Improvement in strength and toughness of austempered ductile cast iron by a novel two-step austempering process

Austempered ductile cast iron (ADI) has emerged as an important engineering material in recent years because of its excellent mechanical properties. These include high strength with good ductility, good wear resistance, fatigue strength and fracture toughness. It is therefore considered as an economical substitute for wrought or forged steel in several structural applications especially in the automotive industry. In this investigation, a nodular or ductile cast iron with predominantly pearlitic as-cast structure was processed by a novel two-step austempering process. Two batches of samples were prepared. All the specimens were initially austenitized at 927 degreesC (1700 degreesF) for 2 It. The first batch of samples were processed by conventional single-step austempering process at several temperatures such as 260 degreesC (500 degreesF), 273 degreesC (525 degreesF), 288 degreesC (550 degreesF), 302 degreesC (575 degreesF), 316 degreesC (600 degreesF), 330 degreesC (625 degreesF), 343 degreesC (650 degreesF), 357 degreesC (675 degreesF), 371 degreesC (700 degreesF), 385 degreesC (725 degreesF) and 400 degreesC (750 degreesF) for 2 h, whereas the second batch of samples were processed by the two-step austempering process. These samples were initially quenched for 5 min in a salt bath maintained at 260 degreesC (500 degreesF) and then austempered for 2 It at several austempering temperatures. These temperatures were 288 degreesC (550 degreesF), 302 degreesC (575 degreesF), 316 degreesC (600 degreesF), 330 degreesC (625 degreesF), 343 degreesC (650 degreesF), 357 degreesC (675 degreesF), 371 degreesC (700 degreesF), 385 degreesC (725 degreesF) and 400 degreesC (750 degreesF). Influence of this two-step austempering process on microstructure and mechanical properties of ADI was examined. Test results show that this two-step austempering process has resulted in significant improvement in yield and tensile strengths and fracture toughness of the material over the conventional single-step austempering process. (C) 2003 Elsevier Ltd. All rights reserved.