Austempered ductile iron (ADI) has found enormous applications in recent years due to its high strength and hardness, coupled with substantial ductility and toughness. The high strength and hardness of ADI have caused many researchers and engineers to doubt the machinability of this material. In the present work a series of test were conducted in order to evaluate the machinability ADI bars that were austempered at various temperatures and times and the machinability is investigated by adopting too] life, tool wear rate, cutting forces, and surface finish produced on a job as general criteria. Machinability tests were carried out according to ISO 3685: 1993 (E) standard "Tool Life Testing with Single Point Turning Tools" on eight different ADI structures, austempered at 250, 300, 350 and 400 degrees C for I and 2 h. The cutting tools used in the tests were coated carbide inserts, ISO SNMG 120408 (K10), clamped on tool holders CSBNR 2525 M12. No cutting fluid was used during the turning operations. Cutting forces, flank wear and surface roughness values were measured throughout the tool life and the machining performance of ADI having different structures were compared. In the machinability tests structures austempered at 300 degrees C for I It and 2 h were observed to produce unexpected results. That is to say, structures having less hardness values seemed to wear the tool faster than the harder structures. In order to investigate the grounds of this case, some more tests on these structures were conducted. (c) 2007 Elsevier Ltd. All rights reserved.