Relative density, or specific gravity, is the ratio of the density of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest; for gases, air at room temperature is the reference.
Relative Density has no Units.

Cast iron is used to manufacture machine beds because it resist more compressive load and has good damping characteristics.

The main constituents of HSS are Tungsten, Chromium, Vanadium