A influência do nióbio na resistência à fadiga de liga de ferro fundido nodular austemperado

Abstract

This master's thesis investigates the influence of niobium addition on the mechanical properties and fatigue strength of austempered ductile iron (ADI), with the aim of evaluating its potential as a partial substitute for molybdenum and exploring industrial applications. Three alloys were produced with niobium contents of 0 %, 0.23 %, and 0.29 %, and subjected to austempering heat treatment. The samples were characterized in terms of chemical composition, microstructure, mechanical properties (tensile strength, impact strength, hardness), and rotating bending fatigue strength. The addition of niobium reduced nodularity, and nodule density and promoted the formation of niobium carbide (NbC), confirmed by SEM and EDS analyses. A significant increase in yield strength was observed with 0.23 % Nb, along with a decrease in elongation at 0.29 % Nb. Toughness showed a non-monotonic trend, improving at 0.23 % and declining at 0.29 %, while hardness increased with niobium content, although with greater dispersion in the alloy with the highest concentration. In fatigue tests, the alloy without niobium showed high data dispersion and a low correlation coefficient (R² = 0.3), whereas the niobium containing alloys exhibited greater uniformity and predictability (R² = 0.80 and 0.76). However, the endurance limit was reduced by 0.23 % Nb and was partially recovered with 0.29 % Nb. The results indicate that niobium addition contributes to increased mechanical strength and reduced variability in fatigue behavior for finite life, with 0.23 % Nb being the most promising content for applications requiring higher reliability. The results suggest that niobium, as an alloying element, can enhance the mechanical properties of austempered ductile iron. Nevertheless, the agglomeration of primary NbC carbides tends to impair toughness, ductility, and endurance limit. [resumo fornecido pelo autor]