PLAIN CARBON STEELS 1 Plain carbon steels are steels in which the alloying elements do not play a significant role in determining the properties of the metal. The two systems used are both based on carbon content. The phase diagram of the iron/carbon alloy indicates that at 0.8% carbon a eutectoid composition exists, and a pearlitic structure forms upon slow cooling. With carbon content less than 0.8%, steels are sometimes termed "hypo-eutectoid" and with carbon content greater than 0.8% we have "hypereutectoid" steels. Unfortunately the classification based on eutectoid structure is coarse and little used for practical purposes. Instead, low, medium and high carbon steel are the preferred groupings used.