Browsing by Subject "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 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 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 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 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.
- ItemOpen AccessCritical analysis of simulated thermomechanical processing of aluminium can body stock(2015) Hyde, Chase Kennedy; Knutsen, Robert D; George, Sarah LHot Plane Strain Compression (PSC) testing is a thermomechanical testing method used to simulate the deformation condition of industrial rolling. Thermomechanical processing (TMP) factors such as the amount of strain, strain rate and temperature all influence the microstructural evolution. The geometry of the PSC test sample and anvil are important factors in order to achieve the plane strain condition and acceptable strain distribution within deformed sample. Geometrical factors such as the breadth ratio (BR) relates the the samples breadth (b) to anvils face width (w) and this ratio has a significant effect on the breadth spread of the sample. The height ratio (HR) relates w to the samples height (h) and this ratio has a significant effect on the strain distribution. Two different geometric PSC testing configurations were investigated for this study, the one configuration had less favourable geometric ratios with a BR of 3 and a HR of 1 and the other configuration had more favourable ratios, with the BR of 4.62 and the HR of 1.3. This investigation is to evaluate the feasibility of a newly installed TMP machinery, the Gleeble 3800, to simulate the hot finishing rolling conditions by the use of hot PSC tests for the production of the can body stock (CBS) aluminium alloy AA3104. Single hot PSC tests were carried out at temperatures of 300, 350 and 400 ⁰C at strain rates of 10, 30 and 100 sec-1 and multi-pass hot PSC tests were carried out to simulate the different rolling passes experienced on the hot finishing rolling mill of the production of the aluminium alloy AA3104. The strain rate, temperature control, flow stress and microstructural flow were investigate to establish whether PSC testing is feasible on the Gleeble 3800.
- ItemOpen AccessThe development of abrasive-corrosive wear resistance of steels by microstructural control(1988) Barker, Keith Cecil; Ball, AnthonyThe performance of developmental alloyed steels with improved abrasive-corrosive wear resistant properties has been evaluated. The synergistic effect of abrasion and corrosion in the accelerated wear of steels is examined and the main parameters identified. A model of the process is proposed. The model is used to develop the optimum abrasive-corrosive wear resistance in steels for applications in the gold mines of South Africa. A wide range of engineering steels, both commercially available and experimental, has been evaluated in laboratory simulated abrasive and abrasive-corrosive wear tests. An appraisal of the wear tests and the applicability of the results to in-service conditions has led to the development of an additional abrasive-corrosive wear test. It has been established that both the microstructure and chemical composition determine the resistance of a material to wear. Control of the microstructure by alloying and heat treatment is attempted in order to optimise the abrasive-corrosive wear resistant properties for each class of microstructure whilst maintaining adequate formability and weldability. Abrasion of a metal surface has been shown to accelerate the rate of corrosion. Three categories of corrosion behavior are defined. A model of the abrasive-corrosive wear process is proposed to account for the behavior. The model adequately predicts the outcome to a change in system parameter, namely: an increase in the corrosivity of the water, an increase in the frequency of abrasive events, a change in the chemical composition and the degree of passivity inherent in the material. Recommendations are made to maximize the abrasive-corrosive wear resistant properties without resorting to expensive highly alloyed steels. To satisfy the needs of the mining industry, two microstructures of note are identified: a metastable austenitic (TRIP type) steel and a 0.25% carbon lath martensitic alloyed steel. A basic chemical composition is proposed with each microstructure. The austenitic steel is shown to achieve its abrasion resistance through the high degree of work hardening it undergoes during abrasion and the high ultimate strength of the strained material. The lath martensitic steel has the necessary strength to toughness ratio for good abrasion resistance. A 20% degree of work hardening in conjunction with a bulk hardness in excess of 500 HV is prescribed for superior abrasion resistant properties in the wear system of the mines. The life time cost of the martensitic alloyed steel recommends it for applications in the gold mines of South Africa.
- ItemOpen AccessDevelopment of high performance and efficient coating repair systems for offshore tropical marine environment(2018) Agostinho, Francisco José; Knutsen, Robert DRehabilitation 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.
- ItemOpen AccessDevelopment of the small punch test platform to evaluate the embrittlement of power plant materials(2017) Tshamano, Lavhelesani Oliet; Sonderegger, Bernhard; Knutsen, Robert D; Becker, ThorstenLife assessment of critical components and piping is performed in the electrical power plants in order to prevent structural/component failure and prolong safe operation of the equipment. In the event that these components fail, the consequences can be very costly since this may result in power supply disruptions, component replacements, environmental damages and the loss of human life. Regulations, standards and codes are designed to ensure the safe operation of the power plants. However, on their own, they are not adequate to account for aging power plants that have been in service for more than half of their originally designed lifespans, since failures have been experienced due to in-service aging mechanisms (i.e. temper embrittlement, creep, etc.) and poor engineering and maintenance practises. Mechanical, metallurgical and non-destructive techniques are used to evaluate the condition of the in-service materials in order to aid in these life assessments. The structural integrity assessments utilise material toughness properties as determined through fracture toughness testing, which requires a significant quantity of material, and is therefore cumbersome and expensive. Consequently, several other material property testing techniques are used to aid in structural integrity assessments, such as impact energy, tensile and hardness testing. Through empirical correlations, these test results are used to estimate fracture toughness properties and, consequently, the error bands are expected to be as high as 50%. Due to its small size, the small punch test (SPT) technique can be regarded as a quasi-non-destructive test, and is therefore a preferred method for determining the fracture toughness in aid of structural assessment. The SPT technique involves a compression load from the punch to a sample (ϕ8mm x 0.5mm thick) clamped between clamping and receiving dies. This study aims to develop a test rig that will be used to perform the SPT in order to quantify the level of embrittlement on the ex-service, low-pressure steam turbine material (NiCrMoV steel). The data results acquired from the SPT technique are the reaction load of the punch and the deformed displacement of the sample performed at a constant displacement rate according to CWA 15627:2007. Two SPT rigs were designed, manufactured and commissioned. These two were commissioned using FEM and tensile test results for validations. The steel was subjected to three different conditions: as received (AR), de-embrittled (DE) and hardened (HD). The three types of steel illustrated that the SPT can quantify embrittlement levels through the correlation of tensile, Charpy impact energy and fracture toughness testing.
- ItemOpen AccessDuctility in high chromium super-ferritic alloys(1989) Wolff, Ira M; Ball, AnthonyThe competition between microfracture and plastic flow has been studied in relation to the thermomechanical processing parameters and minor element chemistry of wrought super-ferritic alloys based on a composition of Fe-40wt% Cr. These alloys have been developed for corrosion-resistant applications, specifically by micro-alloying with platinum group metals to induce cathodic modification, but their use has been hampered by inadequate toughness at ambient temperatures. Brittle cleavage of the alloys is a consequence of the high resistance to plastic flow required to accommodate local stresses, such as those found ahead of a loaded crack. Once initiated, a crack propagates in a brittle manner with minimal ductility. The impact toughness therefore relies on the ability of the alloys to withstand crack initiation. The frequency of the crack initiation events is related to the distribution of secondary phases within the matrix and at the grain boundaries. A direct means of improving the toughness and the ductility is accordingly via annealing cycles and minor alloying additions to control the precipitation of second phases. The ductility is enhanced by raising the mobile dislocation density, and this may be achieved by pre-straining recrystallised material, or increasing the number of dislocation sources in the otherwise source-poor material. The generation of mobile dislocations by prismatic punching at second phase particles in response to local or tessellated stresses was found to increase the ductility and the impact toughness of the alloy. The addition of nickel also increases the brittle fracture resistance by promoting stress accommodation at the crack tip, a result which can, in principle, be explained on the basis of enhanced dislocation dynamics. The tendency of the alloys to form a stable recovered substructure was identified as a critical parameter for both the mechanical and corrosion properties. The low-angle dislocation sub-arrays contribute to overall strain-hardening, but destabilise the passivity of the alloys in acid media. In practice, rationalisation of the microstructural parameters has enabled the practicable fabrication of tough, corrosion-resistant alloys, suitable for commercial development.
- ItemOpen AccessThe effect of alloy chemistry and strain rate on the Md30 temperature of metastable austenitic stainless steels(1994) Papo, Jones Malesela; Knutsen, Robert DThe work covered in this thesis provides a comprehensive discussion of the transformation behaviour of Type 304 metastable stainless steels with small' variations in alloy composition. The study focuses mainly on the austenite stability with respect to alloy composition, rate of deformation and temperature. To achieve these objectives, uniaxial tensile tests at 0.3 true strain were performed at low and high strain rates (10-3s-1 and 3 x 10-2s-1 respectively), in the temperature range of -60 to 55Â°C under isothermal testing conditions.
- ItemOpen AccessThe effect of drawing strain on the fatigue behaviour of stainless and carbon steel wires(2001) Topic, Miroslav; Ball, Anthony; Allen, Colin; Tait, Robert BA study has been made of the fatigue crack initiation and fatigue crack growth behaviour of three different steels in wire form, namely, an austenitic AISI 304 stainless steel, a corrosion resistant ferritic steel, 3CR12, and pearlitic high carbon steel. The stainless steel wires were produced in the laboratory at a drawing speed of 50 mm min-1, without intermediate annealing, whilst the high carbon pearlitic steel was manufactured commercially. Studies were made on stainless steel wires as a function of drawing strain between 0.09 and 0.585. Fatigue testing was carried out on an ESH servo hydraulic testing machine on both notched and unnotched samples and the S-N curves were used to evaluate the fatigue properties of the steels. Tests were performed with sinusoidal loading and load ratios of R= 0.048 and R=0.22 at a frequency of 2Hz. The microstructural evolution during drawing was characterised by optical and transmission optical microscopy, and x-ray diffraction. Fatigue crack growth and fracture surfaces were studied using scanning electron microscopy. In general, the fatigue limit was enhanced by increased drawing strain, but such strain also increased the subsequent crack propagation rates. The highest value of fatigue limit of 630 MPa was exhibited by the commercial pearlitic steel despite of its high notch sensitivity. Both shot peening of the steel wire surface and reducing the surface roughness by manual polishing increased the fatigue limit between 40 and 25 % respectively. The fatigue limit of AISI 304 stainless steel wire was improved from 215 MPa to 650 MPa after drawing to 0.585 strain. This improvement is attributed to the deformation-induced phase transformation of (ϒ) austenite to α'-martensite. X-ray diffractometer traces show that the amount of strain-induced martensite varied from 8% in the wires drawn at low strain (0.09) to 36% in the wire samples drawn to 0.585 strain. This study has established that approximately 20% of deformation-induced martensite, through drawing strain, is a critical amount which determines the subsequent fatigue response of this steel. If the amount of previously developed martensite is less than the critical amount of 20%, the martensite formed during the fatigue process will act beneﬁcially by retarding fatigue cracking, raising the fatigue limit and resulting in a ductile fatigue fracture surface. However, in the presence of more than 20% of martensite, any martensite induced by cyclic strain will encourage more rapid crack initiation compared to a material containing less than 20% martensite which leads to more brittle fracture surface characteristics. The fatigue limit of 3CR12 steel wire was also improved from 130 MPa to 310 MPa (maximum stress) after drawing to 0.68 strain. The experimental results indicate that the use of drawn 3CR12 ferritic steel for wire application under cyclic conditions is restricted to low stress levels. However, the application of heat treatment and the resultant development of a dual-phase microstructure, improved the fatigue limit to 470 MPa. Based on the findings in this study, recommendations regarding material selection and drawing process optimisation for wire production to improve the fatigue performance of AISI 304 stainless steel is given.
- ItemOpen AccessThe effect of functional electrical stimulation on abdominal muscle strength and gross motor function in children with cerebral palsy a randomised control trial(2014) Joffe, Jessica Robyn; Jelsma, JenniferIncludes abstract. Includes bibliographical references.