Estudo do efeito dos parâmetros do patamar isotérmico de austêmpera em um aço bainítico isento de carbonetos
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Dissertação
Data
2024
Autores
Basso, Guilherme Arruda
Orientador
Magnabosco, R.
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Citação
BASSO, Guilherme Arruda. Estudo do efeito dos parâmetros do patamar isotérmico de austêmpera em um aço bainítico isento de carbonetos. 2024. 231 p. Dissertação (Mestrado em Engenharia Mecânica) - Centro Universitário FEI, São Bernardo do Campo, 2024. Disponível em: https://doi.org/10.31414/EM.2024.D.131727.
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Palavras-chave
Bainita,Transformação bainítica,Austenita
Resumo
Os aços bainíticos isentos de carbonetos apresentam microestrutura bainítica refinada isenta de carbonetos precipitados, com austenita retida entre os feixes de ferrita bainítica. São interessantes na substituição de aços martensíticos revenidos ou perlíticos em indústrias como a automotiva, ferroviária e de mineração. Sendo assim, o objetivo deste trabalho foi o de se obter um aço de microestrutura bainítica isenta de carbonetos por meio de tratamentos térmicos de austêmpera variando-se o tempo e a temperatura do patamar isotérmico, avaliando a influência destes parâmetros na transformação bainítica e na microestrutura. Para isso, foram produzidos lingotes e conduzidos tratamentos térmicos de homogeneização, após simulação em software Dictra®, a 1000°C por 10 horas. Em seguida, foram realizados tratamentos térmicos de austêmpera em escala laboratorial, em dilatômetro de têmpera Bähr DIL 805A/D do LTF Poli-USP. Um ciclo térmico inicial de têmpera foi aplicado para determinação das temperaturas de transformações de fases. As temperaturas obtidas foram: Ac1=805°C, Ac3=855°C e Ms=233°C para o início e fim de transformação austenítica e início de transformação martensítica, respectivamente. Em seguida, foram conduzidos tratamentos térmicos de austêmpera com patamares isotérmicos entre 250°C e 300°C mantidos por até 8,5 horas. Além dos resultados de dilatometria, foram obtidas medidas de dureza, imagens por microscopia óptica e eletrônica, e resultados de ferritoscópio e difração de raios-X (estas duas técnicas para avaliar e quantificar a presença de austenita retida). Os tratamentos térmicos com patamares isotérmicos mantidos a 270°C e 300°C por 8,5 horas foram reproduzidos em escala piloto. O que se nota é que na medida em que se aumenta a temperatura e tempo de patamar isotérmico, aumenta a fração volumétrica de bainita e diminui a de martensita formadas, como resultado da maior estabilização da austenita, evidenciada pela redução da temperatura Ms, até que, com 8,5 horas a 270°C, a transformação bainítica se completa e não mais existe martensita formada durante o resfriamento até temperatura ambiente. Com aumento da temperatura de patamar isotérmico há redução tanto no tempo de incubação da bainita (de 2,4 horas a 250°C para 0,8 hora a 300°C) quanto no tempo para fim da transformação (de 8,1 horas a 280°C para 7,2 horas a 300°C) e aumento na taxa de transformação. Nas curvas de dilatometria, o perfil sigmoidal característico de transformações de fases difusionais foi observado. A microestrutura obtida em escala laboratorial foi reproduzida com sucesso em escala piloto. No entanto, nas amostras de escala piloto foram obtidas, no geral, quantidades de austenita inferiores às das amostras de dilatometria. Por ferritoscópio, as frações volumétricas obtidas de austenita foram maiores que por DRX, mas por ambas as técnicas as maiores temperaturas de patamares isotérmicos (280°C e 300°C) resultaram em maiores quantidades de austenita retida, evidenciando maior estabilização desta fase, o que se reflete em durezas menores e, pelo diagrama TTT simplificado obtido, possivelmente são as temperaturas que resultam em maior cinética de transformação bainítica para a liga. Os resultados, portanto, apontam para um mecanismo de nucleação e crescimento da bainita fortemente dependente de difusão
Carbide-free bainitic steels have a carbide-free refined bainitic microstructure, with retained austenite between the bainitic ferrite laths. Their use is interesting in replacing tempered martensitic or perlitic steels in the automotive, railway and mining industries. Therefore, the goal of this work was to obtain steel with bainitic and free from carbides microstructure through the use of austempering heat treatments varying in the isothermal shelf both time and temperature, evaluating the influence of these parameters in bainitic transformation and microstructure. For that, ingots were produced, and homogenization heat treatments were conducted at 1000°C held for 10 hours, as defined after simulation on Dictra® software. Subsequently, austempering heat treatments were performed on a laboratory scale using a Bähr DIL 805A/D quenching dilatometer from LTF Poli-USP. At first, a quenching heat cycle was applied to determine the transformation temperatures. The obtained temperatures were Ac1=805°C, Ac3=855°C and Ms=233°C for the austenitic transformation start and end and martensitic transformation start, respectively. Afterwards, austempering heat treatments were conducted with an isothermal shelf between 250°C and 300°C held up to 8.5 hours. Besides the dilatometry results, hardness measures, optical and electron microscope images, ferritscope and X-ray diffraction results were obtained (the last two techniques for evaluating and quantifying the presence of retained austenite). The heat treatments with isothermal thresholds held at 270°C and 300°C for 8.5 hours were reproduced on a pilot scale. It can be noted that as the temperature and time of the isothermal threshold are raised, bainite content is increased. Martensite volumetric fractions are decreased due to the higher stabilization of austenite, evidenced by the decrease of the Ms temperature, until, with 8.5 hours at 270°C, the bainitic transformation is completed, and there is no more martensite formed during the cooling down to ambient temperature. With the increase of the isothermal threshold temperature, there is a decrease in both bainitic incubation time (from 2.4 hours at 250°C to 0.8 hour at 300°C) and transformation end time (from 8.1 hours at 280°C to 7.2 hours at 300°C) and increase in transformation rate. The typical sigmoidal shape from diffusional phase transformations was observed in dilatometric curves. The microstructure obtained on a laboratory scale was successfully reproduced on a pilot scale. However, in the pilot scale, samples generally obtained lower amounts of retained austenite when compared to the dilatometry samples. By ferritscope, obtained austenite volumetric fractions were higher than the ones by XRD. Still, by both techniques, the highest isothermal threshold temperatures (280°C and 300°C) resulted in higher amounts of retained austenite, emphasizing higher stabilization of this phase, which is reflected in lower hardness and, by the simplified TTT obtained, possibly these are the temperatures which result in highest bainitic transformation kinetics for the alloy. The results, therefore, point to a strongly diffusion-dependent bainite nucleation and growth mechanism
Carbide-free bainitic steels have a carbide-free refined bainitic microstructure, with retained austenite between the bainitic ferrite laths. Their use is interesting in replacing tempered martensitic or perlitic steels in the automotive, railway and mining industries. Therefore, the goal of this work was to obtain steel with bainitic and free from carbides microstructure through the use of austempering heat treatments varying in the isothermal shelf both time and temperature, evaluating the influence of these parameters in bainitic transformation and microstructure. For that, ingots were produced, and homogenization heat treatments were conducted at 1000°C held for 10 hours, as defined after simulation on Dictra® software. Subsequently, austempering heat treatments were performed on a laboratory scale using a Bähr DIL 805A/D quenching dilatometer from LTF Poli-USP. At first, a quenching heat cycle was applied to determine the transformation temperatures. The obtained temperatures were Ac1=805°C, Ac3=855°C and Ms=233°C for the austenitic transformation start and end and martensitic transformation start, respectively. Afterwards, austempering heat treatments were conducted with an isothermal shelf between 250°C and 300°C held up to 8.5 hours. Besides the dilatometry results, hardness measures, optical and electron microscope images, ferritscope and X-ray diffraction results were obtained (the last two techniques for evaluating and quantifying the presence of retained austenite). The heat treatments with isothermal thresholds held at 270°C and 300°C for 8.5 hours were reproduced on a pilot scale. It can be noted that as the temperature and time of the isothermal threshold are raised, bainite content is increased. Martensite volumetric fractions are decreased due to the higher stabilization of austenite, evidenced by the decrease of the Ms temperature, until, with 8.5 hours at 270°C, the bainitic transformation is completed, and there is no more martensite formed during the cooling down to ambient temperature. With the increase of the isothermal threshold temperature, there is a decrease in both bainitic incubation time (from 2.4 hours at 250°C to 0.8 hour at 300°C) and transformation end time (from 8.1 hours at 280°C to 7.2 hours at 300°C) and increase in transformation rate. The typical sigmoidal shape from diffusional phase transformations was observed in dilatometric curves. The microstructure obtained on a laboratory scale was successfully reproduced on a pilot scale. However, in the pilot scale, samples generally obtained lower amounts of retained austenite when compared to the dilatometry samples. By ferritscope, obtained austenite volumetric fractions were higher than the ones by XRD. Still, by both techniques, the highest isothermal threshold temperatures (280°C and 300°C) resulted in higher amounts of retained austenite, emphasizing higher stabilization of this phase, which is reflected in lower hardness and, by the simplified TTT obtained, possibly these are the temperatures which result in highest bainitic transformation kinetics for the alloy. The results, therefore, point to a strongly diffusion-dependent bainite nucleation and growth mechanism