Quiet and light spoked wheel centres made of Austempered Ductile Iron

Monobloc wheels are today the main choice of all manufacturers and system operators (trains and metros) thanks to their low weight, simple maintenance and easy approval processes. However, tyred wheels could be competitive again in some situations if a proper redesign is performed, considering that nowadays disk braking is used in almost all vehicles. Both wheel centres and tyres can be manufactured differently in order to eliminate the critical features of their old fashioned design. Materials and shapes can be managed to optimize both mass and maintenance. In this paper, the design process of new wheel centres made of Austempered Ductile Iron (ADI) for a Diesel Multiple Unit (DMU) is described, with particular reference to casting simulations, static and fatigue structural assessment. Mechanical properties of ADI are also introduced and compared to those of steels normally used for railway wheels. The use of unconventional or innovative materials for wheelsets is strongly limited in Europe by the current standardization frame. EU Directives and Regulations on safety and interoperability on railways of the Union allow only the use of materials described in the EN standards “supporting” them. Steel grades for rails and wheels are prescribed as these elements are the interface between two subsystems (infrastructure and rolling stock) and their compatibility must be ensured across Europe. On the opposite, standards that define the performances and not the materials are, for example, those about bogie frames and carbodies, in which acceptance tests are defined but the designer has the freedom to choose the architecture and the materials according to the needs. In this research, a new material for tyred wheel centres is evaluated. As long as wheels centres are not considered in the current EU regulation frame, different materials could be used. Multi-material tyred wheels have been proposed in the past in order to achieve a lower noise emission [1] and lower mass [2]. However, in both cases, aluminium wheel centres were used and to the author’s knowledge no prototypes were tested in service. An innovative tyred wheel with spoked wheel centre casted with Austempered Ductile Iron (ADI) is developed in this research to cope with both problems of unsprung mass and noise. ADI is a recently developed material, standardized according to [4] and [5], with mechanical properties similar to steels, while aluminium suffers from low Young’s modulus and an undefined fatigue endurance limit. ADI consists of a ductile iron subjected to an isothermal heat treatment called austempering whose peculiar properties make this material a serious competitor of alloyed high strength steels (e.g. 42CrMo4), with the advantages to be lighter than steel (7.25 kg/dm3 instead of 7.85 kg/dm3) and easily castable with excellent castings quality [3]. Widespread NDT can be used similarly to steel to check sanity and integrity of castings.