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In this paper, a grey iron was submitted to low temperature isothermal heat treatment, austempering, in order to obtain austempered grey iron (AGI). Tensile strength, Brinell hardness and tribological properties were determined for the as-cast condition and for two austempering conditions. The tribological properties were evaluated by pin-on-disk testing in dry and wet environments. The results show that AGI exhibit better mechanical properties than the as-cast grey iron. Also, the mechanical properties of AGI improve as the isothermal heat treatment temperature decreases. The wear resistance of AGI is better than the wear resistance of as-cast grey iron in both dry and wet conditions. In dry conditions the wear resistance of AGI increases as the isothermal heat treatment temperature decreases. On the other hand, the wear resistance of AGI in wet conditions increases as the isothermal heat treatment temperature increased. The results are explained in terms of the microstructural differences found between the materials under evaluation.

In this work, the formation of austenite in the intercritical interval under continuous heating and isothermal holding in ductile iron with various chemistries was investigated using high-resolution dilatometry and quantitative metallographic analysis. The study was conducted using fully ferritic and fully pearlitic matrices as initial microstructures. Subsequently, a mathematical model based on Avrami's equation, that describes the formation of austenite at the intercritical range, was proposed and adjusted to the experimental data. The results show that austenite formation at the intercritical range is faster and happens at lower temperatures when the initial microstructure is pearlitic. Additionally, the kinetic of austenite formation did not change by adding Cu and it is accelerated by adding Ni to the alloy. Finally, the Avrami's equation allowed to model the austenite formation under continuous heating followed by isothermal holding with a good adjustment to the experimental data, which contributes to the understanding of the kinetic of austenite formation in ductile iron at the intercritical range.

Carbo-austempered steels with a nanobainitic microstructure in the case are established as an exciting alternative in applications demanding high stability and reliability against wear and fatigue. In this work, the effect of heat treatment conditions, i.e. austenitizing and isothermal transformation temperatures, on the wear behavior under rolling/sliding conditions of a carburized cast steel with a nanobainitic structure in the surface was examined. The results show that wear damage is mainly due to contact fatigue, with a small extent of oxidative wear, abrasion and adhesion. The results were compared with the conventional tempered martensite plus retained austenite microstructure obtained by means of carburizing followed by quenching and tempering. The nanobainitic microstructures show a better wear resistance than the quenched and tempered microstructures. Also, the specific wear rate in carbo-austempered steels was lower as the isothermal heat treatment temperature decreased. Results are explained in terms of the microstructural differences, hardening behavior and plasticity of the materials under study.

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.

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.

Se plantea, mediante la utilización de las ecuaciones básicas de transferencia de calor unidimensionales en estado estacionario, un método para la evaluación del radio crítico de aislamiento en un sistema cilíndrico y se presentan los resultados obtenidos en la determinación del radio crítico del horno de cubilote de la planta ecoeficiente de la Universidad de Antioquia.