Nanoscale bainite is a remarkable microstructure that exhibits a very promising combination of high strength with good ductility and toughness. The development of these types of microstructures has been focused on wrought materials, and very little information is available for steel castings. In this work, a specially designed cast steel with 0.76 wt % C was fabricated, and the heat treatment cycles to develop bainitic nanostructures were determined by studying the kinetics of the bainitic transformation using high-resolution dilatometry. The effects of isothermal holding temperature and time on the final microstructure and mechanical properties were thoroughly characterized in order to evaluate a future industrial implementation of the process in an effort to contribute to enhance and widen the potential applications for cast steels.
Intercritical austenitizing is a key step on the production of dual-phase austempered ductile iron. Therefore, understanding the formation of austenite at the intercritical range should provide critical information for the future development of this family of alloys. In this work, a ductile iron alloyed with copper and nickel (3.4 C, 2.6 Si, 0.9 Ni, 0.6 Cu, wt%) was studied. The as-cast alloy was submitted to ferritic annealing and normalizing in order to obtain fully ferritic and fully pearlitic microstructures, respectively. The effect of microsegregation, initial microstructure (ferrite or pearlite) and nodule count on the formation of austenite in the intercritical range under continuous heating was studied using electron probe micro-analyzer-EPMA-high-resolution dilatometry, optical microscopy and scanning electron microscope-SEM-. The results showed that silicon, copper and nickel segregate around the graphite nodules and manganese segregates to the last freezing zones. Also as nodule count increases the segregation level decreased. Regarding the rate of austenite formation, the results showed that it increases as nodule count increases. Additionally, austenite formation is faster when the starting microstructure is pearlitic and it increases as the pearlite interlaminar spacing decreases. Finally, the results showed that the critical temperatures for austenite formation depend mainly on the starting microstructure (ferrite or pearlite).
Carburizing implies the existence of a carbon gradient from the surface to the core of the steel, which in turn will affect both the critical temperature for austenite formation and the kinetics of the bainitic transformation during the austempering treatment. Therefore, for future development of carbo-austempered steels with nanobainitic microstructures in the case, it is key to understand the effect of such carbon gradient has on the final microstructure and the mechanical properties reached by the heat treatments used. This work was divided into two parts, firstly two alloys with similar carbon content to those at the surface and center of the carburized steel were used to establish the optimal heat treatment parameters and to study bainite transformation kinetics by high resolution dilatometry. In a second step, a carburized alloy is produced and subjected to the designed heat treatments, in order to evaluate the microstructure and mechanical properties developed. Results thus obtained are compared with those obtained in the same carburized alloy after following the most common quench and temper treatment.
Este es un artículo publicado en acceso (Open Access) abierto bajo la licencia Creative Commons Attribution NonCommercial, que permite su uso, distribución y reproducción en cualquier medio, sin restricciones siempre que sin fines comerciales y que el trabajo original sea debidamente citado.Recibido: 25 Set., 2014 Aprobado: 16 Mar., 2015 E-mail: sara.aguilar@udea.edu.co (SMAS)Resumen: En este trabajo se estudian los fenómenos metalúrgicos que ocurren en la soldadura SMAW de un acero inoxidable ferrítico AISI 430 con un acero inoxidable austenítico AISI 316L. Para el estudio se utilizaron dos tipos de electrodos: austenítico AWS E309L y dúplex AWS E2209-16, ambos con un diámetro de 3,2 mm. Las uniones soldadas se realizaron con un solo pase y se variaron simultáneamente la corriente y la velocidad de soldadura; las condiciones fueron 49 A y 2,4 mm.s -1 como valores bajos y 107 A y 4,3 mm.s -1 como valores altos. Se evaluó la influencia del tipo de electrodo y de los parámetros de soldadura en la evolución microestructural de las zonas afectadas por el calor y de las zonas de fusión, encontrando diferencias en la morfología y cantidad de ferrita delta para todas las condiciones estudiadas. Se evidenció crecimiento y refinación de grano ferrítico y formación de martensita en la zona afectada por el calor del metal base ferrítico. Se evaluó también la resistencia a la tensión hallando similitudes en todas las soldaduras.Palabras-clave: Soldadura de aceros inoxidables; SMAW; Metalurgia de la soldadura; Acero inoxidable austenítico; Acero inoxidable ferrítico. Microstructural Transformations in Dissimilar Welds between Austenitic Stainless Steel and Ferritic Stainless SteelAbstract: This research studies the metallurgical transformations taking place the SMAW welding of AISI 316L austenitic stainless steel with AISI 430 ferritic stainless steel. To perform the study were used two different electrodes, AWS E309L austenitic and AWS E2209-16 duplex stainless steels 3.2 mm diameter. The joints were made with a single-pass welding including current and welding speed variations; the low values were 49 A and 2,4 mm.s -1 and the high values were 107 A and 4,3 mm.s -1 . This study evaluated the influence of the type of electrode and the welding parameters on the microstructural evolution of heat affected and fusion zones. Also, differences were found on morphology and delta ferrite amount for all weld metals. The heat affected zone of the ferritic side showed grain coarsening and grain refinement with martensite at the grain boundaries. Tensile strength was similar for all welded joints. Key-words:Stainless steel welding; SMAW; Welding metallurgy; Austenitic stainless steel; Ferritic stainless steel. IntroducciónLos aceros inoxidables (AI) son ampliamente usados en aplicaciones a elevadas temperaturas cuando los aceros al carbono y los aceros de baja aleación no presentan una adecuada resistencia a la corrosión. Los aceros inoxidables austeníticos (AIA) tienen mejor resistencia a la corrosión que los aceros inoxidables ferríti...
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