Building Design and Construction Handbook - Merritt - Ventech!

TEMPERATURE, DEG F
3000
2000
1000
0
IRONS
AUSTENITE
SOLID SOLUTION OF
CARBON IN GAMMA
IRON
AUSTENITE
A1
AND CEMENTITE
FERRITE &
A1,3
AUSTENITE
A3
FERRITE
PLUS
PEARLITE
HYPO-
EUTECTOID
STEELS
EUTECTOID
PEARLITE
Acm
BUILDING MATERIALS 4.53
PEARLITE
PLUS
CEMENTITE
HYPER-
EUTECTOID
STEELS
FIGURE 4.2 Iron-carbon diagram.
AUSTENITE, LEDEBURITE
AND CEMENTITE
CEMENTITE, PEARLITE AND
TRANSFORMED LEDEBURITE
CAST IRONS
EUTECTIC
LEDEBURITE
IRON CARBIDE
CEMENTITE
0.80
4.30
1.0 2.0 3.0 4.0 5.0
4.40.1 Iron-Carbon Equilibrium Diagram
PERCENT CARBON
The iron-carbon equilibrium diagram in Fig. 4.2 shows that, under equilibrium
conditions (slow cooling) if not more than 2.0% carbon is present, a solid solution
of carbon in gamma iron exists at elevated temperatures. This is called austenite.
If the carbon content is less than 0.8%, cooling below the A 3 temperature line causes
transformation of some of the austenite to ferrite, which is substantially pure alpha
iron (containing less than 0.01% carbon in solution). Still further cooling to below
the A 1 line causes the remaining austenite to transform to pearlite—the eutectoid
mixture of fine plates, or lamellas, of ferrite and cementite (iron carbide) whose
iridescent appearance under the microscope gives it its name.
If the carbon content is 0.8%, no transformation on cooling the austenite occurs
until the A 1 temperature is reached. At that point, all the austenite transforms to
pearlite, with its typical ‘‘thumbprint’’ microstructure.
At carbon contents between 0.80 and 2.0%, cooling below the A cm temperature
line causes iron carbide, or cementite, to form in the temperature range between
A cm and A 1,3. Below A 1,3, the remaining austenite transforms to pearlite.
4.40.2 Types of Irons
Metals containing substantially no carbon (several hundredths of 1%) are called
irons, of which wrought iron, electrolytic iron, and ‘‘ingot’’ iron are examples.
Wrought iron, whether made by the traditional puddling method or by mixing
very low carbon iron and slag, contains a substantial amount of slag. Because it
contains very little carbon, it is soft, ductile, and tough and, like low-carbon ferrous
metals generally, is relatively resistant to corrosion. It is easily worked. When broken,
it shows a fibrous fracture because of the slag inclusions. ‘‘Ingot’’ iron is a
very low carbon iron containing no slag, which is also soft, ductile, and tough.