Bibliography: pages 96-104. / A hypoeutectoid steel has been heat treated to produce a range of different hardnesses and microstructures. A simulative field test rig has been employed to determine the relationship between microstructural parameters, material properties and soil constitution. Attempts have been made to correlate the ranking order for the wear resistance of similar treated steels in field and laboratory tests. It has been established that wear resistance is a function of soil constitution, steel carbide morphology and hardness. Explanations have been advanced for the differences in the wear resistance of similar steels in different soils based on the mechanism of material removal. Recommendations have been made regarding the selection of steel microstructure for varying soil conditions.

This work focuses on optimizing the sintering process of Ti-6Al-4V using Al-V master alloy powder to achieve a cost efficient product with acceptable mechanical properties. Reference was also made to Ti-6Al-4V products produced by Clinning (University of Cape Town 2012) using elemental Al and V instead of the master alloy addition proposed in this work. Commercially pure titanium (CP-Ti, Alfa Aesar) was used in this study as a reference, since its behavior during processing is more predictable. Relative density and the microstructural feature were studied in-depth in order to understand their impact on mechanical properties. The two starting materials were sintered under vacuum (approximately 10-5 mbar) at 1000⁰C (6hrs), 1100⁰C (2, 4, 6hrs), 1200⁰C (1, 2, 4hrs) and 1300⁰C (1, 2hrs). Relative density, microstructure analysis and mechanical property measurements characterized on the as-sintered material before specimens were hot deformed at 800⁰C to 0.69 strain and subsequently annealed at 870⁰C for 1hr. Similar measurements and analyses were conducted on the samples post deformation and annealing.

The relationship between microstructure and electrical resistivity has been studied in palladium-tungsten and palladium-molybdenum alloys, which exhibit an anomalous increase in resistivity after annealing. The effect of dislocations and structural order on resistivity has been investigated in order to determine the mechanisms whereby changes in resistivity occur in these alloys. The electrical resistivity of palladium-tungsten and palladium-molybdenum alloys has been determined as a function of annealing temperature, using a sensitive electronic apparatus purpose-designed for the present work. For alloys of low solute concentration a minimum in the electrical resistivity with respect to annealing temperature, which has not previously been reported, was observed. Specimens subjected to annealing experiments have been studied by means of transmission electron microscopy, in order to determine the effect of annealing on microstructure and structural order. The information obtained bas been used to relate the measured changes in electrical resistance and microhardness to the structural evolution of the alloys. It is concluded that the unusual resistance effects observed arise from competition between changing degrees of structural order and dislocation density. Which of these mechanisms predominates under given conditions varies as a function of solute concentration and annealing temperature. In alloys containing more than 6 at.% solute, recovery and recrystallisation processes are accompanied by an increase in resistivity and a decrease in microhardness; after recrystallisation is complete, further annealing bas no significant effect on either property. In alloys containing less than 6 at.% solute, recovery and recrystallisation are accompanied by a decrease in both resistivity and microhardness; but further annealing results in an increase in resistivity. Consideration of this data leads to the conclusion that microbardness is more sensitive to changes in dislocation density than structural order, whereas electrical resistivity is more sensitive to structural order than dislocation density. On this basis it is shown that short-range order increases rapidly at low annealing temperatures in alloys containing more than 6 at.% solute, but slowly in alloys containing less than 6 at.% solute in which short-range order increases rapidly only at higher annealing temperatures.

Bibliography: pages 133-136. / A survey has been made of the quality and type of materials used for tillage tools in South Africa. Conclusions have been drawn regarding the inadequacy of the manufacturing processes used and the resultant quality of the tool material. A rig has been designed for the abrasion testing of materials in soil. The reproducibility of the method has been shown to be high and an evaluation has been made of the relative wear resistance of a series of ·heat treated steels. A medium carbon boron steel has been shown to have great promise as a tillage tool material because of its high wear resistance and toughness. The deformed surface layers and the mechanisms of wear of steels subjected to field and laboratory abrasive testing has been examined. The removal of material through predominantly ploughing or cutting mechanisms has been shown to be dependent on the heat treatment and composition of the steels together with the nature of the abrasive. White surface layers have been observed to form on medium and high carbon steels subjected to soil abrasion. Suggestions have been advanced for their formation. Attempts have been made to assess the transferability of data between field and laboratory testing.

The erosion of ashlock hopper valves in coal gasification systems, and the maintainance downtime resulting from this erosion, is a cause of considerable concern to the coal conversion industry. After an assessment of the factors considered relevant to solid particle erosion as presented in literature, a design for a laboratory test apparatus to closely simulate the in-service problem was proposed. A test rig prototype, reproducing service conditions of temperature, pressure and geometry, was constructed and preliminary testing conducted. Various problems impairing the efficient operation of the test facility became apparent and these, together with the proposed solutions and modifications to the prototype rig, are discussed. Both room temperature and elevated temperature (400°C) erosion tests were conducted on the hardfacing seat material presently used on site as well as a on selection of common engineering materials. Optical and scanning electron microscopy (SEM) studies revealed a close correlation between in-situ and laboratory erosion damage, suggesting that the test apparatus is a valid simulation of the service problem.

An investigation has been carried out into the effect of rapid solidification on the microstructure and structural order present in dilute Au-Ti alloys, and the subsequent evolution of these properties on post-solidification heat treatment. Alloys of compositions lwt.% Ti, 2wt.% Ti, 3wt.% Ti and 5wt.% Ti have been rapidly solidified by a technique known as chill block melt spinning (CBMS). The microstructure and structural order present in the alloys both directly on solidification and after post- solidification heat treatment have been characterised using optical microscopy, scanning electron microscopy and transmission electron microscopy; the evolution of the mechanical properties on post-solidification heat treatment has been determined by means of microhardness tests. The flow characteristics of the molten alloys are observed to deteriorate with increasing Ti content resulting in an increase the cooling rate experienced by the alloys during rapid solidification with increasing solute concentration. The as-solidified alloy microstructures are therefore rationalised on the basis of variations in both cooling rate during CBMS and solute content. TEM examination of the as-solidified ribbons demonstrates that alloys containing up to 3wt.% Ti exhibit little evidence of either solute segregation or the formation of the equilibrium, long-range-ordered (Dla) Au₄Ti phase. In a 5wt.% Ti alloy the (Dla) Au4Ti phase is observed to nucleate during processmg. Long-exposure electron diffraction patterns from 2wt.% Ti, 3wt.% Ti and 5wt.% Ti alloys reveal diffuse intensity maxima consistent with the presence of <1½0> special-point order, a state of order which has not been identified previously in Au-Ti alloys. On the basis of electron diffraction patterns taken from these alloys the incorporation of elements of both DO₂₂ and Dla structures within the lattice is appropriate in the description of the structural order giving rise to special-point reflections. The state of order present in the as-solidified 2wt.% Ti and 3wt.% Ti alloys is shown to be best described by incorporating both elements of <1½0> special-point order and elements of the (D1a) long-range-ordered structure. In addition, the nature and distribution of the three-dimensional diffuse streaking observed in zone-axis patterns from a variety of different orientations is discussed and interpreted. This state of order is observed to be stable up to a temperature of 335°C. The lwt.% Ti alloy contained only 0.65wt.% Ti after processing. This loss of Ti results in extensive grain growth on heat treatment at temperatures above 350°C with no detectable second phase formation; as a result the alloy microhardness decreases on heat treatment. In the 2wt.% Ti and 3wt.% Ti alloys no grain growth is observed to occur on heat treatment at temperatures of up to S00°C. On heat treatment at 350°C the Au₄Ti phase is shown to precipitate in these alloys with a commensurate increase in the alloy microhardness. However, extended heat treatment at 500°C results in the coarsening of the Au₄Ti precipitates and is associated, in some instances, with a loss of precipitate coherency and an annealing out of orientational variants of the Au₄Ti phase.

The hardening behaviour of three cold-worked platinum alloys, Pt 5 at% Mo, Pt 5 wt% Ru and Pt 5 wt% Cu, has been investigated through a systematic series of heat treatments. All three of the experimental alloys showed a hardness increase during annealing within a specific temperature range. The hardness of the Pt-Mo and Pt-Ru alloys was found to increase rapidly at annealing temperatures above the recrystallisation temperature, with the final hardness similar to the original coldworked hardness. The hardness of Pt-Cu showed an increase of up to 30% at low annealing temperatures of between 200°C and 500°C. In addition, the Pt-Cu alloy also showed the increased hardness found in Pt-Mo and Pt-Ru at high annealing temperatures, but the hardness increase was not to the same extent. Specimens subjected to the annealing treatments were studied by means of optical, scanning electron and transmission electron microscopy, in order to determine the effect of annealing on microstructure and structural order. Resistivity, XRD and OTA techniques were employed in order to study the mechanisms of ordering with temperature, but these techniques did not produce any significant results. It was concluded that the most likely cause for the hardness increase observed in all three experimental alloys was due to a change in structural order upon annealing. The Pt-Mo and Pt-Ru alloys hardened through an increase in short-range order at annealing temperatures above the recrystallisation temperature. The Pt-Cu alloy hardened through the development of long-range order on annealing between 200°C and 500°C. This increase in hardness was in· addition to the high dislocation density in the alloy specimen due to prior cold-work.

A laboratory carburising furnace in which metal dusting conditions are simulated has been designed and constructed. This furnace has been used to simulate and study the metal dusting corrosion of four iron-based alloys viz. 9Cr Mo 45 steel, Incoloy 800H, AISI 310 stainless steel and Chromanite - an experimental high-nitrogen Cr-Mn stainless steel (HNSS). Tests conducted on the carburising furnace show that the rig is capable of heating a flowing gas environment to temperatures of 800°C in the horizontal ceramic tube. The design allows the testing of up to thirty-six test specimens in a constanttemperature test zone. Systems for the safe heating and disposal of gases such as hydrogen and carbon monoxide have been incorporated into the design. Four twenty-four hour exposures were performed on specimens of the CrMo steel and as received samples of AISI 310. This was followed by a series of seven week-long exposures of Incoloy 800H, AISI 310 and the high nitrogen stainless steel (HNSS). These specimens were tested in an annealed and polished condition in order to increase their susceptibility to metal dusting. In a third test series, specimens of these three alloys were tested in an annealed and abraded condition in order to determine the effect of grain size and surface roughness on metal dusting resistance. Exposures of the CrMo specimens resulted in general metal loss and massive carbon deposition after the first exposure of 24 hours. Filamentous carbon deposits containing metal particles showed that metal dusting corrosion of the specimens had taken place. The as received AISI 310 specimens showed no signs of metal dusting attack over the same exposure time. This was attributed to a protective surface chromia layer that prevented carburisation of the specimens. During the second test series, specimens of Incoloy 800H in the sensitised condition showed a high susceptibility to metal dusting. Carburisation of the matrix carburisation was accompanied by large carbon protrusions growing from the specimens' surfaces. Large pits were observed on the specimens after five weeks. Sensitised AISI 310 specimens also showed signs of metal dusting but at a slower rate. The difference in performance between these two alloys was attributed to the difference in alloying contents, notably chromium, nickel and silicon. The HNSS specimens showed a high resistance to carburisation, carbon deposition and metal loss during the first six weeks. Small amounts of carbon deposition and pitting were observed after the seventh exposure. The good resistance to metal dusting of this alloy was attributed to its alloying contents, which included chromium, manganese, sulphur and nitrogen. The results of the third test senes showed that resistance to metal dusting was significantly improved by increasing the surface roughness and decreasing the grain size of the specimens. A new alloy, Fe-25Cr-12Ni-9Mn-4.5Al-2Si-0.5N is proposed for fabrication and exposure to metal dusting environments to evaluate its suitability for use in industrial applications. It is also recommended that further work be carried out in evaluating the effect of increasing the nitrogen, chromium and manganese contents of the Fe-18Cr- 9Mn-0.5N alloy that performed well in this project. Investigations into the effect of aluminising and nitriding components should also be carried out.

The life span of high temperature power plant pipework is principally a function of material creep damage - an irreversible plastic deformation of the material when subjected to temperatures and loads beyond a certain threshold. Within Eskom, the South African parastatal power utility, creep damage is primarily quantified by way of Metallographic Replication (replicas). This is a quasi NonDestructive Examination (NDE) technique that looks at the microstructure of the sample in question. Although well-known and used extensively, replicas, as with any technology, have their shortcomings. Extracting of replicas and their subsequent analysis are manual processes that inherently suffer from subjectivity. Furthermore, storage and archiving of vast quantities of physical replica slides for future reference is cumbersome - a challenge that digitisation can address. The aforementioned vulnerability to analysis subjectivity and benefits of digitisation are areas which a technology known as Digital Image Correlation (DIC) - a non-contact, full field, deformation measurement technique - can potentially address. Some research has been done on using DIC for power plant creep measurement; however literature quantifying its performance in this specific application is scarce. This study thus looks into setting up a DIC system optimised for measuring strain in an area of the pipework welds known as the Heat Affected Zone (HAZ) - the weakest part of the weldment. The achievable accuracy is established and the major parameters that affect DIC accuracy are investigated, elucidating the trade-offs between optimising each. In addition, two scenarios exist for the acquisition of DIC measurement data from a plant: when the plant is operating (online); or when the plant is shut down for maintenance (offline). The encumbrance of imaging a hot surface makes the former scenario the more demanding, and was thus investigated. This data was subsequently used to substantiate whether DIC has the potential to be used online (i.e. at elevated temperatures) or is limited to use during shut downs.

Includes bibliographical references. / This work addresses the problem of erosion of steel heat exchanger coils in a petroleum producing plant by magnetite catalyst particles entrained in the flowing gas stream.