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Applications of Computational Thermodynamics and Kinetics on Transformations in Stainless Steels
KTH, School of Industrial Engineering and Management (ITM).
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Stainless steels are high-alloyed, usually with multiple components and often also dual matrix phases, as for duplex stainless steels. This make predictions and calculations of alloying effects on equilibria and transformations complicated. Computational thermodynamics has emerged as an indispensable tool for calculations within these complex systems with predictions of equilibria and precipitation of phases. This thesis offers examples illustrating how computational methods can be applied both to thermodynamics, kinetics and coarsening of stainless steels in order to predict microstructure and, to some extent, also properties. The performance of a current state-of-the-art commercial thermodynamic database was also explored and strengths and weaknesses highlighted.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2013. , 60 p.
Keyword [en]
Stainless Steels, Duplex, Ferrite, Austenite, Computational Thermodynamics, Thermo-Calc, Dictra
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-121337ISBN: 978-91-7501-714-3 (print)OAI: oai:DiVA.org:kth-121337DiVA: diva2:618463
Public defence
2013-05-16, B2, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20130429

Available from: 2013-04-29 Created: 2013-04-28 Last updated: 2013-04-29Bibliographically approved
List of papers
1. On Phase Equilibria in Duplex Stainless Steels
Open this publication in new window or tab >>On Phase Equilibria in Duplex Stainless Steels
2010 (English)In: STEEL RES INT, ISSN 1611-3683, Vol. 81, no 5, 337-346 p.Article in journal (Refereed) Published
Abstract [en]

The equilibrium conditions of four duplex stainless steels; Fe-23Cr-4.5Ni-0.1N, Fe-22Cr-5.5Ni-3Mo-0.17N, Fe-25Cr-7Ni-4Mo-0.27N and Fe-25Cr-7Ni-4Mo-1W-1.5Cu-0.27N were studied in the temperature region from 700 to 1000 degrees C. Phase compositions were determined with SEM EDS and the phase fractions using image analysis on backscattered SEM images. The results showed that below 1000 degrees C the steels develop an inverse duplex structure with austenite and sigma phase, of which the former is the matrix phase. With decreasing temperature, the microstructure will be more and more complex and finely dispersed. The ferrite is, for the higher alloyed steels, only stable above 1000 degrees C and at lower temperatures disappears in favour of intermetallic phases. The major intermetallic phase is sigma phase with small amounts of chi phase, the latter primarily in high Mo and W grades. Nitrides, not a focus in this investigation, were present as rounded particles and acicular precipitates at lower temperatures. The results were compared to theoretical predictions using the TCFE5 and TCFE6 databases.

Keyword
Duplex Stainless Steel, Phase Equilibrium, Intermetallic Phases, Sigma Phase, Thermo-Calc
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-27708 (URN)10.1002/srin.201000010 (DOI)000278292200002 ()
Note
QC 20101220Available from: 2010-12-20 Created: 2010-12-20 Last updated: 2013-04-29Bibliographically approved
2. Evaluation of the WRC 1992 diagram using computational thermodynamics
Open this publication in new window or tab >>Evaluation of the WRC 1992 diagram using computational thermodynamics
2013 (English)In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 57, no 3, 305-314 p.Article in journal (Refereed) Published
Abstract [en]

Ferrite content of a stainless steel weld metal is a vital parameter for ensuring that the microstructure and thus also corrosion and mechanical properties are adequate. A rapid way of estimating ferrite content and thus microstructure is by using weld metal composition and a Welding Research Council (WRC)-1992 diagram. The ferrite-forming alloying elements are estimated to a chromium equivalent, the austenitic to a nickel equivalent and the ferrite content is estimated by help of isoferrite lines. This diagram was derived by empirical work on a large number of commercial and laboratory stainless weld metals covering a wide alloy span. The present paper uses computational thermodynamics, i.e. Thermo-Calc, to evaluate the influence of temperature and key stainless alloying elements. The alloying range for duplex stainless weld metal was studied and the WRC 1992 diagram was compared with results from calculations. The chromium and nickel equivalents were evaluated and coefficients for Mo, Nb, C, N and Cu assessed. The results showed that while the coefficients for Mo and N proved accurate, the values for Nb, C and Cu would require an adjustment. Alternative diagrams with adjusted chromium and nickel equivalents and iso-ferrite lines in both ferrite numbers and volume percent were suggested.

Keyword
Stainless steel, Simulating, Thermodynamics
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-121333 (URN)10.1007/s40194-013-0025-7 (DOI)000317621800004 ()
Note

QC 20130603

Available from: 2013-04-28 Created: 2013-04-28 Last updated: 2017-12-06Bibliographically approved
3. Computational thermodynamics study of the influence of tungsten in superduplex stainless weld metal
Open this publication in new window or tab >>Computational thermodynamics study of the influence of tungsten in superduplex stainless weld metal
2012 (English)In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 56, no 11-12, 79-87 p.Article in journal (Refereed) Published
Abstract [en]

Tungsten is used by some duplex stainless steel producers for partial substitution of molybdenum, both elements enhancing the corrosion resistance of duplex stainless steel. The negative aspect of both molybdenum and tungsten alloying is that they increase the tendency to precipitate intermetallic phases, which may have a detrimental effect on corrosion and mechanical properties. The temperature region for intermetallic phase precipitation is about 700-1000 degrees C, depending on alloy composition, and the time for precipitation is within minutes for superduplex steels. There has been scientific discussion on the relative effects of Mo alone or Mo-W on intermetallic precipitation behaviour in superduplex steels for the past two decades. While the base material response to ageing and precipitation of intermetallic phases has been satisfactorily assessed, weld metal has proved more of a challenge. The main reason for this is that welding is a very complex process introducing many parameters to the assessment which not have to be considered in studies of base material. For example superduplex weld metal typically solidify fully ferritic but may in case excessive nitrogen is added solidify as a mixture of ferrite and austenite. The solidification mode may vary also between weld passes as a consequence of minor variations in composition. Ferritic solidification is the preferred mode, giving the well known Widmanstatten austenite, which forms in the solid state during cooling. Mixed mode solidification gives a vermicular appearance, which is known to increases the tendency to intermetallic formation. A comprehensive study using computational thermodynamics was done to investigate this matter. This study included equilibrium calculations, Scheil-Gulliver solidification simulations, and calculations of the driving force for intermetallic phase precipitation and further a study of diffusion of Mo and W in these alloy systems. The different approaches were applied on model superduplex weld metals with nominal compositions matching commercial superduplex fillers available today. The principal conclusion is that all thermodynamic calculations clearly indicates the W containing filler to show a more pronounced sensitivity to heat treatments by precipitation of intermetallic phases.

Keyword
Computation, Thermodynamics, Calculation, Duplex stainless steel, Weld metal
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-121332 (URN)10.1007/BF03321398 (DOI)000311195500009 ()
Note

QC 20130502

Available from: 2013-04-28 Created: 2013-04-28 Last updated: 2017-12-06Bibliographically approved
4. Evaluation of Austenite Reformation in Duplex Stainless Steel Weld Metal using Computational Thermodynamics
Open this publication in new window or tab >>Evaluation of Austenite Reformation in Duplex Stainless Steel Weld Metal using Computational Thermodynamics
2014 (English)In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 58, no 2, 217-224 p.Article in journal (Refereed) Published
Abstract [en]

This paper provides a theoretical study of austenite reformation during cooling of duplex stainless weld metal using computational thermodynamics and kinetics. Model alloys of the superduplex 2509 weld metal and 2205 base material were used for the study and austenite reformation was calculated at equilibrium conditions and at cooling rates from 0.01 to 2,000 K/s. The calculations also provided the possibility to study the distribution of the alloying elements at ferrite and austenite phase boundaries. The importance of nitrogen for austenite reformation at rapid cooling was studied by comparing with simulations for a low-nitrogen alloy. The results were also compared with experimental and model results from the literature providing good correlation and the unique possibility to study the distribution of alloying elements between the growing austenite and the shrinking ferrite.

Keyword
Duplex stainless steel, Weld metal, Austenite, Mathematical models, Thermodynamics
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-121334 (URN)10.1007/s40194-013-0104-9 (DOI)000331002600007 ()2-s2.0-84898992877 (Scopus ID)
Note

QC 20140128. Updated from submitted to published Epub ahead of print

Available from: 2013-04-28 Created: 2013-04-28 Last updated: 2017-12-06Bibliographically approved
5. On the effect of nickel substitution in duplex stainless steel
Open this publication in new window or tab >>On the effect of nickel substitution in duplex stainless steel
Show others...
2008 (English)In: Materials Science and Technology, ISSN 0267-0836, E-ISSN 1743-2847, Vol. 24, no 3, 348-355 p.Article in journal (Refereed) Published
Abstract [en]

The present experimental and theoretical study investigates the effect of nickel on the phase balance and resulting properties of a 22Cr duplex stainless steel. The decrease in nickel was balanced by nitrogen and manganese additions. It was found that a minimum nickel content was required to maintain mechanical and corrosion properties at technically relevant levels. The influence of increasing nitrogen content on resulting phase composition and properties is discussed.

Keyword
duplex, stainless, steel, nitrogen, low nickel
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-27712 (URN)10.1179/174328408X276116 (DOI)000255903900015 ()
Note
QC 20101220Available from: 2010-12-20 Created: 2010-12-20 Last updated: 2017-12-11Bibliographically approved
6. Microstructure and properties of stainless steel strip welds
Open this publication in new window or tab >>Microstructure and properties of stainless steel strip welds
2008 (English)In: Proceedisngs, Stainless Steel - Science and Market, Helsinki, Finland, 2008Conference paper, Published paper (Refereed)
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-27713 (URN)
Note
QC 20101220Available from: 2010-12-20 Created: 2010-12-20 Last updated: 2013-04-29Bibliographically approved
7. An Experimental and Theoretical Evaluation of Microstructure Coarsening in Duplex Stainless Steels
Open this publication in new window or tab >>An Experimental and Theoretical Evaluation of Microstructure Coarsening in Duplex Stainless Steels
2013 (English)In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 84, no 11, 1126-1137 p.Article in journal (Refereed) Published
Abstract [en]

The coarsening of duplex stainless steel microstructures at temperatures above 1000C was investigated experimentally and theoretically. Hot-rolled duplex 2205 and superduplex2507 samples were heat treated isothermally at 1100C up to 700 h, and also between 1000 and 1250C for 24 h. The ferrite content and the microstructure coarseness, quantified by the measured austenite spacing, AS, were evaluated. It was found that theobserved coarsening could be described using Ostwald ripening theory expressed as AS**n-AS0**n=kt, where AS0 is austenite spacing of the as-delivered material, k the rateconstant, n the exponent, and t the heat treatment time. Using regression analysis, n and k were evaluated and the resulting predictions compared with literature theories andresults. The results were also calculated according to the original Lifshitz, Slyozov,Wagner (LSW) theory with parameters taken from literature, as well as with the Dictra software for diffusion and growth in multicomponent systems. The calculations indicated that while the classical LSW theory failed to give a good description of the experimentalresults, the Dictra simulations proved very accurate, even though systems of equal ferrite and austenite content are considerably outside the intended range of applicability of the software.

Keyword
Austenite spacing, Duplex stainless steel, Heat treatment, Microstructure, Ostwald ripening
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-121336 (URN)10.1002/srin.201200323 (DOI)000328148100007 ()2-s2.0-84887318813 (Scopus ID)
Funder
Vinnova
Note

QC 20140617. Updated from In press to Published.

Available from: 2013-04-28 Created: 2013-04-28 Last updated: 2017-12-06Bibliographically approved

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  • Other locale
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Output format
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