Machinery Repairman

Figure 15-14.—Microstructural constituents of slowly cooled carbon steel (all etched with either picral or nital).
absorb a considerable amount as pure austenite, the
maximum being about 2 percent at 2,065°F (fig. 15-11,
point E). The solid solution of carbon in delta iron is
called delta ferrite, and the solid solution of carbon in
alpha iron is called alpha ferrite, or, more simply, ferrite.
The physical process by which iron-carbon alloys,
especially those containing less than about 0.6 percent
of carbon, solidify is rather complicated. All you really
need to know, however, is that all iron-carbon alloys
containing less than 2 percent of carbon (steel) will,
immediately or soon after solidification is complete,
consist of single-phase austenite. Cast irons containing
greater than 2 percent carbon will consist of two phases
immediately after solidification-austenite and
cementite. Under some conditions this cementite
formed on cooling through the temperature 2,065°F
15-15
(ECF in fig. 15-11) will decompose partly or completely
into austenite and graphite.
The part of the iron-carbon phase diagram that is
concerned with the heat treatment of steel is reproduced
on an expanded scale in figure 15-13. Regardless of the
carbon content, steel exists as austenite above line
GOSE. Steel of 0.83 percent carbon is designated as
eutectoid steel, and those with lower or higher carbon
as hypoeutectoid and hypereutectoid, respectively.
An eutectoid steel, when cooled at very slow rates
from temperatures within the austenitic field, undergoes
no change until the temperature reaches 1,330°F (line
PSK) (fig. 15-13). At this temperature (known as the A 1
temperature), the austenite transforms completely to a
mixture of ferrite and cementite having a typical
lamellar structure (fig. 15-14, view E). The mixture is