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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.

Austempered ductile iron (ADI) castings provide a unique combination of high strength and toughness coupled with excellent design flexibility for chassis applications. This paper describes the development of the upper control arm for the 1999 Ford Mustang Cobra as an austempered ductile iron casting. The full service development process used is described from initial design through finite element analysis (FEA), design verification, casting production, heat treatment, nondestructive evaluation and machining. To achieve significant weight savings, an austempered ductile iron casting was chosen for this application instead of an as-cast SAE J4341, Grade D4512 ductile iron casting or a steel forging. This is believed to be the first application of an austempered ductile iron casting for a safety critical, automotive chassis application.