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Evaluation of the η (Eta) nitride with three laboratory melts
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2015 (English)Independent thesis Advanced level (degree of Master (One Year)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

η (eta) nitride, Cr3Ni2SiN, is a precipitate found in high temperature austenitic stainless steel and is not yet included in Thermo-Calc steel database TCFE7. The aim of this thesis is to collect thermodynamic data to enable the addition of η nitride in the databases. Three laboratory melts with varying levels of silicon, chromium and nickel have been aged at 700-1000 °C for 75 h, 300 h and 1200 h and examined by Light Optical Microscopy, Scanning Electron Microscopy, Energy Dispersive Spectroscopy, Wavelength Dispersive Spectroscopy, Electron Backscattered Diffraction and X-ray Powder Diffraction.

η nitride is in the studied alloys an equilibrium phase stabilized with nitrogen. Presence of η nitride was confirmed by Energy Dispersive Spectroscopy and X-ray Powder Diffraction. It was found to precipitate in four different ways, at primary grain boundaries, intragranularly, as a "skeleton-like" precipitate and as a border around the occurring Cr2N precipitates. The area fraction of η nitrides increases with longer aging times and is favored by silicon and nickel. The composition of η nitride is not changing regardless of material composition, aging temperature and aging time. The composition of η nitride in all three materials are 8.7-9.7 wt.% silicon, 47-54 wt.% chromium, 1.4-4.1 wt.% iron and 33-36 wt.% nickel. The nitrogen content 2

determined by Wavelength Dispersive Spectroscopy is 2.8-3.2 wt.%. No complete equilibrium was achieved and together with incomplete mixing of the alloying elements during melting, the microstructure is difficult to evaluate. Other precipitates found are Cr2N, π nitride, σ phase and two unidentified phases, M and N. Of these phases at least Cr2N is not an equilibrium phase as it dissolves during aging. Further aging to achieve complete equilibrium is necessary.

Place, publisher, year, edition, pages
2015. , 155 p.
Keyword [en]
η (Eta) nitride, high nitrogen, aging, equilibrium condition, precipitations, nitrides, π (Pi) nitride, High Temperature Austenitic Stainless Steel
Keyword [sv]
η (Eta) nitrid, hög kvävehalt, åldring, jämviktstillstånd, utskiljningar, nitrider, π (Pi) nitrid, austenitiska rostfria högtemperaturstål
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-176065OAI: oai:DiVA.org:kth-176065DiVA: diva2:865850
Educational program
Master of Science in Engineering - Materials Design and Engineering
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Available from: 2015-11-23 Created: 2015-10-29 Last updated: 2015-11-23Bibliographically approved

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