Abstract This work aims to evaluate the influence of laser power and reinforcement feeding rate on the microstructure, hardness, and wear behavior of NbC-reinforced Hastelloy C276TM alloy composite coatings. From a dual feeding system, one-step metal-matrix composite coatings were deposited with 10, 30, and 50% mass feeding of NbC powder with laser powers of 1.5 and 3.0 kW. Coatings deposited with 1.5 kW and 30% NbC showed some degree of porosity due to a combination of NbC feeding rate and melting pool temperature. Laser power and NbC feeding rate altered the melting efficiency and substrate burn-in shape, remarkably influencing the dilution. The composite microstructure was comprised of Ni-γ (FCC) dendrites with interdendritic network carbide which, in turn, ranged from lamellar-like M6C to blocky-like conjugated MC-M23C6 carbide. Primary petaloid-like MC [Nb] carbide was formed from a 30% reinforcement rate on, whilst a considerable number of unmelted particles was observed for 50%. The higher the reinforcement feeding rate, the higher the carbide fraction and better wear performance of low heat-input specimens. Synthesis with high heat-input sheds a light on the deleterious effect of the dilution and is not an option to enhance the wear performance.

Resumo: O presente trabalho teve como objetivo determinar parâmetros de processamento a laser para deposição de revestimentos de Stellite 6 livres de descontinuidades e com geometria adequada sobre aço inoxidável austenítico. Revestimentos foram depositados com uma fonte de laser de diodo, equipada com um cabeçote coaxial contínuo, utilizando diferentes potências do feixe e velocidades de varredura. A geometria (largura, altura, ondulação e diluição) dos revestimentos foi medida por meio de análise da secção transversal. Esses dados obtidos foram utilizados para a modelagem do processo (superfície de resposta) e para estimar parâmetros otimizados (função desejabilidade), que foram posteriormente verificados experimentalmente. A microestrutura dos revestimentos depositados com os parâmetros otimizados foi caracterizada por microscopia óptica e eletrônica de varredura. A seleção adequada dos parâmetros resultou em revestimentos com bom acabamento, livre de descontinuidades superficiais e baixa diluição. Os resultados mostram que a potência do laser teve efeito significativo sobre a diluição, enquanto a velocidade influenciou na altura da camada depositada. A microestrutura resultante do revestimento é composta por dendritas ricas em cobalto e a região interdendrítica com carbonetos de cromo. Os resultados de dureza estão de acordo com o previsto na literatura.

Abstract: The present work aimed to determine laser processing parameters for deposition of defect-free Stellite 6 coatings and with suitable geometry on austenitic stainless steel. Coatings were deposited with a diode laser source, equipped with a continuous coaxial head, using different beam powers and scanning speeds. The geometry (width, height, undulation and dilution) of the coatings was measured by analyzing the cross section. These obtained data were used to model the process (response surface) and to estimate optimized parameters (function desirability), which were later verified experimentally. The microstructure of the coatings deposited with the optimized parameters was characterized by optical and scanning electron microscopy. The proper selection of parameters resulted in coatings with good surface finish, free of superficial defects and low dilution. The results show that the laser power had a significant effect on the dilution, while the speed influenced the height of the deposited layer. The microstructure resulting from the coating is composed of dendrites rich in cobalt and interdendritic region with chromium carbides. The hardness results are in line with the predicted in the literature.

Abstract Laser power influences the range of dilution with the substrate and thus, the microstructure and properties of the coatings. This work evaluated the effect of laser power on the dilution, microstructure, hardness and wear behavior of Hastelloy C276TM alloy coatings deposited on AISI 304L stainless steel and GGG40 ductile iron. The microstructure was comprised of γ (Ni-FCC) dendrites and molybdenum-rich interdendritic regions containing carbides. The coatings showed similar second phase nature and fraction and equivalent hardness and wear behavior for the lowest laser power condition. Otherwise, higher power on 304L induced to a lower fraction of blocky-like M23C6/MC carbides; whilst on GGG40 led to a higher fraction of a predominantly lamellar M6C type carbide. Largely diluted coatings on ductile iron entailed differences in the strengthening mechanisms which led to higher hardness and lower wear rate. Moreover, wear tracks showed surface oxidation which in turn negatively influenced the performance of specimens with low dilution on both substrates. On the other way, with high dilution, this feature was only observed on stainless steel.

Nesse trabalho, foram estudados aspectos cinéticos da transformação bainítica incompleta em ligas de ferro nodular, contendo 2,36 e 2,68% Si. As amostras foram austenitizadas a 900°C, durante 90 minutos, e austemperadas a 320 e 370ºC, em tempos entre 1,5 e 60 minutos. A caracterização microestrutural foi realizada pelas técnicas de microscopia óptica (MO), microscopia eletrônica de varredura (MEV), difração de Raios X (DRX) e microdureza Vickers. Foi desenvolvido um método de quantificação das frações transformadas por microscopia óptica. Devido ao elevado teor de silício, as ligas apresentaram o fenômeno de estase da transformação bainítica. As amostras tratadas a 320°C apresentaram microestrutura mais refinada e maiores frações transformadas para o início da estase da reação. As frações transformadas obtidas por DRX foram menores que as encontradas por MO. Esse fenômeno foi mais intenso para a menor temperatura de austêmpera. Curvas de transformação isotérmica foram obtidas para todas as condições estudadas. A cinética de transformação bainítica incompleta foi analisada segundo o modelo de Johnson-Mehl-Avrami-Kolmogorov (JMAK). Os valores do expoente n da equação JMAK variaram entre 0,15 e 0,67.

This work aimed at studying the kinetic aspects of the incomplete bainite transformation of ductile cast iron containing 2.36 and 2.68% silicon (in wt %). Samples were initially austenitized at 900°C during 90 minutes and then austempered at 320 and 370°C in times ranging from 1.5 to 60 minutes. Microstructural characterization was performed by light optical microscopy (LOM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Vickers microhardness tests. A LOM-based method for transformed fractions quantification has been developed. Due to its high silicon content, both alloys presented the bainite transformation stasis phenomenon. Samples austempered at 320°C displayed more refined microstructures and higher transformed volume fractions at the reaction stasis beginning. Low resolution is the main drawback for using light optical microscopy in the phase quantification. Transformed volume fractions determined by XRD showed smaller values than those obtained by LOM, mainly for the lower austempering temperature. Kinetic transformation curves have been determined for all studied conditions. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) model was used to study the incomplete bainite transformation kinetics. Avrami exponents varied from 0.15 to 0.67.