Scielo RSS <![CDATA[Journal of the Southern African Institute of Mining and Metallurgy]]> vol. 116 num. 10 lang. pt <![CDATA[SciELO Logo]]> <![CDATA[<b>Journal Comment</b>]]> <![CDATA[<b>Relevance or Extinction?</b>]]> <![CDATA[<b>Exploring differences in various Nb(V)- and Ta(V) tropolonato complexes by a solid-state structural comparison study</b>]]> An in-depth crystallographic comparison of the crystal structures of tetrakis(tropolonato-;k²O,O')niobium(V) chloride ([Nb(Trop)4]Cl) and tetrakis(tropolonato-k²O,O')tantalum(V) chloride ([Ta(Trop)4]cl) was undertaken in an attempt to relate differences in complex stability previously noted in solution with solid-state characteristics such as coordination geometry, ligand bite angles and bond lengths. Significant differences of these parameters for the complexes of the two metal centres could translate to an ideal starting point for improved separation of tantalum and niobium. It was noted that the less stable Nb(V) compound, experiences a higher average degree of strain (71.39(2)°) within the bidentate chelate rings vs. the Ta(V) analogue (72.04(2)°). When considering the higher degree of strain, the Nb(V) complex more readily offers up one of its tropolone ligands to yield the more stable [NbO(Trop)3]. The [Ta(Trop)4]+ cation also exhibits a more distorted square antiprismatic geometry than [Nb(Trop)4]+. This coordination geometry is quite distorted and could be a reason that [Ta(Trop)4]+ is inclined towards an energetically favourable dodecahedral geometry, contributing to a more stable conformation. <![CDATA[<b>High-temperature thermal plasma treatment of monazite followed by aqueous digestion</b>]]> Monazite is a chemically inert, radioactive phosphate mineral that contains the light rare earth metals (Ce, La, Nd, Pr and Y), thorium and uranium. A new process, which improves the extraction efficiency of monazite, is under development. The equilibrium mole composition, in conjunction with the Gibbs free energy, indicates that as monazite is heated in the presence of carbon to a temperature above 1400°C, it decomposes into the rare earth oxides. These temperatures are within the operational limits of a thermal plasma. When monazite is fed to a DC plasma it forms amorphous plasma-treated monazite (PTM), which is more susceptible to chemical attack. This increased chemical reactivity permits dilute mineral acids to digest the PTM, which leads to higher extraction efficiencies of the rare earth elements than when monazite is digested using the conventional processes. When the conventional process for digestion of monazite is adapted to PTM, the extraction of rare earth elements, thorium and uranium is increased significantly. <![CDATA[<b>Investigation of the 'tiger skin' defect on indefinite chill rolls</b>]]> Hot strip mills (HSMs) are used to roll steel slabs down from their initial size to plate or sheet, which are then used to manufacture a wide variety of goods for various industrial markets. The surface quality of the rolls that contact and deform the strip must be maintained during rolling, as roll imperfections can be imposed onto the strip, affecting the quality. For many years various roll producers have experienced the occurrence of a surface defect, commonly referred to as 'tiger skin' (TS), on indefinite chill (IC) work rolls, but the cause of the defect has not been thoroughly investigated. The characteristics of the TS defects on the roll surface were studied in an attempt to identify the cause and prevent its occurrence. In this paper, the characterization results of the defects are highlighted. Various techniques were utilized in the analysis: visual inspection, chemical analysis, optical microscopy, hardness testing, scanning electron microscopy and X-ray diffraction. The purpose of the testing was to identify any variations in the roll microstructure that could give rise to the surface variation. The results confirmed that the TS defects appeared as a pattern that is not related to a variation in the surface profile, surface cracking, chemical segregation, graphite characteristics, carbide characteristics, primary phase characteristics, bulk hardness, or microhardness. Further investigations are recommended. <![CDATA[<b>The separation of zirconium and hafnium from (NH<sub>4</sub>)<sub>3</sub>Zr(Hf)F<sub>7</sub> using amine-based extractants</b>]]> The suitability of the amine-based extractants Alamine 336, Aliquat 336 and Uniquat 2280 (10 wt%) for the selective extraction of zirconium (Zr) and hafnium (Hf) from a (NH4)3Zr(Hf)F7 solution was investigated. The extractant-containing organic phase consisted of cyclohexane and 5 v/v% 1-octanol as phase modifier. The (NH4)3Zr(Hf)F7 was dissolved in either hydrochloride acid or sulphuric acid (0.01-8.0 mol/dm³). The following variables were investigated: (i) the type and concentration of the acid, (ii) the equilibrium time, (iii) the organic/aqueous phase ratio and (iv) the extractant to metal ratio. Subsequently, using the optimized extraction conditions, the stripping of extracted metals from the organic phase was investigated using sodium chloride and sulphuric acid. The metal content in the aqueous phase was determined using ICP-OES before and after extractions. When using sulphuric acid solutions, extractions of >80% were reached both for Zr and Hf. In all cases equilibrium was achieved in less than 25 minutes of contact between the organic and the aqueous phase. Stripping of metals was achieved with >40% separation and 50% extraction using a sodium chloride (0.7 mol/dm³) stripping solution. <![CDATA[<b>Corrosion characteristics of mild steel storage tanks in fluorine-containing acid</b>]]> The hydrofluoric acid (HF) industry in South Africa uses normalized mild steel (SA 516 Gr 70) for the storage and distribution of its technical-grade product (70% HF). The technical-grade acid is split from the anhydrous hydrogen fluoride (AHF) product during distillation just after HF is produced, in a stainless-steel-lined kiln, from the reaction of calcium fluoride (CaF2) with sulphuric acid (H2SO4). Uniform corrosion of the storage tanks is mitigated during commissioning by contacting the steel with 70% HF. A corrosive reaction takes place (2H+ + 2F- + Fe → H2 + FeF2) to form scale inside the tank which diminishes the attack of the steel by fresh HF, thus prolonging the service life expectancy of the vessels. This iron fluoride scale in the vessel grows continually, resulting in corrosion of the vessels continuing at a predictable rate (approx. 0.5 mm/a) since the first commissioning of the tanks at Necsa in 1993. In early 2012, an increase in the average corrosion rate of the tanks to 3 mm/a measured at and below the liquid band in the storage vessels was noted. Three months later the corrosion rate had increased to 30 mm/a, just before the first leak from the tank was detected. The tanks were decommissioned shortly afterwards and an investigation revealed that the rapid corrosion was due to the presence of increased levels (>50 ppm) of nitric acid (HNO3) in the tanks, which attacked the fluoride layer protecting the steel. The oxidation characteristics of high concentrations of HNO3 with low concentrations of HF on stainless steel surface treatment are well documented. The effects of low levels of nitric acid on the corrosion of steels in sulphuric acid are also known, but no suitable published data currently exists on corrosion by high HF concentrations with low HNO3 concentrations. Establishing the rate and mechanism of corrosion by HNO3 (0.1 to 1%) in 70% HF is currently a high priority for the HF industry in South Africa and will become increasingly important in the near future due to depleting fluorite reserves and cheaper, but less efficient, processes producing H2SO4 with higher impurity levels, which were not anticipated in the original plant design. The aim of this study is to simulate the corrosion conditions in the plant on a laboratory scale in order to establish the corrosion characteristics of the steels used in the HF plant, which are increasingly exposed to the HNO3 impurities that tend to concentrate in the final technical-grade HF acid product. <![CDATA[<b>Cold-spray coating of an Fe-40 at.% Al alloy with additions of ruthenium</b>]]> In previous work by the authors, it was established that additions of 0.2 at.% Ru to an Fe-40 at.% Al alloy improved the corrosion and oxidation resistance of the alloy. The alloy was produced by mechanical alloying and spark plasma sintering and the work showed that non-equilibrium processing was able to significantly refine the grain size of the material. The sintered material had a higher hardness than the as-cast material and the change in grain size did not significantly affect the oxidation and corrosion. In the present research, the mechanically alloyed powder was coated onto a mild steel substrate using cold-spray coating at a gas pressure of 10 bar and a temperature of 500°C. The coatings were found to be 5-10 μm thick, although thicknesses of up to 30 μm were observed. The coated materials were subjected to oxidation and corrosion tests to determine the effectiveness of the coating in increasing the oxidation and corrosion resistance of mild steel. This was done to determine the effectiveness of cold-spray coating as a technique to coat these mechanically alloyed powders. <![CDATA[<b>The influence of particle size distribution on the properties of metal-injection-moulded 17-4 PH stainless steel</b>]]> Metal injection moulding (MIM) is a near-net-shaping powder metallurgy technique suitable for the cost-effective mass production of small and complex components. In this paper, the effects of the metal powder particle size on the final properties of 17-4 PH stainless steel are reported. Three different particle sizes (d50 33.0, 7.91 and 4.04 μm) were used to prepare the injectable MIM feedstocks using a CSIR-developed wax-based binder system at a fixed solids loading of 60 vol.%. The distribution slope parameter and the rheological flow index behaviour were used to predict the flowability of the feedstocks. The effects of the particle size on MIM products were established. The results are discussed in terms of the subsequent microstructural and mechanical properties of MIM 17-4 PH stainless steel against the established MPIF Standard 35. <![CDATA[<b>Spheroidisation of iron powder in a microwave plasma reactor</b>]]> Plasma-assisted spheroidisation of metal powders offers several technical advantages with respect to both the ease of materials handling and powder-metallurgical item manufacturing. Advantages include improved flowability, increased powder packing density, elimination of internal component cavities and fractures, changes in morphology resulting in decreased friction between particles and contamination during pneumatic transport and enhanced particle purity. In this empirical study, spherical iron particles were produced using a microwave plasma operating at atmospheric pressure and characterized using optical microscopy and SEM techniques. Iron powders were fed into the system at fixed operating conditions, resulting in particles with spherical structures. The theoretical estimate of the time required for melting to occur is 1.6 ms, an order of magnitude smaller than the actual residence time. <![CDATA[<b>Effect of yield strength on wear rates of railway wheels</b>]]> Excessive wear rates on railway wheels can result in rolling-stock derailments. If wear is detected early the possible derailments can be prevented by prematurely replacing wheelsets, although the inventory cost and maintenance downtime remain a challenge. In the current study, wheels were introduced and monitored in-service for two years to investigate the cause of excessive wear rates. The wear rates and stresses were calculated for the wheels suitable for a maximum load of 26 t per axle. Microstructural and mechanical properties were analysed. As expected, the general microstructure of all the wheels tested was pearlitic. Although all the wheels complied with the tensile strength requirements, stress calculations confirmed material distortion on excessively worn wheels whereas trial wheels revealed yield strength exceeding the yield criterion. High wear rates observed on the wheels were a result of low yield strength relative to the load per axle. The typical wear mechanism found was due to a combination of rolling contact fatigue and abrasive brake wear. Despite compliance of the wheels with existing requirements regarding material and mechanical properties (hardness, ultimate tensile strength), it is recommended that the yield strength must also be taken into consideration as a critical parameter. <![CDATA[<b>Premature failure of re-sulphurized steel studs</b>]]> Studs are rods threaded on both ends and are used as mechanical fasteners for two or more components. Stud finds usage in industries such as automotive, construction and aviation. The advantage of studs is that they allow for disassembly of components during maintenance. Studs are continuously subjected to both tensile and shear stresses, depending on the application, therefore they should be manufactured from suitable materials that are capable of withstanding all these external forces. Studs are sometimes manufactured from re-sulphurized steel due to its improved machinability; however, there is a certain limit on the amount of sulphur, above which failure may occur. Premature failure of studs has been a common problem, but failed studs are commonly replaced with new ones without conducting failure investigations. The present work investigates the premature failure of two re-sulphurized steel studs used on a rocker arm assembly and recommends alternative materials. The investigation was carried out by conducting visual examination, microstructural analysis, hardness measurements, chemical analysis and a detailed scanning electron microscopy (SEM) investigation on the fracture surface. <![CDATA[<b>Deformation behaviour of aluminium low-micron MMCs and MMNCs at warm working temperatures (0.3-0.5 T<sub>m</sub>)</b>]]> This work evaluates the deformation behaviour, at warm working temperatures, of green particle-reinforced aluminium composites produced by powder blending in a high-energy ball mill. The work focuses on metal matrix composites (MMCs) based on the 2124-Al alloy, reinforced with 10 or 15 vol.% SiC and metal matrix nanocomposites (MMNCs) based on the 2124-Al alloy, reinforced with 5 or 10 vol.% Al2O3. Three batches for each powder were blended and powder properties such as particle size distribution (PSD) and shape were consistent after blending. It was observed that a more uniform distribution of the reinforcement phase in the aluminium alloy matrix was achieved in 2124-Al/Al2O3 than in 2124-Al/SiC composites. The powders (unreinforced 2124-Al and blended) were initially over-aged at 350°C for 2 hours to reverse any natural ageing that may have occurred prior to use. The over-ageing was incorporated to improve compressibility of the powders with the aim of achieving green compacts with higher integrity. Uniaxial compression tests performed at ambient temperature on a Gleeble® 3500 thermomechanical simulator were unsuccessful as the green compacts fragmented. Engineering stress-strain curves showed that green compacts of unreinforced 2124-Al, 10%SiC MMC and 5%Al2O3 MMNC deformed in a similar manner at ambient temperature and had the same compressive fracture stress of approximately 170 MPa. When the deformation temperature was increased from ambient to warm working temperatures (170-280°C) it was observed that electrical resistance heating (the heating mode of the Gleeble®) of unreinforced Al alloy, MMC and MMNC green compacts did not occur. This was attributed to the high electrical conductivity of aluminium, which resulted in poor heat generation due to the low electrical resistance in the samples. It was presumed that the small sample size (d=8 mm, A=12 mm) also caused rapid heat loss. After further experimentation, the green compacts were heated successfully by insulating the samples to retain heat. It was found that at 280°C, increasing the soaking time from 6 to 20 minutes decreased flow stress and improved plastic flow in the 2124-Al/10%SiC green compact. <![CDATA[<b>Grain refinement of 25 wt% high-chromium white cast iron by addition of vanadium</b>]]> Mill liner wear is a major cost item in the mining industry and there is continuous research to prolong the life of the liners. Over the years it has become apparent that even though high-chromium white cast irons are highly efficient as abrasion-resistant materials, a combination of wear resistance and fracture strength remains difficult to achieve. Increasing the hardness of the high-chrome white cast iron (HCWCI), which improves the resistance to abrasion wear, is often accompanied by a deterioration in fracture strength. Operational conditions inside the mill require that the liner should be made of highly wear-resistant material with some fracture strength. Vanadium additions ranging from 0.2 to 3 wt% were made to HCWCI in an attempt to refine the microstructure. It was found that an increase in vanadium content promotes grain refinement. A content of 1.5-3 wt% gave the best results as measured by the maximum breaking strength. <![CDATA[<b>X-ray computed microtomography studies of MIM and DPR parts</b>]]> Parts manufactured through power metallurgy (PM) typically contain pores that can be detrimental to the final mechanical properties. This paper explores the merits of 3D X-ray computed tomography over traditional microscopy for the characterization of the evolution of porosity in metal injection moulding (MIM) and direct powder rolling (DPR) products. 17-4 PH stainless steel (as-moulded, as-debound and sintered) dog-bone samples produced via MIM and Ti-HDH strips (as-rolled and sintered) produced via DPR and were analysed for porosity. 3D micro-focus X-ray tomography (XCT) analysis on specimens from both processes revealed spatial variations in densities and the existence of characteristic moulding and roll compaction defects in agreement with traditional microscopic microstructural analysis. It was concluded that micro-focus XCT scanning can be used to study MIM and DPR parts for the characterization of the amount, position and distribution of porosity and other defects. However, the majority of the sub-micron sized pores could not be clearly resolved even at the highest possible instrument resolution. Higher-resolution scans such as nano-focus XCT could be utilized in order to fully study the porosity in MIM and DPR parts. <![CDATA[<b>Effect of niobium on the solidification structure and properties of hypoeutectic high-chromium white cast irons</b>]]> The most commonly used high-chromium white cast iron (HCWCI) is the hypoeutectic white cast iron that contains 2-3.5 wt.% C and 10-30% Cr. This type of material relies on hard, brittle M7C3 carbides to impart good wear resistance. Due to its good wear resistance, this material has found application in the mining industry and cement industry. However, it has low fracture strength and this challenge led to research aimed at refining the microstructure. This was done by the addition of niobium in the range between 0.1 and 0.6 wt.%. In the as-cast condition, the addition of niobium was found to increase the fracture strength due to the finer eutectic microstructure. The hardness was also found to have increased slightly compared to the reference alloy with no niobium additions. Low fracture strength after heat treatment was observed in the Nb-added HCWCI, which is believed to have been caused by the precipitation of secondary carbides and the transformation of the matrix from austenite to martensitic.