Gray iron

[3] Its former widespread use[clarify] on brakes in freight trains has been greatly reduced in the European Union over concerns regarding noise pollution.

[4][5][6][7] Deutsche Bahn for example had replaced grey iron brakes on 53,000 of its freight cars (85% of their fleet) with newer, quieter models by 2019—in part to comply with a law that came into force in December 2020.

[1][11] Rapid cooling partly or completely suppresses graphitization and leads to the formation of cementite, which is called white iron.

The tips of the flakes act as preexisting notches at which stresses concentrate and it therefore behaves in a brittle manner.

Higher strength gray irons, up to class 40, have lower carbon equivalents and a pearlite matrix.

Gray iron above class 40 requires alloying to provide solid solution strengthening, and heat treating is used to modify the matrix.

Gray iron is a common engineering alloy because of its relatively low cost and good machinability, which results from the graphite lubricating the cut and breaking up the chips.

The graphite also gives gray iron an excellent damping capacity because it absorbs the energy and converts it into heat.