O ADI constitui-se no desenvolvimento mais recente na família dos ferros fundidos nodulares. Com o tratamento de austêmpera, consegue-se produzir uma microestrutura única, constituída de ferrita acicular e austenita estável rica em carbono, a ausferrita, proporcionando alta resistência mecânica aliada à ductilidade e tenacidade, além de boa resistência à fadiga e ao desgaste. Neste trabalho estudou-se o efeito dos elementos de liga Cu, Ni e Mo nas propriedades mecânicas e austemperabilidade do ADI. Foram produzidas barras cilíndricas de ferros fundidos nodulares nos diâmetros de Ø2, Ø3 e Ø4 ligadas com Cu, Cu-Ni, Cu-Ni- Mo respectivamente. Os corpos de prova utilizados nos ensaios de tração, impacto e microdesgaste foram retirados a meio-raio das barras. Posteriormente, cada conjunto de corpo de prova foi austenitizado a 890°C durante 2 horas, sendo em seguida submetido a uma condição de tratamento de austêmpera específica, combinando temperaturas de austêmpera de 300 e 360°C e tempos de 1, 2, 3 e 4 horas. Realizou-se análises microestruturais ópticas e por microscopia eletrônica de varredura para correlacionar propriedades mecânicas com a microestrutura. Na análise de austemperabilidade, as barras cilíndricas foram austemperadas para verificar a variação de microestrutura em função do raio da barra. Verificou-se que a temperatura de austêmpera exerce forte influência na microestrutura do ADI e consequentemente nas propriedades de tração, ductilidade, tenacidade e resistência ao desgaste. Nos tratamentos a 300°C obteve-se uma microestrutura mais refinada, com maior quantidade de ferrita acicular, responsável pela maior resistência mecânica e resistência ao desgaste; enquanto que no tratamento realizado a 360°C obteve-se uma microestrutura mais grosseira, com maior quantidade de austenita estável, responsável pela melhor ductilidade e tenacidade. Nos tempos de austêmpera analisados, não ocorreram variações significativas nas microestruturas e propriedades mecânicas. A resistência mecânica e a tenacidade decresceram com a adição de Mo, provavelmente devido à segregação deste elemento, mas a resistência ao desgaste a seco e a austemperabilidade foram mais efetivas em comparação com ligas contendo Cu e Cu-Ni. No ensaio de desgaste usando-se lubrificante, o ADI ligado com Cu-Ni austemperado a 360°C apresentou uma resistência ao desgaste um pouco inferior a de um aço 17CrNiMo6 cementado. / The ADI is the most recent development in the nodular iron family. With the austempering treatment, a unique microstructure, consisting of acicular bainite ferrite and stable austenite rich in carbon, named ausferrite is produced. This microstructure provides high mechanical strength combined with ductility, toughness and good fatigue and wear resistances. In this work, the effect of alloying elements Cu, Ni and Mo on the mechanical properties and austemperability of the ADI were studied. To conduct these studies, cylindrical bars with diameters of Ø2\", Ø3\" and Ø4\" were cast with Cu, Cu-Ni and Cu-Ni-Mo alloying, respectively. The samples were then manufactured by removing specimens from the bar midradius position. Subsequently, each set of the samples was austenitized at 890°C for 2 hours, and then subjected different conditions of austempering treatment. These treatments were developed by combining austempering temperatures of 300 and 360°C with austempering times of 1, 2, 3 and 4 hours. Microstructural analysis was carried out using optical and scanning electron microscopy to correlate mechanical properties with the microstructure. In the austemperability analysis, cylindrical bars were austempered (at 360°C for 3 hours) to correlate microstructure and radius. The experimental results showed that the austempering temperature exerts strong influence on the ADI´s microstructure and consequently on the mechanical properties. The austempering temperature of 300°C produced the best results, a refined microstructure, with a greater amount of acicular ferrite, responsible for greater strength and wear resistance. The austempering temperature of 360°C produced a coarse microstructure, with larger amount of austenite and responsible for better ductility and toughness. The austempering times used in this work did not produce significant variations in the properties and microstructures. Regarding alloying elements, the mechanical strength and toughness decreased with the addition of Mo, probably due to the segregation of this element, but the wear resistance and austemperability were more effective when compared with the alloys containing Cu and Cu-Ni. In the lubricated wear test, the ADI alloyed with Cu-Ni and austempered at 360°C presented a wear resistance only a fraction lower than a cemented 17CrNiMo6 steel.

The aim of this dissertation work is the evaluation of the influence of nickel alloying on the structure and mechanical properties, both monotonic and dynamic, of nodular cast iron with ferritic and bainitic matrix. Two chock melts with 0.5 and 2.7 % Ni were used to study the nickel influence. The quantitative evaluation of structure of these melts using image analysis was done and basic tensile mechanical properties were determined. Subsequently, the time optimization of two-stage ferritic annealing and isothermal austempered heat treatment at 375 °C was performed with the aim to obtain optimal ferritic and bainitic structures with best static and dynamic mechanical properties. After ferritic annealing the nickel alloying contributes to substitution hardening of ferritic matrix which positively affects its strength and other mechanical properties. The higher nickel content in the bainitic structure causes the shift of phase transformation times to longer times which results in restricted production of small carbides and in bigger volume of retained austenite. These features were confirmed by observation in transmission electron microscope. Precise tensile and low cycle fatigue tests at temperatures 23 and – 45 °C were performed on the optimized structures of both nodular cast irons. As a result of the notch effect of graphite nodules, microplastic deformation of both nodular cast irons was observed at stresses which were lower than the yield stress. The Hollomon's equation very well describes the individual parts of tensile curves for both nodular cast irons including their mutual comparison. From the low cycle fatigue tests, the cyclic hardening/softening curves, the evolution of elastic modulus and hysteresis loop shape parameters, cyclic stress-strain curves and fatigue life curves were obtained for both temperatures and materials. Moreover, the decrease of retained austenite volume was measured by neutron diffraction and the evolution of surface relief was characterized during cyclic straining for both austempered nodular cast irons at both temperatures. On the basis of these results both cyclic plasticity and fatigue degradation mechanisms in relation to the cyclic strain localization were described for both nodular cast irons.