Publication: Obtención de aleaciones maestras mediante molinda mecánica para la modificación de aceros de baja aleación
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2009-10
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2009-10-06
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Abstract
El Mn ha sido un elemento ampliamente usado en la metalurgia convencional dado su bajo precio y la mejora que supone en las propiedades mecánicas de los sistemas. No obstante, su uso en la pulvimetalurgia, aunque ha sido estudiado de manera extensa desde mediados del siglo XX, no se ha podido implementar de manera plena en la industria, debido principalmente a su alta afinidad por el oxígeno y su elevada presión de vapor. Dichos fenómenos hacen que tanto la fabricación de polvos con Mn como su procesado estén dificultados debido tanto a la oxidación como a la sublimación del Mn. Por este motivo, el porcentaje de Mn en los aceros de baja aleación en la industria está limitado a 0.7%, aunque a escala laboratorio se han obtenido sistemas con contenidos de Mn superiores a 4% en peso. La necesidad de introducir el Mn en los sistemas de aceros de baja aleación ha estado impulsada tanto por factores tecnológicos como económicos que conciernen a los elementos de aleación empleados comúnmente en estos sistemas. Por un lado, el Ni ha sido catalogado como un elemento potencialmente cancerígeno y se pretende eliminar su uso en modo de partículas, y por otro lado el Mo ha experimentado un importante aumento de su precio en los últimos 4 años. En el presente trabajo de tesis doctoral se propone un método alternativo para introducir el Mn en el sistema de aleación evitando los problemas de oxidación, y aprovechando las peculiaridades del fenómeno de sublimación del Mn. Gracias al proceso de aleación mecánica se han conseguido procesar aleaciones maestras con 50% en peso de Mn de tres sistemas base Fe diferentes (Fe, FeCr y FeMo). Además, eligiendo diferentes tiempos de molienda, se han obtenido aleaciones maestras con diferentes propiedades. Tiempos cortos dan partículas de Fe deformadas que embeben las partículas frágiles de Mn, mientras que tiempos largos de molienda, suponen la formación de partículas de solución sólida Fe-Mn muy deformadas plásticamente. En cualquier caso, la concentración de defectos va a favorecer en gran medida los mecanismos de transporte de masa con lo que el proceso de sinterización se activa. Para la caracterización de las aleaciones maestras se han llevado a cabo análisis químicos, estudios de la evolución de la formación de la solución sólida mediante el ajuste de los máximos de difracción, evolución de la morfología de la partícula y mecanismo de molienda mediante microscopía electrónica y además, se ha realizado un estudio del endurecimiento inducido con el proceso de molienda de alta energía empleando la técnica de nano-indentación. Con el fin de validar los sistemas de aleación modificados por la incorporación de dichas aleaciones maestras, se estudió tanto la sinterización como los tratamientos térmicos de endurecimiento y temple bainítico de los sistemas base propuestos con la adición de 2% de sus respectivas aleaciones maestras. Además de la caracterización tanto metalográfica como mecánica de estos sistemas, se estudió la respuesta al desgaste tras los tratamientos térmicos. ______________________________________________
Manganese has been widely used in wrought steels because of its low price and the improvements in the mechanical properties. Nevertheless, in the powder metallurgy industry, although it has been studied since 1950, its implementation has not been complete basically due to its high affinity for oxygen and its high vapour pressure. These phenomena carry some difficulties in the fabrication and the processing of these systems. An excessive oxygen pick-up and Mn losses because of its sublimation are associated with these systems and, hence they have hindered their industrial development and have limited the Mn content to only 0.7 % wt., although at laboratory scale, systems with up to 4% of Mn have been successfully obtained. The need to introduce Mn to low alloy sintered steels has been influenced by both technological and economical reasons which concern the commonly used alloying elements. On one hand, Ni has been categorised as a carcinogenic element when manipulated in the powder form and its used is about to be prohibited. On the other, Mo has experienced a price rise during the last 4 years, increasing the final product price. In the present PhD work is proposed an alternative method to add Mn to the system putting by the oxidation problems and taking the advantage of its sublimation. Thanks to the mechanical alloying process, it was possible to make Mn-Fe (50:50) master alloys of three different Fe-based systems (Fe, FeCr and FeMo). Besides, by choosing different milling times, it was possible to obtain master alloys with different properties. Short milling times produce deformed Fe particles which trap the brittle Mn particles, whereas longer milling times promote the formation of a Fe-Mn solid solution .In any case, the master alloy particles possess a high lattice defects concentration induced by the continuous collision with the grinding media. Such concentration favours the mass transport mechanisms and the nucleation, and hence the sintering process is activated. The characterisation of the master alloys consisted of chemical analyses, diffraction studies of the evolution of the formation of the solid solution, the study of the evolution of particle shape and milling mechanism with scanning electron microscopy and furthermore, a study of the hardening induced to the milled particles that has been performed considering the nano-indentation techniques. In order to validate the master alloys previously obtained, systems containing 2% of the corresponding master alloys were sintered and heat treated. Two different heat treatments were considered, sinter-hardening and austempering. Besides the microstructure and mechanical characterisation of the processed systems, the wear response after the heat treatments was studied.
Manganese has been widely used in wrought steels because of its low price and the improvements in the mechanical properties. Nevertheless, in the powder metallurgy industry, although it has been studied since 1950, its implementation has not been complete basically due to its high affinity for oxygen and its high vapour pressure. These phenomena carry some difficulties in the fabrication and the processing of these systems. An excessive oxygen pick-up and Mn losses because of its sublimation are associated with these systems and, hence they have hindered their industrial development and have limited the Mn content to only 0.7 % wt., although at laboratory scale, systems with up to 4% of Mn have been successfully obtained. The need to introduce Mn to low alloy sintered steels has been influenced by both technological and economical reasons which concern the commonly used alloying elements. On one hand, Ni has been categorised as a carcinogenic element when manipulated in the powder form and its used is about to be prohibited. On the other, Mo has experienced a price rise during the last 4 years, increasing the final product price. In the present PhD work is proposed an alternative method to add Mn to the system putting by the oxidation problems and taking the advantage of its sublimation. Thanks to the mechanical alloying process, it was possible to make Mn-Fe (50:50) master alloys of three different Fe-based systems (Fe, FeCr and FeMo). Besides, by choosing different milling times, it was possible to obtain master alloys with different properties. Short milling times produce deformed Fe particles which trap the brittle Mn particles, whereas longer milling times promote the formation of a Fe-Mn solid solution .In any case, the master alloy particles possess a high lattice defects concentration induced by the continuous collision with the grinding media. Such concentration favours the mass transport mechanisms and the nucleation, and hence the sintering process is activated. The characterisation of the master alloys consisted of chemical analyses, diffraction studies of the evolution of the formation of the solid solution, the study of the evolution of particle shape and milling mechanism with scanning electron microscopy and furthermore, a study of the hardening induced to the milled particles that has been performed considering the nano-indentation techniques. In order to validate the master alloys previously obtained, systems containing 2% of the corresponding master alloys were sintered and heat treated. Two different heat treatments were considered, sinter-hardening and austempering. Besides the microstructure and mechanical characterisation of the processed systems, the wear response after the heat treatments was studied.
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Acero, Aleaciones, Metalurgia, Manganeso