Publication: Contribución de la microestructura en la fractura de aceros sinterizados
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2014-09
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2014-09-15
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Abstract
En el ámbito de los aceros sinterizados, son numerosos los estudios cuyo foco de interés se ha centrado en explicar el papel de la microestructura en el comportamiento mecánico. Este interés nace de la necesidad de obtener aceros con un nivel de propiedades más competitivo, que propicie la expansión de esta familia de aceros frente a procesos de fabricación convencionales.
En este sentido, el análisis de la relación microestructura-propiedades en los aceros pulvimetalúrgicos supone un reto, ya que en general presentan microestructuras muy heterogéneas y con abundante porosidad, lo que dificulta el estudio. Resulta especialmente interesante poder conocer qué microconstituyentes son los que colaboran en soportar la deformación plástica que se opone a la fractura del material, así cómo identificar en qué zonas nuclean las grietas y por donde propagan a través de la microestructura. Este conocimiento ayudará a un mejor diseño microestructural a través del procesado, y la principal consecuencia será una mejora en las prestaciones. La mayoría de los estudios reportados hasta el momento y que tratan de acercarse al problema de esta manera, han expuesto los aceros a ensayos cíclicos (ensayos de fatiga) y a un posterior análisis de la superficie de fractura. Además, se han empleado técnicas de elementos finitos o análisis de la superficie de fractura con SEM, con tal de obtener información sobre la propagación de la grieta y las zonas preferentes de nucleación de las primeras grietas. Sin embargo, no existen estudios del análisis in-situ de la contribución de diferentes aspectos microestructurales en la localización de la deformación plástica o los mecanismos de fractura de este tipo de aceros, y menos aún estudios donde se combinen de forma simultánea los distintos enfoques.
En este trabajo se lleva a cabo una monitorización in-situ del fallo de aceros sinterizados comerciales sometidos a una carga uniaxial con el fin de identificar los aspectos microestructurales más críticos que contribuyen y condicionan la fractura del material. El estudio se realiza a partir de la combinación de dos técnicas que actualmente se utilizan en la caracterización de aceros convencionales: 1) análisis in-situ (dentro del microscopio electrónico de barrido) de muestras sometidas a ensayos de tracción y 2) análisis de deformación plástica local de mediante la técnica de Correlación Digital de Imágenes “Digital Image Correlation” (DIC). El estudio es reforzado con el análisis de la superficie fracturada una vez que se produce el fallo en los materiales.
Para ello, se utilizan cuatro tipos de aceros sinterizados obtenidos a partir de polvos base Fe comerciales que permiten obtener aceros con distinto grado de homogeneidad: 1) Acero al C a partir de polvo de hierro atomizado en agua, 2) Acero al Mo a partir de polvo de hierro prealeado con Mo, 3) Acero al Mo, Ni y Cu a partir de polvo de hierro aleado por difusión con Mo, Ni y Cu y 4) Acero al Mo, Ni y Cu a partir de polvo de hierro prealeado con Mo y aleado por difusión con Ni y Cu. El empleo de las tres técnicas conjuntamente ha permitido esclarecer la contribución de cada microconstituyente en la localización de la deformación y la progresión de grietas en materiales sinterizados utilizados en la industria.
In the technical and scientific literature which concerns about sintered steels, it could be found a wide range of approaches so as to understand the microstructure behavior during tensile test. This interest could be explained given the need to develop PM steels to more competitive materials compared to conventional steels. It turned out to be a big challenge because of sintered steel heterogeneity and the abundance of pores, which difficult it’s analysis. Thus far, most of the technical papers study the progress of the main crack after tensile or impact test, or analysis the fracture surface. Most of them manage to understand the effect of the microstructure with fine element tools such as FEM or analysis of the fracture surface with SEM after tensile test. As far as the contribution of microstructure in mechanical behavior is concerned, there are no significant published works which explain the contribution of the present phases on plastic deformation of the whole system and the role of each phase in the progress of the cracks after a tensile test. Therefore, in this work it is made some research about this issue putting into practice three different techniques: 1) in-situ tensile test (inside the scanning electron microscope), monotorizing the initation and evolution of the main crack through the microstructure, 2) analysis of fracture surface after the tensile test, and 3) look further at individual phases contribution in avoiding the crack propagation, using “Digital Image Correlation” (DIC) technique, which is a software that is able to detect which individual microconstituent is responsible for the plastic deformation of the material under an axial stress. To achieve this aim, four different PM sintered steels with different microstructures are used: 1) Steel with C from atomized iron powder (ASC100.29, por Höganäs AB) 2) Steel with Mo from a prealloyed Fe-Mo-C powder (Astaloy Mo grade, from Höganäs AB), 3) Steel with Mo, Ni and Cu, from a diffusion alloyed Fe-Ni-Cu-Mo-C powder, (Distaloy AE, from Höganäs AB) and 4) Steel with Mo, Ni and Cu, from a prealloyed and diffusion alloyed Fe-Ni-Cu-Mo-C powder, (Distaloy HP1, from Höganäs AB) After putting into practice the three techniques together, better understanding of the contribution of each phase in the mechanical behaviour is achieved. As a result, there is a good knowledge of which alloying technique is suitable for each application depending on the mechanical conditions.
In the technical and scientific literature which concerns about sintered steels, it could be found a wide range of approaches so as to understand the microstructure behavior during tensile test. This interest could be explained given the need to develop PM steels to more competitive materials compared to conventional steels. It turned out to be a big challenge because of sintered steel heterogeneity and the abundance of pores, which difficult it’s analysis. Thus far, most of the technical papers study the progress of the main crack after tensile or impact test, or analysis the fracture surface. Most of them manage to understand the effect of the microstructure with fine element tools such as FEM or analysis of the fracture surface with SEM after tensile test. As far as the contribution of microstructure in mechanical behavior is concerned, there are no significant published works which explain the contribution of the present phases on plastic deformation of the whole system and the role of each phase in the progress of the cracks after a tensile test. Therefore, in this work it is made some research about this issue putting into practice three different techniques: 1) in-situ tensile test (inside the scanning electron microscope), monotorizing the initation and evolution of the main crack through the microstructure, 2) analysis of fracture surface after the tensile test, and 3) look further at individual phases contribution in avoiding the crack propagation, using “Digital Image Correlation” (DIC) technique, which is a software that is able to detect which individual microconstituent is responsible for the plastic deformation of the material under an axial stress. To achieve this aim, four different PM sintered steels with different microstructures are used: 1) Steel with C from atomized iron powder (ASC100.29, por Höganäs AB) 2) Steel with Mo from a prealloyed Fe-Mo-C powder (Astaloy Mo grade, from Höganäs AB), 3) Steel with Mo, Ni and Cu, from a diffusion alloyed Fe-Ni-Cu-Mo-C powder, (Distaloy AE, from Höganäs AB) and 4) Steel with Mo, Ni and Cu, from a prealloyed and diffusion alloyed Fe-Ni-Cu-Mo-C powder, (Distaloy HP1, from Höganäs AB) After putting into practice the three techniques together, better understanding of the contribution of each phase in the mechanical behaviour is achieved. As a result, there is a good knowledge of which alloying technique is suitable for each application depending on the mechanical conditions.
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Keywords
In-situ test, Técnica DIC, Fractura, Microestructura, Ensayo de materiales, Metales