Efecto del tiempo de recocido en una fundición gris ASTM A48 clase 40 sobre la resistencia a la tracción, tensión de fluencia, ductilidad, resistencia al impacto, dureza y microestructura
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Date
2024
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Universidad Nacional de Trujillo
Abstract
La presente investigación estudió el efecto del tiempo de recocido, considerando: 2, 5, 8, 10, 12, 15 y 18 horas, a una temperatura de 900ºC en la fundición gris ASTM A48 - clase 40 con enfriamiento lento (dentro del horno); sobre la resistencia a la tracción, tensión de fluencia, ductilidad, resistencia al impacto y dureza. Las variables dependientes fueron evaluadas utilizando: un ensayo mecánico de tracción uniaxial, según norma ASTM E-8, para obtener de forma automatizada la resistencia a la tracción, tensión de fluencia y ductilidad; el método Charpy, según norma ASTM E23-98, para obtener la resistencia al impacto; y técnicas de identación, según norma ASTM E-10, para obtener la dureza. Inicialmente, la microestructura del material de estudio está conformada por una matriz de perlita y hojuelas de grafito tipo A que son una estructura frágil; y conforme aumenta el tiempo de recocido hasta las 18 horas, la perlita desaparece y a su vez aumenta la ferrita debido a la transformación de la austenita a 900ºC, obteniendo finalmente una estructura con matriz ferrítica y hojuelas de grafito tipo A, el cual durante todo el proceso del tratamiento térmico es estable. Debido a este fenómeno, la resistencia a la tracción, tensión de fluencia y dureza disminuyen; y la resistencia al impacto y la ductilidad aumentan ya que la estructura final es dúctil y tenaz. A un tiempo de recocido de 12 horas se logró tener una matriz ferrítica al 100% con hojuelas de grafito tipo A, y a tiempos mayores lo único que sucede es un engrosamiento de las hojuelas de grafito producto de la difusión de carbono proveniente de la austenita. Los resultados obtenidos se consideran bajos en comparación con otros materiales similares de fundición que contienen grafito, debido a que las hojuelas actúan como concentradores de tensión y tienen efecto de entallas que no permiten el deslizamiento. Los resultados obtenidos fueron contrastados mediante análisis estadístico con un nivel de confianza del 95%, que demuestra la confiabilidad del estudio realizado.
The present research examined the effect of annealing considering durations 2, 5, 8, 10, 12, 15, and 18 hours, at temperature of 900ºC on ASTM A48 gray cast iron - class 40 with slow cooling (inside the furnace). It focused on tensile strength, yield strength, ductility, impact resistance, and hardness. Dependent variables were assessed using uniaxial tensile mechanical test, following ASTM E-8 standards, to automatically obtain tensile strength, yield strength, and ductility; the Charpy method, following ASTM E23-98 standards, for impact resistance;indentation techniques, as per ASTM E-10 standards, to determine hardness. Initially, the material's microstructure consists of a matrix of pearlite and type A graphite flakes, forming a brittle structure. As the annealing time increases up to 18 hours, pearlite disappears, and simultaneously, ferrite increases due to the transformation of austenite at 900ºC. Eventually, a structure with a ferritic matrix and type A graphite flakes is obtained, which remains stable throughout the thermal treatment process. Due to this phenomenon, tensile strength, yield strength, and hardness decrease, while impact resistance and ductility increase because the final structure becomes ductile and tough. At an annealing time of 12 hours, a 100% ferritic matrix with type A graphite flakes was achieved. Further increases in time only result in thickening of the graphite flakes due to the diffusion of carbon fromaustenite. The obtained results are considered low compared to other similar graphite-containing cast iron materials, as the flakes act as stress concentrators and notch effects that hinder sliding. The results were validated through statistical analysis with a 95% confidence level, demonstrating the reliability of the conducted study.
The present research examined the effect of annealing considering durations 2, 5, 8, 10, 12, 15, and 18 hours, at temperature of 900ºC on ASTM A48 gray cast iron - class 40 with slow cooling (inside the furnace). It focused on tensile strength, yield strength, ductility, impact resistance, and hardness. Dependent variables were assessed using uniaxial tensile mechanical test, following ASTM E-8 standards, to automatically obtain tensile strength, yield strength, and ductility; the Charpy method, following ASTM E23-98 standards, for impact resistance;indentation techniques, as per ASTM E-10 standards, to determine hardness. Initially, the material's microstructure consists of a matrix of pearlite and type A graphite flakes, forming a brittle structure. As the annealing time increases up to 18 hours, pearlite disappears, and simultaneously, ferrite increases due to the transformation of austenite at 900ºC. Eventually, a structure with a ferritic matrix and type A graphite flakes is obtained, which remains stable throughout the thermal treatment process. Due to this phenomenon, tensile strength, yield strength, and hardness decrease, while impact resistance and ductility increase because the final structure becomes ductile and tough. At an annealing time of 12 hours, a 100% ferritic matrix with type A graphite flakes was achieved. Further increases in time only result in thickening of the graphite flakes due to the diffusion of carbon fromaustenite. The obtained results are considered low compared to other similar graphite-containing cast iron materials, as the flakes act as stress concentrators and notch effects that hinder sliding. The results were validated through statistical analysis with a 95% confidence level, demonstrating the reliability of the conducted study.
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Resistencia
, Metalurgia