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High-Temperature Phase Equilibria of Duplex Stainless Steels Assessed with a Novel In-Situ Neutron Scattering Approach
Swerea KIMAB AB, POB 7047, S-16407 Kista, Sweden.
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. Vol. 48, no 4, p. 1562-1571
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-227297DOI: 10.1007/s11661-016-3953-1ISI: 000396057300009Scopus ID: 2-s2.0-85009889679OAI: oai:DiVA.org:kth-227297DiVA, id: diva2:1204220
Funder
VINNOVA, 2013-03258
Note

QC 20180523

Available from: 2018-05-07 Created: 2018-05-07 Last updated: 2018-05-23Bibliographically approved
In thesis
1. Precipitation in Advanced Stainless Steels
Open this publication in new window or tab >>Precipitation in Advanced Stainless Steels
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:nbn:se:kth:diva-227303 (URN)978-91-7729-774-1 (ISBN)
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

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