Case History: Lower suspension arm in ADI900-8

Truck suspension arm in ADI austempered ductile iron

Lower suspension arm in ADI900-8: the alternative to forged steel for a lighter, stronger and better performing component

This case history describes the manufacture of a lower suspension arm in ADI900-8 austempered ductile iron.

Traditionally, manufacturers of wheel suspension systems, like our customer, design this type of lower suspension arm in forged steel (42CrMo4 Q&T) which is a much better known material among designers. Our customer opted to design this oscillating arm suspension in ADI900-8 austempered ductile iron from the outset, going “against the grain”, and taking full advantage of its features, both mechanical and productive, disregarding forged steel.

Industrial applications of an oscillating arm

The oscillating arm suspension is intended to absorb shocks and accelerations endured by the vehicle when on the road, thus increasing driving comfort and ensuring passenger safety.

It can be used on trucks, buses, industrial vehicles or special vehicles with a load capacity of more than 9 tons per axle. Using ADI, instead of forged steel, means that the overall dimensions can be compacted, the weight of the vehicle reduced and the load-bearing capacity increased, thus guaranteeing a longer vehicle life.

According to the Consolidated Resolution on the Construction of Vehicles (R.E.3), Revision 6[1], these vehicles fall into the following macrocategories: 2.2 M: vehicles used for the carriage of passengers 2.3 N: vehicles used for the carriage of goods.

Benefits (quality and quantity) of ADI austempered ductile iron compared to steel in the design of a lower suspension arm

Weight reduction

The ADI900 cast iron weighs 10% less than the forged steel alternative, which allowed the weight of the final component to be optimised

Improved vehicle performance

The ADI solution made it possible to increase the vehicle’s load capacity and reduce wheel wear.

Reduced risk of crack formation

The risk of the formation of cracks in ADI cast parts (potentially leading to breakage) during the austempering heat treatment is non-existent or, in any case, less than 42CrMo4 steel solutions after heat treatment.

Design freedom (complex and precise shapes)

Less machining required to obtain the finished product resulting in lower costs. The foundry process ensures improved manufacturing precision for the unfinished component, with a shape very close to the machined component.

Greater production process flexibility

The forging process requires larger production batch sizes (pieces/batch) while the casting process is more flexible and capable of producing smaller batches in line with customer requirements.

Reasons for choosing ADI for the lower suspension arm of a commercial truck

During the design stage, our customer needed to create a lower suspension arm with mechanical features that would guarantee a level of fatigue resistance suitable for the long life cycles to which this type of component is subjected. The resulting fatigue limit σAG; PS50% at 430 Mpa of ADI900-8 is, therefore, within the accepted range.

This was the key technical requirement to also ensure a level of safety in line with the standards required for this type of application (suspension arms are automotive and safety components) while at the same time ensuring less wear on the vehicle’s wheels.

Since this is a safety component, one of the factors that influenced the customer’s choice of ADI as the material for the design of the suspension arm was the need to eliminate, or at least minimise, the formation of surface cracks during the production process. Cracks often result in the breakage of a component that must withstand fatigue such as a suspension arm.

The austempering heat treatment uses a less aggressive salt bath quenching (1) technique compared to water or oil quenching to which quenched and tempered steels are generally subjected (2). Consequently, the level of induced residual stresses is very low and this significantly reduces the risk of surface cracks forming compared to 42CrMo4 steel solutions. This is a significant and important design benefit for anyone designing safety components.

1 – Austempering

“Soft” isothermal hardening: is achieved by using a liquid salt bath (consisting of water, nitrates and nitrites) between 250 °C and 400 °C as a quenching medium. This allows the martensite start to remain above the martensite formation.

2 – Quenching

hardening in water or oil: in these cases, the steel is cooled rapidly using water around 50°C or oil around 200°C as a hardening medium. In both cases, under the martensite start temperature which thus leads to the formation of martensite and the resulting risk of crack formation (which is why hardening is always followed by tempering).

Material used for the lower suspension arm before the use of ADI

Technology
Material
Rp0,2
Rm
A5
HB
Fatigue limit
σAG; PS50%
Forging
42CrMo4
1000 MPa
1100 Mpa mini
13%
302
500 MPa

New version: material in ADI900-8

Technology
Material
Rp0,2
Rm
A5
HB
Fatigue limit
σAG; PS50%
Melting
EN-GJS-900-8 (ADI900-8)
600 MPa
900 MPa
8%
280 - 340
430 MPa

Tensile and fatigue test performed on test specimens obtained from separately cast samples Ø25 mm. The fatigue limit on smooth surfaces (R-1) was obtained by testing Ø6.5 mm smooth test specimens machined with the Short Stair Case method (run out at 5ML of cycles). In reality, the workpiece has a variable thickness.

Construction methods and techniques in the comparison with forged steel (42CrMo4 Q&T), and ADI900-8

The lower suspension arm in ADI900-8 for trucks, described in this case history, weighs about 47 kg.

The ADI austempered ductile iron casting is obtained on an automatic green sand horizontal line and then subjected to austempering heat treatment. Lastly, treatment is followed by the machining of the cast parts, which is required to obtain the finished component. The ADI solution of a wheel suspension oscillating arm minimises the machining stages, thanks to the design freedom offered by cast iron resulting in the development of a rough casting with a morphology very close to the shape of the finished component (net-to-shape).

The same suspension arm, weighing about 50 kg, could have been manufactured in forged steel, just using a powerful press, but involving a very energy-intensive forging process. This would have entailed, however, the removal of more material to obtain the finished machined component, because, unlike a foundry casting, the resulting forged component has a rough shape that is still far removed from the shape of the final component after machining.

Case History

Real cases explaining the development of client projects