Cast iron brake discs are commonly used for road and race applications. The graphite flake arrangement of grey cast iron matches the high thermal conductivity requirements of brake discs, although with the brittleness characteristic of this material. Therefore the design of cast iron brake discs is a compromise between a thermally efficient design to reduce the operating temperature and a design generating a controlled tensile stress level to prevent crack failure, with as little mass penalty as possible.
The most critical failure mode on competition brake discs is catastrophic crack propagation in the early stages of service life. Dynamometer testing has shown that the initial bedding process greatly reduces the likelihood of catastrophic disc failure. This fact leads to the hypothesis that a heat treatment process occurs on the discs during bedding, increasing their crack resistance.
A visual microstructure analysis using conventional and scanning electron microscopy has revealed that the braking face temperature locally exceeds the critical phase transformation temperature during bedding. Moreover, a breakdown of pearlite occurs around graphite flakes, leading to a more ductile and crack resistant ferritic structure.
This paper finally investigates the compared influences of heat treatment and friction interface in the strengthening of the brake disc during bedding.