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°C (1700°F) for 2 h. The first batch of samples were processed by conventional single step austempering process at several temperatures such as 260°C (500°F), 273°C (525°F), 288°C (550°F), 316°C (600°F), 330°C (625°F), 343°C (650°F), 357°C (675°F), 371°C (700°F), 385°C (725°F) and 400°C (750°F) 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°C (500°F), 273°C (525°F) 288°C (550°F), 316°C (600°F), 330°C (625°F), 343°C (650°F), 357°C (675°F) and 371°C (700°F), and while being kept at these temperatures in a salt bath, the temperature of the salt bath was raised by 14°C (25°F) 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.

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

http://ftp.esrf.eu/pub/UserReports/22854_B.pdf

High Resolution X-ray Tomography of Short Fatigue Crack Nucleation in Austempered Ductile Cast Iron

Many of the properties of austempered ductile iron depend on the austenite which is retained following the bainite reaction. A neural network model within a Bayesian framework has been created using published data to model the retained austenite content. The model allows the quantity of retained austenite to be estimated as a function of the chemical composition and heat treatment parameters. The computer programs associated with the work have been made freely available ( http //www.msm.cam.ac.uk/map/mapmain.html )

Estimation of the amount of retained austenite in austempered ductile irons using neural networks

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

The primary goal of this investigation was to create austempered ductile cast iron (ADI) with a fully ferritic microstructure without compromising its mechanical properties. This was achieved by applying a novel heat treatment process. This process consists of austempering and subsequent isothermal tempering below the inter critical ( A 1 ) temperature of ductile cast iron. Ductile cast iron samples were initially austenitized at 927 °C (1700 °F) for 2 h and then austempered at three different temperatures 260 °C (500 °F), 316 °C (600 °F) and 385 °C (725 °F) and finally isothermally tempered at 484 °C (900 °F) for 2 h. This resulted in a fully ferritic microstructure. The effect of this tempering on the physical and mechanical properties of the material was examined and compared with conventionally processed ADI. Tests results show that when ADI is austempered at 260 °C (500 °F) and subsequently tempered at 484 °C (900 °F) it has significantly better mechanical properties than the samples initially austempered at other two temperatures (i.e. 316 °C (600 °F) and 385 °C (725 °F)).

Development of austenite free ADI (austempered ductile cast iron)

Rotating bending fatigue tests were carried out on austempered ductile iron containing 1.5 wt% nickel and 0.3 wt% molybdenum. The ductile iron was austenitized at 900 or 1050 °C and then austempered at 280 or 400 °C for different lengths of time to obtain different microstructures. The fatigue strength was correlated with the amount of retained austenite and its carbon content, which were both determined by X-ray diffraction technique. While the tensile strength decreased with increasing retained austenite content, the fatigue strength was found to increase. Carbide precipitation was found to be detrimental to fatigue strength. Lower austenitizing temperature resulted in better fatigue strength.

N. Venkataraman

Papers

Effect of microstructure on the fatigue strength of an austempered ductile iron