. It was therefore concluded that the main purpose of the study, namely to achieve a less time-consuming model suitable for production applications, was fulfilled. The final part of the paper presents how the simplified models can provide engineers or operators with off-line or on-line guidelines on performing the vacuum degassing operation such that quality requirements regarding sulphur and hydrogen contents in the steel product are met.

  • 23.
    Hallström, Samuel
    KTH, Superseded Departments, Metallurgy.
    Thermodynamic assessment of the Ni-Ru system2004In: Journal of phase equilibria (Print), ISSN 1054-9714, E-ISSN 1544-1032, Vol. 25, no 3, p. 252-254Article in journal (Refereed)
    Abstract [en]

    The Ni-Ru system is assessed using the CALPHAD technique. The only available experimental information for this system concerns phase equilibria, whereas no thermochemical information has been found. The THERMO-CALC software is applied to obtain Gibbs energy (G) functions for the individual phases. The phase diagram calculated from the new G functions is in excellent agreement with the experimental diagram, and all the thermodynamic parameters are given in the Appendix. In addition, tentative activity diagrams and the metastable miscibility gaps in fcc and hcp phases are calculated from the new thermodynamic description.

  • 24. Hillert, M.
    et al.
    Odqvist, J.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Comparison between solute drag and dissipation of Gibbs energy by diffusion2001In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 45, no 2, p. 221-227Article in journal (Refereed)
    Abstract [en]

    The solute drag and the dissipation of Gibbs energy in a partitionless phase transformation are compared numerically. They agree if the solute drag is compared with the driving force over the interface and the solute spike in front of the interface. The dissipation can be evaluated over this whole region or over the interface alone.

  • 25. Hillert, M.
    et al.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Effect of surface free energy and surface stress on phase equilibria2002In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 50, no 9, p. 2429-2441Article in journal (Refereed)
    Abstract [en]

    Due to a call for modification of the graphical method of describing phase equilibria for systems with a second-phase inclusion using a common tangent, as previously presented, the thermodynamic basis for such diagrams has now been examined in detail but only for isotropic surface proper-ties. For an incompressible second phase with composition independent surface properties there seems to be no need of modification. For compressible liquids and solids under constant pressure the common tangent construction can still be used but the evaluation of how much the Gibbs energy curve for the second phase should be lifted is more complicated. For spherical inclusions of compressible solids there will be an additional factor containing the compressibility. The equilibrium condition is modified for all cases if the surface properties change with composition and that will cause severe complications for the common tangent construction. There may be a particular composition dependence of the specific surface free energy for an interstitial phase but it can be treated as any other phase. Although the partial volumes appear in the equilibrium conditions for each component, they play only a minor role for the two-phase equilibrium in binary systems. An exception is the equilibrium between a particle and its own vapor.

  • 26.
    Hillert, Mats
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Höglund, Lars
    KTH, Superseded Departments, Materials Science and Engineering.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Diffusion-controlled lengthening of Widmanstatten plates2003In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 51, no 7, p. 2089-2095Article in journal (Refereed)
    Abstract [en]

    Various treatments of the diffusion-controlled lengthening of plates during precipitation from a supersaturated solid solution are compared numerically. Serious doubts are raised about the validity of Trivedi's treatment which has been generally accepted. A recent treatment based directly on Ivantsov's solution gives results in between those of Trivedi's equation and Zener's equation, as modified by Hillert.

  • 27.
    Hillert, Mats
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Höglund, Lars
    KTH, Superseded Departments, Materials Science and Engineering.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Role of carbon and alloying elements in the formation of bainitic ferrite2004In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 35A, no 12, p. 3693-3700Article in journal (Refereed)
    Abstract [en]

    One approach to the prediction of the carbon content of austenite, remaining after the precipitation of bainitic ferrite, is based on the assumption that bainitic ferrite during growth inherits the carbon content of the parent austenite. An alternative approach is based on the assumption that bainitic ferrite grows with a low carbon content and there is no major difference between Widmanstatten ferrite and bainitic ferrite. The two approaches are now compared using information from alloyed steels with considerable amounts of Si, where the formation of cementite is retarded. The former approach does not account for the effect of Mn and fails severely at low alloy contents. The latter approach seems more promising but is not without difficulties. In particular, in order to explain the effects of Cr and Mo, it seems necessary to introduce a kinetic effect, presumably caused by solute drag.

  • 28.
    Hillert, Mats
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Discussion on local equilibrium at solid/liquid interfaces during melting2002In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 46, no 6, p. 455-457Article in journal (Refereed)
    Abstract [en]

    Melting experiments were recently used to support the assumption of considerable deviation from local equilibrium. It is now claimed that the observed velocities of melting were low enough to allow diffusion in the solid to give a well developed spike of the solute in front of the migrating interface making a strong deviation from local equilibrium unlikely.

  • 29.
    Hillert, Mats
    et al.
    KTH, Superseded Departments, Metallurgy.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    On the definitions of paraequilibrium and orthoequilibrium2004In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 50, no 5, p. 697-699Article in journal (Refereed)
    Abstract [en]

    Hultgren's terminology of paraferrite and paracementite in alloy steels and his definition of paraequilibrium are reviewed. They are not completely compatible due to the possibility of forming ferrite with the initial alloy content (paraferrite) even under full local equilibrium (local orthoequilibrium). That has caused some confusion.

  • 30.
    Höglund, Lars
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Analysis of the Kirkendall effect, marker migration and pore formation2001In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 49, no 8, p. 1311-1317Article in journal (Refereed)
    Abstract [en]

    Some aspects of Kirkendall and lattice plane migration are studied by means of numerical simulations. The simulations are performed using individual mobilities assessed in this work and thermodynamic data available in literature. The simulations show that the position and stability of the Kirkendall plane as well as a number of other interesting features can be predicted using numerical simulation software in combination with assessed thermodynamic and kinetic data.

  • 31.
    Jonsson, Lage
    KTH, Superseded Departments, Metallurgy.
    Mathematical modeling of selected ladle operations1998Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    The traditional approach to modeling ladle processes involvea critical survey of the thermodynamic aspects of the processincluding mass and heat balantes at the macroscopic level. Onthe other hand, the macrokinetics, specific to the reactor haveoften been studied separately, using computational fluiddynamits. Over the years, it has been realized that traditionalmethods have obvious limitations in providing an understandingof heterogeneous processes. In a long range perspective, acomplete description of ladle metallurgy and a fundamentalunderstarrding of the underlying process phenomena require aconsideration of all the individual reactions involved andtheir interrelations. This, in tum, clearly requiressimultaneous solution of thermodynamics, heat and fluid flow.The salient features of the present study are the considerationof fundamental transport equations and establishedthermodynamic relationships so that mathematital models takinginto account both thermodynamics and fluid flow could beachieved.

    A technique to' incorporate the slag phase in two-phasefluid-flow models for ladle treatment is introduced in thisthesis. This development enabled, for the first time,predictions of fluid flow conditions in the important steelklagregion. Good agreement is shown between predictions andphysical modeling reported in literature, regarding thecriteria for initiation of entrapment of a lighter phase into aheavier phase.

    Furthermore, a method to combine fluid dynamits andthermodynamics to model slag/metal reactions is introduced andis demonstrated in the case of sulphur refining during gasstirring. By considering the volume of mixing between slag andmetal and the thermodynamic equilibrium in the two-phase zone,the calculations of interfacial area and the mass-transfercoeffcients for different elements are avoided. In this way,the combined effect of fluid-flow and thermodynamic conditionscould be predicted from fundamental transport equations.Comparison between predicted data and plant data demonstratesthat the method could be used to predict the desulphurizationwith an acceptable accuracy.

    A new method to model ladle processes involving gas-liquidreactions is also introduced in this thesis. In the cases wherethe interfacial reaction and mass transfer in the gas-phase arerapid, the overall mass transfer resistance could be expressedby the mass-transfer in the melt. The hydrogen refinementprocess during vacuum degassing in an ASEA-SKE ladle furnacehas been taken as an example. The predictions have beencompared with data from production scale experiments carriedout at Ovako Steel. Good agreement between the calculatedhydrogen toncentrations and the hydrogen tontent in the sampletaken after vacuum degassing substantiates the feasibility ofthis technique.

    Key words:Slag-Metal Reactions, Metal-Gas Reactions,Ladle Metallurgy, Sulphur Refining, Hydrogen Refining, VacuumDegassing, Combined Kinetics and Thermodynamics, 3-Phase Model,Gas Stirring, Modeling, CAS-OB, CFD

  • 32.
    Jonsson, Lage
    et al.
    KTH, Superseded Departments, Metallurgy.
    Grip, Carl-Erik
    Johansson, Arne
    Jönsson, Pär
    Three-phase (steel, slag, gas) modeling of the CAS-OB process1997Conference paper (Other academic)
  • 33.
    Jonsson, Lage
    et al.
    KTH, Superseded Departments, Metallurgy.
    Jönsson, Pär
    Modeling of the fluid flow conditions around the slag/metal interface in a gas-stirred ladle1996In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 36, no 9, p. 1127-1134Article in journal (Refereed)
    Abstract [en]

    A three-phase model of a gas-stirred steel bath covered with a slag layer has been developed. Predicted steel surface velocities have been shown to be in at least five times greater agreement with experimental data compared to predicted velocities from a model which excludes the effect of a slag phase. It has also been shown that it is possible to predict the amount of slag dispersed into the steel.

  • 34.
    Jonsson, Lage
    et al.
    KTH, Superseded Departments, Applied Process Metallurgy.
    Jönsson, Pär
    KTH, Superseded Departments, Applied Process Metallurgy.
    Seetharaman, Seshadri
    KTH, Superseded Departments, Metallurgy.
    Sichen, Du
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Micro-Modelling.
    Process Models for Ladle Refining - A FundamentalApproach2000In: 6th Japan-Nordic Countries Joint Symposium, Nagoya, 2000, p. 77-84Conference paper (Refereed)
  • 35.
    Jonsson, Lage
    et al.
    KTH, Superseded Departments, Metallurgy.
    Jönsson, Pär
    Sichen, Du
    Prediction and experimental verification of hydrogen refining parameters during vacuum degrassing in a  ladle furnace: Part 1 - theoretical model1998In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460Article in journal (Other academic)
  • 36.
    Jonsson, Lage
    et al.
    KTH, Superseded Departments, Metallurgy.
    Jönsson, Pär
    Sichen, Du
    Prediction and experimental verification of hydrogen refining parameters during vacuum degrassing in a  ladle furnace: Part 2 - Plant trials and verifications1998In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460Article in journal (Other academic)
  • 37.
    Jönsson, Pär
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    Guozhu, Ye
    Jonsson, Lage
    KTH, Superseded Departments, Metallurgy.
    A model of oxygen flow from slag to steel in the ladle1996In: Proceedings of the International conference MSMM, 1996, p. 309-313Conference paper (Other academic)
  • 38.
    Jönsson, Pär
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    Jonsson, Lage
    KTH, Superseded Departments, Metallurgy.
    Sichen, Du
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Micro-Modelling.
    Viscosities of ld slags and their impact on ladle refining1997In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 37, no 5, p. 484-491Article in journal (Other academic)
  • 39. Kaptay, G.
    et al.
    Matsushita, Taishi
    KTH, Superseded Departments, Metallurgy.
    Mukai, K.
    Ohuchi, T.
    On different modifications of the capillary model of penetration of inert liquid metals into porous refractories and their connection to the pore size distribution of the refractories2004In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 35, no 3, p. 471-486Article in journal (Refereed)
    Abstract [en]

    Different modifications to the classical capillary model of penetration of liquid metals into porous refractories are presented; (1) with capillaries having different radii, (2) with zigzag capillaries, and (3) with capillaries, having periodically changing capillary radius along the path of penetration. All the modified capillary models were checked against our experimental results of measuring the penetration of liquid mercury into three types of alumina refractories, having different microstructure and pore size distribution. The maximum penetration height was measured by X-ray radiography, as a function of applied outside pressure. The model with periodically changing capillary has been found to describe the experimental data satisfactorily. This model divides the process of penetration into two stages. During the first period of pre-penetration, the maximum penetration height changes very slowly (but not linearly) as the outside pressure is increased in the interval between the minimum threshold pressure and the maximum threshold pressure. In the second, bulk penetration period, appearing above the maximum threshold pressure, the maximum height of penetration increases rapidly with outside pressure, according to the classical capillary model of penetration. The three structural model parameters of the model (minimum pore radius, maximum pore radius, and period of pore structure) were connected with the measured pore size distribution curves of the refractories through semiempirical equations. As a result, our complex semiempirical model is able to predict penetration diagrams for any inert liquid metal into any refractory of a similar type.

  • 40.
    Larsson, Henrik
    et al.
    KTH, Superseded Departments, Metallurgy.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Gas nitriding of high vanadium steels: Experiments and simulations2004In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 35A, no 9, p. 2799-2802Article in journal (Refereed)
    Abstract [en]

    Four experimental high vanadium alloys were gas nitrided in an ammonia-nitrogen atmosphere. The microstructure and concentration gradients have been investigated by means of several techniques. The nitriding process has been tentatively simulated using the DICTRA software. A precise process simulation does not seem possible at present; the reason for this is discussed. Instead, bounds for the carbon and nitrogen concentration profiles were obtained by applying different simulation conditions.

  • 41. Loginova, I.
    et al.
    Amberg, Gustav
    KTH, Superseded Departments, Mechanics.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Phase-field simulations of non-isothermal binary alloy solidification2001In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 49, no 4, p. 573-581Article in journal (Refereed)
    Abstract [en]

    A phase-field method for two-dimensional simulations of binary alloy solidification is studied. Phase-field equations that involve both temperature and solute redistribution are formulated. The equations are solved using the finite element method with triangular elements on unstructured meshes, which are adapted to the solution. Dendritic growth into a supersaturated melt is simulated for two temperature regimes: (a) the temperature is prescribed on the boundary of the computational domain; and (b) the heat is extracted through the domain boundary at a constant rate. In the former regime the solute redistribution is compared with the one given by an isothermal model. In the latter case the influence of the size of the computational domain and of the heat extraction rate on dendritic structure is investigated. It is shown that at high cooling rates the supersaturation is replaced by thermal undercooling as the driving force for growth.

  • 42. Loginova, I.
    et al.
    Odqvist, J.
    Amberg, Gustav
    KTH, Superseded Departments, Mechanics.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    The phase-field approach and solute drag modeling of the transition to massive gamma ->alpha transformation in binary Fe-C alloys2003In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 51, no 5, p. 1327-1339Article in journal (Refereed)
    Abstract [en]

    The transition between diffusion controlled and massive transformation gamma --> alpha in Fe-C alloys is investigated by means of phase-field simulations and thermodynamic functions assessed by the Calphad technique as well as diffusional mobilities available in the literature. A gradual variation in properties over an incoherent interface, having a thickness around 1 nm, is assumed. The phase-field simulations are compared with a newly developed technique to model solute drag during phase transformations. Both approaches show qualitatively the same behavior and predict a transition to a massive transformation at a critical temperature below the T-0 line and close to the alpha/alpha + gamma phase boundary. It is concluded that the quantitative difference between the two predictions stems from different assumptions on how the properties vary across the phase interface yielding a lower dissipation of Gibbs energy by diffusion in the phase-field simulations. The need for more detailed information about the actual variation in interfacial properties is emphasized.

  • 43.
    Loginova, I.
    et al.
    KTH, Superseded Departments, Mechanics.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Arnberg, G.
    On the formation of Widmanstdtten ferrite in binary Fe-C - phase-field approach2004In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 52, no 13, p. 4055-4063Article in journal (Refereed)
    Abstract [en]

    A phase-field method, based on a Gibbs energy functional, is formulated for gamma --> alpha transformation in Fe-C. The derived phase-field model reproduces the following important types of phase transitions: from C diffusion controlled growth through Widmanstatten microstructures to massive growth without partitioning of C. Applying thermodynamic functions assessed by the Calphad technique and diffusional mobilities available in the literature, we study two-dimensional growth of ferrite side plates emanating from an austenite grain boundary. The morphology of the ferrite precipitates is defined by a highly anisotropic interfacial energy. As large values of anisotropy lead to an ill-posed phase-field equation we present a regularization method capable of circumvent non-differentiable domains of interfacial energy.

  • 44.
    Lucas, Carlos
    et al.
    KTH, Superseded Departments, Metallurgy.
    Szewczyk, Dariusz
    KTH, Superseded Departments, Metallurgy.
    Blasiak, Wlodzimierz
    KTH, Superseded Departments, Metallurgy.
    High Temperature Air And Steam Gasification Of Wood Pellets2002Report (Other academic)
  • 45.
    Lucas, Carlos
    et al.
    KTH, Superseded Departments, Metallurgy.
    Szewczyk, Dariusz
    KTH, Superseded Departments, Metallurgy.
    Blasiak, Wlodzimierz
    KTH, Superseded Departments, Metallurgy.
    Mochida, S
    High-temperature air and steam gasification of densified biofuels2004In: BIOMASS BIOENERG, 2004, Vol. 27, no 6, p. 563-575Conference paper (Refereed)
    Abstract [en]

    An experimental study was carried out to investigate gasification of densified biofuels using highly preheated air and steam as a gasifying agent. Preheat of air and steam is realised by means of the newly developed high-cycle regenerative air/steam preheater. Use of highly preheated feed gas provides additional energy into the gasification process, which enhances the thermal decomposition of the gasified solids. For the same type of feedstock the operating parameters, temperature, composition and amount of gasifying agent, were varied over a wide range. Results of experiments conducted in. a high-temperature air/steam fixed bed updraft gasifier show the capability of this technology of maximising the gaseous product yield as a result of the high heating rates involved, and the efficient tar reduction. Increase of the feed gas temperature reduces production of tars, soot and char residue as well as increases heating value of the dry fuel gas produced. Overall, it has been seen that the yield and the lower heating value of the dry fuel gas increase with increasing temperature.

  • 46.
    Nordquist, A.
    et al.
    KTH, Superseded Departments, Metallurgy.
    Tilliander, Anders
    KTH, Superseded Departments, Metallurgy.
    Jönsson, Pär
    KTH, Superseded Departments, Metallurgy.
    A laboratory study of reactions in the hot spot area and in droplets during top lance blowing2004Conference paper (Refereed)
    Abstract [en]

    The reactions in the hot spot area and in droplets during top lance blowing in a laboratory induction furnace were investigated. The experiments were performed in alumina crucible, with an inner diameter of 150mm and a height of 300mm. In hot spot sampling, the lance was lowered with a flow of inert gas through it to detect where the hot spot was going to be situated. In metal droplet sampling, the droplets were captured with an alumina cup that had a quartz tube as grip. A difference in size, appearance and shape were noted between droplets from melts with and without silicon addition.

  • 47. Odqvist, J.
    et al.
    Hillert, Mats
    KTH, Superseded Departments, Materials Science and Engineering.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Effect of alloying elements on the gamma to alpha transformation in steel. I2002In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 50, no 12, p. 3211-3225Article in journal (Refereed)
    Abstract [en]

    A newly constructed computer program was used to simulate partitionless growth of alpha in gamma of Fe-Ni-C alloys, taking into account so-called solute drag by evaluating the dissipation of Gibbs energy due to diffusion inside the interface and in the nickel spike being pushed in front of it. It could be shown how the conditions at the alpha/gamma interface vary with the velocity. A continuous change from paraequilibrium to quasi-paraconditions could be illustrated in the phase diagram. By combination with an approximate analytical growth equation, it was possible to derive the thickness of a as function of time. The growth velocity was assumed to start at very high values but decreased due to the pile-up of carbon. For alloy compositions outside the limit for quasi-paraconditions and just inside it the growth was predicted to stop suddenly when critical conditions are approached during the reaction. For alloy compositions further inside the limit, there was no such stop, except by the action of impingement of diffusion fields for carbon. Somewhere between the lines for paraequilibrium and quasi-paraconditions there is a rather rapid drop of the final thickness of a. In order to explain experimental information on this drop one must accept an appreciable tendency of segregation of nickel to the alpha/gamma interface.

  • 48. Odqvist, J.
    et al.
    Sundman, Bo
    KTH, Superseded Departments, Materials Science and Engineering.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    A general method for calculating deviation from local equilibrium at phase interfaces2003In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 51, no 4, p. 1035-1043Article in journal (Refereed)
    Abstract [en]

    A general method to calculate the deviation from local equilibrium at phase interfaces in multicomponent systems is suggested. The deviation is caused by solute drag and finite interfacial mobility. In the limit of low transformation rates the new method degenerates to the well-known local equilibrium condition. The phase interface is divided into three zones, each with a finite thickness. In each zone a variation in thermodynamic properties and diffusional mobilities is assumed.

  • 49.
    Petersson, Anders
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Constitutive behaviour of WC-Co materials with different grain size sintered under load2004In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 52, no 7, p. 1847-1858Article in journal (Refereed)
    Abstract [en]

    The deformation during sintering under an applied uniaxial load was investigated experimentally for two WC-Co cemented carbides with different grain size. The lower applied loads changed only the relative magnitude of axial and radial shrinkages, whereas the higher loads also affected the volumetric strain. Local maxima in shrinkage rate were found at relative densities close to 0.8 for all sintering conditions. The constitutive parameters uniaxial viscosity, Poisson's ratio and sintering stress were empirically modelled as functions of temperature and relative density.

  • 50. Sahajwalla, V.
    et al.
    Hilding, T.
    Von Oelreich, Anne
    KTH, Superseded Departments, Metallurgy.
    Gupta, S. K.
    Björkman, B.
    Wikström, J. -O
    Fredriksson, Patrik
    KTH, Superseded Departments, Metallurgy.
    Seetharaman, Seshadri
    KTH, Superseded Departments, Metallurgy.
    Structure and alkali content of coke in an experimental blast furnace and their gasification reaction2004Conference paper (Refereed)
    Abstract [en]

    Coke samples excavated from LKAB's Experimental Blast Furnace (EBF) at MEFOS in Luleå, Sweden were used to observe the influence of in-furnace reactions on the changes in chemical stracture of cokes and their influence on kinetics of CO 2 reactivity. In addition to growth of carbon crystallite of coke, alkali concentration particularly potassium and sodium were found to increase as coke descended towards lower part of the EBF. The increase in carbon structure could be linearly related to measured temperature profiles of EBF. Isothermal and non-isothermal TGA measurements are shown to indicate that CO 2 reactivity of coke becomes progressively faster as it moves towards lower part of blast furnace. The study suggests that alkali enrichment of cokes in an operating blast furnace could have a strong catalytic effect on the CO 2 reactivity. Further research is expected to clarify the mechanisms of influence of coke minerals on reactivity and their implications in a blast furnace.

12 1 - 50 of 68
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