Page 143 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

|22 1000 Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment

.3 8O0_
The annealing process may be carried out in an
formed at lower temperatures to minimize or prevent
A 900 _. 2 \ inert or a controlled atmosphere, or it may be per-
Normalizing
O Acm surface oxidation.
An annealing temperature may be higher than
12/ Full the material’s recrystallization temperature, depend-
annealing
A3
ing on the degree of cold work. For example, the
recrystallization temperature for copper ranges be-
I9 700- A tween 200° and 300°C, whereas the annealing tem-
perature needed to fully recover the original
Spheroidizing
properties ranges from 260° to 65 0°C, depending on
1
the degree of prior cold work (see also Section 1.6).
6000 0.2 0.4 0.6 0.8 1.0 1.2 1_4 1.6
Full annealing is a term applied to the annealing of
Composition (% C)
ferrous alloys. The steel is heated to above A1 or A3
(Fig. 4.23), and the cooling takes place slowly [typi-
FIGURE 4.23 Heat-treating temperature ranges for plain-
carbon steels, as indicated on the iron-iron carbide phase cally at 10°C per hour], in a furnace, after which it is
turned off. The structure obtained through full an-
diagram.
nealing is coarse pearlite, which is soft and ductile
and has small, uniform grains.
800
70 To avoid excessive softness from the annealing
700- _ Quenehed of steels, the cooling cycle may be done completely in
ED. eoo - it 60 still air. This process is called normalizing, to indicate
that the part is heated to a temperature above A3 or
EE soo - »
Acm in order to transform the structure to austenite.
§ 400- so FQ
Normalizing results in somewhat higher strength and
Q soo- mm,@g».1,@¢ hardness, and in lower ductility, than does full an-
I 200- 40 nealing (Fig. 4.24). The structure obtained is fine
pearlite, with small, uniform grains. Normalizing is
ioo -
generally carried out to refine the grain structure,
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 obtain uniform structure (homogenization), decrease
residual stresses, and improve machinability. The
Carbon content (%)
structure of spheroidites and the procedure for
FIGURE 4.24 Hardness of steels in the quenched and obtaining it were described in Section 4.7 and shown
normalized conditions as a function of carbon content. in Figs. 4.14 and 4.23. Splaeroidizing annealing
improves the cold workability (Section 4.7) and the
machinability of steels (Section 21.7).

Process Annealing. During process annealing (also called intermediate annealing,
subcritical annealing, or in-process annealing), the workpiece is annealed to restore
its ductility, part or all of which may have been exhausted by cold working.
Afterwards, the part can be worked further into the final desired shape. If the tem-
perature is high and/or the time of annealing is long, grain growth may result
(Section 1.6), with adverse effects on the formability of the annealed parts.

Stress-relief Annealing. To reduce or eliminate residual stresses, a workpiece is
generally subjected to stress-relief annealing, or simply stress relieving. The temper-
ature and time required for this process depend on the material and on the magni-
tude of the residual stresses present. The residual stresses may have been induced
during forming, machining, or other shaping processes, or they may have been
caused by volume changes during phase transformations.
For steels, the part is not heated to as high as A1 in Fig. 4.23, in order to avoid
phase transformations; slow cooling, such as occurs in still air, is generally em-
ployed. Stress relieving promotes dimensional stability in situations where subse-
quent relaxing of residual stresses may cause distortion of the part when it is in