Tasdiqlayman“Chet tiilar” kafedras imudiri M. B. Ganikhanova

Solid-state Transformations in Ferrous Alloys

Steel is unique in its ability to exist as a soft ductile material that can easily be

formed or machined and then, as a result of a heat treatment, assume the role of a

hard, tough material that resists changing shape. There are two reasons for this

behavior. The first is the fact that iron undergoes an allotropic change at 1330°F.

Carbon has little solubility in the body-centered cubic lattice that is characteristic of iron at room temperature, but up to 2 percent carbon is soluble in the facecentered cubic lattice which is stable above 1330°F. The second is the solid-state eutectoid reaction in which a solid solution at a certain temperature can react to form two new solid phases. In the case of carbon steels, the -phase, austenite, transforms to -iron (ferrite) and cementite, (Fe3C) by the eutectoid reaction. The eutectoid reaction is common in many materials, but only steel exhibits such a marked change in properties.

Thus, when iron with 0.8 percent carbon is heated above 1330°F, the carbon dissolves and the resulting solid solution is called austenite or -iron. If the austenite is suddenly quenched in water, the carbon cannot escape and thus is trapped within the lattice structure as an interstitial atom that strains the lattice because of the increased volume it must occupy in the new body-centered tetragonal structure called martensite, which is characterized by a needlelike microstructure.