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  • 1. Arai, Y.
    et al.
    Emi, Toshihiko
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Casting of Metals.
    Fredriksson, Hasse
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Casting of Metals.
    Shibata, H.
    In-situ observed dynamics of peritectic solidification and delta/gamma transformation of Fe-3 to 5 At. Pct Ni alloys2005In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 36A, no 11, p. 3065-3074Article in journal (Refereed)
    Abstract [en]

    The characteristics of liquid -> delta ferrite and delta ferrite -> gamma austenite transformation in Fe-3.50 at. pct Ni alloy, and of the peritectic solidification of Fe-4.83 and 5.01 at. pct Ni alloys, have been dynamically observed in situ with a confocal scanning laser microscope (CSLM) combined with an infrared image furnace (IIF). The unique sequence, morphologies, and rates of these transformations have been determined at a temperature gradient of 4 K/mm and growth rates of 2 to 3 mu m/s, in unidirectional solidification experiments. These observations have been discussed in terms of the constitutional undercooling description of solidification theory.

  • 2.
    Borgenstam, Annika
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Hedström, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Hillert, Mats
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Kolmskog, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Stormvinter, Albin
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    On the Symmetry Among the Diffusional Transformation Products of Austenite2011In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 42A, no 6, p. 1558-1574Article in journal (Refereed)
    Abstract [en]

    Information on the diffusional transformation products of austenite in high-carbon steels is reviewed and supplemented with new microscopic studies. A comparison with transformation products in low-carbon steels indicates that there is a symmetry with pearlite in the middle, where ferrite and cementite are equal partners, and with acicular ferrite or cementite on each side. They both form with a surface relief, and at lower temperatures, each one is the leading phase in a eutectoid microstructure, bainite and inverse bainite, respectively. However, there is an asymmetry because at low temperatures bainite appears in high-carbon steels but inverse bainite never appears in low-carbon steels. At a constant high carbon content, there is another kind of symmetry, which is related to temperature. At intermediate temperatures the eutectoid reaction results in spherical nodules in which the cementite constituent originates from Widmanstatten plates. It turns spiky at both higher and lower temperatures with the leading phase in the spikes being cementite at higher temperatures and ferrite at lower temperatures. In the first kind of symmetry, there is an abrupt change among the three reaction products; in the second kind of symmetry, there is a gradual change. Accepting that all the eutectoid microstructures form by diffusion of carbon, one may explain the existence of both symmetries by the variation of the ratio of the supersaturations of ferrite and cementite with carbon content and with temperature.

  • 3.
    Borgenstam, Annika
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Hillert, Mats
    KTH, Superseded Departments, Materials Science and Engineering.
    Bainite in the light of rapid continuous cooling information1996In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 27, no 6, p. 1501-1512Article in journal (Refereed)
    Abstract [en]

    Rapid continuous cooling of pure iron can produce three different transformations yielding acicular structures: Widmanstätten a, lath martensite, and lenticular martensite. The information on their extensions into binary systems with carbon, nickel, and chromium has been reviewed, and admittedly rough methods have been used for estimating growth rates in order to examine the role of diffusion. The effect of alloying elements on their plateau temperatures and growth rates indicates that Widmanstätten a in Fe-C alloys grows under conditions close to local equilibrium for carbon, and it is suggested that the same should hold for edgewise growth of bainite. In Fe-Ni alloys, there are indications that Widmanstätten α grows under a considerable solute drag, an effect which may also occur for bainite. In Fe-Cr alloys, the solute drag effect seems to be weaker but may increase with the carbon content.

  • 4.
    Bratberg, Johan
    et al.
    Computational Thermodyn. Res. Group, Swedish Institute of Metals Research.
    Frisk, Karin
    Computational Thermodyn. Res. Group, Swedish Institute of Metals Research.
    An experimental and theoretical analysis of the phase equilibria in the Fe-Cr-V-C system2004In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 35A, no 12, p. 3649-3663Article in journal (Refereed)
    Abstract [en]

    New experimental phase equilibria information about the Fe-Cr-V-C system are presented and used to modify the thermodynamic description of the system. The main interest was focused on the composition of the MC and M7C3 carbides and the necessary adjustment of the Cr and V distribution between carbides and matrix. A new set of thermodynamic parameter values describing the Gibbs energy of different phases was obtained. A number of calculated sections of the Cr-C, Cr-V-C, Fe-Cr-C, and the Fe-Cr-V-C systems are presented and compared against both new and old experimental information. Calculations on commercial alloys to verify the improvements in multicomponent systems are also presented. The present calculation using the Thermo-Calc software shows much better agreement with the new experimental results than previous assessments.

  • 5. Bustnes, J. A.
    et al.
    Viswanathan, N. N.
    Sichen, Du
    KTH, Superseded Departments, Materials Science and Engineering.
    Seetharaman, Seshadri
    KTH, Superseded Departments, Materials Science and Engineering.
    Investigation on reduction of CoAl2O4 by hydrogen gas using TGA2000In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 31, no 3, p. 540-542Article in journal (Refereed)
  • 6. Dahle, A. K.
    et al.
    Hillert, Mats
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Discussion of "nucleation mechanism of eutectic phases in aluminum-silicon hypoeutectic alloys"2005In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 36, no 6, p. 1612-1613Article in journal (Refereed)
    Abstract [en]

    A previous study tried to prove that each plate of Si in the ordinary Al-Si eutectic is nucleated individually by the action of a small iron-containing particle. The study justified the need for a more complete understanding of the Al-Si eutectic reaction in the absence of chemical modifiers in order to shed light on the complicated mechanisms operating in chemically modified Al-Si alloys. Reacting to this claim, the present authors state that the mechanisms of the ordinary eutectic reaction of Al-Si alloys have been well understood for several decades, thus a very strong evidence is required to justify a revision of that understanding. For their part, the authors of the first study justified their claim for a revision by considering results of thermal analyses, optical microscopy, scanning and transmission electron microscopy, and selected area electron diffraction (SAED) analyses and elemental X-ray mapping performed on samples of precisely controlled chemistry.

  • 7. Dahle, A. K.
    et al.
    Hillert, Mats
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Reply to discussion on "Nucleation mechanism of eutectic phases in aluminum-silicon hypoeutectic alloys'2006In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 37A, no 4, p. 1353-1353Article in journal (Refereed)
  • 8. Dhindaw, B. K.
    et al.
    Antonsson, Tomas
    KTH, Superseded Departments, Materials Science and Engineering.
    Tinoco, J.
    Fredriksson, Hasse
    KTH, Superseded Departments, Materials Science and Engineering.
    Characterization of the peritectic reaction in medium-alloy steel through microsegregation and heat-of-transformation studies2004In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 35A, no 9, p. 2869-2879Article in journal (Refereed)
    Abstract [en]

    In the present work, the phenomenon of the peritectic reaction was characterized in a medium-ailoy steel. Several directional solidification and thermal-analysis experiments were done to investigate the reaction process. Directional solidification experiments carried out did not tend to show any direct evidence of a peritectic reaction. Microsegregation studies on the directionally solidified samples and those solidified under isothermal conditions bring out some interesting features. It has been documented that if the segregation ratio for Ni is higher than that for Cr, there is a correlation that the peritectic reaction had occurred in that region. On the other hand, a higher Cr segregation ratio as compared to Ni showed the possibility that the liquid had directly transformed to gamma-austenite without undergoing a peritectic reaction. Measurement of energies of transformations and the analysis of their values in different segments of the cooling-curve differential thermal analysis (DTA) experiments have helped in understanding the peritectic reaction. It is revealed that the transformation is more like diffusionless transformation, where gamma-austenite directly precipitates from delta-ferrite. Indeed, this proposition is also supported by the segregation patterns for Cr and Ni obtained in the solidified samples of this steel during directional solidification and DTA experiments and also by calculations to show the presence of enough lattice defects or vacancies to aid the aforementioned transformation.

  • 9. Divakar, M.
    et al.
    Hajra, J. P.
    Jakobsson, A.
    Seetharaman, Seshadri
    KTH, Superseded Departments, Materials Science and Engineering.
    Thermodynamics of surfaces and adsorption in the Fe-C-S-O system2000In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 31, no 2, p. 267-276Article in journal (Refereed)
    Abstract [en]

    Surface tension experiments have been conducted on Fe-C-S-O alloys using the sessile drop technique at 1823, 1873, and 1923 K in a purified argon atmosphere. The experimental data indicate lowering of surface tension values for alloys with low carbon and oxygen contents relative to the corresponding ternary Fe-S-O system of comparable compositions. The data exhibit an increasing trend of the surface tension values as a function of carbon and oxygen contents and temperature of the melt, Modified Butler's equations have been used in conjunction with the consistent thermodynamic relations for a dilute quaternary system to calculate the surface tensions and adsorption functions of the system, The calculated values are in excellent agreement with those of the experimental data for compositions having lower oxygen at 1823 K, however, the values for the alloys containing higher oxygen contents depart from those of the experimental data. The calculated surface tension values agree within 150 mN/m with those of the experimental data at 1873 K. The agreement, in general, has been favorable despite the constraints involved in the calculations as discussed in the text.

  • 10. 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.

  • 11. Engström, Anders
    et al.
    Höglund, Lars
    KTH, Superseded Departments, Materials Science and Engineering.
    Ågren, John
    COMPUTER-SIMULATION OF DIFFUSION IN MULTIPHASE SYSTEMS1994In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 25, no 6, p. 1127-1134Article in journal (Refereed)
    Abstract [en]

    A general model to treat multicomponent diffusion in multiphase dispersions is presented. The model is based on multicomponent diffusion data and basic thermodynamic data and contains no adjustable parameters. No restriction is placed on the number of components or phases that take part in the calculations, as long as the necessary thermodynamic and kinetic data are available. The new model is implemented into the DICTRA software, which makes use of THERMO-CALC to handle the thermodynamics. The model is applied to carburization of Ni alloys and heat treatment of welded joints between dissimilar materials. In both cases, the diffusion is accompanied by carbide formation or dissolution. A good agreement between experiments and calculations is found, despite the fact that no adjustable parameters are needed.

  • 12. Erneman, J.
    et al.
    Schwind, Martin
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. AB Outokompo Stainless, Sweden.
    Nylöf, L.
    Nilsson, J. O.
    Andrén, H. O.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Comparison between quantitative metallography and modeling of sigma-phase particle growth in AISI 347 stainless steel2005In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 36A, no 10, p. 2595-2600Article in journal (Refereed)
    Abstract [en]

    A comparison was made between two experimental methods to determine the (T-phase volume fraction and three methods to model a-phase growth in a niobium-stabilized stainless steel (AISI 347). The a-phase volume fraction and precipitate size were determined in material statically aged and creep deformed at 700 degrees C with both KOH etched specimens using bright field optical microscopy (OM/BF) (conventional method) and specimens etched with oxalic acid using scanning electron microscopy and backscattered electrons (SEM/BSE) (new method). Both experimental methods used manual thresholding together with digital image analysis. The calculations were made with DICTRA software, using both the TCFE database and the SSOL database with some modification concerning the effect of silicon on the stability of sigma-phase particles. The modeled sigma-phase volume- fractions showed rather good agreement with the measured results from statically aged material using the new method. It was found that the stabilizing effect of silicon on sigma phase should be included in the thermodynamic database used for modeling.

  • 13.
    Esfahani, Hosein Bartar
    et al.
    Shahid Bahonar Univ Kerman, Sch Engn, Dept Met & Mat Sci, Jomhoori Eslami Blvd, Kerman, Iran..
    Raiszadeh, Ramin
    Shahid Bahonar Univ Kerman, Sch Engn, Dept Met & Mat Sci, Mat Sci, Jomhoori Eslami Blvd, Kerman, Iran..
    Doostmohammadi, Hamid
    Shahid Bahonar Univ Kerman, Sch Engn, Dept Met & Mat Sci, Jomhoori Eslami Blvd, Kerman, Iran..
    The Effect of Strontium on the Strength of Layers of Double Oxide Film Defects2016In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 47A, no 3, p. 1331-1338Article in journal (Refereed)
    Abstract [en]

    The change in the strength of layers of oxide film (bifilm) defects when held in Al-0.05 and 0.1 wt pct melts was evaluated indirectly by comparing the porosity, average pore roundness, and pore number density results obtained from a reduced pressure test of the commercial purity and Sr-containing Al melts subjected to different holding and vibration regimes. Any changes in the composition of the pores were also studied using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results showed that the Al2O3 layers of a newly formed oxide film defect, when held in Al-0.05 or 0.1 wt pct Sr melt, transformed gradually into an Sr-containing oxide. This transformation caused the strength of the oxide layers to increase and the rate of the expansion of its atmosphere and removal of the defect from the melt to decrease. This might influence the increase in the porosity of Sr-modified Al castings.

  • 14.
    Hedström, Peter
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Lindgren, L. E.
    Almer, J.
    Lienert, U.
    Bernier, J.
    Terner, M.
    Odén, M.
    Load Partitioning and Strain-Induced Martensite Formation during Tensile Loading of a Metastable Austenitic Stainless Steel2009In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 40A, no 5, p. 1039-1048Article in journal (Refereed)
    Abstract [en]

    In-situ high-energy X-ray diffraction and material modeling are used to investigate the strain-rate dependence of the strain-induced martensitic transformation and the stress partitioning between austenite and alpha' martensite in a metastable austenitic stainless steel during tensile loading. Moderate changes of the strain rate alter the strain-induced martensitic transformation, with a significantly lower alpha' martensite fraction observed at fracture for a strain rate of 10(-2) s(-1), as compared to 10(-3) s(-1). This strain-rate sensitivity is attributed to the adiabatic heating of the samples and is found to be well predicted by the combination of an extended Olson-Cohen strain-induced martensite model and finite-element simulations for the evolving temperature distribution in the samples. In addition, the strain-rate sensitivity affects the deformation behavior of the steel. The alpha' martensite transformation at high strains provides local strengthening and extends the time to neck formation. This reinforcement is witnessed by a load transfer from austenite to alpha' martensite during loading.

  • 15.
    Hillert, M.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Discussion of "A Commentary on Reaction Kinetics in Processes of Nucleation and Growth"*2011In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 42A, no 11, p. 3241-3241Article in journal (Refereed)
  • 16.
    Hillert, Mats
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Discussion of "A Personal Commentary on Transformation of Austenite at Constant Subcritical Temperatures"2011In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 42A, no 3, p. 541-542Article in journal (Refereed)
  • 17.
    Hillert, Mats
    KTH, Superseded Departments, Materials Science and Engineering.
    Nature of massive transformation2004In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 35A, no 1, p. 351-352Article in journal (Refereed)
    Abstract [en]

    A description of massive transformation is presented. Massive transformation is defined as a kind of composition-invariant nucleation-and-growth formation of a solid phase from another solid phase. The nature of massive transformation is often described as diffusional. The concentration of nucleation to grain boundaries in the parent phase is a significant feature of massive transformation.

  • 18.
    Hillert, Mats
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Centennial of the Diffusionless Paradigm of Bainite2012In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 43A, no 12, p. 4487-4495Article in journal (Refereed)
    Abstract [en]

    The diffusionless growth model of bainite formation was included in an international consensus in 1912 that all transformation products of austenite form by an initial transformation to martensite. The introduction of isothermal treatment revealed that all the products form directly. However, for bainite, the idea of some relation to martensite survived and developed into the diffusionless paradigm. Zener introduced the T (o) concept for predicting the start temperature of lower bainite, but also described diffusional growth of upper bainite. The present description of the diffusionless growth model is now examined and criticized.

  • 19.
    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.

  • 20.
    Holländer Pettersson, Niklas
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Swerim AB, Isafjordsgatan 28A, S-16440 Stockholm, Sweden.
    Lindell, D.
    Swerim AB, Isafjordsgatan 28A, S-16440 Stockholm, Sweden..
    Lindberg, F.
    Swerim AB, Isafjordsgatan 28A, S-16440 Stockholm, Sweden..
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Formation of Chromium Nitride and Intragranular Austenite in a Super Duplex Stainless Steel2019In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 50, no 12, p. 5594-5601Article in journal (Refereed)
    Abstract [en]

    Precipitation of chromium nitrides and formation of intragranular austenite were studied in detail for the super duplex stainless steel grade 2507 (UNS S32750). The situation of multipass welding was simulated by heat treatment at 1623 K (1350 degrees C) and quenching followed by short heat treatments at 1173 K (900 degrees C). The microstructural evolution was characterized using transmission and scanning electron microscopy, electron backscatter, and transmission Kikuchi diffraction, and it was observed that the interior of the ferrite grains contained chromium nitrides after quenching. The nitrides were predominantly of CrN with a cubic halite-type structure and clusters of CrN-Cr2N where rod-shaped trigonal Cr2N particles had nucleated on plates of CrN. After heat treatment for 10 seconds at 1173 K (900 degrees C), the nitride morphology was transformed into predominantly rod-shaped Cr2N, and finely dispersed intragranular secondary austenite idiomorphs had formed in the nitride-containing areas within the ferrite grains. After 60 seconds of heat treatment, both the Cr2N nitrides and the secondary austenite were coarsened. Analysis of electron diffraction data revealed an inherited crystallographic relationship between the metastable CrN and the intragranular austenite. The mechanism of chromium nitride formation and its relation to secondary austenite formation in duplex stainless steels are discussed.

  • 21.
    Hulme-Smith, Christopher
    et al.
    Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge, CB3 0FS, UK.
    Ooi, Shgh Woei
    Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge, CB3 0FS, UK.
    Bhadeshia, Harshad Kumar Dharamshi Hansraj
    Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge, CB3 0FS, UK.
    Thermally Stable Nanocrystalline Steel2017In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 48, no 10, p. 4957-4964Article in journal (Refereed)
    Abstract [en]

    Two novel nanocrystalline steels were designed to withstand elevated temperatures without catastrophic microstructural changes. In the most successful alloy, a large quantity of nickel was added to stabilize austenite and allow a reduction in the carbon content. A 50 kg cast of the novel alloy was produced and used to verify the formation of nanocrystalline bainite. Synchrotron X-ray diffractometry using in situ heating showed that austenite was able to survive more than 1 hour at 773 K (500 ˚šC) and subsequent cooling to ambient temperature. This is the first reported nanocrystalline steel with high-temperature capability.

  • 22.
    Huyan, Fei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Hedström, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Höglund, Lars
    Borgenstam, Annika
    A Thermodynamic-Based Model to Predict the Fraction of Martensite in Steels2016In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 47A, no 9, p. 4404-4410Article in journal (Refereed)
    Abstract [en]

    A thermodynamic-based model to predict the fraction of martensite in steels with undercooling has been developed. The model utilizes the thermodynamic driving force to describe the transformation curve and it is able to predict the fraction of athermal martensite at quenching to different temperatures for low alloy steels. The only model parameter is a linear function of the martensite start temperature (M (s)), and the model predicts that a steel with a higher M (s) has a lower difference between the martensite start and finish temperatures. When the present model is combined with a previously developed thermodynamic-based model for M (s), the model predictions of the full martensite transformation curve with undercooling are in close agreement with literature data.

  • 23.
    Huyan, Fei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Yan, Jia-Yi
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Höglund, Lars
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Simulation of the Growth of Austenite from As-Quenched Martensite in Medium Mn Steels2018In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 49A, no 4, p. 1053-1060Article in journal (Refereed)
    Abstract [en]

    As part of an ongoing development of third-generation advanced high-strength steels with acceptable cost, austenite reversion treatment of medium Mn steels becomes attractive because it can give rise to a microstructure of fine mixture of ferrite and austenite, leading to both high strength and large elongation. The growth of austenite during intercritical annealing is crucial for the final properties, primarily because it determines the fraction, composition, and phase stability of austenite. In the present work, the growth of austenite from as-quenched lath martensite in medium Mn steels has been simulated using the DICTRA software package. Cementite is added into the simulations based on experimental observations. Two types of systems (cells) are used, representing, respectively, (1) austenite and cementite forming apart from each other, and (2) austenite forming on the cementite/martensite interface. An interfacial dissipation energy has also been added to take into account a finite interface mobility. The simulations using the first type of setup with an addition of interfacial dissipation energy are able to reproduce the observed austenite growth in medium Mn steels reasonably well.

  • 24. Izadi, Hossein
    et al.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Gerlich, Adrian P.
    Grain Growth Behavior and Hall-Petch Strengthening in Friction Stir Processed Al 50592014In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 45A, no 12, p. 5635-5644Article in journal (Refereed)
    Abstract [en]

    Friction stir processing (FSP) of Al 5059 is studied in which subsequent heat treatment is conducted to investigate its effects on grain size and hardness. It was found that mainly elongated and rhomboidal morphologies of Al-6(Mn,Fe) particles are present in the alloy both before and after FSP, where the rhomboidal particles are more effective in pinning grain boundaries during heat treatment. The stir zone reached a temperature of 705 K (432 A degrees C), and ThermoCalc modeling confirmed that the Al-6(Mn,Fe) particles will remain stable at this temperature. Negligible grain growth was observed during FSP of the Al 5059 due to low grain boundary mobility resulting from slow diffusion associated with a high Mg content in the alloy. During heat treatment at 448 K (175 A degrees C) grain growth could be correlated with time using a particle-controlled grain growth model. Microhardness values indicate that Hall-Petch behavior occurs in the processed alloy, while dislocation density and particle dispersion play a minor role in strengthening.

  • 25. Jaradeh, Majed
    et al.
    Carlberg, Torbjörn
    DTA and DSC Studies of aluminium 3003 alloys with Zn and Cu additions2007In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 38, no 9, p. 2138-2147Article in journal (Refereed)
    Abstract [en]

    The effect of Zn additions in the range of 0 to 5 wt pct and Cu contents of 0.15 to 1.0 wt pct on the solidification and precipitation behavior of aluminum alloys of AA 3003 type was investigated with differential thermal analysis (DTA), differential scanning calorimetry (DSC), optical microscopy, and scanning electron microscopy/energy-dispersive X-ray (SEM/EDX). The results show that the additions of varying Zn and Cu have significant effects on the crystallization process, phase precipitation, and microstructure formation of the alloys. Phases formed have been identified and morphological differences have been discussed.

  • 26.
    Kaplan, Bartek
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics.
    Markström, Andreas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Norgren, Susanne
    Sandvik Mining R&D, Stockholm.
    Selleby, Malin
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics.
    Experimental Determination of the Solubility of Co in the Cr-Based Carbides Cr23C6, Cr7C3, and Cr3C22014In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 45, no 11, p. 4820-4828Article in journal (Refereed)
    Abstract [en]

    Thermodynamic calculations based on the CALPHAD method are nowadays often applied in the design of new materials due to increasing demands on shorter lead times for development. However, such calculations rely heavily on the assessed thermodynamic descriptions, which in turn rely on the amount and quality of available experimental data, especially for binary and ternary sub-systems. The ternary Co-Cr-C system is an extremely important subsystem to, e.g., multi-component cemented carbide grades, such as W-Co-Cr-M-C (M = Ti,Ta,Nb,V,Zr,Hf), as well as Cr-containing Co-base alloys. In the case of the Co-Cr-C system, there is a lack of reliable data on the solubility of Co in Cr-carbides. Therefore, the present work concerns an experimental study of the solubility of Co in all three of the Cr-based carbides, i.e., Cr23C6, Cr7C3, and Cr3C2. This was done by synthesizing appropriate samples in the M7C3+M23C6+liquid and M7C3+M3C2+graphite three-phase fields. The results show that a recent thermodynamic description of the Co-Cr-C system is unable to reproduce the experimentally determined solubilities. Therefore, the present study provides important input for future alloy development and improvement of the thermodynamic description of the Co-Cr-C system.

  • 27.
    Kolmskog, Peter
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Eutectoid Transformations in 4.12 Mass Pct Cr 0.88 Mass Pct C Steel2011In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 42A, no 13, p. 3941-3951Article in journal (Refereed)
    Abstract [en]

    The sequence of eutectoid microstructures, obtained by lowering the temperature of the isothermal transformation, was studied in synthetic steel with 4.12 mass pct Cr 0.88 mass pct C. The results were compared with observations on plain carbon steels with 1.65 and 1.67 mass pct C. In both cases, the main features can be explained as an effect of a lowered temperature on the increasing supersaturation of cementite in austenite and an even stronger effect on that of ferrite. One distinction was a continuous change in the pearlite structure toward a more acicular structure. This structure is named acicular pearlite.

  • 28.
    Kolmskog, Peter
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Hillert, Mats
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Hedström, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Babu, Sudarsanam Suresh
    Terasaki, Hidenori
    Komizo, Yu-Ichi
    Direct Observation that Bainite can Grow Below M-S2012In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 43A, no 13, p. 4984-4988Article in journal (Refereed)
    Abstract [en]

    In situ simultaneous synchrotron X-ray diffraction and laser scanning confocal microscopy have confirmed that bainite in steels can grow below the martensite start temperature. This observation suggests that the formation curves for bainite in time-temperature-transformation diagrams should be extended below the martensite start temperature. Furthermore, the implication of this observation on the growth mechanism of bainitic ferrite is discussed.

  • 29.
    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.

  • 30.
    Leach, Lindsay
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Hillert, Mats
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Modeling C-Curves for the Growth Rate of Widmanstatten and Bainitic Ferrite in Fe-C Alloys2016In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 47A, no 1, p. 19-25Article in journal (Refereed)
    Abstract [en]

    When Zener formulated his maximum growth rate criterion for predicting the coarseness of various metallographic objects, he simplified the growth rate equations and predicted that the optimum coarseness should be twice the critical value for which all the driving force would be absorbed by interfacial energy. It is now emphasized that a composition dependence of the diffusion coefficient has a considerable influence and can result in a ratio much larger than two. Various approximations have now been removed from the growth rate equation. When applied to acicular ferrite in the Fe-C system, a C-curve for the growth rate is obtained that resembles the unusually wide C-curve obtained experimentally when information on Widmanstatten ferrite and bainite is combined. It is not necessary to explain that shape as a combination of separate curves for Widmanstatten ferrite and bainite. The main reason for the wide C-curve is the direct effect of the composition dependence of the diffusivity of carbon in austenite.

  • 31.
    Leach, Lindsay
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Kolmskog, Peter
    Sandv Min & Rock Technol, Sandv Ind Area, 6480 RTDRB, S-81181 Sandviken, Sweden..
    Höglund, Lars
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Hillert, Mats
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Critical Driving Forces for Formation of Bainite2018In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 49A, no 10, p. 4509-4520Article in journal (Refereed)
    Abstract [en]

    An empirical equation for predicting bainite start temperatures of steels was recently derived by starting from binary Fe-C alloys and continuing with ternary Fe-C-M alloys. This result is now illustrated with a family of B-S lines in a T,C diagram for a series of constant Mn contents. The critical driving force for the formation of ferrite is calculated for diffusionless or diffusional processes, and these quantities are used as dependent variables with carbon content or temperature as independent variables. Negative critical driving forces are predicted for a diffusionless process in binary Fe-C alloys, showing that this process cannot apply to the formation of bainite. The critical driving force for a diffusional process increases strongly with decreasing temperature and increasing carbon content. Mn and Ni, contrary to Cr, Mo and Si, have remarkably small effects on this critical driving force. The results are discussed by imagining that the magnitude of the critical driving force is governed by the height of an energy barrier that must be surmounted during growth. It is modeled as completely determined by the alloy composition. It is represented with an equation evaluated by fitting to the recent empirical equation and describing the carbon dependence of the barrier.

  • 32.
    Leach, Lindsay
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Kolmskog, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Höglund, Lars
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Hillert, Mats
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Use of Fe-C Information as Reference for Alloying Effects on B-S2019In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 50A, no 10, p. 4531-4540Article in journal (Refereed)
    Abstract [en]

    Many empirical equations of the variation of the critical temperature with alloy content of the start of bainite formation in steels are available. They are often obtained by regression analysis of measured values for a large number of alloyed steels, usually with several alloying elements. However, such equations differ considerably, especially when applied to pure Fe-C alloys, which results in large differences between reported effects of individual alloying elements since they have not been based on the Fe-C system as a reference. Apparently, for the first time, an empirical equation is now derived by starting with information from Fe-C alloys and low alloy steels and then adding the effect of each alloying element separately, using information from ternary Fe-C-M alloys. Sets of information from the same alloy content but different carbon contents proved particularly useful. Lines connecting such points are regarded as B-S lines for the respective alloy content and the effect of alloying elements was evaluated from their distance from the B-S line for Fe-C alloys. Only under this condition can coefficients for alloying elements be expected to represent the physical effect of the elements. The resulting equation was tested with about 600 experimental B-S temperatures.

  • 33.
    Leach, Lindsay
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Höglund, Lars
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Diffusion-Controlled Lengthening Rates of Bainitic Ferrite a Part of the Steel Genome2019In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 50A, no 6, p. 2613-2618Article in journal (Refereed)
    Abstract [en]

    As a step in the further development of models and databases to support design of new steels, i.e., the steel genome, the growth of bainitic ferrite plates is accounted for by a thermodynamic and kinetic approach. The thermodynamic aspects are represented by CALPHAD databases and a Gibbs energy barrier for growth B-m. Experimental information on ferrite-plate growth rates for a number of Fe-C alloys, some of high-purity, are analyzed in terms of a modified Zener-Hillert model and the barrier as well as some kinetic parameters are evaluated. It is found that the barrier varies in a smooth way with carbon content and lengthening rate. In order to improve the agreement with the experimental information it was necessary to adjust the diffusion coefficient of carbon in austenite at low temperatures. It is concluded that the representation of the experimental data is satisfactory.

  • 34.
    Lindwall, Greta
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Campbell, C. E.
    NIST, Mat Measurement Lab, Gaithersburg, MD 20899 USA..
    Lass, E. A.
    NIST, Mat Measurement Lab, Gaithersburg, MD 20899 USA..
    Zhang, F.
    NIST, Mat Measurement Lab, Gaithersburg, MD 20899 USA..
    Stoudt, M. R.
    NIST, Mat Measurement Lab, Gaithersburg, MD 20899 USA..
    Allen, A. J.
    NIST, Mat Measurement Lab, Gaithersburg, MD 20899 USA..
    Levine, L. E.
    NIST, Mat Measurement Lab, Gaithersburg, MD 20899 USA..
    Simulation of TTT Curves for Additively Manufactured Inconel 6252019In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 50A, no 1, p. 457-467Article in journal (Refereed)
    Abstract [en]

    The ability to use common computational thermodynamic and kinetic tools to study the microstructure evolution in Inconel 625 (IN625) manufactured using the additive manufacturing (AM) technique of laser powder-bed fusion is evaluated. Solidification simulations indicate that laser melting and re-melting during printing produce highly segregated interdendritic regions. Precipitation simulations for different degrees of segregation show that the larger the segregation, i.e., the richer the interdendritic regions are in Nb and Mo, the faster the -phase (Ni3Nb) precipitation. This is in accordance with the accelerated precipitation observed experimentally during post-build heat treatments of AM IN625 compared to wrought IN625. The -phase may be undesirable since it can lead to detrimental effects on the mechanical properties. The results are presented in the form of a TTT diagram and agreement between the simulated diagram and the experimental TTT diagram demonstrate how these computational tools can be used to guide and optimize post-build treatments of AM materials.

  • 35.
    Lindwall, Greta
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Flyg, Jesper
    Swerea KIMAB.
    Frisk, Karin
    Swerea KIMAB.
    Sandberg, Odd
    Uddeholms AB.
    Experimental and theoretical investigations of hot isostatically pressed-produced stainless steel/high alloy tool steel compound materials2010In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 42, no 5, p. 1165-1172Article in journal (Refereed)
    Abstract [en]

    Consolidation of tool steel powders and simultaneous joining to a stainless 316L steel areperformed by hot isostatic pressing (HIP). Two tool steel grades are considered: a high vanadiumalloyed carbon tool steel, and a high vanadium and chromium alloyed nitrogen tool steel.The boundary layer arising during diffusion bonding is in focus and, in particular, the diffusionof carbon and nitrogen over the joint.Measurements of the elemental concentration profiles andcorrosion tests by the double loop–electrochemical potentiokinetic reactivation (DL-EPR)method are performed. Comparative calculations with the DICTRA software are performedand are found to be in agreement with the experimental results. It is found that the carbon toolsteel grade has a more critical influence on the corrosion resistance of the stainless 316L steel incomparison to the nitrogen tool steel grade.

  • 36.
    Magnusson, Hans
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Creep strain modelling of 9-12 Pct Cr steels based on microstructure evolution2007In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 38, no 9, p. 2033-Article in journal (Refereed)
    Abstract [en]

    Creep deformation is simulated for 9 pct Cr steels by using the Norton equation with the addition of back stresses from dislocations and precipitates. The composite model is used to represent the heterogeneous dislocation structure found in 9 to 12 pct Cr steels. Dislocation evolution is modeled by taking capturing and annihilation of free dislocations into account. Recovery of immobile dislocations is derived from the ability of dislocation climb. In spite of the fact that the initial dislocation density is high and is reduced during creep, primary creep is successfully modeled for a P92 steel. Subgrain growth is evaluated using a model by Sandström (1977). The long time subgrain size corresponds well to a frequently used empirical relation, with subgrain size inversely proportional to the applied stress.

  • 37.
    Magnusson, Hans
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Modeling Creep Strength of Welded 9 to 12 Pct Cr Steels2010In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 41A, no 13, p. 3340-3347Article in journal (Refereed)
    Abstract [en]

    The influence of weld simulated heat treatments of 9-12% steels is evaluated by a fundamental model for creep. The heat affected microstructure is predicted by considering particle coarsening, particle dissolution, and subgrain coarsening. Particle coarsening is predicted for a multi-component system, showing significant M23C6 coarsening in the BCC matrix. Dissolution simulations of MX and M23C6 are performed by considering a size distribution of particles, indicating that the smallest particles can be dissolved already at relatively low welding temperatures. Recovery in dislocation networks will take place due to the coarser particles. Creep rate modelling is performed based on the heat affected microstructure, showing strength reduction of weld simulated material by 12% at 850ºC and 26% at 900ºC. The main cause of this degradation is believed to be the loss of the smallest carbonitrides.

  • 38.
    Magnusson, Hans
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    The role of dislocation climb across particles at creep conditions in 9 to 12 pct Cr steels2007In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 38A, no 10, p. 2428-2434Article in journal (Refereed)
    Abstract [en]

    The influence of a distribution of particles on creep strength is analyzed. The time it takes for dislocations to climb across the particles is the basis for a model that can describe the effect of particle size distribution, particle area fraction, stress, and temperature on the creep rate. The degradation of microstructure through coarsening is taken into account. The particle size distributions for M23C6 carbides and MX carbonitrides in a 9 pct Cr steel are accurately represented by an exponential function. Coarsening coefficients and phase fractions for MX and M23C6 particles are predicted using thermodynamic modeling, and show good fit to experimental data. The size distributions are used to determine the amount of dislocations, which can either climb across particles or make Orowan loops. The dislocation climb model is integrated into a creep rate prediction model and is used to reproduce experimental creep data for P92 steel.

  • 39.
    Malik, Amer
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.).
    Odqvist, Joakim
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Höglund, Lars
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Hertzman, Staffan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Outokumpu Stainless Research Foundation.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Phase-Field Modeling of Sigma-Phase Precipitation in 25Cr7Ni4Mo Duplex Stainless Steel2017In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 48A, no 10, p. 4914-4928Article in journal (Refereed)
    Abstract [en]

    Phase-field modeling is used to simulate the formation of sigma phase in a model alloy mimicking a commercial super duplex stainless steel (SDSS) alloy, in order to study precipitation and growth of sigma phase under linear continuous cooling. The so-called Warren-Boettinger-McFadden (WBM) model is used to build the basis of the multiphase and multicomponent phase-field model. The thermodynamic inconsistency at the multiple junctions associated with the multiphase formulation of the WBM model is resolved by means of a numerical Cut-off algorithm. To make realistic simulations, all the kinetic and the thermodynamic quantities are derived from the CALPHAD databases at each numerical time step, using Thermo-Calc and TQ-Interface. The credibility of the phase-field model is verified by comparing the results from the phase-field simulations with the corresponding DICTRA simulations and also with the empirical data. 2D phase-field simulations are performed for three different cooling rates in two different initial microstructures. A simple model for the nucleation of sigma phase is also implemented in the first case. Simulation results show that the precipitation of sigma phase is characterized by the accumulation of Cr and Mo at the austenite-ferrite and the ferrite-ferrite boundaries. Moreover, it is observed that a slow cooling rate promotes the growth of sigma phase, while a higher cooling rate restricts it, eventually preserving the duplex structure in the SDSS alloy. Results from the phase-field simulations are also compared quantitatively with the experiments, performed on a commercial 2507 SDSS alloy. It is found that overall, the predicted morphological features of the transformation and the composition profiles show good conformity with the empirical data.

  • 40. Muddle, B C
    et al.
    Aaronson, H I
    Kelly, P M
    Srinivasan, G R
    Laughlin, D E
    Reynolds, W T
    Braun, M
    Purdy, G
    Zhang, W Z
    Howe, J M
    Zhang, M X
    Soffa, W A
    Baskes, M I
    Vasudevan, V K
    DeTavernier, C
    Furuhara, T
    Hillert, M
    Pond, R C
    Shiflet, G J
    General discussion session of the 2004 Hume-Rothery Symposium on "The structure and diffusional growth mechanisms of irrational interphase boundaries"2006In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 37A, no 3A, p. 961-974Article in journal (Other academic)
  • 41.
    Muhmond, Haji Muhammad
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Casting of Metals.
    Fredriksson, Hasse
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Casting of Metals.
    Relationship Between the Trace Elements and Graphite Growth Morphologies in Cast Iron2014In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 45, no 13, p. 6187-6199Article in journal (Refereed)
    Abstract [en]

    The graphite morphology transition was studied using various techniques and additives in ultra-pure binary and ternary alloys with hypo- and hypereutectic compositions. Some of the trace elements were observed to stabilize the flake growth morphology of graphite, while others did not. The distance between the graphite basal planes of spheroidal, flake, and undercooled fine graphite was measured and the lattice fringes were studied using high resolution transmission electron microscope, after preparing a thin lamella of graphite using focused ion beam. Latent heat measurement was performed using differential scanning calorimeter on the pure binary alloy with and without sulfur and oxygen additions. The substitution of various elements under study in a monolayer of graphene was analyzed by considering the binding energies of the elements with C and their bonding nature. Simulations were performed using a molecule editor program and visualizer (Avogadro software), which considers various types of interatomic forces to optimize a monolayer of graphene to a minimum energy. The effect of the type of cyclic C-ring structure and energy of the basal plane of graphite with a connection to the addition of trace elements individually in the monolayer of graphene was studied and simulated to understand the resulting bulk graphite growth morphology.

  • 42.
    Nassar, Hani
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Fredriksson, Hasse
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Process Science.
    Peritectic Reactions and Transformations in Low-Alloy Steels2010In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 41A, no 11, p. 2776-2783Article in journal (Refereed)
    Abstract [en]

    Differential thermal analysis (DTA) experiments on low-alloy steels with varying C, Si, Cr, and Mo contents indicated an increase in the difference between the liquidus and peritectic temperatures during solidification with the decrease in C and increase in Mo contents. In a number of the quenched samples, massive transformations of ferrite to austenite were observed. Electron microprobe analysis of the diffusion across a massive transformation front, along with the high growth rates estimated, gives strong reason to believe that these growths are uncontrolled by diffusion. As ferrite transforms to austenite during the peritectic reaction, shrinkage in volume occurs, causing elastic straining at the interface separating the two phases. It was shown through thermodynamic analysis of the equilibrium at the triple point that the increase in energy of the two phases due to this strain can result in undercooling below the equilibrium peritectic temperature and decreases in the equilibrium peritectic concentrations.

  • 43.
    Nick, Arash S.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Casting of Metals.
    Vynnycky, Michael
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Casting of Metals.
    Fredriksson, Hasse
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Casting of Metals.
    A Theoretical Analysis of the Interaction Between Pores and Inclusions During the Continuous Casting of Steel2016In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 47, no 6, p. 2985-2999Article in journal (Refereed)
    Abstract [en]

    A mathematical model is derived to predict the trajectories of pores and inclusions that are nucleated in the interdendritic region during the continuous casting of steel. Using basic fluid mechanics and heat transfer, scaling analysis, and asymptotic methods, the model accounts for the possible lateral drift of the pores as a result of the dependence of the surface tension on temperature and sulfur concentration. Moreover, the soluto–thermocapillary drift of such pores prior to final solidification, coupled to the fact that any inclusions present can only have a vertical trajectory, can help interpret recent experimental observations of pore-inclusion clusters in solidified steel castings.

  • 44.
    Pettersson, Niklas
    et al.
    Swerea KIMAB AB, S-16407 Kista, Sweden.
    Pettersson, R. F. A.
    Wessman, S.
    Precipitation of chromium nitrides in the super duplex stainless steel 25072015In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 46A, no 3, p. 1062-1072Article in journal (Refereed)
    Abstract [en]

    Precipitation of chromium nitrides during cooling from temperatures in the range 1373 K to 1523 K (1100 C to 1250 C) has been studied for the super duplex stainless steel 2507 (UNS S32750). Characterization with optical, scanning and transmission electron microscopy was combined to quantify the precipitation process. Primarily Cr2N nitrides were found to precipitate with a high density in the interior of ferrite grains. An increased cooling rate and/or an increased austenite spacing clearly promoted nitride formation, resulting in precipitation within a higher fraction of the ferrite grains, and lager nitride particles. Furthermore, formation of the meta-stable CrN was induced by higher cooling rates. The toughness seemed unaffected by nitrides. A slight decrease in pitting resistance was, however, noticed for quenched samples with large amounts of precipitates. The limited adverse effect on pitting resistance is attributed to the small size (~200 nm) of most nitrides. Slower cooling of duplex stainless steels to allow nitrogen partitioning is suggested in order to avoid large nitrides, and thereby produce a size distribution with a smaller detrimental effect on pitting resistance.

  • 45.
    Pettersson, Niklas
    et al.
    Swerea KIMAB AB, POB 7047, S-16407 Kista, Sweden.
    Wessman, S.
    Hertzman, S.
    Studer, A.
    High-Temperature Phase Equilibria of Duplex Stainless Steels Assessed with a Novel In-Situ Neutron Scattering Approach2017In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 48, no 4, p. 1562-1571Article in journal (Refereed)
    Abstract [en]

    Duplex stainless steels are designed to solidify with ferrite as the parent phase, with subsequent austenite formation occurring in the solid state, implying that, thermodynamically, a fully ferritic range should exist at high temperatures. However, computational thermodynamic tools appear currently to overestimate the austenite stability of these systems, and contradictory data exist in the literature. In the present work, the high-temperature phase equilibria of four commercial duplex stainless steel grades, denoted 2304, 2101, 2507, and 3207, with varying alloying levels were assessed by measurements of the austenite-to-ferrite transformation at temperatures approaching 1673 K (1400 °C) using a novel in-situ neutron scattering approach. All grades became fully ferritic at some point during progressive heating. Higher austenite dissolution temperatures were measured for the higher alloyed grades, and for 3207, the temperature range for a single-phase ferritic structure approached zero. The influence of temperatures in the region of austenite dissolution was further evaluated by microstructural characterization using electron backscattered diffraction of isothermally heat-treated and quenched samples. The new experimental data are compared to thermodynamic calculations, and the precision of databases is discussed.

  • 46.
    Pilhagen, Johan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Delaminations by Cleavage Cracking in Duplex Stainless Steels at Sub-zero Temperatures2014In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 45A, no 3, p. 1327-1337Article in journal (Refereed)
    Abstract [en]

    Impact toughness testing was conducted on 10 and 30 mm plates of 2205 together with a 30 mm plate of LDX 2101(A (R)) duplex stainless steel (DSS). The testing temperatures were between 153 K (-120 A degrees C) and room temperature. Interrupted fracture toughness tests of the 10 mm plate and a 50 mm plate of 2205 were also performed. The conclusion from the fractographic investigation was that the delaminations that occur in hot-rolled DSSs were cleavage fractures. The toughness anisotropy can be explained by the cleavage fracture and the appearance of the microstructure. The result from the interrupted fracture toughness test revealed that the delaminations initiated prior to the maximum force plateau and propagated ahead of the stable crack growth during testing. Estimated upper limit for the fracture delamination initiation toughness at sub-zero temperatures for the 2205 base metal according to the crack-tip opening displacement method was 28 to 61 mu m for the 10 mm plate, 70 to 106 mu m for the 30 mm plate and below 100 mu m for the 50 mm plate.

  • 47. Purdy, Gary
    et al.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Brechet, Yves
    Enomoto, Masato
    Furuhara, Tadashi
    Gamsjager, Ernst
    Goune, Mohamed
    Hillert, Mats
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Hutchinson, Christopher
    Militzer, Matthias
    Zurob, Hatem
    ALEMI: A Ten-Year History of Discussions of Alloying-Element Interactions with Migrating Interfaces2011In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 42A, no 12, p. 3703-3718Article in journal (Refereed)
    Abstract [en]

    ALEMI is concerned with the interactions between Alloying Elements and Migrating Interfaces. A first meeting was held in conjunction with the 2000 TMS Fall Meeting in St. Louis, MO. About 22 attendees endorsed the principles contained in the invitation, which envisaged a more collaborative approach to the study of alloying element interactions with transformation interfaces, especially in alloy steels. The meetings were intended to be informal workshops emphasizing the sharing of ideas and plans for research. The development of a shared stock of alloys for research was planned, as well as the publication of summaries of discussions in an open, preferably archival, forum. Eight further meetings were held, often in conjunction with major conferences. An approximate equilibrium developed between discussions of theoretical matters and experimental results and methods. A remarkable number of those who attended the first meeting in St. Louis continued to participate. Research ideas were put forward, issues debated, collaborations fostered, and the science of transformation interfaces advanced.

  • 48.
    Rahaman, Moshiour
    et al.
    Ferritico, Brinellvagen 85, S-10044 Stockholm, Sweden..
    Mu, Wangzhong
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    Odqvist, Joakim
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy. Ferritico, Brinellvägen 85, S-10044 Stockholm, Sweden.
    Hedström, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Ferritico, Brinellvägen 85, S-10044 Stockholm, Sweden.
    Machine Learning to Predict the Martensite Start Temperature in Steels2019In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 50A, no 5, p. 2081-2091Article in journal (Refereed)
    Abstract [en]

    The martensite start temperature (M-s) is a critical parameter when designing high-performance steels and their heat treatments. It has, therefore, attracted significant interest over the years. Numerous methodologies, such as thermodynamics-based, linear regression and artificial neural network (ANN) modeling, have been applied. The application of data-driven approaches, such as ANN modeling, or the wider concept of machine learning (ML), have shown limited technical applicability, but considering that these methods have made significant progress lately and that materials data are becoming more accessible, a new attempt at data-driven predictions of the M-s is timely. We here investigate the usage of ML to predict the M-s of steels based on their chemical composition. A database of the M(s)vs alloy composition containing 2277 unique entries is collected. It is ensured that all alloys are fully austenitic at the given austenitization temperature by thermodynamic calculations. The ML modeling is performed using four different ensemble methods and ANN. Train-test split series are used to evaluate the five models, and it is found that all four ensemble methods outperform the ANN on the current dataset. The reason is that the ensemble methods perform better for the rather small dataset used in the present work. Thereafter, a validation dataset of 115 M-s entries is collected from a new reference and the final ML model is benchmarked vs a recent thermodynamics-based model from the literature. The ML model provides excellent predictions on the validation dataset with a root-mean-square error of 18, which is slightly better than the thermodynamics-based model. The results on the validation dataset indicate the technical usefulness of the ML model to predict the M-s in steels for design and optimization of alloys and heat treatments. Furthermore, the agility of the ML model indicates its advantage over thermodynamics-based models for M-s predictions in complex multicomponent steels. (C) The Author(s) 2019

  • 49. Ryen, Øyvind
    et al.
    Nijs, Oscar
    Sjölander, Emma
    KTH, School of Engineering Sciences (SCI), Physics.
    Holmedal, Björn
    Ekström, Hans-Erik
    Nes, Erik
    Strengthening mechanisms in solid solution aluminum alloys2006In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 37A, no 6, p. 1999-2006Article in journal (Refereed)
    Abstract [en]

    A number of commercial and high-purity non-heat-treatable aluminum alloys are investigated in this work. It is found that both magnesium and manganese in solid solution give a nearly linear concentration dependence of the strength at a given strain for commercial alloys. This deviates from high-purity AlMg binary alloys, where a parabolic concentration dependence is found. Mn in solid solution is found to give a considerably higher strengthening effect per atom than Mg, both in terms of yield stress and initial work hardening rate. This strengthening effect is stronger comparing commercial grades to high-purity alloys. This enhanced strengthening is believed to be a synergy or clustering effect caused by interaction between Mn atoms and trace elements, probably silicon, in solid solution.

  • 50.
    Safara, Nima
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. epartment of Material Science, Dalarna University, Falun, 79188, Sweden.
    Engberg, G.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Modeling Microstructure Evolution in a Martensitic Stainless Steel Subjected to Hot Working Using a Physically Based Model2019In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 50, no 3, p. 1480-1488Article in journal (Refereed)
    Abstract [en]

    The microstructure evolution of a martensitic Stainless steel subjected to hot compression is simulated with a physically based model. The model is based on coupled sets of evolution equations for dislocations, vacancies, recrystallization, and grain growth. The advantage of this model is that with only a few experiments, the material-dependent parameters of the model can be calibrated and used for a new alloy in any deformation condition. The experimental data of this work are obtained from a series of hot compression, and subsequent stress relaxation tests performed in a Gleeble thermo-mechanical simulator. These tests are carried out at various temperatures ranging from 900 to 1200 °C, strains up to 0.7, and strain rates of 0.01, 1, and 10 s −1 . The grain growth, flow stress, and stress relaxations are simulated by finding reasonable values for model parameters. The flow stress data obtained at the strain rate of 10 s −1 were used to calibrate the model parameters and the predictions of the model for the lower strain rates were quite satisfactory. An assumption in the model is that the structure of second phase particles does not change during the short time of deformation. The results show a satisfactory agreement between the experimental data and simulated flow stress, as well as less than 5 pct difference for grain growth simulations and predicting the dominant softening mechanisms during stress relaxation according to the strain rates and temperatures under deformation.

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