Austempered ductile iron (ADI) refers to nodular cast iron, subjected to a special heat treatment process called austempering, to achieve an optimal combination of strength, ductility and toughness. It represents a strong competitor to steel and aluminium alloys due to its lower density (7,2 g/cm3 to 7,85-7,87 g/cm3 for steel), weight saving potential and lower production costs.
One of the main problems in the production of ADI is the heat treatment process, especially in thick-walled castings. The inner parts cool down much slower than outer parts, which can result in a heterogenous microstructure, formed before reaching the isothermal temperature, consisting of parts of pearlite and ferrite fixed in austenite. To overcome this problem, nickel is added, which is one of the elements that is capable to postpone the transformation times of ferrite and/or pearlite, thus giving the inner parts of a casting sufficient time to cool down without the formation of unwanted pearlite.
In order to investigate the influence of nickel on microstructure and mechanical properties of ADI, we made three melts with different nickel contents. Austempering was done at different temperatures using a quenching dilatometer and salt baths in order to determine the transformation points, from which we plotted the TTT diagram. Microstructure was also analysed and mechanical tests were made, followed by thermal analysis.