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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/26079 |
| Date | 06 March 2017 |
| Creators | Knutsen, Robert Douglas |
| Contributors | Ball, Anthony |
| Publisher | University of Cape Town, Faculty of Engineering and the Built Environment, Centre for Materials Engineering |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Doctoral Thesis, Doctoral, PhD |
| Format | application/pdf |
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