Austempering: heat treatment of ductile iron
Austempering: the key strength of Zanardi Fonderie for high-performance, light and sustainable ductile iron castings
Thanks to our austempering furnaces, we also perform heat treatments for third parties
The Austempering heat treatment, which allows the production of cast parts in austempered ductile iron (ADI), has been our key strength for 30 years. Staff at Zanardi Fonderie performs heat treatments on cast parts in spheroidal graphite iron in a dedicated internal division equipped with hi-tech furnaces for the manufacture of austempered ductile irons ADI, ideal for the manufacture of lightweight, high-performing mechanical parts for use in on-road and off-road vehicles, industrial gear reducers, earth moving machinery and agricultural machinery. Thanks to the austempering heat treatment, it is possible to give the ductile iron mechanical properties comparable or superior to those of quenched and tempered steel, maintaining the typical lightness of ductile iron and its natural capacity to adapt to the manufacture of complex moulds.
The Zanardi Austempering brand for outsourced heat treatment
Through the Zanardi Austempering brand, we also offer our heat treatment services to European foundries specialised in the production of grey cast iron, spheroidal graphite iron and construction steel. We assist foundries in defining the most correct chemical composition of the base cast iron, in order that the heat treatment leads to the obtainment of the desired performance levels, offering active consultancy thanks to our expertise and wealth of experience acquired over the years.
The Austempering plant in the Zanardi Fonderie facility
Our austempering heat treatment plant is currently composed of 3 AFC Holcroft UBQA 36” x 72” x 36” furnaces, representing the height of current ductile iron and steel austempering technology. This system allows excellent results in terms of quality of the material, repeatability of the process, range of hardness and safety of the work environment. We can treat castings up to a maximum of 1,000 kg, from very small batches to automotive volumes.
How the austempering heat treatment occurs
Thanks to Zanardi Fonderie’s controlled process and technologically advanced plant, we perform austempering heat treatments in an environment completely isolated from the outside. After loading the treatment baskets and a pre-heating stage, they are inserted into one of our AFC Holcroft UBQA 36″ x 72″ x 36” furnaces. For each austempering heat treatment, our staff defines the times, treatment temperature, number of pieces per basket and their positioning based on the thickness of the casting, chemical analysis and desired ADI grade. Thanks to this procedure, it is possible to achieve and guarantee a constant, high-quality result.
Stages of the austempering heat treatment
The austempering heat treatment is broken down into two macro stages:
Austenization in a controlled endogas environment, during which the casting is brought to around 800-900 °C.
“Soft” isothermal hardening in a liquid salt bath (composed of water, nitrates and nitrites) between 250 °C and 400 °C.
At the end of these two stages, which are strictly controlled, the austempered ductile iron ADI is obtained, a material with an ausferritic matrix. Ausferrite is a structure made up of graphite nodules immersed in a matrix formed by fine-grain acicular ferrite free of carbides and austenite.
How the austempered ductile iron is obtained
Austempered ductile iron is essentially a spheroidal graphite iron enriched with certain alloying elements, mainly copper, nickel and molybdenum. The alloying elements are essential in transforming the matrix and hardenability of the component, which as the thickness increases – in the absence of alloying elements – becomes more and more difficult to harden “to the core”. The temperatures and cycle times are therefore adapted and regulated based on the desired ADI grade, the thickness of the part being treated and alloying element content.
The result and product of the austempering heat treatment
The austempering process applied to a spheroidal graphite iron casting suitably enriched by alloying elements (copper, nickel and molybdenum), therefore leads to the development of a casting in austempered ductile iron, otherwise known as ADI. ADI – which in its various grades maintains the lightness typical of ductile iron and lends to the easy moulding of complex design shapes – is a material with superior mechanical properties with respect to those of classic spheroidal graphite iron, and comparable or superior to those of quenched and tempered steel.