Browsing by Author "Ball, Anthony"

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    Abrasion-corrosion and stress corrosion resistance of a 9Cr-2Ni-0.7Mo steel in simulated mine water
    (1991) Gatzanis, Gustav Ernest; Ball, Anthony
    The locally (RSA) developed 9Cr-2Ni-0.7Mo steel designated 927 formed the subject of this study. Its abrasion-corrosion and stress corrosion performances were assessed in laboratory tests simulating the underground environment in South African gold mines. The results indicate that the alloy performs favourably in abrasive-corrosive applications, outperforming several other higher chromium containing steels which have been designed for the purpose. The alloy is also highly resistant to sec at free corrosion potential in simulated mine water. The good abrasion-corrosion resistance is attributed to the adequate corrosion resistance of the alloy acting in conjunction with the favourable combination of strength and toughness afforded the alloy by its fine grain size and microduplex microstructure of martensite and interlath retained austenite. The production variables of plate thickness and prior cold working were found to exert negligible influence on corrosion-abrasion resistance. This is ascribed to the small influence of these processes on the hardness and associated mechanical properties due the inherent low work hardening ability of the alloy. Slow strain rate (SSR) stress corrosion cracking tests were performed on the alloy in four microstructural conditions viz. as-rolled, tempered, welded and post weld heat treated. The material showed an immunity to sec in all the microstructural conditions for tests conducted at open circuit potential. This apparent immunity is attributed to the difficulty in initiating sec by pitting on the plain specimens over the relatively short test durations. Polarisation to extreme cathodic potentials (-1200m V) resulted in hydrogen embrittlement of this high strength alloy with failure predominantly along prior austenite grain boundaries. Anodic potentials in the excess of OmV induced tunnel-like corrosion pitting attack. Fractographical evidence of sec at the base of these pits indicates the development of the conditions necessary for sec within the pit confines. This is cited as evidence in support of the hypothesis of sec initiation difficulty.
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    The abrasive wear behaviour of mineral-filled polypropylene
    (1997) Sole, Brian Michael; Ball, Anthony
    Polypropylene 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.
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    An abrasive-corrosive wear evaluation of some aluminium alloys
    (1989) Meyer-Rödenbeck, G D; Ball, Anthony
    This 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.
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    An assessment of polymeric materials and surface treated steels as cavitation erosion resistant materials
    (1983) Barletta, A; Ball, Anthony
    The 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.
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    The brittle and plastic response of quartz.
    (1980) Glover, Graham John; Ball, Anthony
    Quartz 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.
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    Cavitation erosion of materials
    (1980) Heathcock, Christopher John; Ball, Anthony
    This investigation was undertaken to find the optimum hydraulic fluids and materials for combating cavitation erosion in hydraulic mining machinery operating,on 5:95 oil-in-water emulsions, Controlled cavitation was generated in the laboratory in a vibratory cavitation apparatus employing the stationery specimen system. The influence of separation distance between the drill tip and specimen and of varying medium temperature on cavitation erosion were also investigated but were of secondary importance to the studies on materials and emulsions, The evaluation of the influence of a number of commercial metal cutting coolants and specific hydraulic emulsions oncavitation erosion,showed that those emulsions with a high mineral oil content suppress erosion the most, This phenomenon is considered to be either the result of a reduction in nucleation of highly damaging surface cavities due to the elimination of surface nuclei by an oil film, or to damping of cavity collapse due to the high dissolved gas content of oil-in-water emulsions, A large number of commercial grade alloys and polymers have been investigated, The alloys ranged from single phase, face centered cubic nickel alloys to complex, multi-phase, cobalt based alloys while the polymeric materials were all linear and crystalline. The modes of erosion and mechanisms of erosion resistance were studied in detail by optical microscopy, scanning and transmission electron microscopy and X-ray diffractometry. The fractographic studies on the alloys revealed, in particular, the role played by second phase particles and other surface features such as protruding grain boundaries and surface steps created by dislocation movement, twinning and phase transformations, Furthermore, X-ray diffractometry coupled with theoretical considerations and the findings of previous investigations indicated the fundamental importance of phase transitions in contributing towards the erosion resistance of alloys, Cobalt based Stellite alloys, nickel based cemented carbides and the intermetallic alloy NiTi have exceptionally high erosion resistance, The Stellites have a high yield stress and also undergo an awe phase transition which results in a high work hardening rate and high strain to fracture, NiTi similarly undergoes a phase transformation which allows absorption of seven per cent reversible strain and also results in a high work hardening rate. The erosion resistance of nickel based tungsten carbides is, in turn, attributable to the very high yield stress of these materials, Ultra high molecular weight polyethylene and nylons have relatively high erosion resistances, Their exceptional performances are due to a high strain to fracture and high yield strength respectively. On the other hand low tensile strength polytetrafluoroethylene has a very poor resistance to erosion. This work has identified various material properties which are required for erosion resistance, These are : (a) a high yield stress, (b) a high work hardening rate and (c) a high strain to fracture.
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    Cavitation erosion of WC-Co
    (1987) Hankey, S E; Ball, Anthony
    An 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.
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    Characterisation and refinement of properties of glass fibre reinforced polyester polymer concrete for use in manhole components
    (1999) Griffiths, Robert; Ball, Anthony
    The 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.
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    A constitutional study of a dual phase steel containing 12% chromium
    (1983) Schaffer, G B; Ball, Anthony
    This 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.
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    Corrosion fatigue of engineering alloys in aqueous environments
    (1990) Harty, Brian Dudley; Ball, Anthony; Noel, R E J
    A 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.
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    Degradation of the beta-alumina electrolyte in a zebra cell
    (1988) Van Heerden, D P; Hutchings, Ron; Ball, Anthony
    Beta-alumina solid electrolytes studied were subjected to charge and discharge cycles in a secondary, high energy density Na/beta-alumina/NaAlCl₄/FeCl₂ cell (known as the Zebra cell) at 250⁰ C. These electrolytes were studied by means of optical microscopy, as well as SEM and EDS analyses to est ablish possible failure modes. After cycling little discolouration, or impurity pickup was found to have occurred in the electrolyte. The forms of degradation of the beta-alumina electrolyte identified appeared to be a result of inherent flaws in the beta-alumina electrolyte tube, problems due to protracted storage of the tubes, or an apparent interfacial film on the cathode/electrolyte interface. A lead wetting agent was used in the cells to enhance the wetting of the beta-alumina electrolyte by the liquid Na. A study of the coating after cycling of the cell showed that the coating was adherent irrespective of the number of cycles completed. The coating did not appear to influence cracking of the electrolyte during cycling. Cracking of the beta-alumina electrolyte was found to have initiated from the Na/electrolyte interface. No evidence of crack initiation nor internal damage was found on the cathode/electrolyte interface. The cracks through the beta-alumina electrolyte wall were found to have sealed by the formation of a plug consisting largely of Na and Cl. On the basis of EDS analyses of the fracture surface of the sealed crack possible sealing mechanisms are proposed.
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    Degradation of the beta-alumina electrolyte in a zebra cell
    (1988) Van Heerden, D P; Hutchings, Ron; Ball, Anthony
    Beta-alumina solid electrolytes studied were subjected to charge and discharge cycles in a secondary, high energy density Na/beta-al~mina/NaA1Cl 4/FeC1 2 cell (known as the Zebra cell) at 250 c. These electrolytes were studied by means of optical microscopy, as well as SEM and EDS analyses to establish possible failure modes. After cycling little discolouration, or impurity pickup was found to have occurred in the electrolyte. The forms of degradation of the beta-alumina electrolyte identified appeared to be a result of inherent flaws in the betaalumina electrolyte tube, problems due to protracted storage of the tubes, or an apparent interfacial film on the cathode/electrolyte interface. A lead wetting agent was used in the cells to enhance the wetting of the beta-alumina electrolyte by the liquid Na. A study of the this coating after cycling of the cell showed that the coating was adherent irrespective of the number of cycles completed. The coating did not appear to influence cracking of the electrolyte during cycling. Cracking of the beta-alumina electrolyte was found to have initiated from the Na/electrolyte interface. No evidence of crack initiation nor internal damage was found on the cathode/electrolyte interfac~. The cracks through the beta-alumina electrolyte wall were found to have sealed by the formation of a plug consisting largely of Na and Cl. on the basis of EDS analyses of the fracture surface of the sealed crack possible sealing mechanisms are proposed.
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    The development of abrasive-corrosive wear resistance of steels by microstructural control
    (1988) Barker, Keith Cecil; Ball, Anthony
    The 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.
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    Ductility in high chromium super-ferritic alloys
    (1989) Wolff, Ira M; Ball, Anthony
    The 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.
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    The effect of drawing strain on the fatigue behaviour of stainless and carbon steel wires
    (2001) Topic, Miroslav; Ball, Anthony; Allen, Colin; Tait, Robert B
    A 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.
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    The effects of hardness, toughness, microstructure and thermomechanical heating on the erosion of ceramic and ultrahard materials
    (1991) Vaughan, R A; Ball, Anthony
    Nine different ceramic and ultrahard materials have been eroded by four different erodents under standardised experimental conditions. The target materials range from the soft stabilised zirconias to the very hard polycrystalline diamond composites. The four erodent particles used were: soft, friable silica, sharp alumina, tough silicon carbide and very hard, synthetic industrial diamonds. The steady state erosion rates of the different target/erodent combinations were measured. The erosion for each combination was studied by examining the target surfaces at progressive stages of erosion and the erodent particles after impact. Special attention has been paid to the morphology of impact sites, the amount of material lost and the mechanisms of erosion. The ceramic materials eroded by an elastic/plastic process: irreversible deformation is followed by lateral fracture. The ultrahard materials erode by a complex process involving deformation and extrusion of the softer phases and fracture and loss of the harder crystallites. The relative hardness of the target and erodent is a determining factor in erosion. When the hardnesses are similar, the ease of initiation and propagation of lateral fracture determines the rate of material removal. When the erodent particles are much harder than the target material, initiation of fracture is inevitable and the ease of crack propagation determines the rate of material removal. This is controlled by the sharpness of the particles and the microtoughness of the target material. The microtoughness is a function of grain size, porosity and defect density. Melting features, seen in many of the eroded surfaces, are thought to arise from a combination of plastic deformation and frictional heating under high contact stresses. The amount of heat dissipated is determined by the erodent and target hardness and friction coefficients. The rate at which the heat diffuses away from the impact zone is determined by the thermal diffusivities, heat capacities and densities of the target and the erodent.
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    The effects of properties, microstructure and phase transformation on the erosion of hard materials
    (1989) Doyle, Richard Alan; Ball, Anthony
    A variety of ceramic and ultrahard materials have been subjected to both solid particle and cavitation erosion. The materials tested include three grain sizes of alumina, stabilised zirconias, sialon, cubic boron nitride and polycrystalline diamond, and these have a range of microstructural, physical and mechanical properties. The damage modes are described for the two types of erosion and the results are critically discussed. It has been shown that different properties and microstructural features control the respective types of erosion. Hardness is the critical property which controls material loss during solid particle erosion. Cavitation erosion in contrast is less sensitive to hardness, but is extremely defect sensitive and preferentially attacks weak or damaged regions on the target. Grain size and shape, and th.e properties of the grain boundary or intergranular phase exert a strong influence on both types of erosion. It has in addition been concluded, that a propensity for a stress induced phase transformation, such as that exhibited by stabilised zirconia, will benefit the resistance of a ceramic to erosion. Ultrahard materials generally outperform the structural ceramics that were tested. While it was not possible to ascertain the effects of grain size conclusively, a large extent of intergrowth between the crystallites during manufacture appears to be beneficial to erosion resistance.
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    The erosion of titanium aluminide intermetallic alloys
    (1995) Howard, Robert Llewellyn; Ball, Anthony; Lang, Candy
    The erosion behaviour of titanium aluminide intermetallic alloys has not been widely reported in the scientific literature and is part of the current international research effort aimed at exploiting these materials for turbine engine and automotive applications. In the present study titanium aluminides have been subjected to both solid particle erosion and cavitation erosion. The erosion rates have been measured and the damage mechanisms have been identified and discussed in terms of the microstructures and mechanical properties of the titanium aluminide alloys. This has been achieved with a variety of investigative techniques, including electron microscopy, mechanical testing and microstructural examination; and, where necessary, the erosion performance of other materials have been evaluated for comparison. In particle erosion, conducted with air blast rigs at room temperature and at elevated temperature, the titanium aluminide alloys exhibit a ductile mode of material removal, and their limited strain to fracture results in higher particle erosion rates than those for 304 stainless steel. Heat treatment to produce changes in microstructure and hardness does not significantly affect particle erosion performance, and elevated temperature tests reveal an increase in particle erosion rate with increasing temperature. In cavitation erosion, the titanium aluminide alloys exhibit a ductile mode of damage accumulation and material loss, and the rates of material loss are lower than those for other engineering materials such as 304 stainless steel and some hardmetal grades which are currently used in erosive environments. The mechanism of cavitation erosion of the Ti₃Al-based alloy involves the accumulation of strain in phase boundary regions and the preferential removal of the more brittle component of the microstructure. For the Ti₃Al-based alloy, cavitation erosion resistance increases with an increase in hardness produced by heat treatment. The TiAl-based alloys exhibit twinning during the initial stages of cavitation, which is characteristic of the high strain rate deformation of TiAl-based alloys, followed by substantial work hardening and preferential material loss from phase interfaces and twinned regions.
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    The fracture and autogenous comminution of quartzite
    (1978) Kanellopoulos, Achilles Constantine; Ball, Anthony
    Comminution is the process which aims at increasing the surface area and the resultant liberation of a particular constituent from the mass of a solid. The autogenous mill uses tumbling to effect comminution, but instead of special milling bodies being added, pebbles of the material to be comminuted are used. The autogenous comminution process utilises less than 0,1 per cent of the energy input. The principal objective of the present work was to analyse autogenous milling behaviour in terms of the individual comminuting mechanisms and to establish the inter-relationships between the main process variables, namely rock petrography, size distribution of the feed, applied load, relative velocity and environment. In this manner the optimisation of the process and an improvement of its efficiency was sought. In addition the establishment of testing procedures to predict the autogenous milling behaviour of a given type of rock was aimed. In the present work the gold bearing Witwatersrand quartzite was used, although the findings are applicable to other types of rocks. Since fracture phenomena are involved in all comminuting mechanisms of impact-compression, chipping and abrasion, slow compression and Brazilian tests were performed. The grain size and the mineral composition of the rock has been found to have a large influence on the local stresses required for these processes. Indeed the results show that the fragility and therefore ease of comminution increases with increasing grain size of the quartzite. Brazilian tests on drill cores of varying diameters may allow the prediction of the critical size of rock of the mill feed which can survive in a mill of given characteristics.
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    The influence of steel microstructure on abrasive wear in soils
    (1987) Scheffler, O; Allen, Colin; Ball, Anthony
    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.