Niobium alloying in grey cast iron for vehicle brake discs

Niobium when alloyed to hypo- as well as hypereutectic iron has several beneficial metallurgical effects. It refines the eutectic cell size, graphite structure and also the lamellae spacing of the pearlite matrix. Nb affects the nucleation of graphite and can hence support inoculation. It also narrows down the temperature hysteresis between austenite and pearlite formation, which is important to cyclic heating. Finally, the ultra hard NbC particles formed lead to a markedly increased wear resistance of the material. The paper elucidates how these effects of Nb can be used to make brake discs with better properties and demonstrates processing benefits in the production of cast iron. Finally some examples of application in heavy trucks and recent passenger car are given.