Gray irons are a group of cast irons that form flake graphite during solidification, in contrast to the spheroidal graphite morphology of ductile irons. This article describes surface hardening of gray irons by flame and induction heating. It provides information on the classification of the gray irons in ASTM specification. The article presents examples that illustrate the use of stress relieving to eliminate distortion and cracking. It describes the three annealing treatments of gray iron: ferritizing annealing, medium (or full) annealing, and graphitizing annealing. The article discusses the parameters of the tensile strength and hardness of a normalized gray iron casting. These include combined carbon content, pearlite spacing, and graphite morphology. The article concludes with a discussion on the induction hardening of gray iron castings.