OpenUCT :: Browsing by Author "Allen, Colin"

Browsing by Author "Allen, Colin"

Now showing 1 - 20 of 23

Open Access
The abrasive wear resistance of austempered spheroidal graphite irons
(1987) Shepperson, S V; Allen, Colin
A 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.
Open Access
Abrasive wear testing of steels in soil
(1987) Quirke, SJ; Allen, Colin
A survey has been made of the quality and type of materials used for tillage tools in South Africa. Conclusions have been drawn regarding the inadequacy of the manufacturing processes used and the resultant quality of the tool material. A rig has been designed for the abrasion testing of materials in soil. The reproducibility of the method has been shown to be high and an evaluation has been made of the relative wear resistance of a series of ·heat treated steels. A medium carbon boron steel has been shown to have great promise as a tillage tool material because of its high wear resistance and toughness. The deformed surface layers and the mechanisms of wear of steels subjected to field and laboratory abrasive testing has been examined. The removal of material through predominantly ploughing or cutting mechanisms has been shown to be dependent on the heat treatment and composition of the steels together with the nature of the abrasive. White surface layers have been observed to form on medium and high carbon steels subjected to soil abrasion. Suggestions have been advanced for their formation. Attempts have been made to assess the transferability of data between field and laboratory testing.
Open Access
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.
Open Access
The effect of operating parameters on the wear behaviour of disc poppet valves in reciprocating slurry pumps
(1988) Joffe,S.H.D; Allen, Colin
An investigation has been carried out to determine the factors controlling the wear characteristics of disc poppet valves in reciprocating slurry pumps used in the transportation of quartzite slurries. A laboratory test rig has been designed and built which closely simulates the operating conditions experienced by slurry pump valves. Experiments have been conducted to determine the effect of the design parameters, namely - slurry constitution, valve closure velocity and valve angle, on the wear resistance of a low alloy steel, as a function of its mechanical properties. It has been shown that the wear of the valves is a sensitive function of the operating parameters. Significant improvements in the life of the valves can be achieved through increasing material hardness, reducing valve closure velocity, slurry density and valve angle. A detailed study has been made of the mechanisms contributing to valve wear. It has been established that both percussive impact and three body abrasion wear mechanisms predominate. The influence of each mechanism has been shown to be a function of the slurry constitution and the material hardness.
Open Access
The effect of pressure and temperature on the microstructure and mechanical properties of polycrystalline graphites
(1995) Van der Riet, Clement David; Allen, Colin
A study has been made of the effects of combinations of pressure and temperature ori six polycrystalline, synthetic graphites, in the high pressure domain (> 1 GPa).The graphites were investigated in three different conditions: (1) the "as received" condition (AR condition),(2) after exposure to pressures of about 3 GPa at room temperature (in a piston-cylinder device – PC condition) and (3) after exposure to temperatures of about 1500°C at pressures of about 5.5 GP a (high temperature- high pressure, or HTHP, condition). Their microstructures have been compared on the basis of X-ray diffraction measurements to determine their crystallite sizes (L˳ and L˳), interplanar spacings (c and a) and textures. Optical and scanning electron microscopy were used to examine their fracture surfaces and macro porosity. Mercury porosimetry provided a means of establishing the pore size distribution of pores of less than 20 1-1m diameter. Bulk and skeletal densities were determined from mercury porosimetry and helium pycnometry respectively. The effects of PC and HTHP conditioning on their mechanical properties, were measured by both uniaxial compression fracture tests, and by electrical resistivity measurements. In addition, the triaxial behavioursof the six graphites in the AR condition were evaluated from piston-cylinder compression tests. All the isopressed graphites were found to have very similar crystallite sizes, interplanar spacings and textures in the AR condition. The extruded graphite had larger crystallite dimensions, and was slightly less isotropic, than the other grades. Fracture occurred due to cleavage of the basal planes of crystallites in the filler particles or binder. The size, shape and orientation of filler particles and porosity with respect to the applied stress field determined whether fracture was intergranular, or trans granular, in nature. Limited basal plane slip and sub-critical microcracking caused uniaxial compressive stress-strain curves typical of those of polycrystalline graphites,i.e. convex with respect to the strain axis. Fracture strengths and strains were related to the proportion of amorphous, intercrystallite bonding and, to a lesser extent, to porosity.
Open Access
The erosion of ultrafine WC-Co
(1999) Pugsley, Victoria Antonietta; Allen, Colin
A series of four ultrafine WC-Co alloys of varying cobalt contents has been sintered from powder produced through the spray conversion process. The materials have been characterised and subjected to slurry erosion and cavitation erosion. A further twelve WC-Co alloys of varying grain sizes and cobalt contents have been produced through conventional processes and subjected to the same tests in order to provide data for comparison. The erosion resistance of all the grades tested was found to increase with decreasing grain size, both under cavitation erosion and slurry erosion conditions. Furthermore, a transition from a localised material removal mechanism to a bulk material removal mechanism was observed in both erosive systems as the WC grain size of the material decreased below about lμm. The erosion resistance of sub-micron materials was found to be considerably more sensitive to bulk deformation parameters than that of coarser materials. As a result, ultrafine grades subjected to slurry erosion or cavitation erosion were found to exhibit lifetimes up to seven times greater than those of the best-performing materials with grain sizes above 1 μm. This increase in erosion resistance was achieved without any significant drop in fracture toupness. Significant differences were observed between the response of WC-Co to the two erosive systems, particularly in those materials exhibiting a localised erosion response. Under slurry erosion conditions, these materials displayed increasing erosion resistance with decreasing cobalt content, and both the cobalt and WC phases responded to erosive attack. Under cavitation erosion conditions, however, the opposite trend was observed, and only the cobalt phase responded to erosive attack. Materials exhibiting a bulk erosion response responded in a similar fashion to both forms of erosive attack, although cavitation erosion produced damage to a greater depth. The effect of defects on material performance was found to depend critically on the erosive system.
Open Access
The erosion of WC-Co coatings
(2002) Ndlovu, Siphilisiwe Nompumelelo; Allen, Colin
A study has been conducted on both the particle and slurry erosive wear behaviour of WC-Co hard metal coatings. The coating compositions were WC-12%Co and WC-10%Co-4%Cr and were produced using both the TAFA JP5000 and the METCO Diamond Jet (DJ) thermal spray systems.
Open Access
Erosion-corrosion resistance of tungsten carbide hard metals with different binder compositions
(1995) Wentzel, Eduard John; Allen, Colin
A study has been made of the slurry erosion resistance of a series of cemented tungsten carbides with different binder compositions consisting of combinations of cobalt, nickel and chromium. Testing was carried out on a specially designed laboratory rig in both tap and salt water using silica sand as an erodent. The synergistic action of erosion and corrosion on WC hard metals results in greatly enhanced wear rates compared to either erosion or corrosion processes alone. Cemented carbides with a 1 0 wt% binder were found to have a better slurry erosion resistance than the corresponding 6 wt% binder grades or the pure metal binder alloys alone. The performance of all the materials was found to be much worse in a salt water medium compared to tap water. The alloying of either pure Nickel or Cobalt binders was found to influence the fracture properties and corrosion resistance and lead to an improvement in the slurry erosion resistance of the cemented carbides. However any improvement in the corrosion resistance of the binder did not directly enhance the slurry erosion resistance of the cermet. Explanations are advanced to explain these differences in behaviour linked to chemical composition, mechanical properties of the binder phase and the dynamic nature of the slurry erosion system. Comparisons are also made between the performance of the Ni-Cr-Co based cermets and the pure alloyed metal binder grades. The acceptability of modelling the slurry erosion and corrosion resistance of cermets based on the behaviour of the binder phase materials is discussed.
Open Access
The fracture toughness of ultrafine WC-Co Alloys
(1999) Sacks, Natasha; Allen, Colin
This thesis examines the fracture toughness behaviour of a series of cemented carbides having carbide grain sizes between 0.35f.Lm and 4f.Lm, cobalt contents ranging from 6 to 15wt%, and vanadium carbide contents from 0 to 0.8wt%. A series of twenty-four ultrafine WC-Co alloys were sintered from powders produced through three different production routes, namely, the spray conversion process and two variations of the conventional powder production methods. A further twelve WC-Co alloys with three different carbide grain size distributions were produced through conventional powder metallurgy processes. Two different fracture toughness test methods have been used to determine the toughness measurements, namely, the Palmqvist Indentation test and the Short Rod test. A Terra Tek Fractometer machine was used to automatically determine the Short Rod toughness values. The Palmqvist testing was carried out on a Vickers hardness machine using indenting loads of 20, 30 and 50kg. The Palmqvist crack lengths were measured by summing the individual crack lengths emanating from the corners of the hardness indentation. These crack lengths were used in a formula developed by Shetty et al., to calculate the Palmqvist fracture toughness. The influence of annealing at 800°C and 900°C on the Palmqvist crack lengths has also been assessed. Microstructural parameters have been determined using ASTM procedures and microscopy techniques and the influence of these parameters on the fracture toughness has also been assessed.
Open Access
High temperature erosive wear of a boiler tube steel
(1997) Suckling, Martin Brian; Allen, Colin
This work is an attempt to evaluate the influence of critical operational parameters on the high temperature erosion of a 1Cr½Mo boiler tube steel. Erosion testing has been carried out in a specially designed and developed unique laboratory apparatus capable of simulating the conditions of temperature, particle velocity and flux as found in the economiser region of pulverised fuel boiler combustors in electricity generation power plants. The work has encompassed the effects of particle type, size, velocity and flux on the erosive wear rates of the 1Cr½Mo boiler tube steel at temperatures of up to 600°C. The response of the target to impacting erodent particles has been analysed using scanning electron and optical microscopy, transmission electron microscopy (TEM) as well as conducting cross-sectional microhardness tests. The change in the mechanical properties of the steel was determined by conducting tensile tests over a range of temperatures from 20°C to 565°C.
Open Access
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.
Open Access
The influence of water composition on the pitting behaviour of newly developed corrosion resistant steels
(1988) Cotterrell, M H; Allen, Colin; Noel, Robert E J
The mechanisation of the working stapes in South African gold mines has required the introduction of a fundamentally new technology, hydro-power, in which machines are powered hydraulically using mine water fed from above ground. Mine water is aggressive and has a variable acidity and pH, and contains high concentrations of sulphate, chloride and nitrate ions. In order to minimise the pitting corrosion of piping and stoping machinery a compromise between selecting a suitable corrosion resistant material and treating the mine water to an acceptable level of corrosiveness is being sought.
Open Access
Micromechanisms of polymer sliding wear
(1993) Marcus, Kashif; Allen, Colin
A study has been made concerning the tribological behaviour of ultrahigh molecular weight polyethylene (UHMWPE) during water-lubricated reciprocating sliding wear. The experimental work has been extended to study also the effect of molecular weight, fillers, lubrication, counterface roughness and sliding configuration on the polymer's transfer characteristics. The wear behaviour of polytetrafluoroethylene (PTFE) has been included for comparative studies. The worn material was studied using stylus profilometry, optical microscopy, scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), Transmission electron microscopy (TEM), X-ray fluorescence (XRF), X-ray diffraction (XRD), differential scanning calorimetry (DSC), infrared spectroscopy (IR) and mass spectrometry. The effect of two fillers, namely glass beads and a titanium-based inorganic filler on the friction and wear behaviour of UHMWPE has been investigated as a function of counterface roughness (Rₐ). It was found that the filled material exhibited lower wear rates than the unfilled material on the rougher counterface. The filled material was also found to be more sensitive to a change in Rₐ and showed higher wear rates than the unfilled polymer on the smooth counterface. A uniform and coherent transfer film is found on the rougher counterface but the transfer film for the titanium-based filler was patchy on the smooth counterface. No coherent transfer film was found when sliding was conducted parallel to the grinding direction on the steel counterface, resulting in relatively high wear rates. Polymer transfer was patchy, the amount increasing as the sliding distance increased. The observed phenomena are explained in terms of mechanical interlocking and chemical bonding of the polymeric material with the metal counterface. An increase in molecular weight did not significantly improve the wear resistance of the UHMWPE. Small variations in counterface roughness values (Rₐ) were found to have a much greater effect on the wear rates than changes in molecular weight. The steady-state wear rate of the polymer was furthermore found to be more dependent on an adherent transfer film rather than a change in bulk morphology. Although PTFE exhibited low friction coefficients, the high wear rates obtained by this polymer is explained by the polymer's inability to form a transfer film under water lubrication, while any film that forms under dry sliding wear is easily peeled off the surface. Significant improvements in wear are found when fillers are added to the polymer. The wear rates for PTFE under dry sliding are similar to those obtained for UHMWPE under water lubrication. Transfet of UHMWPE material to the metal counterface during sliding wear involves interlamellar shear of the polymer and results in the development of a highly oriented transfer film. Significant differences have been found in the degree of crystallinity, crystallite size and orientation in the deformed surface layers of the polymer and debris compared with those of the bulk polymer. The worn surface of the polymer shows slightly increased crystallinity but the crystallinity of the debris is much higher than that of the bulk whilst the crystallite size is much reduced.
Open Access
The particle erosion of steel by magnetite
(2002) Fewell, Sean E; Allen, Colin
This work addresses the problem of erosion of steel heat exchanger coils in a petroleum producing plant by magnetite catalyst particles entrained in the flowing gas stream.
Open Access
Processing and properties of silicon nitride ceramics
(1993) Nel, Jacqueline Margot; Shaw, Martin; Paterson, Adrian; Allen, Colin
Silicon nitride, Si₃N₄, ceramics were produced using either silicon or silicon nitride powder. The silicon was reaction bonded in nitrogen atmosphere to form reaction bonded Si₃N₄,which was then sintered between 1700°C and 1800°C to form a dense Si₃N₄ ceramic. The silicon nitride powder compacts were also sintered between 1700°C and 1800°C. In order to achieve densification Y₂O₃-A1₂O₃ additive combination was used in both processing routes. The physical and mechanical properties of the Si₃N₄ materials was found to be dependent on the processing conditions. The post sintered reaction bonded Si₃N₄ materials had the highest densities and hardness values, while the sintered Si3N4 materials had the highest strength and toughness values. The microstructure was also influenced to a great extent by the processing conditions, and this in tum influenced the mechanical properties of the ceramics.
Open Access
The repetitive impact wear of steels for hydro-powered mining machinery
(1991) Fricke, Roland; Allen, Colin; Ball, Anthony
The repetitive impacting of solid components in industry can result in wear damage which may significantly limit service life. Impact wear problems have been encountered in hydro-powered stoping equipment (eg rockdrills and impact rockbreakers) developed for deep level gold mining in South Africa. This research project was a study of the repetitive impact wear of reciprocating valve components (eg poppet valves within the impact rockbreaker) under simulated conditions. A laboratory apparatus, capable of producing impacts varying in energy from 2 - 5 J and varying in frequency from 5 - 50 Hz in an aqueous environment (distilled water), was designed and built for this purpose. Impact tests were conducted in order to: a) rank materials according to impact wear resistance, b) to determine modes and mechanisms of wear, c) to determine material, microstructural, design and operating parameters of importance in minimising wear, d) to make recommendations concerning the above, to facilitate productivity and longlife of poppet valves within impact rockbreakers. The materials selected for testing (817M40, 1210 and AISI 304, AISI 431 and AISI 440C) are steels currently used by the gold mining industry in different applications and known to perform satisfactorily in service. These materials are not all ideally suited to application in valves. They were chosen in order to illustrate how different steel compositions, microstructures and heat treatments influence the rate and mode of wear.
Open Access
The sliding wear of polymers against steel
(1988) Clarke, C G; Allen, Colin
A laboratory wear testing facility has been developed to generate wear rate data for polymeric materials sliding at constant velocity against a hardened stainless steel base. The polymers investigated were ultra high molecular weight polyethylene (UHMWPE), ultra high molecular weight polyethylene-with a friction reducing additive (UHMWPE/FILL), polyoxymethylene (POM), poly(ethylene terephthalate) (PETP), molybdenum disulphide filled polyamide 6 (PA6/MoS₂) and graphite filled poly(amide-imide) (P(A-I)/GR). Testing was carried out as a function of sliding velocity between 0.13 to 2.27 ms⁻¹, loads of 1, 3 and 5 MP a and counterface roughnesses which varied from 1 micrometre to 0.25 micrometres. An increase in the counterface roughness resulted in a variable increase in the wear rate of the individual polymers except for the filled UHMWPE. These changes in the wear rate have been explained in terms of the mechanism of material removal. A progressive increase in sliding velocity has been shown to result in an initial increase in the wear rate followed by a decrease and finally a rapid increase for all materials under the majority of applied conditions. Explanations for such behaviour have been advanced in terms of the viscoelastic response of the polymers to strain rate and temperature. Low modulus materials however showed a significant drop in wear rate under low loads above a critical velocity which is believed to be due to a transition from boundary to partial el astohydrodynamic lubrication. Generally an increase in load gave an increase in wear rate for all polymers except for UHMWPE and filled UHMWPE at a counterface roughness of 1 micrometre. These conditions have been discussed with reference to the materials response to thermal effects and counterface interactions.
Open Access
Solid particle erosion of ceramics
(1993) Wellman, R G; Allen, Colin
The zeolite ZSM-5 is well-known for its unique intersecting channel system. This channel system has a great bearing on the shape-selective properties and the long life-times of ZSM-5. In this study, ZSM-5 was modified in various ways to eliminate the external acidity of the catalyst to further improve these properties, and the success and effects of these modifications were investigated primarily using temperature programmed desorption techniques. The internal surface of ZSM-5, a medium pore zeolite, plays a major role in the shape selective properties of this catalyst, due to the diffusional restrictions imposed by the channel system on bulky molecules. Even though the number of acid sites on the external surface is small compared to the total number of acid sites, these easily accessible and non-shape selective acid sites may provide a high turnover rate for non-shape selective reactions. Furthermore, the main cause of deactivation of ZSM-5 is thought to be the formation of polyaromatic molecules on the external surface, which block access to the channels of the catalyst.
Open Access
The sliding wear of UHMWPE against ceramics in solutions containing proteins
(1996) Kernick, Melissa Jane; Allen, Colin
The sliding wear behaviour of ultrahigh molecular weight polyethylene (UHMWPE) sliding against partially stabilised zirconia (PSZ) and alumina ceramic counterfaces in various lubricating media was investigated. Wear tests were conducted in order to investigate the effect that the addition of proteins to the lubricating solutions had on the wear behaviour of the UHMWPE against a smooth counterface of PSZ . The lubricants selected were physiological saline solution, distilled water and physiological saline solution containing small additions of synovial fluid and up to 15'% by volume of albumen solution. The effect of the addition of proteins to the lubricating solutions on the production of wear debris was also studied. ii The effect of changing the counterface roughness from Ra of 0.01 μm to 0.06μm was also investigated. The wear behaviour of UHMWPE against alumina ceramics in distilled water was compared to that of UHMWPE against PSZ in similar conditions.
Open Access
Three-body abrasive wear of materials
(2000) Jewell, Gavin; Allen, Colin
This work is an investigation into the phenomenon of three-body abrasive wear. A specially designed three body abrasive wear apparatus has been built, modified and evaluated as part of this overall study. Further, a series of commercially available candidate materials has been evaluated for wear resistance using silica sand as the abrasive on this purpose made rig. The effect of normal load, abrasive particle size, abrasive feed rate and the type of abrasive on three body wear resistance has also been examined. It has been shown that there is little increase in wear with an increase in particle size in the size range from 50µm to 180µm and that above an abrasive particle size of approximately 200µm there is a sharp decrease in the wear with increasing particle size, followed by a levelling off in the wear. The wear was found to increase linearly with increasing load. Varying the abrasive feed rate showed that at lower feed rates the abrasive particles were more efficient at removing materials, so the wear was higher than at higher abrasive feed rates. It has also been shown that although the use of ash from coal-fired power stations as an abrasive produces wear of materials, the volume losses were much smaller than those obtained using silica sand and thus it is considered that the tests using silica gave results which were more reliable. The volume losses of alumina ceramics abraded against ash were insufficient to give reliable wear test data and it was concluded' that ash could not be used to rank materials of high hardness. A number of materials were ranked for wear resistance using silica sand abrasive particles. The alumina ceramics and tungsten carbide composite materials showed the best wear performance.