The present work describes studies about the influence of processing variables on the microstructure and properties of dual phase austempered ductile iron (ADI). The upper and lower critical temperatures of conventional ductile iron melt were determined. Heat treatments involving austenitising within the intercritical interval, followed by austempering, allowed microstructures to be obtained composed of different combinations of free ferrite and ausferrite. Mechanical and fracture toughness tests performed on samples with mixed structures showed interesting combinations of strength and toughness, in comparison with fully ferritic and fully ausferritic matrices, particularly when austempering was carried out at 350uC. The results of the critical crack size, expressed by the relationship (K IC /s YS ) 2 , which indicates the relative toughness of the material, showed the best values for ferritic matrices with y20% ausferrite. This effect is attributed to the location of the ausferrite in the last to freeze regions (the weakest areas in the matrix) where it acted as a reinforcing phase.

The present work aims to evaluate and compare the influence that section size has on the microstructure and properties of fully ferritic, fully ausferritic and dual phase ADI matrices. Samples taken from ‘Y’ blocks of 25, 50 and 75 mm thickness were used to perform metallographic studies and mechanical tests. Cooling rate differences arising from changes in section size promote different microsegregation characteristics affecting solid state transformations and, consequently, the final microstructure and properties. When the section size increases, some properties decrease. The strongest deleterious effect was ascribed to the elongation and impact of ADI samples, where drops of nearly 40% were reached when specimens taken from the thinnest ‘Y’ blocks were compared to those taken from the thickest ones. Regarding fracture toughness, ferritic matrices exhibited the most noticeable detrimental effect. Dual phase ADI samples, on the other hand, presented the least deleterious section size effect on all the studied properties.

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