Gray iron is the most versatile of all foundry metals. The high carbon content is responsible for ease of melting and casting in the foundry and for ease of machining in subsequent manufacturing. The low degree or absence of shrinkage and high fluidity provide maximum freedom of design for the engineer. By suitable adjustment in composition and selection of casting method, tensile strength can be varied from less than 20,000 psi to over 60,000 psi and hardness from 100 to 300 BHN in the as-cast condition. By subsequent heat treatment, the hardness can be increased to HRc 60. If the service life of a gray iron part is considered to be too short, the design of the casting should be carefully reviewed before specifying a higher strength and hardness grade of iron. An unnecessary increase in strength and hardness may increase the cost of the casting as well as increase the cost of machining through lower machining rates. Although the relationship between Brinell hardness and tensile strength for gray iron is not constant, data are shown which will allow use of the Brinell hardness test to estimate the minimum tensile strength of the iron in a casting.