Browsing by Department "Centre for Materials Engineering"
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- ItemOpen AccessAbrasion-corrosion and stress corrosion resistance of a 9Cr-2Ni-0.7Mo steel in simulated mine water(1991) Gatzanis, Gustav Ernest; Ball, AnthonyThe 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.
- ItemOpen AccessThe abrasive wear behaviour of mineral-filled polypropylene(1997) Sole, Brian Michael; Ball, AnthonyPolypropylene is an extremely versatile polymer because its properties can be modified to meet specific requirements. The use of polypropylene in domestic and automobile applications has initiated research focused on the tribological behaviour of the material. In the present study, polypropylene grades have been subjected to both mild and severe abrasive wear conditions with specific emphasis on the surface property of scratch resistance. The experimental work has covered the effect of polymer crystallinity, mineral fillers, and the nature of the abrasive counterface on the wear behaviour of polypropylene. The wear behaviours of polymethylmethacrylate, polycarbonate, acrylonitrile-butadiene-styrene, and high density polyethylene have been determined for comparative purposes. The abrasive wear rates have been measured and the material deformation and removal mechanisms have been identified and characterised in terms of the physical properties of the polymer and the individual fillers, and in terms of the macroscopic mechanical behaviour of the filled composite materials. Investigative techniques used in this study included mechanical testing, optical and scanning electron microscopy, surface profilometry, and differential scanning calorimetry. Under two-body abrasive wear conditions, the unfilled and modified polypropylene materials exhibit a ductile mode of material deformation and removal.
- ItemOpen AccessThe abrasive wear resistance of austempered spheroidal graphite irons(1987) Shepperson, S V; Allen, ColinA study has been made of the structure and abrasive wear resistance of two austempered commercial spheroidal cast irons. Heat treatments have been carried out for different times between 2 and 120 minutes for a range of austenitising temperatures between 850°C and 950°C and austempering temperatures between 250°C and 450°C. The morphology and constitution of the resulting dual phase ferrite/austenite structure has been examined using optical and scanning electron microscopy and x-ray analysis. The maximum quantity of retained austenite in the structure has been shown to vary up to 50% and to be strongly dependent on heat treatment parameters and the composition of the iron. Laboratory abrasive wear testing has been carried out on these austempered irons and compared with the results of similar tests on a range of abrasion resistant carbon steels. All the austempered irons were found to have better abrasion resistance than proprietary abrasion resistant steels. These austempered irons derive their outstanding properties from the morphology of the dual phase ferritic/austenitic matrix coupled to the high work hardening characteristics brought about by the stress induced austenite to martensite transformation during abrasion. The influence of microstructure and mechanically induced transformation has been studied as a function of austempering temperature and time.
- ItemOpen AccessAbrasive wear resistance of ruthenium aluminide intermetallic and ferritic steels containing a sigma intermetallic phase(1998) Ngakane, M; Lang, CandyThe abrasive wear resistance of ruthenium aluminide intermetallic and ferritic steel containing an iron-chrome sigma intermetallic phase have been investigated in this study. A medium carbon wear resistant steel (MCV) was used in the study to facilitate comparison between wear resistances in the materials of interest. Specimens of ruthenium aluminide, MCV and ferritic stainless steels containing a sigma phase were produced. The MCV steel was quenched and tempered to match the bulk hardness of the as-received ruthenium aluminide. Five different grades of thermomechanically worked ferritic steels specimens were heat treated to produce different volume fractions of sigma phase. The mechanical properties of the specimens were investigated by compression testing and microhardness measurements. Abrasion testing was carried out on a pin on belt abrasion apparatus. The surface response of the specimens to abrasive wear was characterised by optical and scanning electron microscopy. Microhardness of the specimens were measured with a digital microhardness machine. The wear resistance of ruthenium aluminide was found to be higher than all materials tested in this project. The wear resistance in some of the ferritic steels containing sigma phase was comparable to that of the wear resistant medium carbon steel. The surfaces of the specimens were shown to work-harden during the abrasion process. The corresponding work-hardening results showed that ruthenium aluminide had the highest work-hardening rate. It can be concluded that the work-hardening ability of the test materials correlates with their respective wear resistance properties.
- ItemOpen AccessAbrasive wear testing of steels in soil(1987) Quirke, SJ; Allen, ColinA 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.
- ItemOpen AccessAn abrasive-corrosive wear evaluation of some aluminium alloys(1989) Meyer-Rödenbeck, G D; Ball, AnthonyThis investigation evaluates the abrasive-corrosive wear behaviour of aluminium alloys with the aim of establishing a data base of performance and guide lines for material optimisation. Wear test apparatus and standard tests developed by previous research programmes were utilised (Noel and Allen, 1981; Barker, 1988). Further tests were then devised for a more detailed characterisation of wear behaviour. Tests conducted showed that aluminium alloys have approximately a quarter to half the abrasion resistance of mild steel. Poor microfracture properties of Al-Si cast alloys were observed as a result of coarse and brittle silicon rich phases contained in the aluminium matrix. Non heat-treatable wrought alloys exhibit ductile micro-deformation characteristics whilst heat-treatable alloys, having the best abrasion resistance, possess better combinations of strength, hardness and toughness. Tests with combined corrosion and wear showed that most aluminium alloys are subject to pitting corrosion due to localised differences in electrode potentials at constituent sites. Higher series alloys with a large number of constituent particles exhibit higher pitting densities. Due to the high electrode potentials of silicon phases and copper and zinc solid solutions, the alloys LM6+Sr, 2014 and 7075 have poor corrosion resistance and are subject to localised and pitting attack. As a consequence the alloys 2014, 7075 and LM6+Sr show a decrease in wear performance under abrasive-corrosive conditions. In contrast the good corrosion resistance of the alloys 5083, 6261 and 7017 provide a significant improvement in wear performance under conditions of long corrosion periods with light abrasive intervals. This study concludes that the abrasion resistance of wrought alloys may be optimised by designing an alloy with a good combination of tensile strength, fracture toughness and hardness together with an intermediate microstructural size distribution of second phase particles in the aluminium matrix. Ageing of heat treatable alloys improves abrasion resistance significantly, peak hardness and strength conditions resulting in optimum abrasion properties.
- ItemOpen AccessAdsorption of oxygen molecules on platinum surfaces modified with subsurface atoms of vanadium : a DFT study(2014) Matengaifa, R; Lang, C I; Sithole, HThe aim of this work was to investigate changes in the electronic structure of platinum as a result of alloying with vanadium, and the effects of these changes on O2 adsorption. This is important for the further development of hydrogen fuel cells, because the oxygen reduction reaction (ORR) presently requires O2 adsorption to occur on pure platinum, which is a prohibitively expensive material. A computational study has therefore been undertaken on alloying platinum (which reduces cost) with vanadium (for which there is plentiful experimental data) and the consequences for O2 adsorption. The first moment of the d-band of platinum alloy DOS was used to represent the d-band centre. The d-band centre of Pt-PDOS became lower as a result of hybridisation between platinum and vanadium. The d-band centre of a pure platinum surface with respect to the Fermi level is -1.99eV, but it is shifted to -3.23eV when vanadium atoms are added to the subsurface layer. The adsorption energies of O2 are sensitive to a combination of calculation parameters used. In this work, the calculations were executed using the CASTEP code. This is a plane wave pseudo potential code. The most stabilised geometry of an adsorbed molecule on pure Pt (111) was at the fcc site and had an adsorption energy of -1,91eV. The adsorption energy at the bridge site of Pt (111) is -1.81eV. When subsurface vanadium atoms were introduced, the equilibrium surface-molecule bond lengths increased. The adsorption energy at the fcc site shifted to -1.37eV, -1.43 for the bridge site and -1.45eV for the hcp site. It was concluded that the presence of vanadium atoms in the surface region destabilises an adsorbed oxygen molecule but a more detailed study is needed to show the effect of the solute atoms on the thermodynamics and kinetics of the whole oxygen reduction reaction chain.
- ItemOpen AccessAnalysis of phase transformations in hydrogenated titanium metals by non-isothermal dilatometry(2011) Abbas, Naseeba; Knutsen, Robert DHydrogen was used as a temporary alloying element in CP Ti and Ti-6AI-4V. The microstructural evolution and phase transformations were monitored, before, during and after hydrogenation with in-situ dilatometric testing. Wrought CP Ti and Ti-6AI-4V specimens were pre-annealed and experienced four consecutive thermal cycles (Cycles 1-4) i.e. hydrogenation, post-hydrogenation, dehydrogenation and post-dehydrogenation, during dilatometric testing. The specimen in each thermal cycle was heated to 1000°C, heating rate 1°C/min (with an isothermal hold at 1000°C for three hours for hydrogenation and dehydrogenation cycles) and then cooled to room temperature at cooling rate of 1°C/min.
- ItemOpen AccessAspects of serrated flow in aluminium alloys(1992) Robinson, Jonathan Mark; Shaw, M PUniaxial tensile testing has been undertaken on a range of aluminium base alloys. Material investigated included commercial binary Al-Mg (5182), ternary Al-Mg-Si (6061) and quaternary Al-Cu-Mn-Si (2014) as well as experimental alloys containing 2at.% additions of Ag, Mg and Zn to commercially pure AI (1070). In addition, composite materials based on both alloys 2014 and 6061, containing 10%, 15% and 20% additions of Ah03 particulate, as well as 20% SiC particulate in the case of 6061, were also tested. Microstructures of materials were varied by prior heat treatments but, for comparison, all materials, were initially tested in the solution treated and quenched condition. Mechanical testing was undertaken at room temperature throughout the course of the work, and at strain rates such that serrated tensile test curves were manifest. The evolution of microstructural features of the deformation was evaluated utilising both optical and electron microscopy. Surface deformation features, including the formation of both type A and type B deformation markings, was examined on pre-polished specimen gauge lengths at various levels of tensile strain. The planarity of slip line traces was correlated with the evolution of related deformation structures in dynamic experiments in a high voltage transmission electron microscope (HVEM). In addition, the formation of slip lines on the surface of the HVEM microtensile specimens compared favourably with those formed on the surfaces of macroscopic tensile specimens. Microscale heterogeneities in the deformation observed during in-situ dynamic HVEM experiments on poly crystalline material correlated with the extent of serrated flow manifest in bulk specimens. All materials deformed in the HVEM displayed inhomogeneous dislocation motion consistent with the macroscopically observable discontinuities. The alloys tested were microstructurally distinguishable during dynamic experiments depending primarily on whether or not they had been deliberately alloyed with magnesium. The alloys containing Mg exhibited the activation of parallel slip traces together with minimal cross-slip in any single micro-yield event. In contrast, the alloys which did not contain Mg exhibited the simultaneous activation of various intersecting slip systems and were characterised by extensive cross-slip during similar yield events. On the basis of these observations, the magnitude of serrations and extent of serrated flow in the alloys has been discussed. The extent to which the different alloys were able to undergo dynamic recovery affected both the evolution of the dislocation structure observed in the conventional transmission electron microscope ( CTEM) as well as the final fracture mode. The existence of a characteristic shear fracture mode was consistently observed to follow tensile deformation which had been dominated by unstable plastic flow. The ready occurrence of dynamic recovery and the associated formation of dislocation cell structures allowed for more fully developed plastic instability during the final stages of tensile deformation and a lower likelihood of final failure by premature shear. Finally, the addition of particulate reinforcement to 2014 and 6061 had different effects that were accounted for by the difference in strength between the two monolithic materials. In the case of the weaker 6061, all particulate additions had a strengthening effect whereas in 2014, increasing the volume percent of reinforcement progressively weakened the composite. Serrated flow properties of both alloys were affected by the addition of the particulate reinforcement. The homogeneity of particle distribution as well as the size of the particulate inclusions affected both the tensile properties and final fracture of the composites.
- ItemOpen AccessAn assessment of polymeric materials and surface treated steels as cavitation erosion resistant materials(1983) Barletta, A; Ball, AnthonyThe object of the research described in this thesis was to optimise the choice of materials used for vital components of hydraulic machinery. Frequently these components are damaged by a process known as cavitation erosion and the operation and efficiency of machines are seriously impaired. Nineteen different polymers which have potential for use in hydraulic components have been eroded by liquid cavitation, employing the stationary specimen system. An attempt has been made to correlate the extent of erosion with the mechanical and chemical properties of the polymers. Modes of erosion of different materials were studied by scanning electron microscopy and a strong correlation was found between these modes and the resistance to erosion. Heterogenous polymers (mixture of two homogenous components), together with the poly amides and polyethylenes, showed the highest erosion resistances. The effect of prior immersion (3 weeks at 70°C) in either a dilute or concentrated form of hydraulic fluid has been investigated for both polyacetal and ultra high molecular weight polyethylene samples in order to simulate service conditions. The polyacetal samples showed improved erosion resistance relative to the samples stored in air or water (3 weeks at 70°C). In contrast, the ultra high molecular weight polyethylene samples failed in a catastrophic manner by solvent stress cracking.
- ItemOpen AccessAssessment of the corrosion behaviour of alloys 825 and 625 in stagnant seawater-effect of temperature and welding(2017) Chicuba, Pedro Claudio Francisco; Knutsen, Robert DAlloy 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.
- ItemOpen AccessThe brittle and plastic response of quartz.(1980) Glover, Graham John; Ball, AnthonyQuartz is the principal mineral constituent of the earth's upper crust and a knowledge of the deformation characteristics of quartz are of geological and technological importance. In common with many ceramic materials, quartz may not satisfy the Von Mises independent slip system criterion and clarification is required on the exact response to an imposed stress system at a given temperature.
- ItemOpen AccessCasting platinum jewellery alloys: the effects of casting variables on fill and porosity(2005) Miller, Duncan; Keraan, Tauriq; Park-Ross, Penny; Husemeyer, Victoria; Brey, Ali; Khan, Irshad; Lang, CandyComparisons are made between platinum-copper and platinum-ruthenium alloys used for jewellery to evaluate the effects of casting variables. The effects of flask temperatures, investments, and centrifugal speeds on microstructure, percentage fill, and porosity were examined over a range of temperatures. Optimum conditions and materials for successful casting of high quality platinum jewellery alloys, using a Hot Platinum induction melting and casting machine, are described. Suitable choice of investment materials and rotational speeds produced good grid fills with Pt-5%Cu and Pt-5%Ru alloys. Metal porosity was more difficult to control, due to the inherently chaotic nature of the casting process, but casting into a relatively cool mould minimised the probability of bad porosity for both alloys. Pt-5%Ru was found to be successful as a casting alloy when used with induction melting technology. It displayed superior uniformity, hardness and colour, compared with cast Pt-5%Cu alloy.
- ItemOpen AccessCavitation erosion of WC-Co(1987) Hankey, S E; Ball, AnthonyAn investigation involving the vibratory cavitation erosion of WC-Co alloys was undertaken in order to determine the mechanisms of material removal. Nineteen grades of WC-Co alloys were studied. These alloys had been previously characterised according to microstructural and mechanical properties. Further characterisation by way of Young's modulus and density of the materials was undertaken. An investigation of the i nfluence of various parameters on cavitation erosion established a binder content dependence on erosion. For two grain sizes, erosion was found to increase to a maximum at 12 vo1-% binder content (1.8 μm grain size) and 23 vol-% binder (2.8 μm grain size). The main mode of material removal was found to be cobalt removal followed by WC grain pull-out. In high binder content alloys, cobalt removal was predominant with little loss of WC grains. X-ray diffraction showed that the allotropic phase transformation of the binder under cavitational attack was beneficial to the erosion resistance of these alloys. The erosion of low binder content alloys was controlled by the contiguity of the WC skeleton. Maximum erosion occurred at binder contents which corresponded to the combination of a fragile WC skeleton and a small volume of available cobalt for strain induced transformation.
- ItemOpen AccessCharacterisation and refinement of properties of glass fibre reinforced polyester polymer concrete for use in manhole components(1999) Griffiths, Robert; Ball, AnthonyThe aim of this investigation is to characterise and refine the physical properties of glass fibre reinforced polyester polymer concrete. This material is currently being employed by AV Mouldings (Pty) Ltd. to manufacture manhole and drain components according to specifications existing for cast iron covers. No specification exists for polymer concrete. In particular it has been found that there is a large market for Type 2A replacement manhole covers and frames due to the current problem in South Africa of the cast iron versions being stolen and sold for scrap metal. It has been found that polymer concrete covers manufactured to replace stolen cast iron covers (in existing cast iron frames) fail occasionally in service. The investigation thus focuses on the characterisation of glass fibre reinforced polymer concrete and analysis of the current standards with a view to establishing a new South African Bureau of Standards (SABS) specification for polymer concrete manhole components. The main testing procedure involved flexural testing of beam specimens. Preliminary tests were carried out to measure strength, toughness, strain rate sensitivity, and the effect of different reinforcing materials. Accelerated degradation tests were then conducted to establish the materials resistance to UV radiation, acids, alkalis, and various solvents. Different resins were evaluated, and experiments were conducted using graded aggregates, in an attempt to reduce the number of voids in the material. Vibratory moulding techniques and postcuring methods were also evaluated. The viability of employing silane coupling agents in polymer concrete was investigated in detail towards the end of the research. Redesign of the Type 2A replacement cover was then undertaken.
- ItemOpen AccessCharacterisation of the Tensile and Metallurgical Properties of Laser Powder Bed Fusion-Produced Ti-6Al-4V ELI in the Duplex Annealed and Dry Electropolished Conditions(Multidisciplinary Digital Publishing Institute, 2023-08-02) Gibbons, Duncan William; Makhetha, William Motsoko; Becker, Thorsten Hermann; van der Merwe, Andre FrancoisMetal additive manufacturing is becoming a popular manufacturing process in industries requiring geometrically complex components, part consolidation, and reductions in material waste. Metals manufactured via additive manufacturing processes such as laser powder bed fusion typically exhibit process-induced defects, material inhomogeneities, and anisotropy in terms of mechanical properties. Post-processing techniques such as heat treatments and surface finishing have been touted as approaches for improving these materials. Although various post-processing techniques have been proposed, the optimal post-processing route remains an active area of research. This research investigates Ti-6Al-4V ELI produced using laser powder bed fusion and post-processed via different routes. The materials in the stress-relieved and duplex annealed material conditions as well as dry electropolished and machined surface conditions were characterised. The duplex annealed Ti-6Al-4V ELI material showed improvements in ductility but at reduced strength when compared with the material in the stress-relieved condition. The microstructure of the duplex annealed material shows little evidence of process-induced defects and features and consists primarily of elongated and acicular α in a lamellar structure with intergranular β and exhibits uniform microhardness throughout the material. A reduced surface roughness due to surface finishing resulted in an improved reduction in area. This research highlights the effects of post-processing treatments and their ability to improve the properties of laser powder bed fusion-produced Ti-6Al-4V ELI.
- ItemOpen AccessA comparative evaluation of manufacturing technologies and their effect on morphology and mechanical properties of poly-(propylene-ethylene) copolymers(2002) Sello, Maitse P; Marcus, Kashif; Sole, Brian MichaelThis project is a technological benchmark exercise for Sasol Polymers where different poly (propylene-ethylene) copolymer grades from five foremost licensed manufacturing PP technologies are evaluated. These copolymer grades were manufactured using the following processes: Novolen PP technology (Sasol grades), Innovene PP technology (BP grades), Spheripol PP technology (Basell grades), Univation PP technology (DOW grades) and Borstar PP technology (Borealis grades). Form this investigation, thirty different grades where evaluated to determine their structural morphology in relation to their physical and mechanical properties. For the ease of analysis, the grades are grouped into three ethylene content ranges; low ethylene contents (below 8%), medium ethylene contents (between 8 -10%) and high ethylene contents (above 10%). Within each of these ethylene content ranges, there is a melt flow index (MFI) range, which increases from low to high. The differences in the MFI added further complications, especially when trying to determine optimum mechanical properties of the copolymer grades. The impact test results show that the toughness is at a maximum at low MFI, and then drops drastically as the MFI increases. Since an addition of ethylene improves the toughness of the copolymer, it was observed that by considering MFI's below 35 g/10 min, the toughness could be enhanced by increasing ethylene content. Grades with MFI's below 35 g/10 min show a drastic increase in impact strength at ethylene content around 8.5%. SEM results show that particle size and spacing are independent of ethylene contents for the grades examined. However, for the BP and DOW grades, the ratio between interparticle distance and particle size decrease steadily with increasing ethylene content. The Basell and BP grades showed a narrow rubber particle size distribution followed by Sasol grades. The DOW grades showed a broad particle size distribution. TEM results show that the internal morphology of the rubber particles differ from grade to grade, with the Sasol and BP grades showing several numbers PE inclusions within the rubber particles, whilst Basell grades having only few PE inclusions. It was observed that those grades with several PE inclusions have a better impact performance compared with those with fewer inclusions.
- ItemOpen AccessA constitutional study of a dual phase steel containing 12% chromium(1983) Schaffer, G B; Ball, AnthonyThis thesis involved a study of the phase transfonnations in a chromium containing corrosion resistant dual phase steel, designated 3CR12. The objectives included the detennination of time-temperature-transformation (TTT) diagrams for the transformations between austenite and ferrite and an investigation into the factors controlling these reactions. The austenite decomposition reaction for a high nickel alloy, 3CR12Ni, and the effect of varying titanium concentrations on the equilibrium phases present in 3CR12, were also examined. Dilatometry was used to determine the transformation temperatures between austenite and ferrite and the Ms temperatures for the alloys investigated. The kinetics of the reactions were investigated by optical microscopy using two different etching techniques while the volume fractions of the various constituents were determined by a point counting method. Transmission electron microscopy was used to study the carbide morphologies and the nucleation and growth modes of the phases during the transformations. The distribution of the alloy elements were determined by microhardness measurements, an electronprobe microanalysis and a Kevex spectrometer attached to a scanning electron microscope. The 3CR12 alloy used in this study did not become fully austenitic above the Ae₃; it lies in the nose of the gamma loop of the Fe-Cr phase diagram. Two temperature regimes were identified on the decomposition of austenite. At 750°c the existing ferrite grains grew into the austenite matrix, while at 650° C and 700°C new ferrite was sympathetically nucleated i.e. it was heterogeneously nucleated on existing ferrite/austenite grain boundaries. Two types of carbide morphologies were formed. These were random precipitation within the ferrite grains and interphase precipitation. The TTI diagram showed conventional "C" curve kinetics. The austenitisation reaction occurred by a para-equilibrium mechanism. The rate controlling process was the structural change from ferrite to austenite; the reaction was not long range diffusion controlled. The speed of the reaction increased continuously with increasjng transformation temperature. No growth of ferrite occurred on isothermal transformation of 3CR12Ni at temperatures below the Ae₁. Increasing the bulk titanium content increased the Ms, Ae₁ and Ae₃ temperatures of 3CR12 due to the removal of carbon from, and the addition of titanium to, solution.
- ItemOpen AccessContinuum Damage Mechanics (CDM) modelling of dislocation creep in 9-12% Cr creep resistant steels(2016) Stracey, Muhammad Ghalib; Knutsen, Robert DThe generation of electricity to meet an ever-growing demand has become a defining characteristic of the modern world for both developed and developing nations alike. This, coupled with the intensifying concern with pollution and its effects on the environment has put immense pressure on how quickly and efficiently power is produced. Being the most prevalent source of electricity generation, coal fired power plants have been subject to increasing scrutiny and study in an effort to improve the efficiency at which they operate. Hence, coal fired power plants are being run at increased temperatures and pressures such as those observed in Super-critical and Ultra-super-critical plants. This has by extension put excessive demand on materials used in these plants specifically within the boiler and superheater pipe sections where the most extreme thermodynamic conditions are experienced. The most commonly used materials for these applications are in the family of ferritic/martensitic 9-12% Cr steels chosen for their superior material properties especially during long-term exposure as coal fired power plants typically operate for over 20 years before being decommissioned. One of the lesser understood aspects of 9-12%Cr steels is with regard to their long-term material properties specifically that of creep degradation and deformation. This has been partially due to the reliance of creep life predictions in the past being based on accelerated creep testing and empirically based modelling. With the relatively recent revelations of empirically based modelling shown to be inaccurate when extrapolated to the long-term, a need has been identified amongst researchers to develop more accurate models based on physical relationships and material microstructure. Moreover, the insight obtained from modern experimental techniques and technologies as well as ever-expanding computing capabilities provide an opportunity to produce microstructurally based models with a high degree of complexity. Thus motivated, the focus of this dissertation was to develop a physically based dislocation creep model using the Continuum Damage Mechanics (CDM) approach. A dislocation CDM model was developed and implemented in the current work for uniaxial creep loading using the numerical modelling software Matlabᵀᴹ. The CDM approach was built upon fundamental dislocation theory as well as other microstructural considerations pertaining to dislocation creep including subgrain coarsening, M₂₃C₆ precipitate coarsening and stress redistribution. The CDM model was found to require calibration in order to be applied to specific 9- 12% Cr steels which was implemented using a parameter optimisation routine. The results obtained were compared with experimentally obtained, long-term creep-time and microstructural data for the 11% Cr steel CB8 and the 9% Cr steel P92. The CDM creep-time predictions were found to vary in accuracy depending upon the experimental data against which the model was calibrated. Upon further investigation, it was hypothesised that the discrepancy observed was due to the formation of the Modified Z-phase in some of the long term creep data but not in others which was based primarily on the differing creep exposure times of the various samples. The CDM creep-time predictions for P92 were found to be accurate when compared with experimental results regardless of creep exposure times. The apparent difference in the approximation of the creep deformation for the two steels was concluded as being due to the formation of the Modified Z-phase in CB8 but not in P92 as Modified Zphase formation is intrinsically linked with the Cr content of the steel.
- ItemOpen AccessCorrosion fatigue of engineering alloys in aqueous environments(1990) Harty, Brian Dudley; Ball, Anthony; Noel, R E JA comparative study of the fatigue crack growth rate (FCGR) behaviour of five alloys in air and in aqueous environments has been performed. The alloys tested include: mild steel as a reference material, a corrosion resistant dual phase steel, 3CR12, a proprietary martensitic stainless steel, AISI 431, a newly developed 8% Cr martensitic steel, Alloy 825, and a newly developed corrosion-abrasion resistant metastable austenitic alloy, 1210. Tests were conducted in laboratory air, distilled water at rest potential, 500 ppm chloride solution at rest potential, 1000 ppm chloride solution at rest potential, and 1000 ppm chloride solution at -1200 m V see; solution temperatures were maintained at 25⁰ C. Crack growth rate tests were performed using sinusoidal loading at a load ratio R = 0.1, a frequency of 3Hz in the laboratory air, and a frequency of 1 Hz in the aqueous environments. At the completion of testing, fracture surfaces were studied using a scanning electron microscope. In air, the mild steel and 3CR12 display comparable rates of cracking and exhibit a greater resistance to fatigue crack propagation than the martensitic AISI 431 and Alloy 825; Alloy 825 shows the least resistance to fatigue crack propagation. The deformation induced transformation in 1210 gives this alloy the greatest resistance to fatigue crack propagation in air. Fatigue crack growth rates were all enhanced in the aqueous environments. The greatest overall rate of environmentally assisted cracking was shown by alloy 825 while the lowest was shown by the mild steel. Although the rate of cracking of 1210 in the aqueous environments was less than that of Alloy 825, 1210 was influenced the most by the aqueous environments. An environmentally assisted cracking index shows that the rate of fatigue crack propagation in 1210 is increased by 32 times in the 500 ppm chloride solution at low stress intensities. The fatigue crack growth rates of mild steel and AISI 431 were significantly influenced by the cathodically polarised conditions in the 1000 ppm chloride solution, compared to the rest potential conditions. In these cases hydrogen was seen to be evolved from the specimen surfaces. Changes in the fatigue crack growth rate behaviour were accompanied by changes in the fracture surface morphologies. The observed changes varied for each alloy and for each environment, and were manifest by the degree of intergranular cracking, cleavage, quasi cleavage, and increased coarseness of the transgranular cracking. The fracture surface morphologies are reported and discussed in detail. In general, the fracture surface morphologies could be directly related to the relative degrees of environmental influence on the rate of cracking; results are explained in terms of existing hypotheses. It is suggested that the environmentally assisted cracking of mild steel and AISI 431 at cathodic potentials in the 1000 ppm chloride solution could only be attributed to hydrogen assisted cracking. Similarly, it is suggested that the large crack growth rate acceleration of 1210 in the aqueous environments could also be attributed to hydrogen. The similar fracture surface morphologies observed on the other specimens after tests in the aqueous environments suggests-that hydrogen could be responsible for the environmentally assisted cracking of all the steels in aqueous environments.