| 1. |
- Borgenstam, Annika, et al.
(författare)
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Activation energy for isothermal martensite in ferrous alloys
- 1997
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 45:2, s. 651-662
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Borgenstam, Annika, et al.
(författare)
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Bainite in the light of rapid continuous cooling information
- 1996
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Ingår i: Metallurgical and Materials Transactions. A. - 1073-5623 .- 1543-1940. ; 27:6, s. 1501-1512
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Borgenstam, Annika
(författare)
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Certain aspects of the bainitic transformation in steel
- 2015
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Ingår 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. - 9780692437360 ; , s. 69-70
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Konferensbidrag (refereegranskat)
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| 4. |
- Borgenstam, Annika, et al.
(författare)
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Critical temperature for growth of martensite
- 1995
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Ingår i: Acta Metallurgica Et Materialia. - Oxford : Pergamon. - 0956-7151. ; 43:3, s. 945-954
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Borgenstam, Annika, et al.
(författare)
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Determination of the critical carbon content for growth of acicular ferrite
- 2005
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Ingår i: Solid-Solid Phase Transformations in Inorganic Materials 2005, Vol 1. ; , s. 105-110
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 6. |
- Borgenstam, Annika, et al.
(författare)
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DICTRA, a tool for simulation of diffusional transformations in alloys
- 2000
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Ingår i: Journal of phase equilibria (Print). - : Springer Science and Business Media LLC. - 1054-9714 .- 1544-1032 .- 1863-7345. ; 21:3, s. 269-280
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Borgenstam, Annika, et al.
(författare)
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Driving force for f.c.c.→b.c.c. martensites in Fe-X alloys
- 1997
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 45:5, s. 2079-2091
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 8. |
- Borgenstam, Annika, et al.
(författare)
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Eutectoids with cementite as the major constituent in Fe-C-M alloys
- 2016
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 103, s. 280-289
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Borgenstam, Annika, et al.
(författare)
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Kinetics of bainite transformation in steels
- 2012
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Ingår i: Phase Transformations in Steels. - : Elsevier. - 9781845699703 ; , s. 468-501
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Bokkapitel (refereegranskat)abstract
- 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.
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| 10. |
- Borgenstam, Annika, et al.
(författare)
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Massive transformation in the Fe-Ni system
- 2000
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 48:11, s. 2765-2775
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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