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Precipitation in Advanced Stainless Steels
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Swerea KIMAB.ORCID iD: 0000-0001-5040-2399
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Stockholm: Kungliga Tekniska högskolan, 2018. , p. 61
National Category
Materials Engineering
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-227303ISBN: 978-91-7729-774-1 (print)OAI: oai:DiVA.org:kth-227303DiVA, id: diva2:1204243
Public defence
2018-06-08, B2, Brinellvägen 23, Stockholm, 10:00
Opponent
Supervisors
Note

QC 20180509

Available from: 2018-05-09 Created: 2018-05-07 Last updated: 2018-05-09Bibliographically approved
List of papers
1. Nanostructure evolution and mechanical property changes during aging of a super duplex stainless steel at 300°C
Open this publication in new window or tab >>Nanostructure evolution and mechanical property changes during aging of a super duplex stainless steel at 300°C
Show others...
2015 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 647, p. 241-248, article id 32735Article in journal (Refereed) Published
Abstract [en]

The nanostructure evolution and the corresponding changes in mechanical properties of a super duplex stainless steel 2507 (UNS S32750) during aging at 300. °C up to 12,000. h have been investigated. Microstructural studies using transmission electron microscopy and atom probe tomography show that subtle Cr concentration fluctuations develop during aging. The amplitude of the concentration fluctuations is proportional to the hardness of the ferrite phase, and it is also proportional to the decrease in room temperature impact toughness during aging. The fracture behaviour of the alloy changes gradually from ductile to cleavage fracture, upon aging. The cracks were found to propagate through the ferrite phase, partly along deformation twin interfaces, and delamination between the austenite and ferrite phases was observed.

Place, publisher, year, edition, pages
Elsevier, 2015
Keywords
Atom probe tomography (APT), Duplex stainless steel, Mechanical properties, Phase separation, Spinodal decomposition, Transmission electron microscopy (TEM), Brittle fracture, Ductile fracture, Electron microscopy, Failure (mechanical), Ferrite, Fracture, Fracture mechanics, High resolution transmission electron microscopy, Nanostructures, Probes, Transmission electron microscopy, Atom-probe tomography, Cleavage fracture, Concentration fluctuation, Cr concentration, Fracture behaviour, Room temperature, Super duplex stainless steel, Stainless steel
National Category
Nano Technology
Identifiers
urn:nbn:se:kth:diva-177210 (URN)10.1016/j.msea.2015.09.009 (DOI)000364250100030 ()2-s2.0-84941633959 (Scopus ID)
Note

QC 20151125

Available from: 2015-11-25 Created: 2015-11-17 Last updated: 2018-05-09Bibliographically approved
2. Experimental and computational study of nitride precipitation in a CrMnN austenitic stainless steel
Open this publication in new window or tab >>Experimental and computational study of nitride precipitation in a CrMnN austenitic stainless steel
2017 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 84, no 27, p. 435-441Article in journal (Refereed) Published
Abstract [en]

The austenitic CrMnN stainless steels are high-strength, tough, and non-magnetic, and are used in oil field applications. The steels have high alloying contents, and precipitation of Cr-nitrides and/or intermetallic phases can occur when cooling through the temperature region 950–700 °C. The nitride precipitates appear in the grain boundaries but can be difficult to observe in the microstructure due to their small size. However, there is an effect of precipitation on corrosion and impact strength and a modelling approach to predict precipitation is valuable for alloy and process development. In the present work precipitation simulations were applied to a CrMnN steel composition, and coupled to experimental investigations after heat treatments at 700 and 800 °C. The early stages, with short heat-treatment times, were studied. The simulations were performed using TC-PRISMA, a software for calculation of multiphase precipitation kinetics, using multicomponent nucleation and growth models. Dedicated thermodynamic and kinetic databases were used for the simulations. The main precipitate was identified by experiments and simulations to be the Cr2N nitride, and the precipitation during isothermal heat treatments was investigated. Isothermal precipitation diagrams are simulated, and the influence of precipitation kinetics on toughness is discussed.

Place, publisher, year, edition, pages
Elsevier, 2017
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-227299 (URN)10.1016/j.msea.2016.12.071 (DOI)000393938300051 ()2-s2.0-85007209096 (Scopus ID)
Note

QC 20180522

Available from: 2018-05-07 Created: 2018-05-07 Last updated: 2018-05-22Bibliographically approved
3. Precipitation of chromium nitrides in the super duplex stainless steel 2507
Open this publication in new window or tab >>Precipitation of chromium nitrides in the super duplex stainless steel 2507
2015 (Swedish)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 46A, no 3, p. 1062-1072Article in journal (Refereed) Published
Abstract [en]

Precipitation of chromium nitrides during cooling from temperatures in the range 1373 K to 1523 K (1100 C to 1250 C) has been studied for the super duplex stainless steel 2507 (UNS S32750). Characterization with optical, scanning and transmission electron microscopy was combined to quantify the precipitation process. Primarily Cr2N nitrides were found to precipitate with a high density in the interior of ferrite grains. An increased cooling rate and/or an increased austenite spacing clearly promoted nitride formation, resulting in precipitation within a higher fraction of the ferrite grains, and lager nitride particles. Furthermore, formation of the meta-stable CrN was induced by higher cooling rates. The toughness seemed unaffected by nitrides. A slight decrease in pitting resistance was, however, noticed for quenched samples with large amounts of precipitates. The limited adverse effect on pitting resistance is attributed to the small size (~200 nm) of most nitrides. Slower cooling of duplex stainless steels to allow nitrogen partitioning is suggested in order to avoid large nitrides, and thereby produce a size distribution with a smaller detrimental effect on pitting resistance.

Place, publisher, year, edition, pages
Springer, 2015
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-227295 (URN)10.1007/s11661-014-2718-y (DOI)000350300200008 ()2-s2.0-84925483443 (Scopus ID)
Funder
VINNOVA
Note

QC 20180508

Available from: 2018-05-07 Created: 2018-05-07 Last updated: 2018-05-09Bibliographically approved
4. Nitride morphology and formation of intragranular austenite in a quenched and reheated super duplex stainless steel
Open this publication in new window or tab >>Nitride morphology and formation of intragranular austenite in a quenched and reheated super duplex stainless steel
(English)Manuscript (preprint) (Other academic)
National Category
Materials Engineering
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-227325 (URN)
Note

QC 20180522

Available from: 2018-05-08 Created: 2018-05-08 Last updated: 2018-05-22Bibliographically approved
5. High-Temperature Phase Equilibria of Duplex Stainless Steels Assessed with a Novel In-Situ Neutron Scattering Approach
Open this publication in new window or tab >>High-Temperature Phase Equilibria of Duplex Stainless Steels Assessed with a Novel In-Situ Neutron Scattering Approach
2017 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 48, no 4, p. 1562-1571Article in journal (Refereed) Published
Abstract [en]

Duplex stainless steels are designed to solidify with ferrite as the parent phase, with subsequent austenite formation occurring in the solid state, implying that, thermodynamically, a fully ferritic range should exist at high temperatures. However, computational thermodynamic tools appear currently to overestimate the austenite stability of these systems, and contradictory data exist in the literature. In the present work, the high-temperature phase equilibria of four commercial duplex stainless steel grades, denoted 2304, 2101, 2507, and 3207, with varying alloying levels were assessed by measurements of the austenite-to-ferrite transformation at temperatures approaching 1673 K (1400 °C) using a novel in-situ neutron scattering approach. All grades became fully ferritic at some point during progressive heating. Higher austenite dissolution temperatures were measured for the higher alloyed grades, and for 3207, the temperature range for a single-phase ferritic structure approached zero. The influence of temperatures in the region of austenite dissolution was further evaluated by microstructural characterization using electron backscattered diffraction of isothermally heat-treated and quenched samples. The new experimental data are compared to thermodynamic calculations, and the precision of databases is discussed.

Place, publisher, year, edition, pages
Springer-Verlag New York, 2017
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-227297 (URN)10.1007/s11661-016-3953-1 (DOI)000396057300009 ()2-s2.0-85009889679 (Scopus ID)
Funder
VINNOVA, 2013-03258
Note

QC 20180523

Available from: 2018-05-07 Created: 2018-05-07 Last updated: 2018-05-23Bibliographically approved

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