Different States of Steel and Their Properties

Question

What are the different states of steel, and their properties?

Answer

A phase diagram for steel has a number of different states. This is because the material can vary both in terms of carbon content and temperature, and a number of mixtures form in different conditions. Ferrite (α) has a very low carbon content (up to 0.005%) and a body-centered cubic structure, which gives it magnetic properties. Austenite (γ) is a high temperature form of steel, with a face-centered cubic structure, which can dissolve considerably more carbon than ferrite – up to 2.1%. Higher carbon content iron, when carbon starts to move out of solution, is called cementite (Fe3C), and forms a hard material at room temperature. Pearlite is a lamellar (layered) mixture of ferrite and cementite, which only forms perfectly when austenite cools slowly at a carbon content of 0.83%. Higher or lower carbon contents will result in clumps of α or Fe3C. When the material is cooled quickly, or quenched, pearlite does not have time to form, and a very hard and brittle material called Martensite forms instead, with a needle-like structure. The toughness and hardness of this material can be increased by tempering the material – that is, raising the temperature to below the critical point and then allowing the material to cool slowly.

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