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  • 1.
    Bonvalet, M.
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Odqvist, Joakim
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Forsberg, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Modelling of prismatic grain growth in cemented carbides2019Inngår i: INTERNATIONAL JOURNAL OF REFRACTORY METALS & HARD MATERIALS, ISSN 0263-4368, Vol. 78, s. 310-319Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A mean-field model dealing with prismatic grain growth during liquid phase sintering of cemented carbides with a Co-rich binder is presented. The evolution of the size of an assembly of non-spherical grains is obtained using a Kampmann-Wagner approach and by introducing a constant shape factor between the characteristic lengths of prisms. This factor is a function of interfacial energies of the two kind of facets, basal and prismatic, considered. The growth model is based on three different mechanisms, that can be rate limiting, taking place in series: 2D nucleation of a new atomic layer, mass transfer across the interface and long-range diffusion. The driving force for coarsening is distributed between the different facets. These equations are solved numerically, and the simulation results reveal that the specific abnormal grain growth phenomena experimentally observed in cemented carbides may be reproduced with this new more realistic description of the grain shape contrary to the spherical approach developed in the past. It is also shown that the initial powder size distribution, and more specifically its shape has a strong influence on the distribution of the driving force between the different rate limiting mechanisms and thus on the occurrence of abnormal grain growth. In that case, the self-similarity of the normalized grain size distribution over time is not achieved.

  • 2.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Certain aspects of the bainitic transformation in steel2015Inngår i: PTM 2015 - Proceedings of the International Conference on Solid-Solid Phase Transformations in Inorganic Materials 2015, International Conference on Solid-Solid Phase Transformations in Inorganic Materials , 2015, s. 69-70Konferansepaper (Fagfellevurdert)
  • 3.
    Borgenstam, Annika
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Nucleation and growth of martensite in steel1997Doktoravhandling, med artikler (Annet vitenskapelig)
  • 4.
    Borgenstam, Annika
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Edmonds, D.
    Hillert, Mats
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Eutectoids with cementite as the major constituent in Fe-C-M alloys2016Inngår i: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 103, s. 280-289Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The addition of a third element to hypereutectoid Fe-C alloys may cause inclusions of a minor constituent in the precipitate of cementite from austenite even if the initial austenite is supersaturated only with cementite. Cementite will often become the major constituent of this kind of microstructure. For Fe-C-Cu alloys this has been explained as the result of precipitation from supersaturated cementite. An alternative mechanism could be that the mixture of cementite and a minor constituent forms by simultaneous and cooperative growth of the two phases, i.e., by a reaction that may be regarded as eutectoid. This mechanism has already been applied to explain the occurrence of eutectoid colonies with cementite as the major constituent and a minor constituent for which there was no supersaturation initially. This phenomenon has been observed in hypereutectoid ternary Fe-C alloys with Al, Mn or Si. The necessary requirements on the ordinary isothermal phase diagram are now examined with a graphical method based on the slopes of tie-lines. It predicts the phenomenon in all cases where it has been observed, including Fe-C-Cu and not in the Fe-C-Ni and Fe-C-Cr systems where it has not been observed. The requirements become more evident when the calculated phase equilibria are plotted as an isothermal phase diagram with the alloy content as a function of the carbon activity instead of carbon content. Finally, a comparison is made with bainite in Fe-C alloys where ferrite is the major and cementite the minor constituent. The same two explanations have been proposed for that case.

  • 5.
    Borgenstam, Annika
    et al.
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Engstrom, A.
    Höglund, Lars
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Ågren, John
    KTH, Tidigare Institutioner                               , Metallurgi.
    DICTRA, a tool for simulation of diffusional transformations in alloys2000Inngår i: Journal of phase equilibria (Print), ISSN 1054-9714, E-ISSN 1544-1032, Vol. 21, nr 3, s. 269-280Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 6.
    Borgenstam, Annika
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Ericsson, Juliette M.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Determination of the critical carbon content for growth of acicular ferrite2005Inngår i: Solid-Solid Phase Transformations in Inorganic Materials 2005, Vol 1 / [ed] Howe, JM; Laughlin, DE; Lee, JK; Dahmen, U; Soffa, WA, 2005, s. 105-110Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The critical carbon content for growth of acicular ferrite has been determined in Fe-Ni-C alloys by using a gradient technique. A carbon gradient has been introduced in binary Fe-Ni alloys which are subsequently heat treated isothermally. The carbon content. at the position where acicular ferrite stops growing, is determined by EMPA (Electron Microprobe Analysis) and microstructural examination with LOM (Light Optical Microscopy). At low Ni contents the experimentally determined values for the critical carbon content can be understood from the hypothesis that the growth of acicular ferrite is controlled by carbon diffusion under paraequilibrium conditions and modified with an extra thermodynamic barrier.

  • 7.
    Borgenstam, Annika
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Hedström, Peter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Hillert, Mats
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Kolmskog, Peter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Stormvinter, Albin
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    On the Symmetry Among the Diffusional Transformation Products of Austenite2011Inngår i: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 42A, nr 6, s. 1558-1574Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 8.
    Borgenstam, Annika
    et al.
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Hillert, M.
    Massive transformation in the Fe-Ni system2000Inngår i: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 48, nr 11, s. 2765-2775Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The critical limit for the massive gamma-->alpha transformation in the Fe-Ni system has been measured by isothermal heat treatment of diffusion couples. The position of the alpha/gamma interface at the end of the treatment could be identified but some growth occurred during the quench. Growth is probably hindered at the beginning of the quench by a redistribution of Fe and Ni at the interface during the isothermal treatment. During the quench it often happens that the alpha/gamma interfaces develop jagged shapes and even very fine plates although there should usually be no orientation relationship. The critical limit agrees fairly well with classical results for the formation of equiaxed ferrite from continuous cooling of homogeneous specimens. At 1023 K it coincides with the alpha/alpha + gamma phase boundary. At lower temperatures it moves inside the alpha + gamma two-phase field. The driving force for the massive transformation increases at decreasing temperature and the limiting composition never approaches the T-0 line.

  • 9.
    Borgenstam, Annika
    et al.
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Hillert, M.
    Nucleation of isothermal martensite2000Inngår i: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 48, nr 11, s. 2777-2785Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Two methods of evaluating an activation energy for the nucleation of martensite from kinetic information on isothermal martensite are discussed. In both methods the nucleation is described as thermally activated growth of an embryo up to a size where rapid growth can get started. The first method is based on the absolute Value of the rate of reaction at each separate temperature and depends on the neglect of the reverse reaction. The second method is based on the temperature dependence of the rate at low temperatures and includes the reverse reaction and it typically yields a lower activation energy. It can be used to define the region of low net driving force where the reverse reaction, which is neglected in the first method, is important and that method should not be used. A unified description of the nucleation of athermal and isothermal martensite is also presented.

  • 10.
    Borgenstam, Annika
    et al.
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Hillert, Mats
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Activation energy for isothermal martensite in ferrous alloys1997Inngår i: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 45, nr 2, s. 651-662Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The experimental information on isothermal α martensite in ferrous alloys is reviewed. From the kinetics one can clearly distinguish between three groups of alloys yielding isothermal martensite. The first group contains high alloy steels with a low Ms temperature. They form isothermal martensite with a temperature dependence corresponding to a very low activation energy, possibly 7 kJ/mol. The second group contains steels with an appreciable amount of carbon. Its rate of formation of isothermal martensite can be explained by assuming that it is triggered by submicroscopic plates of bainite formed with a rate controlled by carbon diffusion. The third group contains Fe---Ni alloys with up to about 25% Ni. There the temperature dependence corresponds to an activation energy of about 80 kJ/mol. It is proposed that their transformation is related to the transformation causing plateau II in experiments with very rapid cooling, a transformation which has previously been proposed to be related to the formation of bainite.

  • 11.
    Borgenstam, Annika
    et al.
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Hillert, Mats
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Bainite in the light of rapid continuous cooling information1996Inngår i: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 27, nr 6, s. 1501-1512Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 12.
    Borgenstam, Annika
    et al.
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Hillert, Mats
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Driving force for f.c.c.→b.c.c. martensites in Fe-X alloys1997Inngår i: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 45, nr 5, s. 2079-2091Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Information on Ms, the starting temperature for formation of martensite, is reviewed and one Ms line each for lath and plate martensite are drawn in a number of Fe-X phase diagrams. A reasonable interpretation of the data indicates the possibility that the distance between the two lines may vary linearly with temperature and be independent of the choice of alloying element. Using thermodynamic descriptions of the binary systems, the driving force for the start of the formation of the two kinds of martensite is calculated from the same interpretation of data. When plotted against temperature the results indicate that the driving force for martensite may not be much affected by solution hardening but may mainly be a function of temperature. For plate martensite it may have a fairly constant value of about 2100 J/mol. For lath martensite it may vary linearly, possibly from 500 J/mol at 800°C to 2100 J/mol at 250°C.

  • 13.
    Borgenstam, Annika
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Hillert, Mats
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Kinetics of bainite transformation in steels2012Inngår i: Phase Transformations in Steels, Elsevier, 2012, Vol. 1, s. 468-501Kapittel i bok, del av antologi (Fagfellevurdert)
    Abstract [en]

    The main kinetic features of the formation of bainite are discussed, firstly in relation to two incompatible hypotheses for the growth mechanism of bainitic ferrite. One is based on diffusionless growth of bainitic ferrite but diffusional growth of Widmanstätten ferrite. The other is based on the assumption that there is only one kind of acicular ferrite and it grows under simultaneous diffusion of carbon into the interior of the parent austenite. The kinetics of the first stage of bainite formation, the growth of acicular ferrite, is treated in detail because it has been subject to the more intensive research. The kinetics of the reactions by which cementite forms and the subsequent reactions, by which the transformation to bainite is completed, have been subject to less research.

  • 14.
    Borgenstam, Annika
    et al.
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Hillert, Mats
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Model for nucleation of isothermal martensite2001Inngår i: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 45, nr 8, s. 917-922Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Kaufman and Cohen's model for thermally activated growth of preexisting embroys of isothermal martensite was defined with a series of obstacles but they only considered jumps in one direction. Jumps in both directions across obstacles were recently considered and the activation energy for the net reaction was described. That nucleation model is now developed further.

  • 15.
    Borgenstam, Annika
    et al.
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Hillert, Mats
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Ågren, John
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Critical temperature for growth of martensite1995Inngår i: Acta Metallurgica Et Materialia, ISSN 0956-7151, Vol. 43, nr 3, s. 945-954Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Ms may be defined as the temperature below which the formation of martensite starts upon cooling. It may also be useful to define Mg, the temperature below which martensite can grow if it is already nucleated. In order to analyze the mechanism of martensite formation, it is essential to know the difference Mg - Ms. We have tried to evaluate Mg - Ms for an Fe-C alloy with a decarburized surface zone in order to induce nucleation. The samples were studied by means of electron microprobe, serial sectioning and optical microscopy. The results indicate that Mg is surprisingly close to Ms. The possibility that Mg is controlled by growth rather than nucleation is discussed.

  • 16.
    Borgenstam, Annika
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Hillert, Mats
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Metallographic evidence of carbon diffusion in the growth of bainite2009Inngår i: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 57, nr 11, s. 3242-3252Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    There are two paradigms regarding the formation of bainite. One is based on the first stage being rapid, diffusionless growth or acicular ferrite and the subsequent formation of carbide occurring by precipitation from the supersaturated ferrite. All assumption that the first stage occurs as a series of subsequent rapid steps resulting in sub-units plays an important role as an explanation of the not so rapid growth observed macroscopically. The other paradigm is based on the first stage being the formation of acicular ferrite under carbon diffusion and on the subsequent growth of carbide and ferrite side by side. Metallographic observations are presented that support the second paradigm. It is difficult to see how they can be accounted for by the first paradigm, in particular the observation of the shapes of sub-units.

  • 17.
    Borgh, Ida
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Hedström, Peter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Odqvist, Joakim
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Effect of carbon activity and powder particle size on WC grain coarsening during sintering of cemented carbides2014Inngår i: International journal of refractory metals & hard materials, ISSN 0958-0611, E-ISSN 2213-3917, Vol. 42, s. 30-35Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Liquid-phase sintering is an important step in the production of cemented carbides. During sintering, the average WC grain size increases, leading to a coarser structure, which affects the performance of the final product. The coarsening occurs by dissolution of small grains and growth of large grains. In the present work, the effect of high carbon activity during sintering on the WC grain coarsening has been evaluated using electron backscattered diffraction (EBSD) and the results have been compared with a previous work where sintering was performed at a lower carbon activity. A more homogeneous grain size distribution was observed in alloys sintered at a high carbon activity. In addition, the effect of the initial powder particle size distribution was investigated. It was found that the coarsening rate of a WC powder with an initial small average grain size is significantly higher as compared to the coarsening rate for a powder with a larger initial average grain size. The results obtained emphasize the importance of considering the complete particle size distribution in order to predict coarsening.

  • 18.
    Borgh, Ida
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Hedström, Peter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Odqvist, Joakim
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Gholinia, Ali
    Winiarski, Bartlomiej
    Withers, Philip J.
    Thompson, George E.
    Mingard, Ken
    Gee, Mark G.
    On the three-dimensional structure of WC grains in cemented carbides2013Inngår i: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 61, nr 13, s. 4726-4733Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In the present work, the size distribution and shape of WC grains in cemented carbides (WC-Co), with different Co contents, have been investigated in three dimensions. Direct three-dimensional (3-D) measurements, using focused ion beam serial sectioning and electron backscattered diffraction (EBSD), were performed and a 3-D microstructure was reconstructed. These measurements were supplemented by two-dimensional (2-D) EBSD and scanning electron microscopy on extracted WC grains. The data from 2-D EBSD collected on planar sections were transformed to three dimensions using a recently developed statistical method based on an iterative inverse Saltykov procedure. This stereological analysis revealed that the assumed spherical shape of WC grains during the Saltykov method is reasonable and the estimated 3-D size distribution is qualitatively in good agreement with the actual distribution measured from 3-D EBSD. Although the spherical assumption is generally fair, the WC grains have both faceted and rounded surfaces. This is a consequence of the relatively low amount of liquid phase during sintering, which makes impingements significant. Furthermore, the observed terraced surface structure of some WC grains suggests that 2-D nucleation is the chief coarsening mechanism to consider.

  • 19.
    Borgh, Ida
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Hedström, Peter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Persson, Tomas
    Norgren, Susanne
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Odqvist, Joakim
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Microstructure, grain size distribution and grain shape in WC-Co alloys sintered at different carbon activities2014Inngår i: International Journal of Refractory Metals and Hard Materials, ISSN 0958-0611, Vol. 43, s. 205-211Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The properties of cemented carbides strongly depend on the WC grain size and it is thus crucial to control coarsening of WC during processing. The aim of this work was to study the effect of sintering at different carbon activities on the final microstructure, as well as the coarsening behavior of the WC grains, including the size distribution and the shape of WC grains. These aspects were investigated for five WC-Co alloys sintered at 1410 C for 1 h at different carbon activities in the liquid, in the range from the graphite equilibrium (carbon activity of 1) to the eta (M6C) phase equilibrium (carbon activity of 0.33). The grain size distribution was experimentally evaluated for the different alloys using EBSD (electron backscatter diffraction). In addition, the shape of the WC grains was evaluated for the different alloys. It was found that the average WC grain size increased and the grain size distribution became slightly wider with increasing carbon activity. Comparing the two three-phase (WC-Co-eta and WC-Co-graphite) alloys a shape change of the WC grains was observed with larger grains having more planar surfaces and more triangular shape for the WC-Co-graphite alloy. It was indicated that in alloys with a relatively low volume fraction of the binder phase the WC grain shape is significantly affected by impingements. Moreover, after 1 h of sintering the WC grains are at a non-equilibrium state with regards to grain morphology.

  • 20.
    Borgh, Ida
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Norgren, Susanne
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Influence of nitrogen Gas pressure on the miscibility Gap in the Ti-Zr carbonitride system2012Inngår i: International Journal of Refractory Metals and Hard Materials, ISSN 0263-4368, Vol. 32, s. 11-15Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The microstructure of cemented carbides with a gradient structure at the surface consists of WC, cubic carbonitrides and a binder phase. The carbonitrides can, for example, consist of Ti(C,N)-Zr(C,N) where it is reasonable to believe that there is a miscibility gap with Ti-rich and Zr-rich carbonitrides. In the present work, the effect of the N-2-gas pressure on the equilibrium composition of the miscibility gap in the (Ti,Zr)(C,N) system has been investigated. In the study, the carbonitride system is in equilibrium with: WC, liquid binder, graphite and, N-2-gas of different pressures. Both Fe and Co are used as binder phase to study the effect of the binder phase. The results verify that there is a miscibility gap in the carbonitride system and that the region of the miscibility gap will change when N is introduced. There is a critical N-2-gas pressure lower than 0.1 bar and above that pressure the compositions of the carbonitride are rather constant as a result of the formation of a surface rim.

  • 21. Chen, Hao
    et al.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Odqvist, Joakim
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Zuazo, Ian
    Goune, Mohamed
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    van der Zwaag, Sybrand
    Application of interrupted cooling experiments to study the mechanism of bainitic ferrite formation in steels2013Inngår i: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 61, nr 12, s. 4512-4523Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    New interrupted cooling experiments have been designed to study the kinetics of bainitic ferrite formation starting from a mixture of austenite and bainitic ferrite. It is found that the kinetics of bainitic ferrite formation during the cooling stage is determined by the isothermal holding time. The formation rate of bainitic ferrite at the beginning of the cooling decreases with increasing prior isothermal holding time. An unexpected stagnant stage during the cooling stage appears when the isothermal holding time increases to a critical point. There are two reasons for the occurrence of the stagnant stage: (i) a solute spike in front of the interface; and (ii) kinetic transition. A so-called Gibbs energy balance approach, in which the dissipation of Gibbs energy due to diffusion inside the interface and interface friction is assumed to be equal to the available chemical driving force, is applied to theoretically explain the stagnant stage. A kinetics transition from a fast growth mode without diffusion of Mn and Si inside the austenite-bainitic ferrite interfaces to a slow growth mode with diffusion inside the interface is predicted. The stagnant stage is caused by the transition to a slow growth mode. The Gibbs energy balance approach describes the experimental observations very well.

  • 22.
    Forsberg, Annika
    et al.
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Ågren, John
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Thermodynamic evaluation of the Fe-Mn-Si system and the γ/ε martensitic transformation1993Inngår i: Journal of phase equilibria (Print), ISSN 1054-9714, E-ISSN 1544-1032, Vol. 14, nr 3, s. 354-363Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The thermodynamic properties of the Fe-Mn-Si system are analyzed by means of thermodynamic models for the individual phases. Special attention is paid to the γ → ε martensitic transition. A complete set of parameters, from which arbitrary sections of the phase diagram as well as the Ms and As temperatures may be calculated, is given.

  • 23.
    Hedström, Peter
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Stormvinter, Albin
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Gholinia, Ali
    Winiarski, Bartlomiej
    Withers, Philip J.
    Karlsson, Oskar
    Hagström, Joacim
    On the Three-Dimensional Microstructure of Martensite in Carbon Steels2012Inngår i: Proceedings Of The 1st International Conference On 3D Materials Science, John Wiley & Sons, 2012, s. 19-24Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The mechanical properties of high-performance steels are often reliant on the hard martensitic structure. It can either be the sole constituent e. g. in tool steels, or it can be part of a multi-phase structure as e. g. in dual-phase steels. It is well-known that the morphology of martensite changes from lath to plate martensite with increasing carbon content. The transition from lath to plate is however less known and in particular the three-dimensional (3D) aspects in the mixed lath and plate region require more work. Here the current view of the 3D microstructure of martensite in carbon steels is briefly reviewed and complemented by serial sectioning experiments using a focused ion beam scanning electron microscope (FIB-SEM). The large martensite units in the Fe-1.2 mass% C steel investigated here are found to have one dominant growth direction, less transverse growth and very limited thickening. There is also evident transformation twinning parallel to the transverse direction. It is concluded that more 3D analysis is required to understand the 3D microstructure of martensite in the mixed lath and plate region and to verify the recently proposed 3D phase field models of martensite in steels.

  • 24.
    Hillert, Mats
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Centennial of the Diffusionless Paradigm of Bainite2012Inngår i: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 43A, nr 12, s. 4487-4495Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 25.
    Hillert, Mats
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Do bainitic and Widmanstatten ferrite grow with different mechanisms?2010Inngår i: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 62, nr 2, s. 75-77Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Caballero et al. recently presented new evidence for different growth mechanisms of Widmanstatten and bainitic ferrite. Their argument was based on Zener's hypothesis of diffusionless growth of bainitic ferrite. It is now demonstrated that Bhadeshia's model, based on Zener's hypothesis, predicts that some of the new measurements, claimed to fall above B-s and to be due to Widmanstatten ferrite, actually fall within the predicted temperature range of bainite, indicating that they cannot be used as new support for Bhadeshia's model.

  • 26.
    Holländer Pettersson, Niklas
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. 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, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Formation of Chromium Nitride and Intragranular Austenite in a Super Duplex Stainless Steel2019Inngår i: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 50, nr 12, s. 5594-5601Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 27. Hou, Ziyong
    et al.
    Linder, David
    Hedström, Peter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Forsberg, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Holmström, E.
    Ström, Valter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Effect of carbon content on the Curie temperature of WC-NiFe cemented carbides2019Inngår i: International Journal of Refractory Metals and Hard Materials, ISSN 0263-4368, Vol. 78, s. 27-31Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We have investigated the effect of the carbon content on the Curie temperature of a cemented carbide composite material with a Ni-Fe alloy as the binder phase and WC as the hard phase. In the carbon concentration range from 5.72 to 5.83 wt% carbon, which covers the interval where WC coexists with fcc Ni-Fe without other phases (the ‘carbon window’), the Curie temperature rises from 200 to 527 °C. This result indicates the possibility to use the Curie temperature to determine the carbon balance in the system. With thermodynamic calculations and kinetic simulations we can quantitatively establish the correlation between the carbon and tungsten content of the binder phase and the Curie temperature. This strong compositional effect on the Curie temperature is quantitatively very different from the conventional Co-based cemented carbides, with Curie temperatures of about 950–1050 °C.

  • 28.
    Hou, Ziyong
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Linder, David
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Hedström, Peter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Ström, Valter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Holmström, E.
    Sandvik Coromant R&D, SE 126 80 Stockholm, Sweden.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Evaluating magnetic properties of composites from model alloys – Application to alternative binder cemented carbides2019Inngår i: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 168, s. 96-99Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The magnetic properties of 85Ni-15Fe model alloys containing Co, W and C have been studied with the intent to isolate the influence of alloy chemistry on quality control measurements of alternative binder cemented carbides. The results show a strong influence of dissolved W on the Curie temperature and the saturation magnetization. The amount of dissolved C, and the presence of WC precipitates, on the other hand, is shown to have negligible effect. Furthermore, the magnetic coercivity is indicated to be entirely dominated by the microstructural features and quite insensitive to composition.

  • 29.
    Huyan, Fei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Hedström, Peter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Modelling of the fraction of martensite in low-alloy steels2015Inngår i: MATERIALS TODAY-PROCEEDINGS, Elsevier, 2015, Vol. 2, s. 561-564Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Thermodynamics-based modelling of the fraction of martensite formed upon quenching in low-alloy steels is developed. The adopted modelling approach has two distinct features: 1) it applies the driving force of the transformation, i.e. the difference of Gibbs energy between austenite and martensite, from thermodynamic calculations; 2) it predicts the sigmoidal shape of transformation to capture also the initial 10-20% of martensite formation, which is distinct from some previous modelling using e.g. the Koistinen-Marburger equation. It is found that the general equation can describe the experimental data of martensite fraction versus quenching temperature for plain carbon steels and low-alloy steels well. Furthermore, the only model parameter that is needed is linearly proportional to the martensite start temperature of the steel, which opens the possibility for a thermodynamics-based simple but yet predictive model if it is coupled with the previously developed thermodynamics-based model for the Ms temperature.

  • 30.
    Huyan, Fei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Hedström, Peter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Höglund, Lars
    Borgenstam, Annika
    A Thermodynamic-Based Model to Predict the Fraction of Martensite in Steels2016Inngår i: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 47A, nr 9, s. 4404-4410Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 31.
    Huyan, Fei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Yan, Jia-Yi
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Höglund, Lars
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Simulation of the Growth of Austenite from As-Quenched Martensite in Medium Mn Steels2018Inngår i: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 49A, nr 4, s. 1053-1060Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 32.
    Huyan, Fei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Yan, Jiayi
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi. Thermo-Calc Software AB, Råsundavägen 18, SE-169 67 Solna, Sweden.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Experimental Determination and Thermodynamic Modeling of Msσ for a Medium Mn SteelManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    Medium Mn steels have attracted worldwide interests recent years due to their excellent mechanical properties and low cost. These steels contain large fraction (~30%) of metastable retained austenite and exhibit good elongation due to transformation-induced plasticity (TRIP). In order to obtain the highest elongation, the mechanical stability of austenite, quantified using Msσ, needs to be optimized. Msσ is defined as the highest temperature at which martensite can form under stress without austenite yielding by slip. The present work aims to formulate a model of Msσ which can be used to design medium Mn steels with optimized elongation. In the present work, an Fe–0.18C–5.08Mn (mass %) steel was intercritically annealed at 650 °C. Based on tensile tests at different temperatures using a single specimen method, the Msσ temperatures were experimentally determined to about 0 °C regardless of intercritical annealing time between 15 min and 3 h. Microstructure observations showed that large austenite grains with a globular shape are more transformed than thin-film ones, and thus the former probably governs the determined Msσ. Msσ was further predicted at the crossing point of yielding by martensite formation and by austenite slip; the former was modeled by expanding an existing model of martensite start temperature and the latter by a constitutive model. The predicted Msσ showed reasonable agreement with the determined values. The model also indicated that a large and a small austenite grain have similar Msσ, which could partly explain why the determined Msσ is rather constant regardless of IA time.

  • 33.
    Huyan, Fei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Yan, Jiayi
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. Thermo-Calc Software AB, Råsundavägen 18, SE-169 67 Solna, Sweden.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Influence of Sub-micron Austenite Grain Size on Martensitic Transformation in a Medium Mn SteelManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    Many previous studies suggest that austenite stability increases with decreasing grain size, i.e. the martensite start temperature, Ms, decreases. Therefore, the influence of small austenite grain size would be high for many modern steels containing fine austenite. Models from literature deviate severely from each other for grain sizes below 1 μm where there is a lack of experimental data. Besides, the experimental data are mostly obtained from a fully austenitic microstructure with equiaxed grains. This raises concerns about the applicability of these models to design medium Mn steels, where the austenite stability is essential for elongation via transformation-induced plasticity. The present work concerns the influence of grain size on martensitic transformation for thin-film austenite in a medium Mn steel. After IA, austenite grains exhibit two morphologies, thin-film like and globular, while the former is dominant. The globular austenite is less stable and responsible for the Ms measured by dilatometry. Similar to particles, the austenite grains are isolated and dispersed, and autocatalysis from surrounding austenite is minimized. Therefore in this work an approach to describe the transformation in small particles has been adopted, where the number fraction of partly or fully transformed austenite grains (F) is phenomenologically formulated as a function of temperature and grain size. Experimental data from the present work and from literature have been used to derive a model of Ms and grain size for thin-film austenite in medium Mn steels, using the cross-sectional area of austenite grain as a variable. The current model provides a practical and useful way of determining austenite stability from 2-dimensional micrograph, and can be used in designing medium Mn steels with optimized austenite stability.

  • 34.
    Huyan, Fei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Yan, Jiayi
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    On the tuning of austenite stability in a medium mn trip steel2015Inngår i: PTM 2015 - Proceedings of the International Conference on Solid-Solid Phase Transformations in Inorganic Materials 2015, International Conference on Solid-Solid Phase Transformations in Inorganic Materials , 2015, s. 933-934Konferansepaper (Fagfellevurdert)
  • 35.
    Jeppsson, Johan
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Mannesson, Karin
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Inverse Saltykov analysis for particle-size distributions and their time evolution2011Inngår i: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, nr 3, s. 874-882Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this work a new method for transforming 2-D to 3-D size distributions is proposed. A representation of the 2-D size distributions is constructed from the data of measured radii with a statistical method called the kernel density estimator. The method yields a smooth density estimation that is more accurate than the classic histogram. The 3-D distribution is optimized from the 2-D density estimate in an iterative manner. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

  • 36.
    Kolmskog, Peter
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Eutectoid Transformations in 4.12 Mass Pct Cr 0.88 Mass Pct C Steel2011Inngår i: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 42A, nr 13, s. 3941-3951Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 37.
    Kolmskog, Peter
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Hillert, Mats
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Hedström, Peter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Babu, Sudarsanam Suresh
    Terasaki, Hidenori
    Komizo, Yu-Ichi
    Direct Observation that Bainite can Grow Below M-S2012Inngår i: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 43A, nr 13, s. 4984-4988Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 38.
    Kolmskog, Peter
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Höglund, Lars
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Hillert, Mats
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Thermodynamic Analysis of the Critical Condition for Acicular FerriteArtikkel i tidsskrift (Annet vitenskapelig)
  • 39.
    Kolmskog, Peter
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Hedström, Peter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Kinetic Study of Transformations ofAustenite in a 4.12 mass% Cr 0.88 mass% C SteelManuskript (preprint) (Annet vitenskapelig)
  • 40.
    Larsson, Henrik
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Trans-interface diffusivity in the Fe-Ni system2007Inngår i: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 56, nr 1, s. 61-64Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The trans-interface diffusivity in the Fe-Ni system has been estimated by fitting simulations to experimental data. For diffusion across the alpha/gamma phase boundary, the mobility of species divided by an interfacial thickness of 1 nm, M-k/delta, is approximately 5% of the phase interface mobility M-int at 873 K and 15% at 973 K.

  • 41. Leach, L.
    et al.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Ågren, J.
    Höglund, L.
    A model for the growth rate of bainitic ferrite2015Inngår i: PTM 2015 - Proceedings of the International Conference on Solid-Solid Phase Transformations in Inorganic Materials 2015, 2015, s. 377-378Konferansepaper (Fagfellevurdert)
  • 42.
    Leach, Lindsay
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Hillert, Mats
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Modeling C-Curves for the Growth Rate of Widmanstatten and Bainitic Ferrite in Fe-C Alloys2016Inngår i: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 47A, nr 1, s. 19-25Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 43.
    Leach, Lindsay
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Kolmskog, Peter
    Sandv Min & Rock Technol, Sandv Ind Area, 6480 RTDRB, S-81181 Sandviken, Sweden..
    Höglund, Lars
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Hillert, Mats
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Critical Driving Forces for Formation of Bainite2018Inngår i: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 49A, nr 10, s. 4509-4520Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 44.
    Leach, Lindsay
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Kolmskog, Peter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Höglund, Lars
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Hillert, Mats
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Use of Fe-C Information as Reference for Alloying Effects on B-S2019Inngår i: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 50A, nr 10, s. 4531-4540Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 45.
    Leach, Lindsay
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Höglund, Lars
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Diffusion-Controlled Lengthening Rates of Bainitic Ferrite a Part of the Steel Genome2019Inngår i: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 50A, nr 6, s. 2613-2618Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 46.
    Leach, Lindsay
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Höglund, Lars
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Representing lengthening rate of bainitic ferrite - a part of the steel genomeManuskript (preprint) (Annet vitenskapelig)
    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 Bm. 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.

  • 47.
    Linder, David
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Hou, Ziyong
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Xie, Ruiwen
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Hedström, Peter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Ström, Valter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Holmström, Erik
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    A comparative study of microstructure and magnetic properties of a Ni–Fe cemented carbide: Influence of carbon content2019Inngår i: International Journal of Refractory Metals and Hard Materials, ISSN 0263-4368, Vol. 80, s. 181-187Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Due to the renewed interest in alternative binders for cemented carbides it is important to understand how the binder composition influences not only mechanical properties but also the microstructure and related measurements for quality control. Microstructure and chemical composition of WC-Co is often evaluated by magnetic measurements. However, when the binder composition deviates significantly from conventional Co-based binders it should not be assumed that the standard measurements can be used to directly evaluate the same parameters. In this paper we investigate the influence of relative C-content on the microstructure and magnetic properties of an alternative binder cemented carbide. It is shown that the saturation magnetization is related to the relative C-content and the magnetic coercivity is related to the microstructure, more specifically to the binder phase distribution, but could not be directly linked to the carbide grain size in the same manner as for standard WC-Co. Furthermore, a direct correlation between Curie temperature and saturation magnetization is observed for this system which means that the Curie temperature potentially could be used for calibration of empirical relations or as a method to accurately determine the binder volume fraction.

  • 48.
    Linder, David
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Walbrühl, Martin
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Borgenstam, Annika
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Martensite transformation in cemented carbides with alternative binders2016Inngår i: World PM 2016 Congress and Exhibition, European Powder Metallurgy Association (EPMA) , 2016Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The recent interest in substitution of cobalt in cemented carbides has led to renewed efforts into finding alternative binders. Promising candidates are Fe and Ni-based systems which generally can be divided into austenitic (fcc) and martensitic (bct) binders. The martensitic transformation may drastically change the properties, thus, when designing an alternative binder it is important to know at what temperature and composition the martensitic transformation takes place. Furthermore, it is of interest to understand how the transformation is affected by the binder mean free path and the stresses in the binder introduced by the carbide grains. Another aspect, that is important for high temperature properties, is the tempering of martensite as well as reversion to austenite. The effect of these processes is here investigated along with how they influence the behavior of the cemented carbides at different temperatures, thereby determining their application range.

  • 49.
    Linder, David
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Walbrühl, Martin
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Yan, Jia-Yi
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Borgenstam, Annika
    KTH, Tidigare Institutioner (före 2005), Materialvetenskap. KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    An energy release rate approach to cemented carbide fracture toughness for computational materials designManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    Integrated computational materials engineering and computational materials design have the potential to greatly accelerate materials development at reduced cost compared to conventional experimentally-based methods. These methodologies, however, require physically-based property models to be truly predictive. Fracture toughness is a critical material property of cemented carbides for high-performance mining and metal cutting tools. In the present work, a fracture toughness model framework based on the energy release rate formalism is presented and applied to conventional and alternative-binder cemented carbides. The framework is physically-based and designed to be modular, where each sub-model can be independently modified or replaced without disturbing the calculation-flow of the overall framework. In the presented examples, the sub-models are based on e.g. finite element simulations and atomistic calculations as well as limited calibration to experimental data. The model framework is intended for integration with previously developed computational tools and models, such as a composite hardness model and a grain growth model, for computational design of novel and improved cemented carbides with the aim to potentially substitute cobalt as the dominating binder phase in cemented carbides.

  • 50.
    Linder, David
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Walbrühl, Martin
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Borgenstam, Annika
    KTH, Tidigare Institutioner (före 2005), Materialvetenskap. KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Indentation behavior of highly confined elasto-plastic materialsInngår i: Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The effect of geometric confinement is well-known from hardness measurements of thin films on stiff substrates and has been modeled both phenomenologically and using e.g. Finite Element Analysis. However, these models are mainly focused on a specific experiment or a certain material family. In the present work, Finite Element Analysis is used to gain a better understanding of the interplay between geometric constraints in various microstructures and a wide range of materials properties. It is shown that a very simple model can be used to replicate thin film hardness data where the film is softer than the substrate as well as how materials properties alter the indentation behavior of materials confined in one to three dimensions. It is shown that qualitative agreement with nanoindentation of the metallic binder phase in the complex 3D-microstructure of a cemented carbide is achieved using an axisymmetric “pill-box” model with classical plasticity. It is also shown that the effect of higher-order confinement can be described by the Korsunsky thin film hardness model by re-optimizing the fitting parameters. 

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