GTBL042-14 GTBL042-Callister-v2 August 29, 2007 8:59
576 • Chapter 14 / Synthesis, Fabrication, and Processing of MaterialsAnnealing of Ferrous Alloys
Several different annealing procedures are employed to enhance the properties of
steel alloys. However, before they are discussed, some comment relative to the label-
ing of phase boundaries is necessary. Figure 14.4 shows the portion of the iron–iron
carbide phase diagram in the vicinity of the eutectoid. The horizontal line at the eutec-
lower critical toid temperature, conventionally labeledA 1 , is termed thelower critical temperature
temperature below which, under equilibrium conditions, all austenite will have transformed into
ferrite and cementite phases. The phase boundaries denoted asA 3 andAcmrepre-
upper critical sent theupper critical temperaturelines for hypoeutectoid and hypereutectoid steels,
temperature respectively. For temperatures and compositions above these boundaries, only the
austenite phase will prevail. As explained in Section 10.21, other alloying elements
will shift the eutectoid and the positions of these phase boundary lines.Normalizing
Steels that have been plastically deformed by, for example, a rolling operation, consist
of grains of pearlite (and most likely a proeutectoid phase), which are irregularly
shaped and relatively large, but vary substantially in size. An annealing heat treatment
normalizing callednormalizingis used to refine the grains (i.e., to decrease the average grain size)
and produce a more uniform and desirable size distribution; fine-grained pearlitic
steels are tougher than coarse-grained ones. Normalizing is accomplished by heating
at least 55◦C (100◦F) above the upper critical temperature—that is, aboveA 3 for
compositions less than the eutectoid (0.76 wt% C), and aboveAcmfor compositions
greater than the eutectoid as represented in Figure 14.4. After sufficient time has
been allowed for the alloy to completely transform to austenite—a procedure termed
austenitizing austenitizing—the treatment is terminated by cooling in air. A normalizing cooling
curve is superimposed on the continuous cooling transformation diagram (Figure
11.27).Full Anneal
full annealing A heat treatment known asfull annealingis often utilized in low- and medium-carbon
steels that will be machined or will experience extensive plastic deformation during
a forming operation. In general, the alloy is treated by heating to a temperature
of about 50◦C above theA 3 line (to form austenite) for compositions less than the1000900800700600
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.61800170016001500140013001200NormalizingFull annealingAcmA 3A 1Composition (wt% C)Temperature (°C)Temperature (°F)Figure 14.4 The
iron–iron carbide
phase diagram in the
vicinity of the
eutectoid, indicating
heat-treating
temperature ranges
for plain carbon
steels. (Adapted
from G. Krauss,
Steels: Heat
Treatment and
Processing
Principles.)