RESUMO O desgaste de componentes e equipamentos industriais e agrícolas representa um grande fator de depreciação de capital e de fonte de despesas com manutenção e reposição de componentes mecânicos. Como o desgaste é um fenômeno essencialmente superficial, envolvendo a remoção mecânica indesejável de material de superfícies, a solução estudada nesse trabalho foi analisar o comportamento do desgaste abrasivo de uma superfície revestida com Carbeto de Tungstênio (WC) aspergido pelo método de aspersão térmica oxi-combustível de alta velocidade, conhecida como HVOF (do inglês High Velocity Oxygen Fuel), sobre um substrato de aço SAE 1020. Para melhorar a adesão entre o revestimento e substrato, diminuir a porosidade presente no revestimento e obter melhores resultados no ensaio de desgaste abrasivo foi realizado um tratamento com laser de CO2, cuja potência utilizada foi de 125 W, na superfície do revestimento de WC. As amostras revestidas via HVOF e HVOF + tratamento com laser de CO2 foram caracterizadas através de Microscopia Eletrônica de Varredura (MEV) para avaliar a morfologia e microestrutura do revestimento e interface, análise de microdureza, ensaio de desgaste e medida da rugosidade foram realizados. Obteve-se como resultado que a amostra revestida pelo método HVOF apresentou um melhor desempenho em ensaios de microdureza, desgaste e rugosidade em comparação com a amostra tratada com o laser.
The laser remelting technique is considered a promising and effective method for improving the surface of thermally sprayed coatings, eliminating microstructural defects such as pores and cracks, increasing the life of parts and equipment by increasing microhardness and increasing anchoring force between coating and substrate. In the present work, tungsten carbide alloy (WC-12Co-4Cr) coatings were deposited on properly prepared SAE 1016 substrates using the high velocity oxy-fuel technique. An ytterbium fiber laser was used to remelt the surface of the coating by accurately varying the scanning speed and laser beam power to achieve a pore and crack-free coating and better metallurgical anchorage to the substrate through the optimization of the studied parameters. The samples were characterized by scanning electron microscopy and microhardness. The results show that it is possible to obtain higher hardness coatings after the laser remelting process, free of pores or pronounced imperfections and metallurgically bonded to the substrate. We also found a processing range for the ytterbium laser remelting of the WC-12Co-4Cr coating aiming at high productivity and microstructural optimization of the coating for different thicknesses.
The effect of abrasive wear is particularly relevant in the industrial areas of agriculture, mining, mineral processing and earth moving. Wear problems occur in mills and silos where grain is stored and moved in large quantities through the process of agro-processing. Machines and equipment that work under direct contact with the grain suffer from severe three-body abrasive wear from the rice husks. The main objective of this study was to investigate the abrasive wear behavior of an Fe-29Cr-4B-1.75Si coating sprayed by electric arc and a WC-10Co-4Cr coating applied by the high-velocity oxygen fuel (HVOF) methods. Both coatings used SAE 1016 carbon steel as the substrate. A dry-sand, rubber wheel abrasion test was performed using ASTM G65 standard that employs sand as an abradant. It was compared to a variation on that test that employed rice husk as the abradant. The coatings were characterized by scanning electron microscopy, X-ray diffraction and microhardness. The microhardness values for coatings and substrate were measured using a microhardness Vickers tester equipped with a diamond pyramid indenter. The results showed that the mechanisms of wear differ by the change of abrasive, characterizing a wear by scratches when using the sand, and by microcutting, ledges and cracks when the rice husk is used as an abradant. The WC-10Co-4Cr deposition by HVOF showed satisfactory performance under abrasive wear with rice husk demonstrating a potential for application in agricultural implements working in the rice culture.
One of the biggest challenges facing industrial sectors such as agriculture, mining, oil and gas, and aerospace today is issues related to component wear. In this context, the laser cladding process has often been chosen as a deposition process for abrasion resistant coatings due to the low dilution and microstructural characteristics found in the deposited material. An option for improving abrasion resistance has been the use of reinforcement phases from carbides. However, it is necessary to have a comparative study of the tribological performance of deposits made with different metallic matrices and different proportions of reinforcements. The state of the art indicates the lack of a detailed tribological study of Metal Matrix Composites (MMC) using the chemical composition of the metallic matrices in this study, as well as the high proportion of tungsten carbide (WC), as a reinforcement element, applied to the ASTM G65 standardized test. Laser processing parameters, hardness and abrasive wear performance were studied by changing the volume fraction of WC by 0, 20 and 30%Vol. in two nickel-based metallic matrices and one iron-based matrix deposited via laser cladding. The results indicate that the increase in the fraction of WC in the metallic matrix provides a smaller volumetric loss and greater resistance to abrasion reaching a volumetric loss of up to 95% lower when compared to the composite material with the matrix in its pure state. This resistance is also related to the microhardness and anchoring performance of the hard WC particles on each metallic matrix. However, there is a processability limit when using a high percentage of reinforcement phase, generating chemical and thermal reactions in the metallic matrix, causing structural defects in the deposited composite coating. In addition, the different mechanisms of abrasive wear are influenced by the hardness and the change in the chemical composition of the metallic matrix, which can lead to adhesive and brittle wear, generating greater volumetric losses during the abrasive test.
Resumo Para obter uma superfície que aumenta a vida útil de peças e componentes de máquinas e que possua propriedades mecânicas que são resistentes ao desgaste abrasivo, devido ao contato com o grão de arroz com casca, uma superfície foi aspergida por meio do método de aspersão térmica oxi-combustível de alta velocidade (HVOF) com pó de carboneto de tungstênio no substrato SAE 1020 e pós-tratamento com laser de CO 2 , desejando obter uma melhor adesão do revestimento com o substrato (interface). Este revestimento tem uma estrutura densa, baixa porosidade, alta adesão ao substrato e é altamente resistente ao desgaste abrasivo. As amostras revestidas foram caracterizadas por microscopia eletrônica de varredura e análise de microdureza. As amostras revestidas com WC apresentaram uma maior dureza em relação as amostras revestidas e tratadas com laser. Palavras-chave:Aspersão térmica;Dureza;Laser;Revestimento. MECHANICAL CHARACTERIZATION OF WC COATING DEPOSITED BY HVOF THERMAL SPRAYING AND CO 2 LASER TREATMENT Abstract In order to obtain a surface that increases the useful life of parts and components of machines and that possesses mechanical properties that are resistant to the abrasive wear, due to the contact with the rice grain with husk, a surface was sprayed by means of the high-velocity oxygen fuel thermal spray method (HVOF) with tungsten carbide powder on the SAE 1020 substrate and post-treatment with CO 2 laser, wishing to obtain better adhesion of the coating to the substrate (interface). This coating has a dense structure, low porosity, high adhesion to the substrate and is highly resistant to abrasive wear. The coated samples were characterized by scanning electron microscopy and microhardness analysis. WC coated samples exhibited a higher hardness compared to coated and laser-treated samples.
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