Improving the Properties of Austempered Ductile Iron

The influence of austenite content on the properties of austempered ductile iron (ADI) was investigated. The “reacted” austenite content in the ADI structure was controlled by varying austenitizing temperature and, thus, the carbon content of the austenite during austenitizing. Phase transformation studies revealed how to vary the carbon content during austenitizing. The investigation included “step-austenitizing”, where the material was first heated above the critical temperature and subsequently cooled and held below the upper critical temperature. Intercritical heat treatments were also performed to produce a mixed ferrite + ausferrite microstructure containing nominally 50% proeutectoid ferrite + 50% ausferrite. Cast plates containing 3.4C-2.7Si-0.3Mn-1.1Ni-0.16Mo were austempered at 725F (385C). The evaluated properties included tensile properties, machinability (in drilling) and thermal expansion coefficient. As austenite content was reduced, tensile strength generally increased, machinability improved significantly, and thermal expansion rate also decreased.