Multiscale Microstructural Analysis of Austempered Ductile Iron Castings

Austempered ductile iron (ADI) is a special class of ductile cast iron that offer a good combination of high tensile and fatigue strength, high toughness and wear resistance. In addition, ADI has good damping characteristics, low cost and low density, the latter of which provides light weighting. Those promising properties of ADI ensures reliable service under heavy conditions of shock, impact and wear; and therefore ADI has been used in many applications including gears, drive wheels, rollers, sliders, suspension parts in automotive, defense, heavy-duty vehicle industries [1]. The outstanding properties of ADI is a result of a special microstructure, which is produced by a specific austempering process. This heat treatment process is composed of 2 steps involving austenitization in the range of 850 – 950 o C followed by isothermal holding in the range of 250 o C-450 o C, which normally should be above the M s and below to B s temperatures [2]. After this treatment the resulting microstructure is composed of spheroidal graphite particles on an ausferrite matrix, which is a mixture of acicular bainitic-ferrite and carbon enriched austenite. In some cases martensite can also be present. The present study compares 6 different ADI samples that exhibit up to 1057 MPa ultimate tensile strength and up to 4.97% elongation. A multiscale microstructure analysis is required to fully understand the microstructure of ADI and then correlate microstructural parameters to mechanical properties. The size, shape, fraction and also the nodularity of graphite particles as well as the matrix microstructure influences the mechanical properties [3]. A simple optical microscope image of as-polished ADI (Figure 1a) is usually enough to characterize the graphite particles,