Tensile properties of austempered ductile irons produced in different casting wall sections

Ductile Irons (DIs) with different nominal contents of nickel were produced into different casting wall sections, that is, 25 mm, 5 mm and 3 mm. The alloys were then austenitized at the same conditions, and austempered for three different combinations of temperatures and times: 250°C for 6 hours; 310 for 3 hours; 380°C for 1 hour. The aim of the investigation was to understand the combined effects of chemical composition, section thickness and austempering conditions on the tensile mechanical properties of ausferrite. So strain hardening analysis of tensile flow curves was carried out, since it has been found more reliable than ductility assessment in determining the optimal austempering conditions. Because of different wall sections, round and flat tensile specimens with geometries complying on ASTM E8/E8M-11 were tested. ADIs from 5 and 3 mm wall sections were tested through flat geometry specimens only, whilst 25 mm wall sections were tested through both geometries. From 25 mm wall section investigation it resulted that the tensile plastic behavior of ADIs was sensitive to the specimen geometry in a consistent way. The influence of chemical composition and wall section thickness on optimal austempering conditions was found through comparing tensile behaviors from the same geometry specimens, indicating that a proper analysis and comparison of tensile properties of ADIs has to take into account the geometry specimens.