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| dc.contributor.supervisor | Olakanmi, E.O | |
| dc.contributor.supervisor | Prasad, R.V.S. | |
| dc.contributor.supervisor | Botes, A. | |
| dc.contributor.supervisor | Pityana, Sisa L. | |
| dc.contributor.author | Ernest, Kyekyere | |
| dc.date.accessioned | 2026-03-24T08:46:45Z | |
| dc.date.available | 2026-03-24T08:46:45Z | |
| dc.date.issued | 2025-09 | |
| dc.identifier.citation | Ernest, K. (2025) Wear resistant coatings for restoring screen plate of Ring Hammer Crusher to a state of improved performance for coal handling applications, Master’s thesis, Botswana International University of Science and Technology: Palapye | en_US |
| dc.identifier.uri | https://repository.biust.ac.bw/handle/123456789/739 | |
| dc.description.abstract | The screen plate, a critical component within a ring hammer crusher (also known as a ring granulator or rolling ring crusher), plays a vital role in the secondary crushing of coal. Functioning both as a platform for coal crushing and as a sieve to achieve the desired coal size, its mechanical and wear properties are essential in the performance of the crusher. During the crusher’s operation, continuous rolling compression of the coal between the ring hammers and screen plate occurs, which damages the plate and enlarges the screen plate’s discharge holes, which regulate the size of coal granules leaving the crusher. Furthermore, the presence of relatively hard minerals such as quartz (1161 HV), feldspar (899 HV), and rutile (934 HV) in the coal accelerates the damage to the screen plates. The enlargement of the discharge holes leads to the production of oversized coal granules (sizes larger than 75 mm) that do not meet customers' specifications. As a result, functional failure of the screen plate occurs much earlier than its expected service life. This increases the frequency and cost of maintenance, the crusher’s downtime, and the coal miner’s operational cost. Hence, identifying various modes of failure and their causative factors, as well as establishing the failure mechanisms associated with the damage seen in the screen plates, is critical in developing mitigating measures to prolong the service life. Furthermore, remanufacturing the damaged screen plate through the formulation of improved wear-resistant coating materials and an appropriate strategy to restore the damaged screen plate to at least its original performance is essential in reducing the mine's operational costs, raw material and energy utilisation, and ensuring environmental protection. To address the identified operational challenges of the screen plate, failure analysis was conducted using characterisation techniques such as optical microscopy (OM), scanning electron microscopy (SEM), Vickers microhardness test, and spectrochemical analysis to identify the failure mechanism and root causes. Findings from the study revealed that A514 and AISI 1045 are the steel grades used, and the prevalent failure modes included discharged hole widening, hole wall break-off, plate edge crack, plate fracture, one-sided edge slimming, and general surface wear of the screen plate. Severe wear also occurred at the central portions of the screen plate relative to the edges, culminating in an 18 – 32% increase in the discharge hole diameter and approximately 16 – 24% reduction in the plate thickness at the centre. The fractographic and wear track analysis identified the principal failure mechanisms of three-body abrasive wear, two-body sliding abrasion wear, shear-induced fatigue fracture and brittle shear fracture. The root causes of the failures are the rotor’s direct impact, defects in the parent material, the presence of hard materials like quartz and feldspar, among others, in the coal and the use of an unsuitable steel grade for the screen plate. The service life of the screen plate can be improved through proper material selection, uniform crusher feeding, surface modification of the “as purchased” screen plate with appropriate wear-resistant materials, and adherence to good maintenance practices. A techno-economic analysis revealed that a hybrid metal inert gas welding (MIG)/laser cladding technique provides the most economically feasible approach for restoring the damaged screen plate to improve performance, culminating in a cost savings of about 20 – 55% compared to purchasing a new plate. MIG welding was applied to rebuild the geometry to the required dimensions only at the central portion due to the extreme wear at that section, while the laser cladding was adopted to coat the entire surface. The effects of welding voltage, wire feed rate, and shielding gas flow rate on the geometric characteristics, microstructure, porosity, and microhardness of gas metal arc MIG overlay welding on A514-grade steel were explored. High MIG welding voltage (27 V) improved the bead width but negatively affected the bead height, penetration depth, and dilution, increasing the porosity in the microstructure. The microhardness of the weld overlay coating was significantly improved at low welding voltage due to the low heat inputs. The low heat input reduces the melt pool temperature, which increases the solidification and cooling rate, favouring the formation of refined grains characterised by high hardness. A welding voltage of 20 V, a wire feed rate of 5 m/min, and a gas flow rate of 22 L/min were selected as optimised process parameters, which improved the microhardness by 62.4% and the wear resistance by 30.9% compared to the substrate. Moreover, the synergic effects of TiC (20 wt.% to 40 wt.%) and SiC (0 to 20 wt.%) on the phase composition, microstructure, geometrical characteristics, and compressive and wear resistance properties of TiC/SiC reinforced 16MnCr5 composites deposited through laser cladding were investigated, aimed at selecting the best material composition with the most desirable quality characteristics. The dominant phases observed in the laser-cladded coatings are α-Fe, TiC, FexSi, Fe3C, and M7C3. As the content of SiC increases, Fe3C and M7C3 phases gradually disappear due to the stabilisation of Fe with Si. The microhardness of the coatings was substantially enhanced, with the average matrix microhardness varying between 778.6 ± 73 HV0.3 and 1003.3 ± 47 HV0.3, compared to the substrate (214.5 ± 9 HV0.3), which constitutes an increase of 263 % to 368 %. The carbides and intermetallics, such as the precipitated TiC, FexSi, and Fe3C, formed from the dissolution of carbides, improved the matrix microhardness of the coating. Beyond 10 wt.% SiC content, a decrease in the matrix microhardness was observed, attributed to the high Si dissolved in the matrix, limiting Fe3C carbide formation. The wear resistance properties of all the coatings exhibited an improvement varying between 2.5 and 6.7 times over that of the substrate, with 5 wt.% SiC/35 wt.% TiC coating achieving the highest wear resistance. This is attributed to the increased matrix microhardness and the high-volume fraction of retained carbides in the coating. Furthermore, the compressive strength of the coating with 5 wt.% SiC was the highest (1128.2±21 MPa), surpassing that of the substrate (992.4±67 MPa) by 14%. By employing hybrid response surface modelling (RSM) and non-dominated sorting genetic algorithm III (NSGA III), the effect of laser power (P), scanning speed (S), and powder feed rate (F) on coatings aspect ratio, dilution ratio, microhardness and wear resistance was investigated and optimised. The influence of the process parameters on aspect ratio, dilution, microhardness, and the wear volume loss is in this order, respectively: S > P > F; S > F > P; P > F >S; and P > S > F and the interactions of the process parameters were significant. An increase in the scanning speed reduces the powder deposition density and laser interaction time, thereby limiting the vertical build-up and reducing the aspect ratio. Based on the NSGA III optimisation, the optimal process parameters identified were P=1550 W, S=500 mm/min, and F=7 g/min. The validation experiment revealed a close agreement with the predicted results, with errors of less than 5% for all objectives. The optimised coating's microstructure consisted predominantly of columnar crystals with minor regions of equiaxed dendrites. Compared to the substrate, the optimised coating's microhardness improved by 350%, its compressive strength was enhanced by 41%, and its wear resistance was 6 times enhanced. The enhanced mechanical and wear-resistance properties will reduce the screen plate's failure frequency, minimising downtime losses and increasing the mine's profitability while reducing energy and raw material utilisation as well as environmental pollution. This study provides insight into developing coatings with enhanced wear-resistance and mechanical properties to improve the surface properties of engineering components subjected to severe loading conditions, such as the screen plate. | en_US |
| dc.language.iso | en | en_US |
| dc.publisher | Botswana International University of Science and Technology | en_US |
| dc.subject | Remanufacture | en_US |
| dc.subject | Laser cladding | en_US |
| dc.subject | MIG welding | en_US |
| dc.subject | Failure Analysis | en_US |
| dc.subject | TiC | en_US |
| dc.subject | SiC | en_US |
| dc.subject | Wear | en_US |
| dc.title | Wear resistant coatings for restoring screen plate of ring Hammer Crusher to a state of improved performance for coal handling applications | en_US |
| dc.description.level | phd | en_US |
| dc.description.accessibility | unrestricted | en_US |
| dc.description.department | mie | en_US |
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