Page 77 - Mechanical Behavior of Materials

76 Chapter 3 A Survey of Engineering Materials

Figure 3.7 Microstructures of gray cast iron (left) and ductile (nodular) cast iron (right).
The graphite flakes on the left are the heavy dark bands, and the graphite nodules on the
right are the dark shapes. In gray iron (left), the fine lines are a pearlitic structure similar
to that in mild steel. (Photos courtesy of Deere and Co., Moline, IL.)

Fe 3 C, also called cementite. This very hard and brittle phase results in the bulk material also being
hard and brittle. For malleable iron, special heat treatment of white iron is used to obtain a result
similar to ductile iron. In addition, various alloying elements are used in making special-purpose cast
irons that have improved response to processing or desirable properties, such as resistance to heat
or corrosion.

3.3.3 Carbon Steels
Plain-carbon steels contain carbon, in amounts usually less than 1%, as the alloying element that
controls the properties. They also contain limited amounts of manganese and (generally undesirable)
impurities, such as sulfur and phosphorus. The more specific terms low-carbon steel and mild steel
are often used to indicate a carbon content of less than 0.25%, such as AISI 1020 steel. These steels
have relatively low strength, but excellent ductility. The structure is a combination of BCC iron,
also called α-iron or ferrite, and pearlite. Pearlite is a layered two-phase structure of ferrite and
cementite (Fe 3 C), as seen in Fig. 3.8 (left). Low-carbon steels can be strengthened somewhat by
cold working, but only minor strengthening is possible by heat treatment. Uses include structural
steel for buildings and bridges, and sheet metal applications, such as automobile bodies.
Medium-carbon steels, with carbon content around 0.3 to 0.6%, and high-carbon steels, with
carbon content around 0.7 to 1% and greater, have higher strengths than low-carbon steels, as a
result of the presence of more carbon. In addition, the strength can be increased significantly by heat
treatment using the quenching and tempering process, increasingly so for higher carbon contents.
However, high strengths are accompanied by loss of ductility—that is, by more brittle behavior.
Medium-carbon steels have a wide range of uses as shafts and other components of machines and