Faculty of Engineering, Built Environment and Information Technology
School of Engineering
Department of Materials Science and Metallurgical Engineering
Selected Highlights from Research Findings
Nitrogen-alloyed stainless steels are rapidly increasing in popularity due to their excellent mechanical properties. Some nitrogen-alloyed stainless steels are currently recognised to offer the best combination of strength and toughness of any material known.
In addition to its beneficial influence on mechanical properties, nitrogen as an alloying element also improves the corrosion resistance of stainless steels. In the fabrication industry, one of the challenges in using these steels is that welding often results in porosity and nitrogen losses.
This significantly weakens the welded joints. In order to control nitrogen losses from welds, a more fundamental understanding of the mechanisms of nitrogen absorption and desorption during welding is required. Research was conducted to quantify the influence of several factors on the nitrogen content of welds.
These included the original nitrogen content of the stainless steel being welded, the sulphur and oxygen contents of the weld metal and the composition of the shielding gas used to protect the molten metal from the atmosphere.
The investigation has yielded a much more complete understanding of the mechanisms of nitrogen absorption and desorption during welding. It was found that small changes in the chemistry of the weld metal can negate nitrogen losses almost completely, while at the same time stabilising the arc and reducing spatter.
Prof M du Toit
Materials Science and Metallurgical Engineering
+27 (0) 12 420 3185
madeleine.dutoit@up.ac.za
Titiana is used as a white pigment in paint and plastics as well as in paper coating. It is also an essential constituent in sunscreen lotions, because of its ability to absorb UV radiation. South Africa produces approximately one-quarter of the world’s titania requirement mainly from the production of high-grade slag. Slag (Ti02) is produced from the reduction of beach sand ilmenite in furnaces at Richards Bay and Saldanha Bay.
Slag is produced when Ilmenite (FeTi03) – a natural occurring black mineral found in beach sand – is concentrated to pure black sand and then fed into an electric furnace where the iron in the ilmenite is reduced to iron metal and a slag containing ~86% Ti02. This molten slag is then tapped out of the furnace and cooled down to ambient temperatures to form a solid material .
All producers of high-grade (>85% titanium oxide) slag encounter the problem of disintegration of the solidified tapped slag at relatively low temperatures when the slag blocks cool down. The disintegrated slag consists of a large proportion of oxidized fine material (<100 micron) that is unsuitable for subsequent use in fluidised-bed chlorination reactors. Titania is produced from the chlorination of impure slag in a reactor where the particles are suspended in an upward-flowing stream of chlorine gas (fluidised-bed reactor).
Titanium tetra-chloride is formed, which is further purified and then oxidised to very pure Ti02. Remedial action consists of rapid water-cooling of the solid slag blocks down to room temperature, but this is not always effective, and in some instances excessive amounts of fine material are still generated.
In this research project, the mechanism of the disintegration was studied, in order to minimise the production of fine-grained material. Research findings indicate that the main cause was an oxidation reaction that takes place at low temperature. The reaction can only be avoided by the exclusion of air or by extremely fast cooling.
Prof JPR de Villiers
Materials Science and Metallurgical Engineering
+27 (0) 12 420 2239
johan.devilliers@up.ac.za
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