The repetitive impacting of solid components in industry can result in wear damage which may significantly limit service life. Impact wear problems have been encountered in hydro-powered stoping equipment (eg rockdrills and impact rockbreakers) developed for deep level gold mining in South Africa. This research project was a study of the repetitive impact wear of reciprocating valve components (eg poppet valves within the impact rockbreaker) under simulated conditions. A laboratory apparatus, capable of producing impacts varying in energy from 2 - 5 J and varying in frequency from 5 - 50 Hz in an aqueous environment (distilled water), was designed and built for this purpose. Impact tests were conducted in order to: a) rank materials according to impact wear resistance, b) to determine modes and mechanisms of wear, c) to determine material, microstructural, design and operating parameters of importance in minimising wear, d) to make recommendations concerning the above, to facilitate productivity and longlife of poppet valves within impact rockbreakers. The materials selected for testing (817M40, 1210 and AISI 304, AISI 431 and AISI 440C) are steels currently used by the gold mining industry in different applications and known to perform satisfactorily in service. These materials are not all ideally suited to application in valves. They were chosen in order to illustrate how different steel compositions, microstructures and heat treatments influence the rate and mode of wear.

Bibliography: pages 92-95. / This study examines the formability of an AISI 301 based metastable austenitic stainless steel, in which nitrogen partially substitutes nickel. In order to understand the formability of the experimental alloys, the tensile behaviour of the alloys is characterised. The tensile properties of metastable austenitic stainless steels are governed by austenite stability which is related to alloy composition and test temperature. At certain alloy compositions, transformation induced plasticity (TRIP) occurs. TRIP depends on the manner in which deformation induced martensite forms in the steels. Incipient necking is resisted if the martensite forms gradually and selectively, preventing propagation of micronecks and microcracks. Tensile tests performed from -5 to 100°C were used to study the effect of TRIP on the ductility of these alloys and optimum tensile properties were obtained at room temperature. In addition, the effect of copper on TRIP and subsequently formability were ascertained using copper alloyed stainless steels. Important stretch formability parameters were obtained from the tensile test which is an intrinsic formability test. TRIP results in improved formability of metastable austenitic stainless steels, and a simulative Engelhardt test was performed to ascertain the effect of TRIP on drawability of the test alloys. It was found that alloys with TRIP characteristics exhibited good drawability and in all cases the test alloys had better limiting drawing ratios than AISI 304 stainless steel. Delayed cracking occurred in alloys with more than 0.2 percent nitrogen content and a low austenite stability, probably as a result of embrittlement of the deformation induced martensite by nitrogen. A study of the cavitation erosion of the test alloys was initiated because it is known that TRIP enhances cavitation erosion resistance in stainless steels. Results indicate that the metastable test alloys demonstrate superior erosion resistance when compared with the stable experimental alloys. Cavitation induced martensite was found in metastable alloys using x-ray diffraction.

Titanium and its alloys have extremely high strength to weight ratio and corrosion resistance. This has made titanium alloys very attractive materials for many structural applications. However, the high price of these alloys has seen their use restricted to very few high performance areas such as aerospace and bio-medical applications. The costs can be significantly reduced by directly converting titanium powder to metal products, particularly via the direct powder rolling process. This present research is based on a parametric study of powder rolling and operational effects including rolling speed, roll gap size, roll surface width and powder feeding rate on the integrity of the metal powder compacted strip that is produced from the direct powder rolling process. The objectives of this work were to predict the powder rolling compaction performance in terms of compaction pressure, roll surface force and rolling torque, and produce the highest possible relative density of CP-titanium green strips by optimized rolling variable setups, and to achieve its full density by additional hot deformation processing. In this work, a purpose-built gravity fed vertical powder rolling mill with a roller diameter of 265 mm and a roller width of 150 mm was used. Johanson's powder rolling model has been implemented to simulate performance and a series of rolling parameter setups have been conducted by the using a purpose-built powder rolling mill. Pre-alloyed, water-atomized stainless steel 316L powder and hydride-dehydride (HDH) CP-titanium powders with a mean particle size of 100 μm were used as the experimental material to validate the simulated results in terms of relative density and strip dimensions. The powder rolling experiments were carried out based on the Johanson's powder rolling model to conduct a parametric experimental study in various setups of powder rolling parameter combinations, including roll gap, roll face width, rolling speed and powder feeding rate.

Bibliography: pages 101-109. / The locally (RSA) developed 9Cr-2Ni-0.7Mo steel designated 927 formed the subject of this study. Its abrasion-corrosion and stress corrosion performances were assessed in laboratory tests simulating the underground environment in South African gold mines. The results indicate that the alloy performs favourably in abrasive-corrosive applications, outperforming several other higher chromium containing steels which have been designed for the purpose. The alloy is also highly resistant to sec at free corrosion potential in simulated mine water. The good abrasion-corrosion resistance is attributed to the adequate corrosion resistance of the alloy acting in conjunction with the favourable combination of strength and toughness afforded the alloy by its fine grain size and microduplex microstructure of martensite and interlath retained austenite. The production variables of plate thickness and prior cold working were found to exert negligible influence on corrosion-abrasion resistance. This is ascribed to the small influence of these processes on the hardness and associated mechanical properties due the inherent low work hardening ability of the alloy. Slow strain rate (SSR) stress corrosion cracking tests were performed on the alloy in four microstructural conditions viz. as-rolled, tempered, welded and post weld heat treated. The material showed an immunity to sec in all the microstructural conditions for tests conducted at open circuit potential. This apparent immunity is attributed to the difficulty in initiating sec by pitting on the plain specimens over the relatively short test durations. Polarisation to extreme cathodic potentials (-1200m V) resulted in hydrogen embrittlement of this high strength alloy with failure predominantly along prior austenite grain boundaries. Anodic potentials in the excess of OmV induced tunnel-like corrosion pitting attack. Fractographical evidence of sec at the base of these pits indicates the development of the conditions necessary for sec within the pit confines. This is cited as evidence in support of the hypothesis of sec initiation difficulty.

Alloy 825 has been extensively used as a cladding alloy in the gas and oil industry for process piping up to and including the manifolds, separators, wellheads, risers and valves. The outstanding corrosion resistance of alloy 825 against general and localised corrosion attack is attributed to its high Ni, Cr and Mo content. However, corrosion failures of alloy 825 equipment have been observed in offshore environments. Alloy 825 has good weldability and for applications that require exceptional resistance to corrosion, Inconel filler metal 625 is used as ''overmatching composition''. Nevertheless, there is always a threat of galvanic corrosion when two dissimilar alloys are electrically connected. In this study, the corrosion behaviour of alloy 825, alloy 625 weld and alloy 825 weldment have been investigated. Potentiodynamic polarization curves for the alloys were recorded in synthetic seawater across a range of temperatures (30 to 60°C). Mixed potential theory was applied to determine corrosion potentials, rates of corrosion and predict the galvanic effect of coupling alloy 825 to alloy 625 filler metal via welding. Three standard methods were considered to determine the critical pitting temperature (CPT) for alloy 825. Lastly, long-term immersion tests in seawater were conducted to determine the relationship between the laboratory accelerated tests results and the performance of the alloys under real service conditions. The results from the experimental tests revealed that alloy 825 and alloy 625 weld exhibit outstanding corrosion resistance to uniform corrosion, despite the effect of temperature on the corrosion rate of both alloys. The galvanic effect of coupling alloy 825 to alloy 625 via welding is insignificant. The corrosion morphology of alloy 825 and its weldment is temperature dependent. At temperatures below 45 °C, grain boundary attack was observed in alloy 825 samples, while pitting corrosion was observed at temperatures higher than 50 °C. Alloy 625 weld exhibited only one mode of corrosion attack, namely the selective dissolution of interdendritic phase throughout the test temperature range. There was no agreement between the CPT results for alloy 825 and its weldment obtained using the three standard methods. No correlation was found between CPT determined by laboratory tests and the temperature above which alloy 825 would suffer pitting corrosion in long term seawater exposure tests.

Rehabilitation coatings of offshore equipment rarely perform as well as the original coating, despite the high cost involved. The performance gap is probably due to high relative humidity, salt contamination and limitations on the use of abrasive blast cleaning. Thus, this research aims to deepen the understanding of surface preparation parameters that affect organic coating performance. Carbon steel samples were subjected to a variety of surface alterations consisting of salt contamination, mechanical (wire brushing) and chemical (rust converter and remover) surface preparations followed by coating application and performance testing. The samples were first pre-corroded in a corrosion chamber to mimic degradation from service then surface preparations were performed after which a coating was applied. Coated new samples (RN) and fully corroded samples (SN) were the reference sets, while other samples were prepared to a variety of surface conditions. Visual inspection and electrochemical impedance spectroscopy (EIS) were performed prior to exposure and periodically during accelerated cycling corrosion testing for a period of 30 days. The visual condition of the samples was used to rank the performance of the prepared samples. These results were used as benchmark to decide the optimum EIS method, either phase angle at high frequency or total impedance at low frequency, for early evaluation of the organic coating performance under the conditions studied. Furthermore, adhesion pull-off testing was performed to rank the effectiveness of the coating over various prepared coating. The reference new samples (RN) proved to be the best surface condition and the corroded samples without preparation (SN) had the worst performance for all tests performed. In addition, it was established that salt contamination had a stronger impact on the coating performance than the amount of corrosion product remaining on the surface. Moreover, it was determined that the best preparation approach after precorrosion of the plates was to apply rust converter to the surface before coating. Adhesion measurement was of secondary concern on the studied coated surfaces as cohesive failure occurred on the pre-treatment layers rather than coating adhesion failure between the coating and the treated surface.

Direct powder rolling (DPR) is thought to be a more cost-effective, more direct route to producing flat product of near final thickness. It is a particularly attractive route for titanium given that the existing wrought method of melting titanium sponge to ingot followed by rolling to slab to plate and finally to thin gauge product is an energy and capital-intensive process. There are several studies that have investigated the operational parameters of DPR, but there has been little assessment of the realization of DPR as a fully operational process producing a commercially viable product. The commercial viability of DPR is a particularly pertinent question for South Africa given the investment, by the Council for Scientific and Industrial Research and the Department of Science and Technology, in the development of an innovative powder manufacturing technology, as well as complementary powder metallurgy methods. A commercial viability assessment of DPR was structured around three analyses: 1.) whether a supply-side market exists to support a commercial enterprise, 2.) how the performance of DPR product compares to the performance of product produced via the conventional wrought route, and 3.) what range of potential product applications could be suitable for DPR product. A systematic review of published research was conducted by extracting and consolidating performance and process data, and a market analysis was conducted by sourcing price points from powder suppliers and wrought product suppliers. The performance of DPR product, in terms of elongation and ultimate tensile strength, was found to be comparable to the typical properties of ASTM grade 3 and 4 wrought product, which contain higher oxygen and are the least ductile of the commercially pure titanium grades. Due to the particulate nature of the starting stock and titanium’s affinity for oxygen, oxidation was found to be the single greatest problem in powder metallurgy. The upper and lower bounds of the oxygen range were identified, and the consolidation of data showed that an oxygen content of less than 0.2 wt% is not commonly achieved for non-hydride derived product. The possibility of producing a weldable product via DPR was found to be low, due to the unacceptable degree of chlorine content, which is typically greater than 0.02 wt% in low-cost (non-melt) commercially available powders, as well as the fact that weldability has not been reliably demonstrated for powder metallurgy product made from these powders. The existing powder market was also found to be inadequately geared towards supporting a commercial enterprise due to the small size of the market and the lack of availability of low-cost quality powders. The comparison of powder prices to wrought product prices showed that the potential for commercial viability is likely to exist only for thin gauge strip of less than 1mm thickness, as this is where cost savings can be attained through direct route processing. Based on the DPR product profile identified, the range of potential product applications was found to be greatly limited. The inability to reliably meet the typical properties of the “workhorse”, grade 2, excludes the largest proportion of applications for which pure titanium in strip form is used (heat exchangers and tubing). Furthermore, the lack of evidence of adequate weldability further restricts the usage of DPR product to applications where welding is not a critical requirement. For these reasons, it was concluded that DPR is not a commercially viable process.

Includes abstract. / Includes bibliographical references. / AA5182 aluminium alloy is used for the manufacturing of can ends for beverage cans. The alloy selection for this part is based on the formability of the material and its resistance to softening over time. Owing to the intricate design of the can end opening tab, it is vital that the material maintains its strength during its shelf life. The mechanical properties of the AA5182 aluminium alloy are dependent on the microstructural evolution of the alloy during processing and forming. Al-Mg alloys, like AA5182, can undergo a low temperature heat treatment, which has the effect of stabilizing the microstructure and minimizing the subsequent recovery processes during and after coil coating. The effects of these heat treatments have been investigated in order to understand the effectiveness of the stabilization heat treatment on the AA5182 alloy. This study investigates various stabilization heat treatment temperature profiles, and then aims to characterize the microstructural evolution of the material during a simulation of the coil coating practice that the material is exposed to as the final step in the rolling mill operation.

Includes bibliographies. / The effects of high energy milling on tin bronze with the composition Cu-24.6wtSn have been examined using hardness testing, optical microscopy, scanning electron microscopy, transmission electron microscopy and x-ray diffraction. High energy milling has caused mechanical alloying of an elemental copper and tin powder blend, and mechanical milling of a cast powder and a melt quenched powder. Nanocrystalline grains with a size between 5 nm and 50 nm have been directly observed in the final milled powder. The powder consist of the a phase and 8 phase and is partially amorphous. An extension of the solid solution solubility has also been detected due to milling. The formation of the metastable tin-rich 11 phase has been observed in the intermediate stage of mechanical alloying of the elemental powder blend due to the higher diffusivity of tin in copper over copper in tin. The formation of the 11 phase during mechanical alloying of tin bronze with the composition Cu-24.6wtSn has not been reported before. The morphological development of the three initial powders has proceeded by different mechanisms during milling due to the different hardness and toughness of the starting powders. Milling of the elemental powder blend and the cast powder proceeds via classic mechanisms for milling of ductile powders and brittle powders respectively, while milling of the tougher melt quenched powder proceeds via a combination of the two mechanisms. An attempt to process the milled powder into a bulk state using various thermomechanical techniques while still retaining a nanocrystalline grain size has not succeeded. The high diffusivity of the material at elevated temperatures has led to grain growth into the micrometer range even at relatively low thermo-mechanical processing temperatures. The milled powders have poor compaction properties due to the highly deformed structure and therefore the processed material has poor properties compared to a cast material.

Bibliography: p.103-116. / Comminution is the process which aims at increasing the surface area and the resultant liberation of a particular constituent from the mass of a solid. The autogenous mill uses tumbling to effect comminution, but instead of special milling bodies being added, pebbles of the material to be comminuted are used. The autogenous comminution process utilises less than 0,1 per cent of the energy input. The principal objective of the present work was to analyse autogenous milling behaviour in terms of the individual comminuting mechanisms and to establish the inter-relationships between the main process variables, namely rock petrography, size distribution of the feed, applied load, relative velocity and environment. In this manner the optimisation of the process and an improvement of its efficiency was sought. In addition the establishment of testing procedures to predict the autogenous milling behaviour of a given type of rock was aimed. In the present work the gold bearing Witwatersrand quartzite was used, although the findings are applicable to other types of rocks. Since fracture phenomena are involved in all comminuting mechanisms of impact-compression, chipping and abrasion, slow compression and Brazilian tests were performed. The grain size and the mineral composition of the rock has been found to have a large influence on the local stresses required for these processes. Indeed the results show that the fragility and therefore ease of comminution increases with increasing grain size of the quartzite. Brazilian tests on drill cores of varying diameters may allow the prediction of the critical size of rock of the mill feed which can survive in a mill of given characteristics.