Reversing Trend Lighter than Aluminum, Harder than Steel
As a portal for light metal casting, we report almost exclusively on light metals as aluminum, magnesium or zinc. But when so-called ADI constructions have the potential to be lighter than aluminum components, we make an exception.
For decades, the use of aluminum, titanium and magnesium components has been regarded as a universal remedy for optimizing the weight of sophisticated structural components. ADI, a new cast iron grade, has been available as a design material for some years now, which, in the triad of costs, resilience and production reliability/availability, repeatedly reduces this trend to absurdity.
When selecting a suitable material, the designer is spoilt for choice: either high-strength, but brittle (ceramic), or tough, but less rigid. An optimization of these opposing properties is not possible with conventional construction materials. The group of ADI cast iron materials is opening up new possibilities in this area of conflict, as: "Strength meets toughness." ADI stands for Austempered Ductile Iron and refers to heat-treated ductile cast iron which is twice as rigid (up to 1,600 N/mm2) as conventional ductile cast iron with spheroidal graphite at the same elongation at break (up to 10 %). The tensile strength is comparable to that of many steel grades (e.g. 16MnCr5 or 42CrMo4) - with exponentially superior shaping properties. Compared to spheroidal graphite iron, the fatigue strength is almost twice as high. As a typical cast-iron material, ADI has a density of about 10 % below that of steel. Due to its high graphite content it also offers the good (noise) damping capacity typical for cast iron.
Typical applications include materials handling, agricultural, construction and mining, rail (traffic) technology, as well as applications for commercial and passenger vehicles, fittings, transmissions and pumps. Apart from wear parts such as plough tips, guide rails, chain links, cutting edges and excavator teeth, highly stressed components such as chassis components and drive components (hollow wheels, axles/axle bridges, brake carriers, camshafts for heavy-duty engines, rollers, wheels) are frequent applications.
However (steel) gears sometimes are replaced by ADI gears for another reason: noise reduction that can only be obtained in this way is a major reason. The additional effect of the constantly renewed hardening of the surface layer under load also increases the service life of the component.
ADI materials (or correctly: ausferitic cast iron materials according to DIN EN 1564) form a group of materials which, due to their combined properties, meet the demands of a wide range of applications. It closes the gap between quenched and tempered steels for massive forming (or cast steel) and the well-known cast iron grades with spheroidal graphite (EN-GJS). The strength range of ADI extends from 800 N/mm2 at high elongations of at least 10 % and high fatigue strengths to wear-resistant grades with 1,600 N/mm2.
The notch sensitivity ratio, which describes the ratio of fatigue strength of unnotched and notched workpieces, is between 1.2 and 1.6 for ADI for the examined notch geometries and between 2.2 and 2.4 for forged steel. ADI is therefore not very notch-sensitive (whereby the significance of traditional notched bar impact tests for cast iron materials can be queried). In contrast to conventional nodular cast iron grades, the fatigue strength of non-notched ADI samples is not proportional to the tensile strength, instead it shows a maximum for those materials which contain a particularly high proportion of stabilized austenite because of temperature control during heat treatment. The tensile strength in ADI is not a measure of the fatigue strength - an assessment must be made using the so-called KIC/ KID values (in accordance with current fracture mechanics concepts such as the instrumented bar impact test to determine dynamic fracture toughness parameters KID or to determine the fracture toughness in the flat elongation state KIC (if necessary, using the mathematical determination of the J integral)).
ADI is produced from spheroidal graphite cast iron by means multi-stage heat treatment. The aim of the heat treatment is to achieve a microstructure of acicular ferrite in an austenite matrix supersaturated with carbon. Due to the high carbon content, the austenite is also stabilized at room temperature and at lower temperatures. Ausferrite has become the established term for this kind of structure (Fig. 1).
The casting is completely austenitized in a protective gas furnace at 840 to 950° C. The casting is then subjected to an austenitizing process. In the second treatment step, a rapid cooling - usually in a moving salt bath - to a transformation temperature between 235 and 425° C takes place. The quality of the ADI is determined by the temperature of the bath. The maximum duration of the cooling process for this step must not exceed 30 s for unalloyed spheroidal graphite cast iron as the base material (Fig. 2). The most important condition for austempering heat treatment is the avoidance of pearlite formation during cooling.
After reaching the temperature for the austenite transformation, the formation of the needle-shaped ferrite begins. This α solid solution in the Fe-C system has a much lower solubility for carbon than austenite. The excess carbon displaced from the α solid solution accumulates in the austenite and impedes its conversion to ferrite. This stabilizes the austenite in a kind of refrigerator effect. As long as the operating temperature of the component does not approach 300 °C, the resulting microstructure remains stable.
Fields of Application
Due to the high strength and elongation values (table), extremely light components can be designed for given loads - ADI designs can even be lighter than aluminum solutions. ADI is therefore much more than just an alternative to EN-GJS-600-3 or St37. At lower costs it competes with cast steel, quenched and tempered forged steels with high strength such as 16 MnCr5, 42CrMo4 and 34CrNiMo6, but also with higher mechanical requirements with so-called typical "lightweight materials" such as aluminum and magnesium.
One aspect often underestimated is the extensive freedom of design of ADI compared to cast steel or forged steel parts. This is because the basic alloy, which is decisive for formability and castability, is spheroidal graphite cast iron. While cast steel flows "like tomato juice" and is extremely prone to shrinkage, spheroidal graphite iron is easy to handle - like orange juice compared to tomato juice. This means that the possibilities for shaping are considerably greater. The proportion of circulating material in the casting is significantly lower and the risk of shrinkage is considerably lower - this greatly increases the design freedom of the designer. I.e. while wall thicknesses from 8 mm can be realized with steel casting, ADI components can be designed much more filigree - even if one should choose 5 mm as the lower limit.
However, this also means that anyone who is offered a steel casting as an ADI component with the same geometry has not really understood the potential offered by the material. This is only a comparison of the workpiece production costs - but not a comparison of the costs of two manufacturing alternatives with the aim of achieving the best possible functional performance at the lowest possible cost. Bionic load-case-adapted designs in ADI are the goal, not clumsy geometries according to the motto: “The more the better”. If the geometry is the same, vague cost information is the result; the technical advantage must then be implemented during the project. And it is only through technical progress that a manufacturing company remains viable in the long term.
Taken to its extreme, one could conclude: "Lightweight design is no longer the foolhardy use of materials with a low specific density, but the intelligent exploitation of material properties with the aim of achieving load-case-adapted design at an acceptable cost level.” The tin foil we used for our daily snack will probably always be made of aluminum, but transmission housings, steering knuckles and other machine and vehicle components subject to higher mechanical loads have a clear tendency towards the ADI material group.