STEEL - Materials for Civil and Construction Engineers - page 90

and manganese, can eliminate the formation of the carbon monoxide bubbles.
Completely deoxidized steels are known as killed steels . Steels that are gen-
erally killed include

Those with a carbon content greater than 0.25%

■

All forging grades of steels

■

Structural steels with carbon content between 0.15 and 0.25 percent

■

Some special steel in the lower carbon ranges

■

Regardless of the refining process, the molten steel, with the desired
chemical composition, is then either cast into ingots (large blocks of steel)
or cast continuously into a desired shape. Continuous casting is becoming
the standard production method, since it is more energy efficient than cast-
ing ingots, as the ingots must be reheated prior to shaping the steel into the
final product.

3.2

Iron-Carbon Phase Diagram

In refining steel from iron ore, the quantity of carbon used must be careful-
ly controlled in order for the steel to have the desired properties. The reason
for the strong relationship between steel properties and carbon content can
be understood by examining the iron-carbon phase diagram.
Figure 3.4 presents a commonly accepted iron-carbon phase diagram.
One of the unique features of this diagram is that the abscissa extends only
to 6.7% rather than 100%. This is a matter of convention. In an iron-rich ma-
terial, each carbon atom bonds with three iron atoms to form iron carbide,
also called cementite. Iron carbide is 6.7% carbon by weight. Thus, on
the phase diagram, a carbon weight of 6.7% corresponds to 100% iron car-
bide. A complete iron-carbon phase diagram should extend to 100% car-
bon. However, only the iron rich portion, as shown on Figure 3.4, is of
practical significance (Callister 2003). In fact, structural steels have a maxi-
mum carbon content of less than 0.3%, so only a very small portion of the
phase diagram is significant for civil engineers.
The left side of Figure 3.4 demonstrates that pure iron goes through two
transformations as temperature increases. Pure iron below 912°C has a BCC
crystalline structure called ferrite. At 912°C the ferrite undergoes a poly-
morphic change to a FCC structure called austenite. At 1394°C, another
polymorphic change occurs, returning the iron to a BCC structure. At
1539°C the iron melts into a liquid. The high and low temperature ferrites
are identified as and ferrite, respectively. Since ferrite occurs only at
very high temperatures, it does not have practical significance for this topic.
Carbon goes into solution with ferrite at temperatures between 400°C
and 912°C. However, the solubility limit is very low, with a maximum of
0.022% at 727°C. At temperatures below 727°C and to the right of the solu-
bility limit line, ferrite and iron carbide coexist as two phases. From 727°C
to 1148°C, the solubility of carbon in the austenite increases from 0.77% to

Fe 3 C,

d a d

a

a