Scielo RSS <![CDATA[Journal of the Southern African Institute of Mining and Metallurgy]]> http://www.scielo.org.za/rss.php?pid=0038-223X20150010&lang=pt vol. 115 num. 10 lang. pt <![CDATA[SciELO Logo]]> http://www.scielo.org.za/img/en/fbpelogp.gif http://www.scielo.org.za http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-223X2015001000001&lng=pt&nrm=iso&tlng=pt <![CDATA[<b>Mining in the global village</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-223X2015001000002&lng=pt&nrm=iso&tlng=pt <![CDATA[<b>Where should the national R&D in materials science fit into South Africa's future nuclear power programme?</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-223X2015001000003&lng=pt&nrm=iso&tlng=pt South Africa recently announced a resurgence in its commercial nuclear power programme. The implications for the development of the necessary high-level manpower within South Africa's tertiary educational system and its national research and development (R&D) capacity in materials science and engineering, as well as in other engineering disciplines, are placed into perspective. An organized national process of developing this manpower by moving away from the previously high-risk and costly 'large programmes' to rather a selection of 'small and better' research projects and a redefinition of what constitutes 'nuclear materials' are proposed as parts of this strategy. <![CDATA[<b>Friction processing as an alternative joining technology for the nuclear industry</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-223X2015001000004&lng=pt&nrm=iso&tlng=pt The process of joining materials by friction is based on generating the heat necessary to create a solid-state mechanical bond between two faying surfaces to be joined. In simple terms, the components to be joined are subjected to frictional heating between rubbing surfaces, causing an increase in interface temperature and leading to localized softening of interface material, creating what is described as a 'third body' plasticized layer. This plasticized zone reduces the energy input rate from frictional heating and hence prevents macroscopic melting. The plasticized layer can no longer transmit sufficient stress as it effectively behaves as a lubricant (Boldyrev and Voinov, 1980; Godet, 1984; Singer, 1998; Suery, Blandin, and Dendievel, 1994). The potential for this solid-state frictional joining process to create high-performance joints between, for example, dissimilar materials with limited detrimental metallurgical impact, and reduced defect population and residual stress level, has had a very significant impact on fabrication and repair in industrial sectors such as transport. This paper presents a brief overview of the advances made within the family of friction processing technologies that could potentially be exploited in the nuclear industry as alternative joining and repair techniques to fusion welding. Modern friction processing technologies can be placed into two main categories: those that make use of a consumable tool to achieve the intended repair or joint (friction stud and friction hydro-pillar processing) and those making use of a non-consumable tool (friction stir welding). The most mature friction joining technology is friction rotary welding, where a joint is formed between original parent materials only. A new addition in this category is linear friction welding, which opens the potential for joining complex near-net-shape geometries by friction heating. The continuous innovation in friction processing over the last 25 years has led to the development of a number of unique processes and applications, highlighting the adaptability of friction processes for specialized applications for high-value engineering components. <![CDATA[<b>Neutron- and X-ray radiography/ tomography: Non-destructive analytical tools for the characterization of nuclear materials</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-223X2015001000005&lng=pt&nrm=iso&tlng=pt A number of important areas in nuclear fuel cycle, at both the front end and back end, offer ideal opportunities for the application of nondestructive evaluation techniques. These techniques do not only provide opportunities for non-invasive testing of ag:irradiated materials, but also play an important role in the development of new materials in the nuclear sector. The advantage of penetrating radiation used as probe in the investigation and testing of nuclear materials makes X-ray and neutron radiography (2D) and tomography (3D) suitable for various applications in the total nuclear fuel cycle. The unique and different interaction modes of the two radiation probes with materials provide several opportunities. Their complementary nature and non-destructive character makes them most suitable for nuclear material analyses, analytical method development, and the evaluation of the performance of existing nuclear material compositions. This article gives an overview of the X-ray and neutron radiography/tomography applications in the field of nuclear material testing, and highlights a few of the success stories. Several selected areas of application in the nuclear fuel cycle are discussed to illustrate the complementary nature of these techniques as applied to nuclear materials. <![CDATA[<b>Non-destructive characterization of materials and components with neutron and X-ray diffraction methods</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-223X2015001000006&lng=pt&nrm=iso&tlng=pt The availability of advanced characterization techniques is integral to the development of advanced materials, not only during development phases, but in the manufactured components as well. At Necsa, two modern neutron diffractometers equipped with in-situ sample environments, as well as complementary X-ray diffraction instruments, are now available as User Facilities within the National System of Innovation in support of the South African research and industrial communities. Neutrons and X-rays, owing to their different interaction mechanisms with matter, offer complementary techniques for probing crystalline materials. Both techniques enable nondestructive investigation of phenomena such as chemical phase composition, residual stress, and texture (preferred crystallite orientation). More specifically, the superior penetration capabilities of thermal neutrons into most materials allows for the analysis of bulk or localized depth-resolved properties in a wide variety of materials and components. Materials that can be investigated include metals, alloys, composites, ceramics, and coated systems. In particular, depth-resolved analyses using neutron diffraction complements surface investigations using laboratory X-rays in many scientific and engineering topics. The diffraction techniques can add significant downstream value to the anticipated nuclear industry development activities. <![CDATA[<b>Fluorine: A key enabling element in the nuclear fuel cycle</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-223X2015001000007&lng=pt&nrm=iso&tlng=pt Fluorine - in the form of hydrofluoric acid, anhydrous hydrogen fluoride, elemental gaseous fluorine, fluoropolymers, volatile inorganic fluorides, and more - has played, and still plays, a major role in the nuclear industry. In order to enrich uranium, the metal has to be in the gaseous state. While more exotic methods are known, the standard and most cost-competitive way of achieving this is by means of uranium hexafluoride (UF6). This compound sublimates at low temperatures, and the vapour is enriched using centrifugal processes. The industrial preparation of uranium hexafluoride requires both elemental fluorine gas and anhydrous hydrogen fluoride (HF). HF is prepared by the reaction of sulphuric acid with fluorspar (CaF2). Fluorine gas in turn is prepared by the electrolysis of HF. Yellowcake is first converted to uranium tetrafluoride (UF4), using HF, after which the compound is treated with fluorine to yield UF6. After enrichment, the UF6 is reduced to UO2 for use in fuel elements in pellet form. South Africa has the largest reserves of fluorspar internationally, and is the third largest producer after Mexico and China. Fluorine technology has many associated difficulties, because of the reactivity of fluorine and the toxicity of HF. The main barriers to entry into the fluorochemical industry are thus the abilities to produce both HF and F2. Both these substances are produced locally, at the industrial scale, at Pelchem SOC Ltd. Should South Africa contemplate developing its own nuclear fuel cycle as part of the awaited new-build nuclear project, it will be imperative to leverage the existing skills with respect to fluorine technology, resident at both Pelchem and Necsa, for this purpose. This paper summarizes the fluorochemical skills developed locally over the past several decades, and suggests strategies for maintaining the technology base and developing it for the next generation of scientists and engineers. <![CDATA[<b>Titanium and zirconium metal powder spheroidization by thermal plasma processes</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-223X2015001000008&lng=pt&nrm=iso&tlng=pt New technologies used to manufacture high-quality components, such as direct laser sintering, require spherical powders of a narrow particle size distribution as this affects the packing density and sintering mechanism. The powder also has to be chemically pure as impurities such as H, O, C, N, and S causes brittleness, influence metal properties such as tensile strength, hardness, and ductility, and also increase surface tension during processing. Two new metal powder processes have been developed over the past few years. Necsa produces zirconium powders via a plasma process for use in the nuclear industry, and the CSIR produces titanium particles for use in the aerospace industry. Spheroidization and densification of these metal powders require re-melting of irregular shaped particles at high temperature and solidifying the resulting droplets by rapid quenching. Spherical metal powders can be obtained by various energy-intensive methods such as atomization of molten metal at high temperatures or rotating electrode methods. Rapid heating and cooling, which prevents contamination of the powder by impurities, is, however, difficult when using these methods for high-melting-point metals. For this reason plasma methods should be considered. Thermal plasmas, characterized by their extremely high temperatures (3000-10 000 K) and rapid heating and cooling rates (approx. 106 k/s) under oxidizing, reducing, or inert conditions, are suitable for spheroidization of metal powders with relatively high melting points. Thermal plasmas for this purpose can be produced by direct current (DC) plasma arc torches or radio frequency (RF) inductively coupled discharges. In order to obtain chemically pure spheroidized powder, plasma gases such as N2, H2, O2, and CH4 cannot be considered, while Ar, Ne, and He are suitable. Neon is, however, expensive, while helium ionizes easily and it is therefore difficult to obtain a thermal helium plasma at temperatures higher than 3000 K. Therefore argon should be used as plasma gas. Residence times of particles in the plasma region range from 5-20 ms, but this is usually sufficient as 7-8 ms is required for heating and melting of titanium or zirconium metal particles in the 30 μm size range at 3500 K. In this study the melting and spheriodization of titanium powders was investigated by DC non-transferred arc and RF induction plasma methods. The powders were characterized before and after plasma treatment by optical microscopy and scanning electron microscopy (SEM) to observe if any melting or spheroidization had occurred. <![CDATA[<b>Plasma technology for the manufacturing of nuclear materials at Necsa</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-223X2015001000009&lng=pt&nrm=iso&tlng=pt New technologies used to manufacture high-quality components, such as direct laser sintering, require spherical powders of a narrow particle size distribution as this affects the packing density and sintering mechanism. The powder also has to be chemically pure as impurities such as H, O, C, N, and S causes brittleness, influence metal properties such as tensile strength, hardness, and ductility, and also increase surface tension during processing. Two new metal powder processes have been developed over the past few years. Necsa produces zirconium powders via a plasma process for use in the nuclear industry, and the CSIR produces titanium particles for use in the aerospace industry. Spheroidization and densification of these metal powders require re-melting of irregular shaped particles at high temperature and solidifying the resulting droplets by rapid quenching. Spherical metal powders can be obtained by various energy-intensive methods such as atomization of molten metal at high temperatures or rotating electrode methods. Rapid heating and cooling, which prevents contamination of the powder by impurities, is, however, difficult when using these methods for high-melting-point metals. For this reason plasma methods should be considered. Thermal plasmas, characterized by their extremely high temperatures (3000-10 000 K) and rapid heating and cooling rates (approx. 106 k/s) under oxidizing, reducing, or inert conditions, are suitable for spheroidization of metal powders with relatively high melting points. Thermal plasmas for this purpose can be produced by direct current (DC) plasma arc torches or radio frequency (RF) inductively coupled discharges. In order to obtain chemically pure spheroidized powder, plasma gases such as N2, H2, O2, and CH4 cannot be considered, while Ar, Ne, and He are suitable. Neon is, however, expensive, while helium ionizes easily and it is therefore difficult to obtain a thermal helium plasma at temperatures higher than 3000 K. Therefore argon should be used as plasma gas. Residence times of particles in the plasma region range from 5-20 ms, but this is usually sufficient as 7-8 ms is required for heating and melting of titanium or zirconium metal particles in the 30 μm size range at 3500 K. In this study the melting and spheriodization of titanium powders was investigated by DC non-transferred arc and RF induction plasma methods. The powders were characterized before and after plasma treatment by optical microscopy and scanning electron microscopy (SEM) to observe if any melting or spheroidization had occurred. <![CDATA[<b>Synthesis and deposition of silicon carbide nanopowders in a microwave-induced plasma operating at low to atmospheric pressures</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-223X2015001000010&lng=pt&nrm=iso&tlng=pt Silicon carbide nanopowders were produced using a microwave-induced plasma process operating at 15 kPa absolute and at atmospheric pressure. Methyltrichlorosilane (MTS) served as precursor, due to its advantageous stoichiometric silicon-to-carbon ratio of unity, allowing it to act as both carbon and silicon source. Argon served as carrier gas, and an additional hydrogen feed helped ensure a fully reducing reaction environment. The parameters under investigation were the H2:MTS molar ratio and the total enthalpy. The particle size distribution ranged from 20 nm upwards, as determined by SEM and TEM micrographs. It was found that an increase in enthalpy and a higher H2:MTS ratio resulted in smaller SiC particle sizes. The adhesion of particles was a common occurrence during the process, resulting in larger agglomerate sizes. SiC layers were deposited at 15 kPa with thicknesses ranging from 5.8 to 15 μm. <![CDATA[<b>A redetermination of the structure of tetraethylammonium <i>mer-</i>oxidotrichlorido(thenoyltrifluoroacetyl acetonat</b><b>0-<i>K<sup>2</sup></i>-0,0')</b><b>niobate(V)</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-223X2015001000011&lng=pt&nrm=iso&tlng=pt The tetraethylammonium salt of the mono-anionic coordination compound me/--oxidotrichlorido(thenoyltrifluoroacetylacetonato-K²O,O')niobate(V) (NEt4)[NbOCl3](ttfa)], has been prepared under aerobic conditions and characterized by single-crystal X-ray diffraction. (NEt4)[NbOCl3](ttfa)] crystallized in the monoclinic .P2i/c space group, with a = 11.483 (5), b = 12.563 (5), c = 17.110(5) Â, and β = 100.838 (5)°. The complex structure exists in a 50.0% (NbA) : 50.0% (NbB) positional disorder ratio. <![CDATA[<b>A theoretical approach to the sublimation separation of zirconium and hafnium in the tetrafluoride form</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-223X2015001000012&lng=pt&nrm=iso&tlng=pt The separation of zirconium and hafnium is essential in the nuclear industry, since zirconium alloys for this application require hafnium concentrations of less than 100 ppm. The separation is, however, very difficult due to the numerous similarities in the chemical and physical properties of these two elements. Traditional methods for separation of zirconium and hafnium rely predominantly on wet chemical techniques, absolvent extraction. In contrast to the traditional aqueous chloride systems, the AMI zirconium metal process developed by Necsa focuses on dry fluoride-based processes. Dry processes have the advantage of producing much less hazardous chemical waste. In the proposed AMI process, separation is effected by selective sublimation of the two tetrafluorides in an inert atmosphere under controlled conditions, and subsequent selective desublimation. Estimates are made for the sublimation rates of the two tetrafluorides based on the equilibrium vapour pressures. A sublimation model, based on the sublimation rates, was developed to determine if the concept of separation by sublimation and subsequent desublimation is theoretically possible. <![CDATA[<b>Glow discharge optical emission spectroscopy: A general overview with regard to nuclear materials</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-223X2015001000013&lng=pt&nrm=iso&tlng=pt Glow discharge optical emission spectroscopy (GD-OES) is an analytical technique used in the analysis of solid, conducting materials. Though primarily of interest as a depth profiling technique on samples with varying layers of both conducting and non-conducting materials, it is also capable of rapid bulk analysis of homogenous, solid samples. GD-OES combines the advantages of ICP-OES (wide detection range, speed, and lack of interferences) with the solid sampling of XRF techniques. This allows the analyst to not only quantify the elemental composition of a sample, but to evaluate it in terms of homogeneity with depth, a field in which auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS) have traditionally been the primary techniques. Although GD-OES does not replace these useful techniques, it does offer various advantages over them, making it an excellent complementary analytical tool. In GD-OES analysis, a low-pressure glow discharge plasma is generated with the sample material acting as a cathode, accelerating the cations in the plasma towards the sample surface. This bombardment causes the sample material to 'sputter', essentially knocking free atoms or molecules of analyte material, which are then drawn into the plasma where they are excited. The light emitted by this excitation is then diffracted to separate the wavelengths emitted by the specific elements and detected by a spectrophotometer. The intensity of the signal is directly proportional to the quantity of analyte element present in the sample, allowing simple calibration and quantitative determination of the elements. Technological improvements made in the past twenty years or so have significantly increased the usefulness of GD-OES for surface analysis. Faster plasma stabilization and start-up allow for the quantification of surface layers as thin as 1 nm. Sputter rates have been accurately measured for many common materials, allowing them to be built into software libraries in the instrument's control software. This dramatically expands the usefulness of this software and the ease of performing analyses. GD-OES analysis of nuclear materials allows for the rapid determination of the elemental composition without the requirement of initial dissolution. The thickness of any corrosion layers on nuclear materials can also be determined. <![CDATA[<b>The influence of niobium content on austenite grain growth in microalloyed steels</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-223X2015001000014&lng=pt&nrm=iso&tlng=pt The relationship between niobium content and austenite grain growth has been investigated through hot rolling simulation on a Bähr dilatometer. The effect of delay time between passes during rough rolling in Nb-microalloyed steels with nitrogen contents typical for electric arc furnace (EAF) melting was studied. The results indicate that the grain growth constants n, Q, and A increase with an increase in Nb content. The activation energy for austenite grain growth Q was found to be in the range of 239 to 572 kJ/mol, the exponential constant n ranged from 2.8 to 6.2, and the material and processing condition constant A from 4.24 χ 1012 to 4.96 χ 10(28), for steels with niobium contents ranging from 0.002% Nb to 0.1% Nb. A general constitutive equation for the prediction of austenite grain growth in these Nb-microalloyed steels under rough rolling conditions has been developed. Good agreement between the experimental and the predicted values was achieved with this constitutive equation. <![CDATA[<b>The influence of thermomechanical processing on the surface quality of an AISI 436 ferritic stainless steel</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-223X2015001000015&lng=pt&nrm=iso&tlng=pt The need to reduce weight while maintaining good mechanical properties in materials used in the automotive industry has over the years seen an increased exploitation of various steels to meet this new demand. In line with this development, the ferritic stainless steel family has seen a wide application in this industry, with the AISI 436 type increasingly being used for automotive trims and mufflers for exhaust systems, as well as a significant part of this steel's application being for the manufacture of wheel nuts and wheel nut caps in trucks, mainly through the deep drawing process. However, there have been reports of some poor surface roughening of this material during deep drawing, with tearing and/or cracking also reported in some instances. This has been suspected to possibly be associated with some local differences in localized mechanical properties between grains and grain clusters of the rolled and annealed material. In order to investigate the poor surface roughness exhibited by AISI 436 ferritic stainless steel (FSS) during deep-drawing, Lankford values (R-mean and Ar), grain size, and microtextures of various sheet samples from this steel were studied. The chemical composition range for the samples was 0.013-0.017% C, 17-17.4% Cr, 0.9-1% Mo, and 0.4-0.5% Nb. The steels were subjected to various hot and cold rolling processing routes i.e. involving industrial direct rolling (DR) or intermediate annealing rolling (IR), and the drawability and final surface qualities of the steels were compared. It was found that the DR route gave an average R-mean and Ar value of 1.9 and -1.4 respectively, while the IR route yielded an average R-mean and Ar value of 1.6 and 0.52 respectively. The high Ar value for the DR processing route had a substantial adverse effect on the drawability. IR samples exhibited a smoother surface finish on visual inspection, while clear flow lines were visible on the DR samples, despite the fact that DR is the preferred industrial processing route due to the reduced production costs it offers. This observation was also confirmed through SEM examinations. The difference in the surface quality was attributed to microtexture. However, the mechanism responsible for this difference still needs to be identified.