What is austempered ductile iron (ADI) and how is it obtained?

Austempered ductile irons are ferrous materials in the form of foundry casting, characterised by the fact that they have an ausferritic ferrous matrix. They are obtained by heat treatment consisting essentially of an austenitization stage, followed by an isothermal quenching stage at a temperature higher than “Martensite start” (“Ms”).

Microstructural characteristics of austempered ductile iron (ADI)

The microstructure of austempered ductile irons consists of a ferrous matrix in which graphite nodules are distributed. Another important difference compared to a steel is the high content of silicon, necessary to give the carbon the diffusivity to prevent, in the solidification phase, the formation of undesired structures such as carbides. The two characteristics, nodules and silicon, determine the difference in mechanical behaviour compared to a steel structure, without diffused discontinuities and characterised by typically lower silicon content. Carbon and silicon act favourably by significantly lowering the melting temperature, allowing the obtaining of sound castings in easier way than a corresponding steel casting.

ADI1050-6 – Ausferritic matrix (x500).

Mechanical characteristics of austempered ductile iron (ADI)

The mechanical characteristics of ductile iron depend on a number of factors. In the quasi-static tensile test (ISO 6892-1), the maximum resistance Rm and the proof stress Rp0.2 are essentially due to the isothermal quenching temperature, while the elongation at fracture A5 depends, in addition to this temperature, also on the other parameters of the heat treatment (austenitization temperature, austenitization time, dwell time at isothermal temperature), as well as the chemical composition, nodularity and, more generally, the quality of the processes for obtaining castings and the heat treatment. Impact strength (ISO 148.1) at room temperature and low temperatures is even more sensitive to these factors.

Table 1 – Static properties

Table 2 – Un-notched impact strength sample

The advantages of ADI cast irons compared to traditional steels and cast irons

Austempered ductile irons (ADI) have numerous advantages when compared to conventional steels and cast irons. We summarise the main ones:

  • Shape: The more complex the shape, the greater the advantage of using a foundry casting instead of a forged and/or welded one. All cast irons, in fact, have a better fluidity (this property is also known as “castability”), compared to cast steels, thus making it possible to obtain more complex geometric shapes even when compared to forged components, for which the use of special machinery (forge) is required to obtain the final shape of the casting, with further limits to design freedom;
  • Number per production batch: the lower the number, even with simple castings, the greater the advantage of using a foundry casting and/or a welded structure compared to a forged one;
  • Thickness: the mechanical characteristics of the ADI material are better the lower the thickness of the casting is (as is generally the case for all casting materials). Beyond a certain thickness, and/or in the presence of important differences of thickness in the same casting, it may be convenient to consider the IDI material instead of ADI;
  • Weight reduction: by using ADI, it is possible to obtain advantageous weight reductions, thanks to the characteristic design freedom of the castings, the mechanical properties and the density of the material (7.2 kg/dm3 compared to 7.8 kg/dm3 of steel);
  • Wear resistance: the austenitic fraction of the ausferrite, although stabilised by the carbon, is susceptible to local transformation into martensite, in the form of isolated needles distributed within an untransformed matrix, when the applied mechanical stress exceeds a threshold value (PITRAM/SITRAM effect, Pressure/Stress Induced Transformation of Residual Austenite into Martensite). This phenomenon makes it possible to exploit a natural attitude of the material in wear resistance, which can essentially manifest itself through two distinct mechanisms: with the “Mechanically Mixed Layer”, in the grades with lower hardness, or with the resistance to abrasive wear in the grades with greater hardness (download Infosheet 2 Material Properties revised by MST as per Wear_2022_06_12 ZFR – ADI wear grades). The transformation described here is different from the massive transformation of the feared residual austenite in steels, which is cause of potential cracking.
  • Vibration damping: all materials containing gamma phase (face-centred cubic, austenite), offer a higher vibration damping capacity than the corresponding alpha phase only materials (body-centred cubic, ferrite/pearlite).
  • Low temperature behaviour: All materials containing gamma phase exhibit a favourable low temperature behaviour (evident in the impact test) better than their competitors consisting of alpha phase alone.

The mechanical characteristics of ADI cast irons are comparable with those of quenched and tempered steels, considering an important limitation: in the tensile test, the fracture of the ADI occurs near the maximum load Rm, typically before the onset of necking. The difference in dislocative motions during the collapse phase also appears in a reduced impact energy for ADI cast irons compared to steels that, on the external surfaces of the specimen, can effectively use the condition of “plane stress” at the edges (shear lips advantage). This difference has no effect in fatigue behaviour, nor in the fracture toughness test in true “plane strain”, i.e. in all operating conditions in linear elastic regime. The difference reappears, as well as in the tensile test, also in fracture mechanics when, as frequently happens, the prevailing stress condition is “plane stress” and not “plane strain”.

Industrial applications of austempered ductile iron (ADI)

Thanks to their mechanical properties, austempered ductile irons (ADI) find space today in various market sectors that also differ greatly from each other in terms of technical requirements and conditions of use. ADI ductile irons now find applications in the industrial sector for the production of high-performance castings for transmission systems and planetary gear units (single-deck satellite carriers, double-deck satellite carriers, triple-deck satellite carriers, gearboxes, differential boxes and gears are increasingly common). In the industrial world, ADI applications have also been developed in recent years for vehicles such as forklift trucks and telehandlers as well as for packaging systems. ADI components for planetary gear units are now increasingly widespread also in the energy sector for the construction of wind power plants. In the automotive, off-road vehicles and on-road vehicle sectors, ADI components for suspension systems and axles are produced (there are increasing examples of upper suspension arms, lower suspension arms, axle stubs, differential housings and differential half housings) there are also components for the engine (such as the crankshaft for off-road vehicles). ADI ductile irons are also used in the earthmoving sector, typically for the production of components for the undercarriage (such as for the drive wheels) and in the agricultural sector where ADI applications find space in coupling systems (hooks and quick couplings), in tools (such as branch cutting blades for forestry use) but also in suspension systems, in the undercarriage and axles (track axles, suspension arms and axle stubs). Given the interesting resistance of ADI ductile irons in wear conditions, the hardest grades of ADI ductile irons (ADI1200, ADI1400 and ADI1600) are also present in the mining and construction sector with components for crushing and grinding systems (armour and hammers are among the most common components). Finally, in the railway sector there is no shortage of ADI components for railway bogies, braking systems and railway switches. This versatility is only possible thanks to the excellent and unique mechanical properties of austempered ductile irons (ADI) that lead these materials to being comparable, often in a winning way, to construction steels, wear-resistant materials (even superficially treated) and traditional ductile irons.

Reference standards for ADI ductile irons

The main reference standards for ADI ductile irons are the following:

  • ISO 17804 (similar to EN 1564)
  • ASTM A897
  • SAE J2477

The classification of grades is similar in all three norms.

The main difference between ISO/EN and American ASTM/SAE is the adoption of the A4 metric of the American standards instead of the A5 adopted in the ISO/EN. The American ones measure the elongation at break on a utilisable length before the test equal to 4 times the diameter, while the ISO/EN ones measure it on a utilisable length equal to 5 times the diameter. With the same material, the American percentage elongation is greater than ISO/EN, as highlighted in ISO 17804 Annex B. Another evidence to be noted is the grade SAE AD 750, not present in the other standards, which explicitly describes this grade as intercritical with the presence of proeutectoid ferrite (see SAE J2477 5.1.1). For more information, see ISO 17804.

Casting thickness and alloy content in austempered ductile iron (ADI)

The basic requirements of austempered ductile irons (ADI) include the thickness of the casting and the content of alloying elements present: each of these factors exerts a certain influence on the material. Austempering heat treatment, in fact, requires that the cooling curve at each point of the casting does not cross over the pearlitic nose of the CCT curves. The cooling curve is physically related to the thickness of the casting and, instrumentally, to the robustness of the treatment plant. CCT curves depend on the material, essentially nodularity (also dependent on the thickness) and on the added alloys. With the same system, the greater the thickness of the casting, the greater the addition of alloying elements must be. For the same thickness, different balance of alloying elements can be chosen which, in combination with the austenitization temperature, determine the same effect in the shifting of the CCT curves, necessary to avoid the pearlitic nose. However, the combination of different chemistry and the austenitization temperature have an important effect on the final ausferritic transformation.