Publication: Diseño de aleaciones maestras para la sinterización con fase líquida de aceros al Mn-Si
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2012-03
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2012-03-22
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Abstract
Los aceros pulvimetalúrgicos han competido con éxito durante años con las piezas fabricadas por moldeo, debido a la posibilidad de fabricar mediante esta técnica piezas complejas, en grandes series y con un bajo coste. En las últimas décadas, el precio de los elementos de aleación comúnmente utilizados en la pulvimetalúrgia de aceros de baja aleación (cómo el Cu, Ni y Mo) ha experimentado continuas fluctuaciones y, globalmente, un aumento creciente. Se plantea en esta situación el reto de encontrar nuevos sistemas de aleación más competitivos que permitan mantener ó, si es posible, incrementar el nivel de propiedades a un menor coste. El Mn y el Si son elementos de aleación utilizados tradicionalmente en los aceros de colada, y se sabe que su potencial para aumentar las propiedades del acero es superior al de los aleantes que se utilizan comúnmente en pulvimetalúrgia. Sin embargo, la gran afinidad por el oxígeno de estos elementos plantea un reto tecnológico a la hora de incorporarlos en el proceso de conformado pulvimetalúrgico ya que la elevada superficie específica de los polvos de partida los hace aún más reactivos con la atmósfera que los rodea. En este trabajo de tesis se plantea la posibilidad de introducir Mn y Si en aceros sinterizados, a partir del uso de “Aleaciones Maestras”. En el contexto de este trabajo, una aleación maestra es un polvo con un elevado contenido en elementos de aleación, que se diseña para ser mezclado con un polvo base, de manera que la composición final del acero se logra tras la sinterización. Al introducir el Mn y el Si prealeados con otros elementos de menor afinidad por el oxígeno, se reduce su actividad química disminuyendo su sensibilidad a la oxidación. La composición de las aleaciones maestras se obtiene a partir un proceso de diseño basado en cálculos termodinámicos y validado mediante estudios con prototipos. La metodología de diseño propuesta permite obtener aleaciones maestras que formen una fase líquida a las temperaturas comunes de sinterización, y además ofrece la posibilidad de predecir el comportamiento de esta fase líquida durante el proceso. La formación de una fase líquida es interesante cuando se utilizan aleaciones maestras, porque promueve la difusión de los elementos de aleación en el polvo base. Este trabajo de tesis se centra principalmente en estudiar la sinterabilidad de este tipo de aceros, analizando fundamentalmente dos aspectos: las características de las fases líquidas formadas y su efecto en las propiedades y en la variación dimensional, y los procesos de reducción y oxidación que tienen lugar como consecuencia de introducir elementos muy afines por el oxígeno (Mn y Si). Los resultados obtenidos permiten comprender los requisitos necesarios para sinterizar estos aceros, lo cual resulta fundamental de cara a poder obtener un producto robusto y fiable que en un futuro pueda llegar a introducirse en el mercado. --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Sintered steels parts have competed favorably with wrought steel machined parts due to the ability of this technique to produce complex parts in large series at a low cost. In the last decades, the increasing and extremely variable cost of alloying elements traditionally used in powder metallurgy (such as Cu, Ni and Mo) has posed the challenge of finding new and more competitive alloying systems which can maintain and even improve the properties of sintered steels, at a lower cost. Mn and Si have been traditionally used in wrought steels and are well‐known for having a high potential for improving the properties of the steel, even higher than the common alloying elements used in powder metallurgy. However, their use in sintered steels brings up some technological challenges due to the very large specific surface of the initial powders which makes them more reactive in contact with the surrounding atmosphere. This work deals with the possibility of confronting some of these problems through the use master alloys. In the context of this work, a master alloy is a powder with a high concentration of alloying elements, designed to be mixed with a base powder so that the final composition of the steel is achieved during sintering. Since Mn and Si are prealloyed with other elements with lower affinity for oxygen, their sensitivity to oxidation is lowered. Master alloy compositions were obtained with a design process based on thermodynamic calculations, and validated with studies performed on prototypes. The design process developed here not only brings the possibility of ensuring the formation of a liquid phase at the common sintering temperatures, but also allows predicting the behavior of this liquid during the sintering process. The formation of a liquid phase when alloying is performed by the master alloy route is interesting for promoting the diffusion of alloying elements in the base powder. This thesis is mainly focused in studying the sinterability of this type of steels, particularly in two facets: liquid phase behavior and its effect on final properties and dimensional stability, and reduction/oxidation phenomena taking place during sintering as a consequence of the introduction of elements with high oxygen affinity. The results obtained help to understand the requirements for sintering these Mn‐Si steels, which is important in order to obtain robust and reliable products likely to be eventually introduced in the market.
Sintered steels parts have competed favorably with wrought steel machined parts due to the ability of this technique to produce complex parts in large series at a low cost. In the last decades, the increasing and extremely variable cost of alloying elements traditionally used in powder metallurgy (such as Cu, Ni and Mo) has posed the challenge of finding new and more competitive alloying systems which can maintain and even improve the properties of sintered steels, at a lower cost. Mn and Si have been traditionally used in wrought steels and are well‐known for having a high potential for improving the properties of the steel, even higher than the common alloying elements used in powder metallurgy. However, their use in sintered steels brings up some technological challenges due to the very large specific surface of the initial powders which makes them more reactive in contact with the surrounding atmosphere. This work deals with the possibility of confronting some of these problems through the use master alloys. In the context of this work, a master alloy is a powder with a high concentration of alloying elements, designed to be mixed with a base powder so that the final composition of the steel is achieved during sintering. Since Mn and Si are prealloyed with other elements with lower affinity for oxygen, their sensitivity to oxidation is lowered. Master alloy compositions were obtained with a design process based on thermodynamic calculations, and validated with studies performed on prototypes. The design process developed here not only brings the possibility of ensuring the formation of a liquid phase at the common sintering temperatures, but also allows predicting the behavior of this liquid during the sintering process. The formation of a liquid phase when alloying is performed by the master alloy route is interesting for promoting the diffusion of alloying elements in the base powder. This thesis is mainly focused in studying the sinterability of this type of steels, particularly in two facets: liquid phase behavior and its effect on final properties and dimensional stability, and reduction/oxidation phenomena taking place during sintering as a consequence of the introduction of elements with high oxygen affinity. The results obtained help to understand the requirements for sintering these Mn‐Si steels, which is important in order to obtain robust and reliable products likely to be eventually introduced in the market.
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Keywords
Aceros pulvimetalúrgicos, Sinterización en fase líquida