Main characteristics of ADI and IDI ductile irons

• Static properties of ADI and IDI ductile irons
• Impact resistance
• Wear resistance
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Mechanical structural components are often designed considering static and cyclic loads in terms of maximum equivalent stress not exceeding the conventional yield strength Rp0.2. However, the design process of a mechanical component begins only after the identification of the suitable material, and it is therefore essential that the designer has all the data he needs. As with ductile irons, also ADI ductile irons differ according to grades, which are used to distinguish cast irons according to their strength level. The grades ADI JS/800-10 (Rp0.2 = 500 Mpa), ADI JS/900-8 (Rp0.2 = 600 Mpa) and ADI JS/1050-6 (Rp0.2 = 700 Mpa), for their mechanical characteristics and excellent machinability, are interesting for making components such as:

• Suspension arms and axle stubs for suspension systems for on-road and off-road vehicles
• Gearboxes and satellite carriers, one, two or three decks for planetary gearboxes for wind and industrial installations
• Differential boxes and axle transmission systems for combines and tractors
• Drive wheels and idle wheels for undercarriages
• Quick couplings and coupling systems for the agricultural and forestry sector.

When it comes to structural applications, the field is very wide both in terms of applications and market sectors. Grade ADI JS1200/3 is positioned in the middle zone between fatigue and wear applications. Over the years, the following have been created:

• Armour and hammers for the mining sector
• Blades for forestry use, given the excellent wear resistance combined with good fatigue behaviour
• Toothed components for transmission systems or damping systems (boxes and covers) for railway bogies.

Grades ADI JS/1400-1 (Rp0.2 = 1100 Mpa), ADI HBW400 (Rp0.2 = 1100 Mpa) and ADI HBW450 (Rp0.2 = 1400 Mpa) are typically chosen for applications where excellent wear resistance is required such as for:

• Protection plates
• Armour and hammers
• Drive wheels for undercarriages.

Grade IDI 800-6 (Rp0.2 = 480 MPa), particularly suitable for use in the manufacture of castings for which a favourable combination of strength and ductility is desired, is optimal especially on large thicknesses and when it is not possible to maintain sufficient their uniformity. The absence of pearlitisers and the obtaining of the structure by heat treatment make it possible, in fact, to have uniform mechanical characteristics with varying wall thickness. By tuning the silicon content (in the range normally allowed for ferritic ductile irons) it is possible to optimise strength or ductility.

ADI – Minimum values from international standard ISO17804:2020 (separately cast samples)
IDI – Zanardi Database (separately cast samples)
Tensile test according to standard EN ISO 6892-1:2009, specimen diameter Φ14 mm; Brinell hardness test according to standard EN ISO 6506-1:2014.

It has been proposed that the difference in energy absorbed during the impact test between the two material families consists of the “Shear Lips Advantage” of the steels, in a state of plane stress on the outer surfaces of the impact specimen for a greater extension than that which characterises ductile irons. From this concept, it can be deduced that the main difference between ductile irons and steels lies in the post-necking behaviour, where common steels show a marked necking, unlike ductile irons where it is prevented by the presence of graphite nodules. Therefore, the impact resistance (unnotched and Charpy) of cast irons is not comparable with that of steels due to the different fracture behaviour under plane stress conditions; the impact resistance is therefore applicable within the same family of materials. In addition to what is described, the impact resistance is useful for determining the ductile-brittle transition temperature and represents a valid tool for process control.

Regarding to ADI austempered ductile irons, designers can consider that in addition to the high level of strength offered, the ductile-brittle transition temperature is very low thanks to the ausferritic matrix. Turning to IDI ductile irons, compared to a pearlitic-ferritic cast iron of equal strength, they offer a moderately higher ductility and are also more stable with temperature variations. By tuning the silicon content (in the range normally allowed for ferritic ductile irons) it is possible to optimise strength or ductility: at a low silicon content the best impact resistance is obtained, even if not at the levels of ADI800-10, ADI900-8 and ADI1050-6; as the silicon content increases, the strength increases at the expense of ductility.

Zanardi experimental database (separately cast samples).
Impact test according to standard UNI EN ISO148-1:2016; specimen’s size 10 x 10 x 55 mm un-notched, striker radius R2 mm, initial energy 450 J. The impact test shall be carried out on four unnotched test pieces; the lowest impact energy value shall be discarded, and the average of the three remaining values shall be used, according to ISO 17804:2020. Same procedure is used for IDI.

Among the ductile irons, the material that is best suited for wear applications is ADI ductile iron: all austempered ductile irons offer a significant improvement in wear resistance when compared with conventional cast irons and also with IDI ductile iron, in particular the high-grade ones.

Minimum values from international standard ISO17804:2020 (separately cast samples).
Tensile test according to standard EN ISO 6892-1:2009, specimen diameter Φ14 mm; Brinell hardness test according to standard EN ISO 6506-1:2014.

The presence of residual austenite brings, in fact, another very important characteristic to ADI ductile irons: it can be subjected to the so-called “stress-induced transformation in operation” in presence of high normal stress conditions.

Stress-induced transformation in operation in ADI ductile irons

This transformation, which allows ADI ductile irons to achieve considerable wear resistance, is more effective than simple hardening. The transformation, in fact, takes place with a localised increase in volume, which causes a state of superficial compression in the affected area. The possible formation and growth of cracks is thus inhibited and the fatigue resistance of the material increases. ADI ductile irons exhibit better wear resistance than steels with martensitic structure of equal hardness. The main uses concern tribological systems in which the wear process consists of abrasion (i.e undercarriage components such as sprockets, hammers for mills, armour) or by sliding-rolling (i.e gears, cam-tappet systems). In the first case, typically, the aspect of hardness prevails and to some extent the impact resistance (≥ ADI1400). In the second case, in addition to wear resistance, high fatigue resistance and good machinability are also required (ADI800-ADI1050-ADI1200); the excellent wear resistance by sliding-rolling is also obtained thanks to the role of graphite nodules, which behave as a solid lubricant interposing themselves between the bodies in contact, reducing friction and at the same time the wear coefficient. The characterisation of the abrasion wear process can be conducted by PAT (Pin Abrasion Test), and it is able to reliably reproduce the abrasive wear mechanism in different load conditions, relative speed between bodies and duration.

In our case, tests were conducted with the aim of comparing different materials, including those of competitors, under the following conditions:

• According to ASTM G132-96 standard;
• Test conditions: pin (material to be tested) φ 8mm, abrasive disc ZrO P100, cooling with water, average sliding speed 1.05 m/s, load 0.7 Mpa;
• Relative wear resistance according to the definition:

where W represents the wear rate of lost material. Ni-Hard cast iron (Wr=100%) has been taken as reference material. The results in terms of relative wear resistance are shown in the following figure.

The characterisation of the sliding-rolling wear process can be reliably carried out by means of the disc on disc test (which is carried out with two discs rolling and sliding on each other at different speeds to reproduce the typical operating conditions of gears), with both lubricated and dry discs (friction and wear of the components in contact strongly depend on the state of lubrication).

In the presence of lubrication, it is possible to reproduce contact fatigue phenomena (pitting) typical of gear coupling wheels at the primitive diameter (Paper PDF).

In our case, by operating dry, it is possible to better analyse the role of the microstructure in the wear behaviour of different materials by replicating the adhesive and triboxidative wear mechanisms (Paper PDF).

For samples machined after heat treatment (ADI800, ADI1050 and IDI) the wear mechanism is adhesion. Since in dry conditions the wear is usually moderate when the mechanism is tribo-oxidation, samples made with a different process (heat treated after machining) have been investigated in order to have a surface oxide layer capable of favouring moderate tribo-oxidative wear (ADI1200- ADI1400-ADIWR2).

The dry rolling-sliding wear test was performed with the aid of a tribometer (Universal tribometer mod. Amsler A135) in disc on disc configuration.

The test conditions reproduced are shown in Figure 7.

One can notice the formation of a surface layer due to the intense plastic deformation in the contact zone. The tribological layer in question is called MML (mechanically mixed layer). It is believed that the MML is formed when a critical deformation is reached, which triggers turbulent motions of material promoted by the intense shear effect, creating a sort of high hardness protection. The samples initially oxidised due to the heat treatment carried out after mechanical processing, wear out by crushing of the oxidised layer. As the hardness of the material increases, a decrease in ductility follows, which no longer makes it possible to form the MML protective hard tribological layer.