AZTERLAN develops an innovative casting process that avoids the austempering heat treatment process in ADI castings
The new system fosters energy savings up to 6% as well as a significant lead-time improvement of production, acting over the cooling process and taking advantage of the energy from spheroidal cast components after solidification.
Austempered Ductile Iron (ADI) is a spheroidal foundry material that offers similar characteristics to those of forged or cast hardened steels: high strength, high mechanical resistance, wear resistance, fatigue and corrosion properties. It is a significantly cheaper material than its competitors being a 20% more economical than steel and 50% cheaper than alumininum. It also allows to obtain complex geometries without the addition of welding processes and/or sub-assemblies associated to other materials.
ADI materials have a wide range of application sectors such as construction, mining or agriculture where the wear and corrosion resistance properties play a significant role. In addition, their excellent resistance/ductility relation makes them appropriate for the transport sector, from railway to industrial vehicles, where it is used in elements with high requirements such as suspension arms, steering knuckles or crankshafts, among others.
In the words of Urko de la Torre, AZTERLAN researcher specialized in iron foundry materials and technologies, “the microstructure of ADI materials is called ausferrite. This is a metallic matrix constituted by two crystallographic phases: austenite and acicular ferrite. The presence of these two phases is responsible of the good relation between resistance and ductility”.
In a conventional manufacturing process, cast components undergo an ulterior austempering heat treatment. Once castings reach room temperature they are heated up to 900°C until a complete standardized austenitic phase is achieved. Afterwards, components are cooled by means of an isothermal tempering (which occurs in temperature ranges between 250-400°C) to avoid the apparition of pearlite phases and foster the ausferritic transformation that will increase the elongation capacity of the material. The aim for the castings is to be hard but not fragile.
Conventional austempering heat treatment to manufacture ADI materials
To achieve an energetically efficient ausferritic matrix, AZTERLAN Metallurgy Research Centre has developed an innovative manufacturing process that avoids this conventional heat treatment. By means of this innovative process when components reach room temperature after solidification, the ausferritic microstructure that provides the desired characteristics is already formed. Said in other words, mechanical properties are achieved in “as cast” conditions.
“The system that we have developed is based on an early shakeout of the castings from the mould, followed by a controlled cooling to a temperature range where ausferrite is formed. From this moment onwards, with the aim of promoting the growth of this new phase, we keep the temperature constant by introducing the components in a material with low thermal conductivity”. Once ausferrite phase is formed, cast components are cooled down to room temperature.
Thermal cooling control to obtain an “as cast” ADI structure and the corresponding mechanical properties avoiding additional heat treatments
Considering that, in a conventional process, components must spend more than 2 hours inside an austempering furnace, followed by 2 hours of isothermal transformation process, the energy savings obtained by means of the new system are obvious. That way, savings up to 6% of the energy cost can be achieved; that means 115KWh per tonne of ADI components served to any customer.
Equally, as ADI heat treatment processes are not usually integrated by the iron foundries, eliminating that step avoids as well unnecessary logistics that increase the carbon print and the final production cost of the ADI components.
Therefore, manufacturing lead time get significantly shorter, with a positive impact in the production plans.
To develop this new system, the Technology Centre has performed thermodynamic studies and has defined a theoretical model that determines the optimal chemical composition and process conditions to achieve the desired or expected final properties. The new methodology has been validated at a laboratory scale in AZTERLAN Pilot Plant facilities. Finally an advanced characterization of these materials has been also performed. The components have been tested to assure that they meet the mechanical requirements.
The results obtained have demonstrated that the materials that have been manufactured by this new process comply with the specifications of ADI materials.
Currently, AZTERLAN works on the development of industrial demonstrators in close collaboration with leading foundry companies.