Development of austempered ductile cast iron (ADI) with simultaneous high yield strength and fracture toughness by a novel two-step austempering process

Austempered ductile cast iron (ADI) has emerged as a major engineering material in recent years because of its excellent mechanical properties. These include high strength with good ductility, good wear resistance and fatigue strength. 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 low-manganese nodular cast iron with a 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 h. 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), 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 two-step austempering process. These samples were initially quenched to the following austempering temperatures, i.e. 260 degreesC (500 degreesF), 273 degreesC (525 degreesF) 288 degreesC (550-F), 316 degreesC (600 degreesF), 330 degreesC (625 degreesF), 343 degreesC (650 degreesF), 357 degreesC (675 degreesF) and 371 degreesC (700 degreesF), and while being kept at these temperatures in a salt bath, the temperature of the salt bath was raised by WC (25 degreesF) per hour for 2 h. The effect of this two-step austempering heat treatment on the microstructure and mechanical properties of the material was examined and compared with the samples processed by conventional single step austempering process. Test results show a significant improvement in mechanical properties and fracture toughness of the material as a result of the two-step austempering process. (C) 2001 Elsevier Science B.V. All rights reserved.