Influence of austenitising and austempering temperatures on microstructure and properties of dual phase ADI

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 350 degrees C. The results of the critical crack size, expressed by the relationship (K-IC/sigma(YS))(2), which indicates the relative toughness of the material, showed the best values for ferritic matrices with similar to 20% 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.