Publication: Estudio de la transformación martensítica, optimización de la microestructura y prevención de la rotura frágil en piezas coladas de acero X38CrMoV5-1 de alta resistencia
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2016-02-15
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Universidad Complutense de Madrid
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Abstract
La presente tesis trata sobre el "Estudio de la transformación martensítica, optimización de la microestructura y prevención de la rotura frágil en piezas coladas de acero X38CrMoV5-1 de alta resistencia”. El acero X38CrMoV5-1/AISI H11 es un acero martensítico al cromo-vanadio-molibdeno para trabajo en caliente fácilmente endurecible, que se fabrica generalmente por moldeo con la consiguiente presencia de porosidad interdendrítica. La primera parte de la tesis consiste en una introducción donde se expone la motivación existente en la industria para la realización de este estudio, ya que tratará de encontrar una alternativa a la fabricación de piezas por moldeo para aplicaciones en sistemas con garantía de uso, tanto a nivel teórico, teniendo en cuenta la teoría de la fractura frágil, como práctico a través de la optimización del tratamiento térmico y, en consecuencia, de la obtención de buenas propiedades mecánicas, tanto de resistencia como de ductilidad y tenacidad. La tesis se desarrolla en nueve capítulos. El primero de ellos presenta una descripción de la martensita y de sus características principales, como su estructura cristalográfica, microestructura, cinética, termodinámica, tratamiento térmico y la influencia de los elementos aleantes, propiedades mecánicas y nuevas técnicas de caracterización. También se trata de dar una visión global y general de los aceros microaleados, la relación existente entre su microestructura y propiedades, la influencia del tipo de aleantes, la solubilidad de carburos y nitruros y su precipitación. Finalmente se recoge un resumen amplio de los estudios realizados hasta la actualidad del acero X38CrMoV5-1. El segundo capítulo explica las características y propiedades de este acero, así como su método de fabricación y aplicaciones más habituales. En los capítulos tercero y cuarto, se estudia en profundidad las transformaciones de fase de este acero haciendo uso de las técnicas de dilatometría y análisis térmico diferencial. Se analiza la microestructura (matriz y segundas fases) mediante microscopía óptica y la variación de la dureza en función de las distintas variables (temperatura de austenización, tamaño de grano austenítico, velocidad de enfriamiento, etc.) que intervienen en los tratamiento térmicos...
This present thesis is titled "Martensistic transformation, optimization of the microstructure and prevention of a brittle fracture of high strength X38CrMoV5-1 steel die cast parts". X38CrMoV5-1 steel is a hot worked martensitic chromium-vanadium-molybdenum steel, easily air hardenable, that it is manufactured by die casting, presenting interdendritic porosity due to the shinkage during solidification. The first part of this study consists on an introduction that exposes the industry motivation to investigate this issue, trying to achieve an alternative manufacturing process in order to avoid die casting for applications that a good mechanical behaviour is mandatory. This point will be treated both theoretically applying brittle fracture calculations, but also, in a practical way, optimizing heat treatment and consequently mechanical properties, such as strength, ductility and toughness. This thesis develops in nine chapters. The first one presents a description of the martensite and its main characteristics, such as crystallograhic structure, kinetics, thermodynamics, heat treatment and the influence of alloying elements on it, mechanical properties and new characterization techniques. The next part of this chapter gives a general approach to microalloyed steels, its relationship between microstructure and properties, the influence of alloying elements, carbides and nitrides solubility and precipitation. Finally, an extent summary of X38CrMoV5-1 steel studies carried out are presented. In the second chapter, the characterictics and properties of this steel are showed, as well as its manufacturing process and its more common applications. An extensive study of phase transformations of this steel is presented using dilatometric and diferential thermal analysis in chapters third and fourth. Microstructure (matrix and second phases) are analysed by optical microscopy and hardness test is performed in function of several variables that are involved in a heat treatment process (austenization temperature, austenitic grain size, cooling rate.etc.). In chapter fifth porosity (size and distribution) measurements were carried out, as well as a microstructure characterization by X-ray diffraction and scanning electron microscopy techniques. This chapter shows matrix and second phases micrographs, depending on the manufacturing process: die-casting or direct mechanization from a steel plate parallel or 262 perpendicular to the rolling direction. Calculations of the precipitate volume fraction are presented in the last part of this chapter...
This present thesis is titled "Martensistic transformation, optimization of the microstructure and prevention of a brittle fracture of high strength X38CrMoV5-1 steel die cast parts". X38CrMoV5-1 steel is a hot worked martensitic chromium-vanadium-molybdenum steel, easily air hardenable, that it is manufactured by die casting, presenting interdendritic porosity due to the shinkage during solidification. The first part of this study consists on an introduction that exposes the industry motivation to investigate this issue, trying to achieve an alternative manufacturing process in order to avoid die casting for applications that a good mechanical behaviour is mandatory. This point will be treated both theoretically applying brittle fracture calculations, but also, in a practical way, optimizing heat treatment and consequently mechanical properties, such as strength, ductility and toughness. This thesis develops in nine chapters. The first one presents a description of the martensite and its main characteristics, such as crystallograhic structure, kinetics, thermodynamics, heat treatment and the influence of alloying elements on it, mechanical properties and new characterization techniques. The next part of this chapter gives a general approach to microalloyed steels, its relationship between microstructure and properties, the influence of alloying elements, carbides and nitrides solubility and precipitation. Finally, an extent summary of X38CrMoV5-1 steel studies carried out are presented. In the second chapter, the characterictics and properties of this steel are showed, as well as its manufacturing process and its more common applications. An extensive study of phase transformations of this steel is presented using dilatometric and diferential thermal analysis in chapters third and fourth. Microstructure (matrix and second phases) are analysed by optical microscopy and hardness test is performed in function of several variables that are involved in a heat treatment process (austenization temperature, austenitic grain size, cooling rate.etc.). In chapter fifth porosity (size and distribution) measurements were carried out, as well as a microstructure characterization by X-ray diffraction and scanning electron microscopy techniques. This chapter shows matrix and second phases micrographs, depending on the manufacturing process: die-casting or direct mechanization from a steel plate parallel or 262 perpendicular to the rolling direction. Calculations of the precipitate volume fraction are presented in the last part of this chapter...
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Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de Materiales, leída el 17-11-2015