| 1. |
- Borgenstam, Annika, et al.
(author)
-
Activation energy for isothermal martensite in ferrous alloys
- 1997
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In: Acta Materialia. - 1359-6454 .- 1873-2453. ; 45:2, s. 651-662
-
Journal article (peer-reviewed)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.
(author)
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Bainite in the light of rapid continuous cooling information
- 1996
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In: Metallurgical and Materials Transactions. A. - 1073-5623 .- 1543-1940. ; 27:6, s. 1501-1512
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Journal article (peer-reviewed)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
(author)
-
Certain aspects of the bainitic transformation in steel
- 2015
-
In: 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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Conference paper (peer-reviewed)
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| 4. |
- Borgenstam, Annika, et al.
(author)
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Critical temperature for growth of martensite
- 1995
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In: Acta Metallurgica Et Materialia. - Oxford : Pergamon. - 0956-7151. ; 43:3, s. 945-954
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Journal article (peer-reviewed)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.
(author)
-
Determination of the critical carbon content for growth of acicular ferrite
- 2005
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In: Solid-Solid Phase Transformations in Inorganic Materials 2005, Vol 1. ; , s. 105-110
-
Conference paper (peer-reviewed)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.
(author)
-
DICTRA, a tool for simulation of diffusional transformations in alloys
- 2000
-
In: Journal of phase equilibria (Print). - : Springer Science and Business Media LLC. - 1054-9714 .- 1544-1032 .- 1863-7345. ; 21:3, s. 269-280
-
Journal article (peer-reviewed)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.
(author)
-
Driving force for f.c.c.→b.c.c. martensites in Fe-X alloys
- 1997
-
In: Acta Materialia. - 1359-6454 .- 1873-2453. ; 45:5, s. 2079-2091
-
Journal article (peer-reviewed)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.
(author)
-
Eutectoids with cementite as the major constituent in Fe-C-M alloys
- 2016
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In: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 103, s. 280-289
-
Journal article (peer-reviewed)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.
(author)
-
Kinetics of bainite transformation in steels
- 2012
-
In: Phase Transformations in Steels. - : Elsevier. - 9781845699703 ; , s. 468-501
-
Book chapter (peer-reviewed)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.
(author)
-
Massive transformation in the Fe-Ni system
- 2000
-
In: Acta Materialia. - 1359-6454 .- 1873-2453. ; 48:11, s. 2765-2775
-
Journal article (peer-reviewed)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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| 11. |
- Borgenstam, Annika, et al.
(author)
-
Metallographic evidence of carbon diffusion in the growth of bainite
- 2009
-
In: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 57:11, s. 3242-3252
-
Journal article (peer-reviewed)abstract
- 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.
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| 12. |
- Borgenstam, Annika, et al.
(author)
-
Model for nucleation of isothermal martensite
- 2001
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In: Scripta Materialia. - 1359-6462 .- 1872-8456. ; 45:8, s. 917-922
-
Journal article (peer-reviewed)abstract
- 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.
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| 13. |
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| 14. |
- Borgenstam, Annika, et al.
(author)
-
Nucleation of isothermal martensite
- 2000
-
In: Acta Materialia. - 1359-6454 .- 1873-2453. ; 48:11, s. 2777-2785
-
Journal article (peer-reviewed)abstract
- 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.
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| 15. |
- Borgenstam, Annika, et al.
(author)
-
On the Symmetry Among the Diffusional Transformation Products of Austenite
- 2011
-
In: Metallurgical and Materials Transactions. A. - : Springer Science and Business Media LLC. - 1073-5623 .- 1543-1940. ; 42A:6, s. 1558-1574
-
Journal article (peer-reviewed)abstract
- 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.
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| 16. |
- Borgh, Ida, 1983-
(author)
-
Aspects of Structural Evolution in Cemented Carbide – Carbide Size, Shape and Stability
- 2013
-
Doctoral thesis (other academic/artistic)abstract
- Cemented carbide is a composite material used in applications like cutting tools and rock drilling inserts. The material commonly consists of WC grains embedded in a Co-rich binder phase and the material properties strongly depend on the WC grain size. Hence, to tailor the properties it is important to understand the fundamental mechanisms of grain coarsening. At the same time, the higher demands on material properties today also require new solutions. In the present work, some different aspects of structural evolutions in cemented carbides have been investigated.The first part of the work considers WC grain coarsening by means of size, size distribution and shape. Some efforts of the work have been to evaluate the effects of C-activity and initial WC powder size and distribution on the coarsening behavior in the material using different characterization techniques, e.g. scanning electron microscopy, and electron backscattered diffraction. Additionally, two earlier developed models are used and evaluated with the experimental data. The results indicate that the C-activity will affect size, size distribution and shape of the WC grains. It was also observed that the initial WC powder size and size distribution will have a large influence on the WC grain coarsening. The statistical shape was found to fit a spherical approximation but for individual grains both faceted and non-faceted shapes was observed. Steps and planar defects were observed supporting that the nucleation of new atomic layers is the main rate limiting mechanism for grain coarsening.The second part of this work considers the carbide phase stability in the (Ti,Zr)C system. The phase stability was investigated after synthesizing and aging a mixed (Ti,Zr)C using X-ray diffraction and different types of electron microscopy techniques. A decomposed lamellar structure was found with a composition variation of approximately 10% between the 50-75 nm thick lamellas. The experimental investigations were supported by computational work and the results were in good agreement. Additionally, two cemented carbide related systems were studied. A miscibility gap was found in the two investigated systems, (Ti,Zr,W)(C,N)-Co or Fe-graphite, and the effect of N2-gas pressure was investigated suggesting a critical N2-gas pressure below 0.1 bar.
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| 17. |
- Borgh, Ida, et al.
(author)
-
Effect of carbon activity and powder particle size on WC grain coarsening during sintering of cemented carbides
- 2014
-
In: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368. ; 42, s. 30-35
-
Journal article (peer-reviewed)abstract
- 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.
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| 18. |
- Borgh, Ida, et al.
(author)
-
Influence of nitrogen Gas pressure on the miscibility Gap in the Ti-Zr carbonitride system
- 2012
-
In: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368. ; 32, s. 11-15
-
Journal article (peer-reviewed)abstract
- 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.
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| 19. |
- Borgh, Ida, et al.
(author)
-
Microstructure, grain size distribution and grain shape in WC-Co alloys sintered at different carbon activities
- 2014
-
In: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368 .- 0958-0611. ; 43, s. 205-211
-
Journal article (peer-reviewed)abstract
- 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 degrees 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. (C) 2013 Elsevier Ltd. All rights reserved.
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| 20. |
- Borgh, Ida, et al.
(author)
-
On the three-dimensional structure of WC grains in cemented carbides
- 2013
-
In: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 61:13, s. 4726-4733
-
Journal article (peer-reviewed)abstract
- 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.
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| 21. |
- Chen, Hao, et al.
(author)
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Application of interrupted cooling experiments to study the mechanism of bainitic ferrite formation in steels
- 2013
-
In: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 61:12, s. 4512-4523
-
Journal article (peer-reviewed)abstract
- 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.
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| 22. |
- Chou, Chia-Ying, et al.
(author)
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Influence of solidification structure on austenite to martensite transformation in additively manufactured hot-work tool steels
- 2021
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In: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 215
-
Journal article (peer-reviewed)abstract
- The microstructure of a hot-work tool steel additively manufactured using laser powder-bed fusion (L-PBF), and its response to post heat treatment, is studied in detail by microstructure characterization and computational thermodynamics and kinetics. The high solidification and cooling rates during the L-PBF process lead to suppression of delta-ferrite and instead solidification of an austenite phase directly containing a cellular substructure where the alloying elements have segregated to the inter-cellular regions and where solidification carbides have formed in the cell junctions. The austenite is then partly decomposed into martensite at lower temperatures. The micro-segregation can be predicted by reducing the complex solidification behavior to a diffusion problem in one dimension enabling detailed comparisons with the measured segregation profiles quantified at a nanometer scale. Martensite start temperature (M-s) calculations along the spatially varying composition show that the M-s temperature decreases in the inter-cellular regions where austenite is observed. The network of austenite in the as-built microstructure can be understood from the combined influence of the composition dependence of the M-s temperature in relation to the build plate temperature and the mechanical stabilization of the small-sized austenite regions. This work demonstrates the power of computational tools based on computational thermodynamics and kinetics for designing tool steels for additive manufacturing by predictions of the steel's response to the L-PBF process and post heat treatments.
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| 23. |
- Chou, Chia-Ying, 1990-
(author)
-
Integrated Computational and Experimental Study of Additively Manufactured Steels
- 2023
-
Doctoral thesis (other academic/artistic)abstract
- The design freedom Additive Manufacturing (AM) offers provides new solutions for improving functionality in industrial applications. It also offers unique opportunities when it comes to materials design.Powder Bed Fusion – Laser Beam (PBF-LB) is currently one of the most popular commercial AM techniques for metallic materials partly due to the relatively low surface roughness and the large design flexibility. However, the number of materials suitable for the PBF-LB process is still rather low and to accelerate the development of grades tailored for this AM process, dedicated computational tools for alloy design are needed. Of importance for materials design, is computational thermodynamics and kinetics coupled with CALPHAD materials descriptions since it enables calculations for multicomponent materials making it possible to predict the effect of varying composition.In this thesis, computational thermodynamic and kinetics coupled with materials characterization are applied to study the microstructure evolution during PBF-LB. Two material classes are in focus – hot-work tool steels and ferritic stainless steels. For the hot-work tool steel, the cooling rates during PBF-LB processing are high enough to induce martensite transformation and in the as-built microstructure a martensitic matrix is observed and some fraction of retained austenite. A solidification sub-structure due to micro-segregation during printing is also observed. Solidification calculations are performed to predict the micro-segregation showing agreement with experimental measurements. The segregation results are then used as input to a semi-empirical martensite start temperature model making it possible to explain the location and amount of retained austenite.In addition to compositional optimization, a computational framework for AM alloy design needs to include the possibility to tailor the AM post heat treatments. An alternative to the conventional hardening treatment is thus studied in the current work. A model for precipitation kinetics is combined with experimental characterization to explore the effect of tempering on the as-built microstructure in comparison to the tempering effect on an austenitized microstructure. The results show that the precipitation kinetics is strongly dependent on the starting structure and that direct tempering of the as-built microstructure changes the precipitation sequence compared to the conventional heat treatment route. The calculations reproduce this result suggesting that it is a thermodynamic effect stemming from different matrix compositions.The other material class, the ferritic stainless steels, is studied in terms of its response to post-heat treatments. The as-built microstructure is characterized by high dislocation density and a fine grain structure in some cases as well as a solidification sub-structure. The mechanical properties of the as-built material are in general good for these steels, however, stress relieving is most often a required post process for the as-built components which may decrease the mechanical properties. To maximize the gained benefits from the unique process condition of PBF-LB, simulations are applied to study the possibility of post heat treatment optimization.To construct a computational framework for AM materials design, multiscale modeling capabilities are needed. This work shows the value of computational thermodynamics and kinetics for understanding the materials behavior on the microscale and hence, contributes to the construction of such a framework. By understanding the physical metallurgy, and enable modeling of the AM processes, the industrialization of AM can be accelerated.
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| 24. |
- Hedström, Peter, et al.
(author)
-
On the Three-Dimensional Microstructure of Martensite in Carbon Steels
- 2012
-
In: Proceedings Of The 1st International Conference On 3D Materials Science. - Hoboken, NJ, USA : John Wiley & Sons. - 9781118470398 - 9781627489331 ; , s. 19-24
-
Conference paper (peer-reviewed)abstract
- 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.
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| 25. |
- Heshmati, Nader, et al.
(author)
-
Correlation between microstructure and fatigue properties of hot-rolled thick-plate complex-phase steel
- 2023
-
In: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 885
-
Journal article (peer-reviewed)abstract
- Complex-phase (CP) steels, with a multiphase microstructure, offer an excellent combination of high strength, ductility, and formability, making them an attractive alternative to conventional high-strength low-alloy (HSLA) steels in the automotive industry. However, the microstructure and fatigue property relation in CP steels is complex. This limits the full exploitation of CP steels in applications, such as heavy-vehicles, where excellent fatigue performance of thick-plates after punching holes is the critical parameter. In this work, we initiate the study of the relation between microstructure and fatigue properties of a commercial CP steel (800CP) and compare it with a conventional HSLA (500MC) steel. Fatigue property, tensile property, and fatigue crack growth rate (FCGR) testing are conducted and the performance of the two steels is rationalized using detailed microstructure characterization, before and after fatigue testing. FCGR testing shows that, despite a higher yield strength of the 800CP, both steels have a similar propagation rate due to a more tortuous crack propagation path and a higher quantity of secondary crack formation in the 800CP microstructure. The high cycle fatigue (HCF) testing shows that the fatigue limit in the 800CP is 25% higher. This increase in fatigue limit is attributed to the improved resistance to fatigue crack initiation in the 800CP due to its larger fraction of bainite.
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| 26. |
- Hillert, Mats, et al.
(author)
-
Centennial of the Diffusionless Paradigm of Bainite
- 2012
-
In: Metallurgical and Materials Transactions. A. - : Springer Science and Business Media LLC. - 1073-5623 .- 1543-1940. ; 43A:12, s. 4487-4495
-
Journal article (peer-reviewed)abstract
- 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.
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| 27. |
- Hillert, Mats, et al.
(author)
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Do bainitic and Widmanstatten ferrite grow with different mechanisms?
- 2010
-
In: Scripta Materialia. - : Elsevier BV. - 1359-6462 .- 1872-8456. ; 62:2, s. 75-77
-
Journal article (peer-reviewed)abstract
- 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.
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| 28. |
- Holländer Pettersson, Niklas, et al.
(author)
-
Formation of Chromium Nitride and Intragranular Austenite in a Super Duplex Stainless Steel
- 2019
-
In: Metallurgical and Materials Transactions. A. - : Springer. - 1073-5623 .- 1543-1940. ; 50:12, s. 5594-5601
-
Journal article (peer-reviewed)abstract
- 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.
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| 29. |
- Holländer Pettersson, Niklas
(author)
-
Precipitation in Advanced Stainless Steels
- 2018
-
Doctoral thesis (other academic/artistic)abstract
- Advanced stainless steels often suffer from complex precipitation behaviour due to the high levels of alloying elements needed to obtain their attractive properties. This demands a correct handling of the material to avoid formation of harmful phases during production, manufacturing and service. An understanding of the precipitation behaviour is thus essential in order to maintain the properties. This thesis has focused on two critical aspects for the duplex stainless steels, namely the precipitation of chromium nitrides and the phase separation in ferrite at low temperatures.Phase separation can compromise the structural stability of duplex stainless steels if they are used in certain temperature ranges for a prolonged period of time. The degradation of mechanical properties can occur slowly and its relation to the evolving nanostructure is of interest for lifetime predictions. The nanostructure evolution and corresponding mechanical property changes during aging has therefore been studied for the duplex stainless steel 2507 by advanced characterisation, property testing and fractography. The loss of toughness and transition in fracture behaviour can be related to a reduced dislocation mobility in the decomposed ferrite as evident by the increased tendency for cleavage failure and deformation twinning. A simple relation between the chromium fluctuations, hardness and impact toughness could be identified.Nitrides are formed isothermally if the material is held at a critical temperature for sufficient time, but the duplex stainless steels are also sensitive to non-equilibrium precipitation during cooling from high temperatures, for example, as in welding. A series of microscopy techniques and heat treatments in a dilatometer have been applied to study the precipitation behaviour of 2507 and its dependence on cooling rate and microstructure. The CrN, in addition to the expected Cr2N, was found to form upon rapid cooling. Electron diffraction studies revealed that CrN is most likely a forerunner to Cr2N which nucleates at the former. The Cr2N in turn acts as nucleation sites for intragranular secondary austenite if a second short-term heat treatment is made, a situation that might occur during multipass welding.The non-equilibrium nitride formation is closely related to the austenite stability and transformation during cooling, and the ferrite grain size. The high-temperature phase equilibria were therefore studied for four duplex stainless steels using in situ neutron scattering. The existence of a fully ferritic state was confirmed and the austenite stability measured. The data can be used for improving the precision of CALPHAD databases for the duplex stainless steels.
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| 30. |
- Hou, Ziyong, 1986-, et al.
(author)
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Evaluating magnetic properties of composites from model alloys – Application to alternative binder cemented carbides
- 2019
-
In: Scripta Materialia. - : Acta Materialia Inc. - 1359-6462 .- 1872-8456. ; 168, s. 96-99
-
Journal article (peer-reviewed)abstract
- 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.
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| 31. |
- Huyan, Fei, et al.
(author)
-
A Thermodynamic-Based Model to Predict the Fraction of Martensite in Steels
- 2016
-
In: Metallurgical and Materials Transactions. A. - : Springer. - 1073-5623 .- 1543-1940. ; 47A:9, s. 4404-4410
-
Journal article (peer-reviewed)abstract
- 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.
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| 32. |
- Huyan, Fei
(author)
-
Computational Materials Design of Medium Mn Steels
- 2018
-
Doctoral thesis (other academic/artistic)abstract
- Medium Mn steels (~ 3–10 mass % Mn), a new category of advanced high strength steels, attracted worldwide research interests recent years due to their excellent mechanical properties and low cost. These steels have fine microstructures and contain large fraction of metastable retained austenite (~ 30 volume %), therefore exhibit excellent strength and elongation. The fine microstructure is mainly introduced by an intercritical annealing process.To accelerate the design of such steels, materials design is applied. The materials design concept is a systematic method. Contrary to conventional methods largely based on trial and error, it is based on the classical processing–structure–properties relationships and a quantitative knowledge of each relation represented by a mathematical model, so-called linkage model. Such models are thus an essential part in materials design.The present thesis aims to develop a framework used for materials design of medium Mn steel. The development of models which serve as linkage tools is thus the focus. Tensile properties, i.e. strength and elongation, are set as the design objectives driven by the industrial application.The major part is concentrated on the linkage tools of processing–structure, i.e. models and simulations to predict the microstructure evolution associated with processing. These linkage tools are based on thermodynamic calculations and kinetic simulations using the commercially available Thermo-Calc and DICTRA software. To be specific, the processing involves austenitization and quenching as well as intercritical annealing and quenching; while the associated structure involves transformation of austenite to martensite and reversion of martensite to austenite. Therefore the following aspects have been studied:martensite fraction with undercooling;austenite reversion during intercritical annealing;influence of austenite grain size on martensite start temperature;mechanical stability of retained austenite.Besides these, prediction of tensile properties is studied in the last part, which serves as an example of a linkage tool of structure–properties.Via integrating these models, to achieve certain tensile properties, the required microstructure and the associated processing can be traced back.
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| 33. |
- Huyan, Fei, et al.
(author)
-
Experimental Determination and Thermodynamic Modeling of Msσ for a Medium Mn Steel
-
Other publication (other academic/artistic)abstract
- 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.
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| 34. |
- Huyan, Fei, et al.
(author)
-
Influence of Sub-micron Austenite Grain Size on Martensitic Transformation in a Medium Mn Steel
-
Other publication (other academic/artistic)abstract
- 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.
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| 35. |
- Huyan, Fei, et al.
(author)
-
Modelling of the fraction of martensite in low-alloy steels
- 2015
-
In: MATERIALS TODAY-PROCEEDINGS. - : Elsevier. - 2214-7853. ; , s. 561-564
-
Conference paper (peer-reviewed)abstract
- 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.
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| 36. |
- Huyan, Fei, et al.
(author)
-
On the tuning of austenite stability in a medium mn trip steel
- 2015
-
In: 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. 933-934
-
Conference paper (peer-reviewed)
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| 37. |
- Huyan, Fei, et al.
(author)
-
Simulation of the Growth of Austenite from As-Quenched Martensite in Medium Mn Steels
- 2018
-
In: Metallurgical and Materials Transactions. A. - : Springer. - 1073-5623 .- 1543-1940. ; 49A:4, s. 1053-1060
-
Journal article (peer-reviewed)abstract
- 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.
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| 38. |
- Jeppsson, Johan, et al.
(author)
-
Inverse Saltykov analysis for particle-size distributions and their time evolution
- 2011
-
In: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 59:3, s. 874-882
-
Journal article (peer-reviewed)abstract
- 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.
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| 39. |
- Kohne, Thomas, et al.
(author)
-
Correlation of Heterogeneous Local Martensite Tetragonality and Carbon Distribution in High Carbon Steel
- 2022
-
In: Materials. - : MDPI AG. - 1996-1944. ; 15:19, s. 6653-
-
Journal article (peer-reviewed)abstract
- A novel approach for the correlation of local martensite tetragonality determined by electron backscatter diffraction and carbon distribution by atom probe tomography (APT) is presented. The two methods are correlated by site-specific sample preparation for APT based on the local tetragonality. This approach is used to investigate the local carbon distribution in high carbon steel with varying local martensite tetragonality. Regions with low tetragonality show clear agglomeration of carbon based on statistical nearest neighbour (NN) analysis, while regions with high tetragonality show only small elongated agglomerations of carbon and no significant clustering using NN analysis. The APT average bulk carbon content shows no quantitative difference between regions with low and high tetragonality, indicating that no significant long-range diffusion of carbon has taken place.
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| 40. |
- Kohne, Thomas, 1990-, et al.
(author)
-
Early Martensitic Transformation in a 0.74C–1.15Mn–1.08Cr High Carbon Steel
- 2022
-
In: Metallurgical and Materials Transactions. A. - : Springer Nature. - 1073-5623 .- 1543-1940. ; 53:8, s. 3034-3043
-
Journal article (peer-reviewed)abstract
- The martensitic transformation in a high carbon steel was studied by a new experimental approach focusing on the nucleation and growth as well as the variant pairing of the early-formed martensite. A mixed microstructure with tempered early-formed martensite and fresh later-formed martensite was achieved by a heat treatment with an isothermal hold below the martensite start temperature. In-situ high-energy X-ray diffraction showed no further transformation of austenite to ferrite/martensite during the isothermal hold. The tempered early-formed martensite was characterized with a combination of light optical microscopy and local tetragonality determination by electron backscatter diffraction. The characterization allowed qualitative as well as quantitative analysis of the tempered early-formed martensite with regard to the prior austenite grain boundaries (PAGB) and variant pairing. The early-formed martensite was shown to grow predominantly along the PAGBs and clustering was observed indicating an autocatalytic nucleation mechanism. The variant pairing of the early-formed martensite had a stronger plate character compared to the later-formed martensite.
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| 41. |
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| 42. |
- Kohne, Thomas, 1990-, et al.
(author)
-
Evolution of Martensite Tetragonality in High-Carbon Steels Revealed by In Situ High-Energy X-Ray Diffraction
- 2023
-
In: Metallurgical and Materials Transactions. A. - : Springer. - 1073-5623 .- 1543-1940. ; 54:4, s. 1083-1100
-
Journal article (peer-reviewed)abstract
- The martensitic transformation was studied by in situ and ex situ experiments in two high-carbon, 0.54 and 0.74 wt pct C, steels applying three different cooling rates, 15 °C/s, 5 °C/s, and 0.5 °C/s, in the temperature range around Ms, to improve the understanding of the evolution of martensite tetragonality c/a and phase fraction formed during the transformation. The combination of in situ high-energy X-ray diffraction during controlled cooling and spatially resolved tetragonality c/a determination by electron backscatter diffraction pattern matching was used to study the transformation behavior. The cooling rate and the different Ms for the steels had a clear impact on the martensitic transformation with a decrease in average tetragonality due to stronger autotempering for a decreasing cooling rate and higher Ms. A slower cooling rate also resulted in a lower fraction of martensite at room temperature, but with an increase in fraction of autotempered martensite. Additionally, a heterogeneous distribution of martensite tetragonality was observed for all cooling rates. © 2023, The Author(s).
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| 43. |
- Kohne, Thomas, 1990-, et al.
(author)
-
Impact of Cooling Rate during High-Pressure Gas Quenching on Fatigue Performance of Low Pressure Carburized Gears
- 2022
-
In: Metals. - : MDPI AG. - 2075-4701. ; 12:11, s. 1917-1917
-
Journal article (peer-reviewed)abstract
- The impact of cooling rate during high-pressure gas quenching on the fatigue performance of low-pressure carburized spur gears was studied for steel grades 20MnCr5 and 17NiCrMo6-4. The results show an increased fatigue limit by 10 to 11% when applying a slower cooling rate for both steel grades. Moreover, for 20MnCr5 the slower cooled gears show an increase in compressive residual stresses by 130 MPa compared to the faster cooling, although no significant difference was observed for 17NiCrMo6-4. It is also seen that the cooling rate affects the core hardness for both steel grades, while other properties like surface hardness, case-hardness depth and martensite variant pairing were unaffected. The results for the retained austenite content and average martensite unit size show no clear effect of the cooling rate. The possible influence of different carbon distributions after quenching for the two used cooling rates on the carbide precipitation and fatigue limit is discussed. View Full-Text
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| 44. |
- Kohne, Thomas, 1990-
(author)
-
Martensitic Transformation in High Carbon Steels - Martensite Tetragonality and Impact of Cooling Rate
- 2022
-
Doctoral thesis (other academic/artistic)abstract
- Case-hardened gears are an essential part of our every day life in their role of power transmission in cars and heavy-duty trucks. The increasing use of low pressure carburizing (LPC) for case-hardening in combination with high pressure gas quenching (HPGQ) for cooling opens new possibilities in the design of heat treatments for improved fatigue performance. The use of slower cooling rates through the martensitic transformation regime of the case-hardened gear surface layer has been shown to result in significant fatigue improvements of up to 20 %. The goal of this work is to understand the underlying mechanism of this fatigue improvement. The work is split into investigations of the martensitic transformation in model carbon steels with a homogeneous carbon content and an industrial scale investigation of fatigue performance of LPC and HPGQ case-hardened gears with an in-depth characterization.The impact of three cooling rates, 15, 5 and 0.5 ºC/s, and martensite start temperature (Ms) on the evolution of the martensite tetragonality and phase fraction during cooling is studied by in-situ high energy X-ray diffraction (HEXRD) and electron backscatter diffraction (EBSD). This was done for two high carbon steels, 0.54 wt% C and 0.74 wt% C, to better understand the transformation in different parts of the case-hardened layer. The martensite tetragonality shows a heterogeneous distribution for all samples with a stronger reduction in tetragonality for slower cooling and higher Ms temperature indicating that autotempering occurs during cooling.The correlation of the local martensite tetragonality determined by EBSD and carbon content in atom probe tomography (APT) is investigated for the 0.74 wt% C samples to clarify if autotempering has taken place during cooling. A novel approach is used for the selection of the site-specific lift-out position of the APT tips based on EBSD maps of local martensite tetragonality with low and high tetragonality values. The average bulk carbon content of low and high tetragonality regions have similar values compared to the nominal composition, but there are clear differences in the carbon distribution. The low tetragonality regions have a stronger heterogeneous carbon distribution with homogeneously distributed carbon enriched or depleted volumes, while the high tetragonality regions show a more homogeneous carbon distribution with few elongated volumes with carbon agglomeration. The clear correlation of the local martensite tetragonality with the local carbon distribution shows that autotempering takes place during cooling.The industrial scale investigation of two steel grades, 20MnCr5 and 17NiCrMo6-4, reproduced the positive effect of slower cooling during HPGQ on the fatigue performance. The surface hardness, case-hardness depth and martensite variant pairing can be excluded as possible mechanisms of the fatigue improvement based on the characterization results, while only small differences are observed for the core hardness, retained austenite content and martensite unit size distribution. The residual stress profile provides no consistent explanation for the improvement since a clear change is only observed for 20MnCr5, whereas 17NiCrMo6-4 shows no effect. The possible effect of different degrees of autotempering for slow and fast cooling on the carbide precipitation kinetics resulting in the fatigue improvement is discussed based on the results from the model steel studies and literature.Finally, the early stages of the martensitic transformation in the 0.74 wt% C samples is studied due to the observation of a two-step process in the transformation kinetics. A heat treatment with isothermal hold at the end of the first stage slightly below Ms temperature is used to achieve a mixed microstructure consisting of tempered martensite formed before the isothermal hold and fresh martensite formed afterwards. The tempered martensite units are determined based on their tetragonality in EBSD and a good agreement with conventional light optical microscopy (LOM) is observed. This opens new possibilities for in-depth statistical analysis. The early formed martensite units are formed and grow dominantly along the prior austenite grain boundary (PAGB). Additionally, the large martensite units are mainly formed in the first transformation step with a strong plate martensite crystallographic character.Overall, this thesis studies the martensitic transformation in high carbon steels with a focus on martensite tetragonality and the impact of the cooling rate during HPGQ. Clear influences of the cooling rate on martensite tetragonality and fatigue performance are observed. In addition, new experimental approaches for the local correlation of martensite tetragonality and carbon content as well as for the determination of tempered martensite in a mixed microstructure are presented.Keywords: High carbon steel, Martensitic transformation, Martensite tetragonality, Fatigue performance
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| 45. |
- Kolmskog, Peter, et al.
(author)
-
Direct Observation that Bainite can Grow Below M-S
- 2012
-
In: Metallurgical and Materials Transactions. A. - : Springer Science and Business Media LLC. - 1073-5623 .- 1543-1940. ; 43A:13, s. 4984-4988
-
Journal article (peer-reviewed)abstract
- 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.
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| 46. |
- Kolmskog, Peter, 1983-
(author)
-
Does Bainite form with or without diffusion? : The experimental and theoretical evidence
- 2013
-
Doctoral thesis (other academic/artistic)abstract
- With the increased interest in bainitic steels, fundamental understanding of the bainite transformationis of major importance. Unfortunately, the research on bainite has been hampered by an oldcontroversy on its formation mechanism. Over the years two quite different theories have developedclaiming to describe the bainite transformation i.e. the diffusionless and the diffusion controlledtheory. In this thesis, attention is directed towards fundamental understanding of the bainitetransformation and both experimental and theoretical approaches are used in order to reveal its truenatureIn the first part of this thesis the symmetry in the Fe-C phase diagram is studied. It is based on ametallographic mapping of microstructures using light optical microscopy and scanning electronmicroscopy in a high carbon steel. The mapping revealed symmetries both with respect to temperatureand carbon content and an acicular eutectoid with cementite as the leading phase was found andidentified as inverse bainite. By accepting that all the eutectoid microstructures forms by diffusion ofcarbon, one may explain the existence of symmetries in the Fe-C phase diagram. Additional supportof its existence is obtained from an observation of symmetries in an alloyed steel. From the performedwork it was concluded that the existence of symmetries among the eutectoid microstructures fromaustenite supports the idea that bainite forms by a diffusion controlled transformation.In the second part the growth of bainite is considered. An experimental study using laser scanningconfocal microscopy was performed and growth rates of the transformation products from austenite ina high carbon, high chromium steel was analysed. The growth rate measurements reveals the kineticrelation between Widmanstätten cementite and the acicular eutectoid previously identified as inversebainite which confirms its existence and the conclusions drawn in the first part. In addition, in-situobservations of bainite formation below Ms provide additional support for the diffusion controlledtheory for bainite formation.The final part of the work is a study of the critical conditions for the formation of acicular ferrite.Based on experimental information found in the literature a thermodynamic analysis is performed inview of the two theories. The results demonstrate that the governing process for Fe-C alloys cannot bediffusionless but both kinds of processes can formally be used for predicting Bs temperatures for Fe-Calloys.
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| 47. |
- Kolmskog, Peter, et al.
(author)
-
Eutectoid Transformations in 4.12 Mass Pct Cr 0.88 Mass Pct C Steel
- 2011
-
In: Metallurgical and Materials Transactions. A. - : Springer Science and Business Media LLC. - 1073-5623 .- 1543-1940. ; 42A:13, s. 3941-3951
-
Journal article (peer-reviewed)abstract
- 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.
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| 48. |
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| 49. |
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| 50. |
- Lamelas, Victor, et al.
(author)
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Broadening of the carbon window and the appearance of core-rim carbides in WC-Fe/Ni cemented carbides
- 2024
-
In: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 999
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Journal article (peer-reviewed)abstract
- Among several separate challenges, the major one for replacing cobalt in cemented carbides is the difficulty to obtain alternative binder materials with a C-window broad enough to be robustly processed under conventional industrial control on the C content. The C-window is defined as the C content range for which phases that are detrimental to the mechanical properties are avoided. The present paper has two main objectives: first, to show that the processing C-window of Fe-Ni based systems is in fact wider than what thermodynamic equilibrium calculations predict, and that its width can be controlled moderately by tweaking the initial WC grain size and the cooling rate used in the material's processing. Secondly, in case those detrimental phases are not avoided, this work gives insight on how to make their appearance less detrimental for the mechanical properties. The morphology, volume fraction and particle size distribution of the detrimental phases, specifically η6-carbides at low C contents, are investigated to explore desirable combination of hardness and toughness of alternative binder cemented carbides. During this study it was also discovered that in samples with carbon contents below the low-C limit of the C window a carbide with hexagonal lattice known as κ, not commonly seen in cemented carbides, appeared and formed the core of a core-rim structure together with the more common η6-phase. It is believed that the κ-carbide form due to local high concentrations of tungsten during solid state sintering and that it has an impact on the precipitation characteristics of the η6-phase.
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