Damage evolution during tensile test of austempered ductile iron partially austenized

Abstract

The goal of this work is to analyze the damage evolution at microstructural scale of an austempered ductile iron partially austenized during tensile loading. The analysis is carried out by means of step-by-step tensile tests and the specimen's surface observation with optical and scanning electron microscopy.The results show that cracks initiate and propagate firstly at the matrix-nodule interface. Then, as the strain is increased, the initiation and propagation in the metallic matrix take places preferentially inside the ausferritic areas and along the ausferritic-ferritic interface. Despite the internodular zones tend to localize the plastic deformation, particularly in the ferritic zones, the ausferrite-ferrite interfaces, and the ausferrite itself acts as preferentially sites of cracks propagation. The final fracture is produced by the coalescence of multiple cracks into a single dominant crack leading to the material final fracture.

Influence of Free Ferrite on the Mechanical Properties of High Strength Intercritical Austempered Ductile Iron

“…It is worthy to note that these heat treatment cycles were performed on samples that were initially ferritized following an annealing heat treatment cycle consisting of an austenitizing stage at 920°C for 60 minutes followed by a slow cooling down to room temperature inside the furnace. This microstructural condition is commonly used to standardize the starting microstructure of the samples to be heat treated to determine the ITI [4][5][8][9][10].…”

“…he continuous need for the development of ductile irons (DI) of improved mechanical properties has led the researchers to investigate the usefulness of the development of complex microstructures. The applicability of metallic matrices formed by free ferrite and ausferrite obtained through the austempering of partially austenitized ductile irons, a material usually called 'IADI' (Intercritically Austempered Ductile Iron), has received some attention [1][2][3][4][5][6][7][8][9][10][11][12][13]. Specific thermal cycles have been developed in order to obtain IADI microstructures.…”

“…The amounts of ferrite and ausferrite are controlled by the austenitizing time: the longer the time, the greater the amount of austenite and, therefore, the larger the amount of ausferrite in the final microstructure. Basso et al [4,5], Kilicli et al [6,7] and Fernandino et al [8][9][10] obtained IADI applying a different methodology. It consists in subjecting a fully ferritic DI to an incomplete austenitization stage, at different temperatures within the ITI, followed by an austempering step in order to transform austenite into ausferrite.…”

Fatigue life assessment of DCI smooth specimens

“…Ductile cast iron (DCI) is a metal matrix composite, made of spherical graphite nodules embedded in a ferrous matrix 1,2 …”