GTBL042-14 GTBL042-Callister-v2 August 29, 2007 8:59
612 • Chapter 14 / Synthesis, Fabrication, and Processing of MaterialsSUMMARY
Forming Operations (Metals)
Casting
Miscellaneous Techniques
This chapter began with a discussion of various fabrication techniques that may be
applied to metallic materials. Forming operations are those in which a metal piece is
shaped by plastic deformation. When deformation is carried out above the recrystal-
lization temperature, it is termed hot working; otherwise, it is cold working. Forging,
rolling, extrusion, and drawing are four of the more common forming techniques.
Depending on the properties and shape of the finished piece, casting may be the most
desirable and economical fabrication process; sand, die, investment, lost foam, and
continuous casting methods were also treated. Additional fabrication procedures,
including powder metallurgy and welding, may be utilized alone or in combination
with other methods.Annealing Processes
Heat treatments that are used to fashion the mechanical properties of metal alloys
were discussed. The exposure to an elevated temperature for an extended time period
followed by cooling to room temperature at a relatively slow rate is termed annealing;
several specific annealing treatments were discussed briefly. During process anneal-
ing, a cold-worked piece is rendered softer yet more ductile as a consequence of
recrystallization. Internal residual stresses that have been introduced are eliminated
during a stress-relief anneal. For ferrous alloys, normalizing is used to refine and
improve the grain structure. Fabrication characteristics may also be enhanced by
full anneal and spheroidizing treatments that produce microstructures consisting of
coarse pearlite and spheroidite, respectively.Heat Treatment of Steels
For high-strength steels, the best combination of mechanical characteristics may be
realized if a predominantly martensitic microstructure is developed over the entire
cross section; this is converted to tempered martensite during a tempering heat treat-
ment. Hardenability is a parameter used to ascertain the influence of composition
on the susceptibility to the formation of a predominantly martensitic structure for
some specific heat treatment. Determination of hardenability is accomplished by the
standard Jominy end-quench test, from which hardenability curves are generated.
Other factors also influence the extent to which martensite will form. Of the
common quenching media, water is the most efficient, followed by oil and air, in
that order. The relationships between cooling rate and specimen size and geometry
for a specific quenching medium frequently are expressed on empirical charts; two
were introduced for cylindrical specimens. These may be used in conjunction with
hardenability data to generate cross-sectional hardness profiles.Fabrication and Processing of Glasses and Glass–Ceramics
The next major section of this chapter discussed the principal techniques used for the
fabrication of ceramic materials. Since glasses are formed at elevated temperatures,
the temperature–viscosity behavior is an important consideration. Melting, working,
softening, annealing, and strain points represent temperatures that correspond to
specific viscosity values. Knowledge of these points is important in the fabrication