Research Articles (Materials Science and Metallurgical Engineering)

Permanent URI for this collectionhttp://hdl.handle.net/2263/1699

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Thermomechanical control of microstructure and precipitation in vanadium microalloyed steel: influence of finish rolling and coiling temperatures
(Wiley, 2024-04) Wu, Zhangqin; Yang, Wen; Tang, Zhenghua; Siyasiya, Charles Witness; Zhang, Jianhua
Six hot compression tests are conducted using the Gleeble3500 thermomechanical simulator to investigate the microstructural evolution and precipitation behavior in low-C–Mn V-microalloyed steel. The specimens are subjected to hot isothermal compression deformation of 87%. The optical microscopy and transmission electron microscopy using carbon extraction replica method are used to characterize the microstructures and precipitation after the simulated thermomechanical controlled process and coiling. The results indicate that increasing the finish rolling temperature benefits the refinement of ferrite grains but has little influence on the refinement of the precipitates. It is also observed that lower coiling temperatures (CTs) promote the formation of fine precipitates. When the CT is 500 °C, the average precipitate size is found to be 86 nm. Furthermore, it is found that the CT significantly influences the nucleation sites of the precipitates inter alia, the matrix, interphase, grain boundaries, and dislocations. As expected, at higher CTs, nucleation is predominantly on the defects rather than the matrix.
Empirical modelling of 2205 DSS flow curves using strain-compensated Arrhenius rate-type constitutive model
(Nature Research, 2024-09-28) Gonya, Elvis M.; Siyasiya, Charles Witness; Makhatha, Mamookho E.
This work predicts, hot flow curves of 2205 DSS using strain-compensated Arrhenius rate-type constitutive model. Twenty-five (25) × Ø10 diameter × 15 mm height cylindrical samples were hot compressed at a temperature between 850 and 1050 °C at an interval of 50 °C and strain rates between 0.001 and 5 s−1, using Gleeble 1500D. After the tests, corrected flow curves were plotted followed by computation of deformations constants at various deformation conditions using steady state stress. The values of the constants were (α = 0.009708, Q = 445 kJ/mol and n = 3.7) and seemed comparable to the previous studies of DSS. Steady state predictive model was then constructed using the calculated constants and showed a reasonably good accuracy with low value of MARE = 7.78%. Furthermore, calculated strain compensated Arrhenius rate type model was used to predict flow curves at various deformation. The model had a good estimation of flow curves of flow curves at 900–1050 °C across all strain rates as reflected by MARE = 5.47%. A notable discrepancy between predicted and experimental flow stress was observed at 850 °C and across all the strain rates. A model refinement using generalised reduced gradient improved the accuracy of the model by 34.7% despite deformation conditions at 850 °C and low strain rates (0.01/ 0.1) s−1 showing minimum improvement. Further modification of Z-parameter by compensating for the strain rate improved the accuracy of the model at 850 °C/0.01 s−1/0.1 s−1. Lastly, a comparison of the current model with the other non-linear model showed that the latter was more accurate in estimation of flow curves since it relied on characteristics flow stress points controlled by underlying active deformation mechanisms.
Gas-phase reactions in nano-strand formation from Al-Fe-Ni powder reacted with CaF2-SiO2-Al2O3-MgO-MnO-TiO2 Flux at 1350 °C : SEM study and diffusion calculations
(MDPI, 2025-03) Coetsee, Theresa; De Bruin, Frederik Johannes; theresa.coetsee@up.ac.za
Please read abstract in the article.
'New mining scheme, same old problems?' ; risk factors and health challenges in underground community mining in Ghana
(IOP Publishing, 2024-12) Aram, Simon Appah; Saalidong, Benjamin M.; Yankyera, Akwasi; Akoto, Akwasi Bamfo; Amoah, Samuel Kofi
Ghana's small-scale mining sector has long been a cornerstone of its economy and a vital source of livelihood for many communities. However, the sector is plagued by significant challenges, including environmental degradation, health and safety risks, and illegal practices. In response, the Ghanaian government introduced the Community Mining Scheme (CMS), a new mining model designed to promote local community participation in artisanal small-scale gold mining (ASGM). This study examines the workplace risk factors and associated health challenges faced by ASGM miners under the CMS, drawing on interviews with fifty underground CMS miners in Ghana. Our findings reveal that miners continue to face numerous health and safety challenges, including inadequate lighting, poor ventilation, exposure to noise and odors, unsupported or poorly supported loose materials, non-use of personal protective equipment, high dust levels, and unsafe blasting practices. Additionally, health concerns such as skin rashes, musculoskeletal injuries, and the risk of mercury poisoning persist. These challenges mirror those reported in the literature on unregulated and illegal ASGM in Ghana, indicating that existing issues remain unresolved under the CMS. To prevent the undermining of this initiative, it is crucial to adopt progressive practices, such as localized coping strategies and the recruitment of trained professionals to lead ASGM operations within the CMS. Enhancing miners' risk perception through targeted outreach and awareness campaigns is also essential. Further recommendations for mitigating safety risks and health challenges are discussed in the article.
Delta ferrite in modified 9Cr–1Mo steel weld metal
(Springer, 2025) Mahlalela, Sibusiso Samuel; Pistorius, Pieter Georg Hendrik; sibusiso.mahlalela@up.ac.za
Please read abstract in the article.
Element transfer behaviors of agglomerated CaF2-ZrO2 fluxes in EH36-shipbuilding steel subject to high-heat input submerged arc welding
(Springer, 2024-10) Chen, Angran; Zhang, Yanyun; Coetsee, Theresa; Kaldre, Imants; Wang, Cong
EH36-shipbuilding steel has been welded by CaF2-ZrO2 fluxes with designed ZrO2 additions. Possible chemical and electrochemical reactions have been postulated to analyze alloying element transfer behaviors. The decomposition of ZrO2 during SAW has been validated by applying the gas–slag–metal equilibrium model and the O supply capacity of ZrO2 has been quantified. For the entire compositional range, O content has been controlled within a well-maintained range from 220 to 400 ppm, and the transferred quantity of Zr content reaches to the maximum value of 120 ppm. It is further demonstrated that ZrO2 addition incurs appreciable Si loss within the weld metal.
Green binder system for taphole clays : a potential for glycerine and phenolic resin
(Elsevier, 2025-06) Cameron, Izak Jian-Pierre; Garbers-Craig, Andrie Mariana
Recent advancements in taphole clay binder development have focused on replacing toxic coal tar/pitch, which contains polycyclic aromatic hydrocarbons (PAH), with greener alternatives. In this study, three different taphole clays were prepared in the laboratory using different binder systems which include phenolic resin: 1) coal tar, 2) glycerine, and 3) petroleum waxy oil. The clays were evaluated and compared to the conventional coal tar and phenolic resin-containing clay used in platinum smelters. The evaluation methods employed included workability and extrusion pressure ageing, hardenability, strength development, and high-temperature properties, which comprised cold crushing strength after ageing at 200 °C, carbon yield, volatile organic compound concentration and apparent porosity. The results indicated that the preferred binder to replace coal tar was glycerine, as this clay retained both its plasticity and hardenability during ageing, while also attaining comparative strength development and high-temperature properties to those of the reference clay containing coal tar.
Plasma soup for the pyrometallurgist's soul
(South African Institute of Mining and Metallurgy, 2025-03) Reynolds, Q.G.; Bowman, B.; Erwee, M.W.; Geldenhuys, I.J.; Sandrock, C.; Venter, G.A.; Xakalashe, B.S.; Zietsman, Johannes Hendrik
Understanding the behaviour of plasma arcs is an important part of the design and operation of direct current electric arc furnace smelting processes, which are used in the industrial production of many metallurgical commodities. In recent years numerical and computational modelling techniques have begun to illuminate the complexity of arc behaviour and how it is deeply connected to the thermodynamic and physical properties of the arc plasma. Plasma properties may be estimated from first principles using statistical mechanics methods, but this requires as input some knowledge of the composition of the gas phase that occurs over the process during smelting. In this paper the authors describe a workflow using thermochemistry software to predict the expected gas phase composition through a given process, followed by plasma property calculations using an open-source software tool that has been in development for several years. This tool, 'minplascalc' has been in development for several years and is also described in detail here. The workflow is then applied to the calculation of plasma properties of interest for a variety of historical and current pyrometallurgical processes, and the results are compared. The plasma property data are published in an open-access database for general use.
Engineering γ-TiAl alloys : the effects of Sn, Si and Mn on densification, microstructure, and mechanical properties
(Elsevier, 2025-06) Ellard, John J.M.; Mathabathe, M.N.; Siyasiya, Charles Witness; Bolokang, A.S.; Vilane, V.N.; Rikhotso-Mbungela, R.; McDuling, C.; Masete, S.
Please read abstract in the article.
Removal of iron from vanadium redox flow battery electrolyte using ion exchange
(South African Institute of Mining and Metallurgy, 2024-12) Makonese, Mufaro; Sole, K.C. (Kathryn); kathy.sole@up.ac.za
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Tracking the degradation of carbon steel mechanical properties due to high-temperature hydrogen attack through strain gauge monitoring
(South African Institute of Mining and Metallurgy, 2024-11) Van Zyl, A.; Pretorius, Christiaan C.E.; Mathoho, Vhutshilo M.; Mostert, R.J. (Roelf); roelf.mostert@up.ac.za
The in-service degradation of the mechanical properties of steel components through the damage mechanism of high temperature hydrogen attack (HTHA), is a topic of concern in the refining and green hydrogen industry. This damage mechanism occurs in susceptible steels operating in environments at high temperatures and hydrogen pressures. The current investigation deals with the indirect monitoring of mechanical degradation via tracking of the swelling strain in affected structures. An autoclave with an AISI 316 shell was utilized to simulate accelerated HTHA damage at 550 °C and 46 bar for exposure times ranging from zero to 700 hours. The progress of the HTHA damage was tracked using encapsulated high-temperature strain gauges. The correlation between the swelling strain and mechanical property degradation was studied to develop a methodology for the continuous monitoring of embrittlement. The tensile sample orientation of the carbon steel plate was included as a variable, i.e., samples parallel, transverse, or perpendicular to the plate rolling direction were included, since it has been shown that the sample orientation influences the HTHA damage features. For the through-thickness orientation, and upon exceeding a threshold value of exposure time, all tensile properties were severely degraded, with values in the order of a 90 per cent reduction being observed. It was found that the degradation of carbon steel mechanical properties can be correlated to the swelling strain measured during exposure. The critical point for mechanical property degradation in the plate through-thickness orientation, whereafter a severe decrease in the ductility of the material occurs, was found to be in the order of 1% of the total swelling strain measured during exposure, equivalent to 65 microstrain. This threshold was found to be significantly lower than that expected from the literature, where limits of 400 to 1000 microstrain were postulated.
Thermochemical analysis of the behaviour of Cu in Ti nano-strand formation from low-temperature reaction of Al-Fe-Cu powder with CaF2-SiO2-Al2O3-MgO-MnO-TiO2 flux
(Elsevier, 2025-03) Coetsee, Theresa; De Bruin, Frederik Johannes; theresa.coetsee@up.ac.za
Please read abstract in the article.
Decomposition of sulfide phases and subsequent matte collection in the black top of a platinum group metal smelter
(Taylor and Francis, 2025) Rivera Li Kao, Oscar; Garbers-Craig, Andrie Mariana; andrie.garbers-craig@up.ac.za
This study explored, on a laboratory scale, how matte separates from the gangue in the black top of a PGM smelter. Two PGM concentrates, one with high sulfide content (17.3 mass%, Platreef concentrate) and the other with low sulfide content (1.3 mass%, UG-2 concentrate), were studied in the temperature range of 800°C to 1480°C, which is the temperature gradient across the black top. The results showed that effective matte separation occurs at temperatures when the matte is completely molten, and a substantial amount of liquid silicate phase has formed. The main matte separation mechanism is the coalescence of sulfide prills and their gravitational settling through the continuous path created by the liquid silicate phase. Complete matte separation occurs at lower temperatures in Platreef concentrate (1400°C in Platreef concentrate vs. 1480°C in UG-2 concentrate), due to the higher concentration of liquid silicate phase of lower viscosity, as well as the higher amount of sulfide minerals in this concentrate. PGMs already dissolve and collect in a nickel-iron-based alloy associated with the matte at temperatures as low as 900°C.
The role of niobium during heat treatment of high-vanadium high-speed steel rolls for improved mechanical properties
(Springer, 2024-12) Chauke, Rivoningo E.; Siyasiya, Charles Witness; Nyabadza, Davisn
High-speed steel (HSS) provides superior wear resistance and high surface quality of rolled-steel products. In this study, the influence of niobium addition on the solidification behavior and microstructure of high-speed steel rolls and their impact on the roll’s hardness and wear resistance were investigated by a combination of thermodynamic simulations of solidification using ThermoCalc, optical and scanning microscopy and hardness testing. The study was conducted on two centrifugal-cast commercial high-vanadium high-speed steel sleeves whose compositions differed in niobium content. Thermodynamic simulations showed that niobium promotes the precipitation of proeutectic Nb-rich MC carbide during solidification. Microscopic observations showed that the proeutectic carbide promotes carbide dispersion in the microstructure. This was found to improve the microhardness of the martensitic matrix and counteract softening during repeated tempering treatment. The addition of less than 2 wt.% Nb in the HSS roll yielded better hot hardness as a result of the roll’s better microstructural integrity and response to HT. The conclusion was that Nb-containing HSS rolls can be produced for the hot-rolling stands that operate at higher rolling force and temperature if the cost-benefit analysis is favorable.
First-principle study on tailoring the martensitic transformation of B2 Nb50−xTixRu50 shape-memory alloy for structural applications
(MDPI, 2024-09) Nkomo, Duduzile; Shen, Yu-Nien; Mostert, R.J. (Roelf); Yamabe-Mitarai, Yoko; Phasha, Maje
NbRu has a potential as a high-temperature shape-memory alloy (HTSMA) because it has a martensitic transformation temperature above 1000 ◦C. However, its shape-memory properties could be improved for consideration in the aerospace and automotive industry. The unsatisfactory shape-memory properties could be associated with the presence of a brittle tetragonal L10 martensitic phase. Therefore, in an attempt to modify the transformation path from B2→L10 in preference of either B2→orthorhombic or B2→monoclinic (MCL), an addition of B2 phase stabiliser, titanium (Ti), has been considered in this study to partially substitute niobium (Nb) atoms. The ab initio calculations have been conducted to investigate the effect of Ti addition on the thermodynamic, elastic, and electronic properties of the Nb50−xTixRu50 in B2 and L10 phases. The results showed that the B2 and L10 phases had comparable stability with increasing Ti content. The simulated data presented here was sufficient for the selection of suitable compositions that would allow the L10 phase to be engineered out. The said composition was identified within 15–30 at.% Ti. These compositions have a potential to be considered when designing alloys for structural application at high temperatures above 200 ◦C.
Effect of coiling temperature on microstructures and precipitates in high-strength low-alloy pipeline steel after heavy reduction during a six-pass rolling thermo-mechanical controlled process
(MDPI, 2024-02-18) Lei, Yicong; Yang, Wen; Siyasiya, Charles Witness; Tang, Zhenghua; charles.siyasiya@up.ac.za
Nb-Ti high-strength low-alloy pipeline steel was subjected to a six-pass rolling process followed by the coiling process at different temperatures between 600 and 650 ◦C using the thermomechanical testing system Gleeble 3500 (Gleeble, New York, NY, USA). This experimental steel was subjected to 72% heavy reduction through a thermos-mechanical controlled process. Thereafter, the microstructures were observed using optical microscopy, scanning electron microscopy, electron backscatter scanning diffraction, and transmission electron microscopy coupled with energy dispersive spectrometry and selected area electron diffraction. For the selected three coiling temperatures of 600, 625, and 650 ◦C, acicular ferrite, polygonal ferrite, and pearlite were observed, and morphology and statistical analysis were adopted for the study of precipitates. Based on the estimation by the Ashby–Orowan formula, the incremental strength through precipitation strengthening decreases with coiling temperatures and reaches 26.67 Mpa at a coiling temperature of 600 ◦C. Precipitationtime- temperature curves were obtained to explain the transformation of precipitates. The (Nb, Ti)(C, N) particles tended to precipitate in the acicular ferrite with [011](Nb, Ti)(C, N)//[011]α-Fe orientation. The lower coiling temperature provided enough driving force for the nucleation of precipitates while inhibiting their growth.
Correlation between elastic properties and phase transformation of Zr50Pd50-xRux high-temperature shape memory alloys designed by DFT
(Springer, 2024-09) Shen, Yu-Nien; Nkomo, Duduzile; Matsunaga, S.; Phasha, M.J.; Yamabe-Mitarai, Y.
The effect of Ru addition on the martensitic transformation and mechanical properties of Zr50Pd50 xRux SMAs shape memory alloys has been studied. Strain–temperature experiments, differential scanning calorimeter, and scanning electron microscope analyses were performed to validate the theoretical predictions by DFT calculation regarding the influence of the C¢ parameter on martensitic stability. Despite the absence of an observed shape memory effect under the tested conditions, the findings suggest that ZrPdRu alloys hold promise for high-temperature applications.
Incompressible versus compressible fluid flow models : a case study on furnace tap-hole lancing
(Southern African Institute of Mining and Metallurgy, 2024-03) Erwee, Markus Wouter; Reynolds, Q.G.; Zietsman, Johannes Hendrik
Pyrometallurgical furnaces, essential for metal extraction, operate at temperatures exceeding 1600°C and represent complex multiphase systems that challenge direct industrial research. Multiphysics models play a key role in shedding light on their intricate behaviours, supporting the refinement of design and operational strategies. Integral to the operation are the tap-holes, which facilitate the removal of molten products and are routinely opened by lancing, a process comparable to the use of a cutting torch, where high temperatures result from oxygen reacting with an iron lance. When the lance pierces the clay, oxygen gas enters the furnace, which could influence the behaviour of the molten material inside. In this work, a multiphase fluid flow model was used to investigate bulk flow dynamics, with a focus on the effects of the lancing process on the inside of the furnace, immediately behind the tap-hole. Incompressible and compressible multiphase fluid solvers were used and compared with respect their performance – the intention was to assess whether using a compressible solver would yield a different solution to the incompressible one. It was concluded that there are negligible disparities in bulk fluid flow behaviour between the solvers for the case studies examined, indicating that solver selection might be less consequential for certain aspects of oxygen lancing.
Extraction of rare earth elements from Phalaborwa phosphogypsum
(Southern African Institute of Mining and Metallurgy, 2024-10) Wildenboer, R.A.; Sandenbergh, Roelf
Rare earth elements (REE) are present at concentrations of approximately 0.36 % in phosphogypsum stacks located in Phalaborwa, South Africa. The REE are present in the phosphogypsum in solid solution in the gypsum and as fluorine and aluminium rich precipitates. Recovery of the REE offers opportunities for valorization and reducing the environmental impact of the phosphogypsum waste. This paper presents the results of investigations into the leaching behaviour of rare earth elements and other impurities from phosphogypsum using aqueous sulfuric acid. The effects of acid concentration, temperature, residence time, and the influence of impurities were evaluated for their impact on the solubility of rare earth elements in a sulfate medium. It was found that a significant fraction of the REE can be leached from the phosphogypsum in a range of conditions much less severe than traditional hard rock rare earth processing conditions. The REE were leached at a sulfuric acid concentration of 110 g/l, temperature of 30°C – 40°C and a residence time of approximately 8 hours.
Investigating optimum hot working window of 2205 duplex stainless steel using modified dynamic material modeling
(Springer, 2024-12) Gonya, Elvis M.; Makhatha, Mamookho E.; Siyasiya, Charles Witness; Silubane, Ndumiso M.; Kibambe, Ngeleshi M.
Please read abstract in the article.

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