A dissertation submitted to the faculty of Engineering, University of the
Witwatersrand, Johanuesburg, in fulfilment of the requirements for the degree of
Master of Science in Engineering. / Ferritic stainless steels possess a number of properties which render them
superior to austenitic stainless steels in certain applications. The ferritic stainless
steels are highly resistant to stress-corrosion cracking and are generally of lower
cost. Previous work had shown that the corrosion properties of the ferritic steels
were optimised at a chromium content of forty percent. Extensive research had
already characterised the mechanical and corrosion behaviour of the Fe"40Cr
alloy in the cast and wrought form. This investigation involves the development
of the material by powder metallurgy processing. Particular emphasis is placed
on a proprietary powder production technique as well as on powder
consolidation by Hot Isostatic Pressing. The effect of Ni, Mo, Ru, Nb, Al and Fe
additions and well as various techniques of introducing these additions are
examined. The unique effect of powder metallurgy manufacture on
microstructure and the kinetics of sigma phase formation is highlighted.
Corrosion tests in various concentrations of sulphuric acid and an industrial field
trial were performed. It was found that combined Ni and Mo contents exceeding
four percent resulted in severe embrittlement due to widespread formation of
sigma phase during the HIP process. The use of powder metallurgy techniques
was also found to enhance the kinetics of sigma phase formation in a particular
alley when compared to the as-cast state. Direct additions of Fe powder were
successful in inhibiting embrittlement, possibly due to a mechanism of Fe
diffusion into Cr-rich regions. This lowered of the Cr content in these regions,
thereby reducing the tendency for sigma precipitation. Corrosion tests indicate
satisfactory resistance for the Fc-40Cr-2Ni-2Mo alloys in 70 weight percent
sulphuric acid at 50°C. Alloys with 0.2 percent Ru additions were found to be
especially resistant, due to the role of Ru as a cathodic modifier. The method of
mixing and diffusion bonding metallic powders of varying composition was
found to be feasible and certain novel combinations of Ni and Fe-rich alloys
exhibited satisfactory corrosion resistance. A mathematical diffusion model was
found to provide an order of magnitude approximation of the time required for
hornogentsation in a certain mixture of powders. / Andrew Chakane 2018
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/26148 |
| Date | January 1998 |
| Creators | Kian, Michael Christopher Wong. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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