chapter  2
30 Pages

Classification and Mechanisms of Steel Transformation

References .......................................................................................................................... 118

The microstructure of steels consists of a spatial arrangement of crystalline aggregates of

different phases. The size, shape, distribution, composition, and crystal structure of these

phases essentially control the final properties of any given steel, including hardness, strength,

ductility, impact toughness, and creep strength. Steel is the most versatile alloy among all the

industrial alloys. It exhibits a diverse range of microstructures that possess different combin-

ations of strengths and toughnesses. In a majority of the steels, this versatility is made possible

cubic [bcc] crystal structure) to high-temperature austenite phase (face-centered cubic [fcc] crystal struc-

ture) and then decomposition of austenite to a low-temperature a-ferrite (bcc) phase by changing the composition and cooling rate. In low-alloy steels, the most important phase

change is from austenite to a-ferrite. For example, for a given composition of low-alloy steel, a ferrite-pearlite microstructure can be obtained by slow cooling. With an increase in cooling

rate, Widmantsta¨tten or upper bainite microstructure can be obtained from austenite. With a

further increase in the cooling rate, a hard martensite microstructure can be obtained. Pearlite

contains a lamellar aggregate of ferrite and cementite phases, upper bainite contains ferrite

platelets separated by austenite or carbides, and martensite contains carbon-supersaturated

ferrite platelets with a high density of dislocations or twin boundaries.