Figure 15-2.--Microscopic structure called austenite magnified
Figure 15-3.--Microscopic structure called martensite magnified
500 times.
2,500 times.
wolfran(W), columbium, and ferrite (or alpha iron)
Recall that steel heated to its hardening temperature
below its hardening temperature.
becomes a face-centered cubic material called austenite.
If austenite is quenched at its hardening temperature and
Face-Centered Cubic Lattice
cooled rapidly to a temperature of less than 400F, it
body-centered terragonal arrangement. During the
contains 14 atoms, one at each corner of the cube and
transformation from austenite to martensite, the steel
one at the center of each face of the cube. In this
becomes supersaturated with carbon. Because of its
arrangement the atoms are more dense (closely packed)
extremely high carbon content and the distortion of its
than in the body-centered arrangement. Metals that
structure, martensite is the hardest and most brittle form
have a face-centered cubic lattice structure include
of steel.
nickel, aluminum, copper, lead, gold, and silver.
When steel is heated to the hardening temperature,
Hexagonal Close-Packed Lattice
the space lattice units in the grain structure transform
from the body-centered cubic form to the face-centered
cubic form. In this form it is called austenite or gamma
D) contains 17 atoms. This structure does not have the
high degree of symmetry evident in the cubic structure,
austenite forms, the carbon in steel decomposes from
and as a result this type of structure is very difficult to
its combined state as cementite (iron carbide) to free
deform. Metals that have this structure have little
carbon. The free carbon then dissolves into the solid hot
iron to form a solid solution of uniformly dispersed
plasticity and are very difficult to work cold. Some
carbon in iron. This form of iron will dissolve up to a
examples of this type of metal are cadmium (Cd), cobalt
maximum of 2 percent carbon. In contrast, the
(Co), magnesium (Mg), titanium (Ti), zinc (Zn), and
body-centered (alpha) form of iron (ferrite) will
beryllium (Be).
dissolve a maximum of about 0.05 percent carbon.
Of the four lattices mentioned, the cubic types are
the most important. If you understand the role these
Body-Centered Tetragonal Lattice
structures play in the heat-treating process, you will be
able to understand better how desired characteristics
view C) contains nine atoms and looks like a
are given to various forms of steel. The relation-
body-centered cubic lattice stretched in one dimension.
ship between atomic structure, carbon content, and
15-4