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Process Parameters for Creation of Porous Stainless Steel Surfaces
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2011 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [sv]

Detta examensarbete ämnar att undersöka värmebehandlingsmetoder för att skapaporösa austenitiska rostfria stål samt utvärdera ifall dessa ytor medför förbättringargentemot referensmaterialen. De porösa ytorna skapades genom en två-stegs metod.Prover oxiderades initialt i en fuktig eller torr argon atmosfär för att sedan reducerasi ren vätgas. De material som använts i denna studie är austenitiska rostrifa stål ienhlighet med standardena 1.4301 (304), 1.4404 (316L), 1.4435 (316L) och F-138 (316LVM).Oxidationsprocessen genomfördes under temperaturer över 630 ◦ C under 150 - 1200 s.Efteråt reducerades proverna i ren vätgas under 150 - 2400 s.För att karakterisera processen undersöktes proverna med svepelektronmikroskopsamt elektron-dispersiv röntgenspektrometer. Vissa prover har även analyserats medhjälp av ljusoptiskt mikroskop, bildanalys samt värmeledningsförmåga. Resultaten visarklart att alla undersökta material kan uppvisa porösa strukturer, dock kan inte allaprocessparametrar möjliggöra uppkomsten av dessa. Generellt sett beror porositeten påden underliggande strukturen skapade under oxidationen. Det har ännu inte varit möjligtatt karaterisera några kristallstrukturer, kemisk analys tyder dock på närvaro av kromoxid,(Cr,Fe) spineller samt järnoxider. Resultaten konstaterar att en kort oxidationstid normaltbildar kromoxid medan längre oxidation medför uppkomsten av järnoxider. En ökadreduktionstid leder till större och färre porer jämfört med kortare reduktionstider. Kemiskaanalyser visar enhälligt att porösa strukturer innahåller till största del järn. En trolig orsaktill detta beteende kan kopplas till det fenomen där krom förångas under oxidation i fuktigaatmosfärer. Följaktikligen blir även ytan känslig för korrosion då majoriteten av krom harförsvunnit.Utvärdering av egenskaperna hos dessa ytor visar en tydlig förbättring i värmeledning vianaturlig konvektion. Värmeväxlingen mellan två uider uppvisar dock inte någon störreskillnad förutom under låga ödeshastigheter då en porös yta tenderar att transporterabort mer värme

Abstract [en]

This work aim to investigate a novel process by means of process parameters for creationof porous austenitic stainless steels surfaces as well as investigate if they permit anyimprovements compared to reference materials. Porous surfaces were created by a two-step method; samples were initially oxidised in a wet argon atmosphere and subsequentlyreduced in a pure hydrogen. The materials used in this investigation are all stainless steelswith specications according to steel grades 1.4301 (304), 1.4404 (316L), 1.4435 (316L)and F-138 (316LVM). The oxidation process is performed above 630 ◦ C for 150 - 1200 s in aow of wet argon or dry air and reduction is performed with a ow hydrogen for 150 - 2400 s.In order to understand the heat treatment processes, samples were characterised by usinga scanning electron microscope together with an electron dispersive x-ray spectrometer.However, light optical microscopy, image analysis and heat transfer measurement werealso used. Results show that all materials can obtain porous structures even though itis evident some process parameters cannot support successful creation of porosity. Ingeneral, the porosity depends on the structure created upon oxidation. It has not beenpossible to identify any crystal structures but compositional analysis as well as previousliterature suggest presence of chromia, (Cr,Fe) spinels and iron oxides. It is concluded thatshort time oxidation frequently creates chromia layers and prolonged oxidation inducesiron oxides due to breakaway oxidation. An increased reduction time results in larger andfewer pores. Compositional analyses show that all porous morphologies are created withina very iron rich layer. It is suggested that wet oxidation promote chromium evaporation,which consequently reduces the amount of chromium in the surface and enables iron oxideto rapidly form. Furthermore, results show that the porous layer is very susceptible tocorrosion as almost no chromium is present after the heat treatment. Evaluation of the properties of porous surfaces reveals a signicant improvement in heattransfer due to natural convection. Heat exchange between uids is though not enhancedgreatly by porous surfaces, a small improvement can be seen for low ow rates but for largerows no improvement is found.

Place, publisher, year, edition, pages
2011.
Keyword [en]
surface porosity, stainless steel, heat treatment, heat transfer, corrosion, oxidation
Keyword [sv]
ytporositet, rostfritt stål, värmebehandling, värmetransport, korrosion, oxidation
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-145121ISRN: KTH/MSE--11/52--SE+CER/EXOAI: oai:DiVA.org:kth-145121DiVA: diva2:716308
External cooperation
Linde AG
Educational program
Master of Science in Engineering - Materials Design and Engineering
Supervisors
Examiners
Available from: 2014-05-21 Created: 2014-05-09 Last updated: 2014-05-21Bibliographically approved

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