A microstructural examination of duplex ferrite -martensite corrosion resisting steels

 

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dc.contributor.advisor Ball, Anthony en_ZA
dc.contributor.author Knutsen, Robert Douglas en_ZA
dc.date.accessioned 2017-11-09T09:04:59Z
dc.date.available 2017-11-09T09:04:59Z
dc.date.issued 1989 en_ZA
dc.identifier.citation Knutsen, R. 1989. A microstructural examination of duplex ferrite -martensite corrosion resisting steels. University of Cape Town. en_ZA
dc.identifier.uri http://hdl.handle.net/11427/26079
dc.description.abstract This thesis reports a study of the microstructural evolution of chromium containing duplex ferrite-martensite steels and examines the effects of the microstructure on the mechanical properties. Emphasis has been placed on determining the microstructural factors responsible for the persistent occurrence of anisotropy in a modified 12 wt% Cr steel designated 3CR12. in addition an investigation has been carried out in order to refine the grain structure of a ferritic steel containing 16-17 wt % Cr by inducing a duplex ferrite-martensite phase structure. The microstructural evolution of 3CR12 was studied during cooling from a solution heat treatment at 1380°C and the natures of the phase transformations evident were investigated. Energy dispersive X-ray spectroscopy (EDS), in association with a scanning electron microscope (SEM), was used to determine the composition of the phases arising from the solid state δ-ferrite to austenite transformation. It is shown that the high temperature δ-ferrite phase partially decomposes to austenite via a Widmanstatten growth mechanism and consequently a banded two phase structure is produced after hot rolling. The element partitioning which arises during the solid state δ-ferrite decomposition ieads to compositional banding with an indelible nature. A model is proposed for the events leading to the generation of the banded phase structure and the formation of an elongated ferritic microstructure in 3CR12 after sub-critical annealing. The type and distribution of non-metallic inclusions occurring in 3CR12 has also been assessed. Characteristic fracture modes developed during impact testing have been related to the grain morphology and the occurrence of non-metallic inclusions. It is shown that splits form parallel to the rolling plane when Charpy specimens are subjected to impact testing and that both impact energy and mode of fracture are dependent on the directional properties of the 3CR12 microstructure. Splitting is predominantly caused by the low energy crack path provided by long, undulating grain boundaries parallel to the rolling plane, and inclusions, particularly manganese sulphides (MnS), facilitate low energy modes of fracture associated with the splitting phenomenon. MnS inclusions are also found to affect the corrosion resistance of 3CR12 and careful control of the chemistry of the steel permits these inclusions to be restricted to levels at which acceptable impact and corrosion properties are maintained. Refinement of the grain structure of ferritic steels containing 16-17 wt % Cr was carried out by modifying the ratio of ferritising elements to austenitising elements in the steel chemistry. Suitable ruckel additions have been determined which provide alloys with sufficient austenitising ability to refine the high temperature δ-ferrite phase and consequently a duplex ferrite-martensite microstructure is produced. Tempering of these alloys at 700°C results in a lamellar ferrite-martensite structure which gives rise to an attractive combination of impact and tensile properties which may provide a stainless steel with superior cost effectiveness to austenitic grades. en_ZA
dc.language.iso eng en_ZA
dc.subject.other Materials engineering en_ZA
dc.subject.other Ferritic steel - Analysis en_ZA
dc.title A microstructural examination of duplex ferrite -martensite corrosion resisting steels en_ZA
dc.type Doctoral Thesis
dc.date.updated 2017-03-06T10:32:34Z
uct.type.publication Research en_ZA
uct.type.resource Thesis en_ZA
dc.publisher.institution University of Cape Town
dc.publisher.faculty Faculty of Engineering and the Built Environment
dc.publisher.department Centre for Materials Engineering en_ZA
dc.type.qualificationlevel Doctoral
dc.type.qualificationname PhD en_ZA
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uct.type.filetype Text
uct.type.filetype Image
dc.identifier.apacitation Knutsen, R. D. (1989). <i>A microstructural examination of duplex ferrite -martensite corrosion resisting steels</i>. (Thesis). University of Cape Town ,Faculty of Engineering & the Built Environment ,Centre for Materials Engineering. Retrieved from http://hdl.handle.net/11427/26079 en_ZA
dc.identifier.chicagocitation Knutsen, Robert Douglas. <i>"A microstructural examination of duplex ferrite -martensite corrosion resisting steels."</i> Thesis., University of Cape Town ,Faculty of Engineering & the Built Environment ,Centre for Materials Engineering, 1989. http://hdl.handle.net/11427/26079 en_ZA
dc.identifier.vancouvercitation Knutsen RD. A microstructural examination of duplex ferrite -martensite corrosion resisting steels. [Thesis]. University of Cape Town ,Faculty of Engineering & the Built Environment ,Centre for Materials Engineering, 1989 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/26079 en_ZA
dc.identifier.ris TY - Thesis / Dissertation AU - Knutsen, Robert Douglas AB - This thesis reports a study of the microstructural evolution of chromium containing duplex ferrite-martensite steels and examines the effects of the microstructure on the mechanical properties. Emphasis has been placed on determining the microstructural factors responsible for the persistent occurrence of anisotropy in a modified 12 wt% Cr steel designated 3CR12. in addition an investigation has been carried out in order to refine the grain structure of a ferritic steel containing 16-17 wt % Cr by inducing a duplex ferrite-martensite phase structure. The microstructural evolution of 3CR12 was studied during cooling from a solution heat treatment at 1380°C and the natures of the phase transformations evident were investigated. Energy dispersive X-ray spectroscopy (EDS), in association with a scanning electron microscope (SEM), was used to determine the composition of the phases arising from the solid state δ-ferrite to austenite transformation. It is shown that the high temperature δ-ferrite phase partially decomposes to austenite via a Widmanstatten growth mechanism and consequently a banded two phase structure is produced after hot rolling. The element partitioning which arises during the solid state δ-ferrite decomposition ieads to compositional banding with an indelible nature. A model is proposed for the events leading to the generation of the banded phase structure and the formation of an elongated ferritic microstructure in 3CR12 after sub-critical annealing. The type and distribution of non-metallic inclusions occurring in 3CR12 has also been assessed. Characteristic fracture modes developed during impact testing have been related to the grain morphology and the occurrence of non-metallic inclusions. It is shown that splits form parallel to the rolling plane when Charpy specimens are subjected to impact testing and that both impact energy and mode of fracture are dependent on the directional properties of the 3CR12 microstructure. Splitting is predominantly caused by the low energy crack path provided by long, undulating grain boundaries parallel to the rolling plane, and inclusions, particularly manganese sulphides (MnS), facilitate low energy modes of fracture associated with the splitting phenomenon. MnS inclusions are also found to affect the corrosion resistance of 3CR12 and careful control of the chemistry of the steel permits these inclusions to be restricted to levels at which acceptable impact and corrosion properties are maintained. Refinement of the grain structure of ferritic steels containing 16-17 wt % Cr was carried out by modifying the ratio of ferritising elements to austenitising elements in the steel chemistry. Suitable ruckel additions have been determined which provide alloys with sufficient austenitising ability to refine the high temperature δ-ferrite phase and consequently a duplex ferrite-martensite microstructure is produced. Tempering of these alloys at 700°C results in a lamellar ferrite-martensite structure which gives rise to an attractive combination of impact and tensile properties which may provide a stainless steel with superior cost effectiveness to austenitic grades. DA - 1989 DB - OpenUCT DP - University of Cape Town LK - https://open.uct.ac.za PB - University of Cape Town PY - 1989 T1 - A microstructural examination of duplex ferrite -martensite corrosion resisting steels TI - A microstructural examination of duplex ferrite -martensite corrosion resisting steels UR - http://hdl.handle.net/11427/26079 ER - en_ZA


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