Papers by Author: Rian Dippenaar

Abstract: Transverse surface cracks in low carbon steel slabs are invariably inter-granular and follow the soft ferrite films outlining the grain boundaries of exceptionally large prior-austenite grains often found at the roots of oscillation marks in continuously cast low-carbon steel slabs. Plastic deformation is concentrated in the ferrite films and cracks initiate in the ferrite films, leading to crack propagation along the austenite grain boundaries. Hot-ductility is significantly reduced by an increase in austenite grain size and in situ observations revealed that depending on the cooling rate, austenite can nucleate and grow by diffusional mechanisms or forms by a massive type of reaction. The delta-ferrite transformation has also been studied by using neutron diffraction techniques and high-energy X-rays in a synchrotron.

Abstract: The dissolution of different sulphides, carbides, carbo-sulphides and nitrides during re-heating of hot rolled low carbon, low manganese, titanium added steel have been studied using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) analysis. In addition, the chemical composition and size distribution of the different precipitates have been determined before and after reheating to analyze the modification of these precipitates in the course of the reheating cycle. The TEM and EDS analyses showed the presence of a wide variety of simple and/or complex precipitates in as rolled samples. The reheating of these samples to temperatures as high as 1350 °C, caused dissolution of most particles, although titanium nitride (TiN) did not dissolve even after reheating. By decreasing the reheating temperature more and more precipitates remained un-dissolved, but some spherodization occurred at higher temperatures.

Abstract: The densification behavior of WC composites based on iron aluminide binder was investigated using laser scanning confocal mi¬croscopy (LSCM). Doped Fe60Al40 alloys with boron levels ranging from 0 to 0.1 wt% were used as the aluminide binders. The aluminide binders were prepared using controlled atmosphere ring grinding and then blended with WC powder. The composite powder compacted in an alumina crucible and held in a platinum holder in the confocal microscope. The temperature increased from ambient temperature up to 1500 °C under high purity argon. The presence of boron was found to facilitate compaction of the composites and improve the wetting between WC and FeAl binder during liquid phase sintering. Increasing the amount of boron in the binder resulted in the melting of binder at lower temperature and increasing of the compacting of the intermetallic tungsten carbide composites.

Abstract: The strip casting of steel, whereby liquid steel is solidified between twin water cooled copper rolls directly into its final shape, is a radical, energy efficient, cost effective route for the production of steel products that also provides exciting opportunities for the development of new products. An experimental program is currently underway to study phenomena associated with rapid solidification of steel using levitating droplet techniques and Gleeble®3500 thermo-mechanical processing. For example, studies have been conducted to investigate the heat transfer, nucleation behaviour and microstructure development during solidification of a low carbon steel and a peritectic steel on copper substrates hard coated electrolytically or using Filtered Arc Deposition (FAD). It was found that peak and average heat fluxes were significantly higher for steels solidified on the first substrate than for the FAD coated substrates. Maximum heat flux on the respective substrates was 36.5 to 39.0 MW/m2 and 8.3 to 9.4 MW/m2. The average heat flux on the respective coated substrates ranged between 9.6 to 12.5 and 5.5 to 6.6 MW/m2.

Abstract: The Zincalume hot-dip coating process is a well-established technique for excellent corrosion protection of steel products. This paper describes the study of two intermetallics based on the Fe- Al-Si-Zn alloy system, α-AlFeSi and α-AlFeSi(+Zn), pertinent to the Zincalume process. These intermetallics are difficult to characterize in-situ due to their formation at high temperature and because they form as very thin intermetallic layers on a steel substrate, which inhibits accurate quantitative analysis. Controlled magneto-ball milling and hot-pressing have been employed in an attempt to synthesise these intermetallic compounds. Magneto-ball milling, under a He atmosphere, was conducted in shearing mode to enable controlled milling of elemental powders, namely Zn and Al, without the excessive cold-welding often associated with milling of ductile powders with high coefficients of surface friction. XRD analysis indicated that uniform mixtures of highly reactive fine-structured powders were produced, as indicated by diffuse elemental peaks of low intensity. Hot-pressing was utilized to sinter the powders into compact intermetallic compounds without sintering aids. The intermetallics were characterized by DTA, XRD and elemental contrast mapping performed on an SEM with EDS.