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Experimental study of phase separation in Fe-Cr based alloys
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Duplex stainless steels (DSSs) are important engineering materials due to their combination of good mechanical properties and corrosion resistance. However, as a consequence of their ferrite content, DSSs are sensitive to the so-called ‘475°C embrittlement’, which is induced by phase separation, namely, the ferrite decomposed into Fe-rich ferrite (α) and Cr-rich ferrite (α'), respectively. The phase separation is accompanied with a severe loss of toughness. Thus, the ‘475°C embrittlement’ phenomenon limits DSSs’ upper service temperature to around 250°C.

In the present work, Fe-Cr binary model alloys and commercial DSSs from weldments were investigated for the study of phase separation in ferrite. Different techniques were employed to study the phase separation in model alloys and commercial DSSs, including atom probe tomography, transmission electron microscopy and micro-hardness test.

Three different model alloys, Fe-25Cr, Fe-30Cr and Fe-35Cr (wt. %) were analyzed by atom probe tomography after different aging times. A new method based on radial distribution function was developed to evaluate the wavelength and amplitude of phase separation in these Fe-Cr binary alloys. The results were compared with the wavelengths obtained from 1D auto-correlation function and amplitudes from Langer-Bar-On-Miller method. It was found that the wavelengths from 1D auto-correlation function cannot reflect the 3D nano-scaled structures as accurate as those obtained by radial distribution function. Furthermore, the Langer-Bar-On-Miller method underestimates the amplitudes of phase separation.

Commercial DSSs of SAF2205, 2304, 2507 and 25.10.4L were employed to investigate the connections between phase separation and mechanical properties from different microstructures (base metal, heat-affected-zone and welding bead) in welding. Moreover, the effect of external tensile stress during aging on phase separation of ferrite was also investigated. It was found that atom probe tomography is very useful for the analysis of phase separation in ferrite and the radial distribution function (RDF) is an effective method to compare the extent of phase separation at the very early stages. RDF is even more sensitive than frequency diagrams. In addition, the results indicate that the mechanical properties are highly connected with the phase separation in ferrite and other phenomena, such as Ni-Mn-Si-Cu clusters, that can also deteriorate the mechanical properties.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , vi, 28 p.
Keyword [en]
Duplex stainless steels, Ferritic stainless steels, Spinodal decomposition, Phase separation, Atom probe tomography, Clustering, Radial distribution function (RDF)
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-119230ISBN: 978-91-7501-632-0 (print)OAI: oai:DiVA.org:kth-119230DiVA: diva2:609996
Presentation
2013-03-22, N111, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20130308

Available from: 2013-03-08 Created: 2013-03-08 Last updated: 2013-10-31Bibliographically approved
List of papers
1. 3D analysis of phase separation in ferritic stainless steels
Open this publication in new window or tab >>3D analysis of phase separation in ferritic stainless steels
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2012 (English)In: Proceedings of the 1st International Conference on 3D Materials Science, John Wiley & Sons, 2012, 221-226 p.Conference paper, Published paper (Refereed)
Abstract [en]

The embrittlement of ferritic stainless steels during low temperature aging is attributed to the phase separation with Fe and Cr demixing. The small scale of the decomposed structure with only minor compositional fluctuations and short distances between the enriched and depleted regions has been a challenge for quite some time. A wide selection of experimental and modeling tools have been used to quantify these types of structures. These analyses often focus on rather late stages of decomposition where the mechanical properties are already seriously affected. The recent advance in 3D tools like phase-field and atom probe tomography have created a need for good quantitative procedures of evaluating the structure and also to link results from the continuum approach to the individual atom measurements. This work aims at addressing this need.

Place, publisher, year, edition, pages
John Wiley & Sons, 2012
Keyword
Phase separation, Atom Probe Tomography, Phase-field modeling
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-119237 (URN)000324176000034 ()2-s2.0-84889868799 (Scopus ID)978-1-11847-039-8 (ISBN)
Conference
1st International Conference on 3D Materials Science (3DMS),Seven Springs, Pennsylvania, Date: Jul 08-12, 2012
Note

QC 20130308

Available from: 2013-03-08 Created: 2013-03-08 Last updated: 2013-12-06Bibliographically approved
2. Quantitative Evaluation of Spinodal Decomposition in Fe-Cr by Atom Probe Tomography and Radial Distribution Function Analysis
Open this publication in new window or tab >>Quantitative Evaluation of Spinodal Decomposition in Fe-Cr by Atom Probe Tomography and Radial Distribution Function Analysis
2013 (English)In: Microscopy and Microanalysis, ISSN 1431-9276, E-ISSN 1435-8115, Vol. 19, no 3, 665-675 p.Article in journal (Refereed) Published
Abstract [en]

Nanostructure evolution during low temperature aging of three binary Fe-Cr alloys has been investigated by atom probe tomography. A new method based on radial distribution function (RDF) analysis to quantify the composition wavelength and amplitude of spinodal decomposition is proposed. Wavelengths estimated from RDF have a power-law type evolution and are in reasonable agreement with wavelengths estimated using other more conventional methods. The main advantages of the proposed method are the following: (1) Selecting a box size to generate the frequency diagram, which is known to generate bias in the evaluation of amplitude, is avoided. (2) The determination of amplitude is systematic and utilizes the wavelength evaluated first to subsequently evaluate the amplitude. (3) The RDF is capable of representing very subtle decomposition, which is not possible using frequency diagrams, and thus a proposed theoretical treatment of the experimental RDF creates the possibility to determine amplitude at very early stages of spinodal decomposition.

Keyword
atom probe tomography (APT), spinodal decomposition, radial distribution function (RDF), phase separation, stainless steels
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-124038 (URN)10.1017/S1431927613000470 (DOI)000319126300018 ()2-s2.0-84877973197 (Scopus ID)
Funder
Swedish Research Council, 621-2009-5289Vinnova
Note

QC 20130628. Updated from "Accepted" to "Published"

Available from: 2013-06-28 Created: 2013-06-25 Last updated: 2017-12-06Bibliographically approved
3. A study of duplex stainless steels aged at 325°C under applied tensile load
Open this publication in new window or tab >>A study of duplex stainless steels aged at 325°C under applied tensile load
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2011 (English)In: 7th European Stainless Steel Conference: Science and Market, Proceedings, Associazione Italiana di Metallurgia , 2011Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
Associazione Italiana di Metallurgia, 2011
Keyword
Duplex stainless steels, Embrittlement, Load effect, Mechanical properties, Microstructure, Phase separation, Welding
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-85479 (URN)2-s2.0-84909606309 (Scopus ID)978-888529884-2 (ISBN)
Conference
7th European Stainless Steel Conference: Science and Market, Como, Italy, 21 September 2011 through 23 September 2011
Note

QC 20120411

Available from: 2012-02-13 Created: 2012-02-13 Last updated: 2015-04-23Bibliographically approved
4. Observations of copper clustering in a 25Cr-7Ni super duplex stainless steel during low-temperature aging under load
Open this publication in new window or tab >>Observations of copper clustering in a 25Cr-7Ni super duplex stainless steel during low-temperature aging under load
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2012 (English)In: Philosophical Magazine Letters, ISSN 0950-0839, E-ISSN 1362-3036, Vol. 92, no 7, 336-343 p.Article in journal (Refereed) Published
Abstract [en]

Atom-probe tomography was used to investigate phase separation and copper (Cu) clustering in the ferrite phase of a 25Cr-7Ni super duplex stainless steel. The steel was subjected to a tensile load during aging at 325 degrees C for 5800 h. The degree of phase separation into alpha (Fe-rich) and alpha' (Cr-rich) was small, but still, it was the highest in the steel subjected to the highest load. Cu was found to cluster, and the number density of clusters increased with increasing load. In the material subjected to the highest load, Cu was enriched in regions that were neither Fe-rich nor Cr-rich. These regions also had the highest number density of Cu clusters.

Keyword
3DAP, stainless steels, phase decomposition, clusters, copper, duplex steels
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-85491 (URN)10.1080/09500839.2012.672773 (DOI)000305682200004 ()2-s2.0-84863190061 (Scopus ID)
Funder
Swedish Research Council, 621-2009-5289Vinnova
Note

QC 20140910

Available from: 2012-02-13 Created: 2012-02-13 Last updated: 2017-12-07Bibliographically approved
5. Concurrent phase separation and clustering in the ferrite phase during low temperature stress aging of duplex stainless steel weldments
Open this publication in new window or tab >>Concurrent phase separation and clustering in the ferrite phase during low temperature stress aging of duplex stainless steel weldments
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2012 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 60, no 16, 5818-5827 p.Article in journal (Refereed) Published
Abstract [en]

The concurrent phase separation and clustering of alloying elements in the ferrite phase of duplex stainless steel weldments after stress aging at 325 degrees C have been investigated by atom probe tomography analysis. Both phase separation, into Fe-rich and Cr-rich ferrite, and solute clustering were observed. Phase separation in the heat-affected zone (HAZ) is most pronounced in the high alloyed SAF 2507, followed by SAF 2205 and SAF 2304. Moreover Cu clustering was observed in the HAZ of SAF 2507. However, decomposition in the weld bead (25.10.4L) was more pronounced than in the HAZs, with both phase separation and clustering of Ni-Mn-Si-Cu. The observed differences in the decomposition behaviors in the HAZ and weld bead can be attributed to the high Ni content and the characteristic microstructure of the weld bead with high internal strains. In addition, an applied tensile stress during aging of weldments has been found to further promote the kinetics of phase separation and clustering.

Keyword
Duplex stainless steels, Phase separation, Atom probe tomography, Spinodal decomposition, Clustering
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-104382 (URN)10.1016/j.actamat.2012.07.022 (DOI)000309438500016 ()2-s2.0-84866129827 (Scopus ID)
Funder
Swedish Research Council, 621-2009-5289Vinnova
Note

QC 20121105

Available from: 2012-11-05 Created: 2012-11-01 Last updated: 2017-12-07Bibliographically approved

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