STEEL - Materials for Civil and Construction Engineers - page 91

1600

d

d Liquid

1538

Liquid

1400

1394

g d

Liquid g

g , Austenite

1200

Fe 3 C

Liquid

2.11% 1148 C 4.3%

Upper critical
temperature
lines

1000

g Fe 3 C

912

800

Lower critical temperature line

a g

0.77% 727 C

a , Ferrite

100% Fe 3 C

600

a Fe 3 C

400

0 1 2 3 4 5 6 6.7

Percent weight of carbon

FIGURE 3.4 The iron-iron carbide phase diagram.

2.11%. The solubility of carbon in austenite is greater than in a ferrite be-
cause of the crystalline structure of the austenite.
At 0.77% carbon and 727°C, a eutectoid reaction occurs; that is, a solid
phase change occurs when either the temperature or carbon content changes.
At 0.77% carbon, and above 727°C, the carbon is in solution as an interstitial
element, within the FCC structure of the austenite. A temperature drop to
below 727°C, which happens slowly enough to allow the atoms to reach an
equilibrium condition, results in a two-phase material, a ferrite and iron car-
bide. The ferrite will have 0.022% carbon in solution, and the iron carbide
will have a carbon content of 6.7%. The ferrite and iron carbide will form as
thin plates, a lamellae structure. This eutectoid material is called pearlite.
At carbon contents less than the eutectoid composition, 0.77% carbon,
hypoeutectoid alloys are formed. Consider a carbon content of 0.25%.
Above approximately 860°C, solid austenite exists with carbon in solution.
The austenite consists of grains of uniform material that were formed when
the steel was cooled from a liquid to a solid. Under equilibrium temperature
drop from 860°C to 727°C, ferrite is formed and accumulates at the grain
boundaries of the austenite. This is a proeuctoid ferrite. At temperatures
slightly above 727°C, the ferrite will have 0.022% carbon in solution and
austenite will have 0.77% carbon. When the temperature drops below 727°C,

a

a