GTBL042-11 GTBL042-Callister-v3 October 4, 2007 11:59
2nd Revised PagesSummary • 453function of temperature and elapsed time is expressed for a specific alloy at constant
temperature and for continuous cooling treatments, respectively. Diagrams of both
types were presented for iron–carbon steel alloys, and their utility with regard to the
prediction of microstructural products was discussed.
Several microconstituents are possible for steels, the formation of which depends
on composition and heat treatment. These microconstituents include fine and coarse
pearlite, and bainite, which are composed of ferrite and cementite phases and result
from the decomposition of austenite via diffusional processes. A spheroidite mi-
crostructure (also consisting of ferrite and cementite phases) may be produced when
a steel specimen composed of any of the preceding microstructures is heat treated at
a temperature just below the eutectoid. The mechanical characteristics of pearlitic,
bainitic, and spheroiditic steels were compared and also explained in terms of their
microconstituents.
Martensite, yet another transformation product in steels, results when austen-
ite is cooled very rapidly. It is a metastable and single-phase structure that may be
produced in steels by a diffusionless and almost instantaneous transformation of
austenite. Transformation progress is dependent on temperature rather than time,
and may be represented on both isothermal and continuous cooling transforma-
tion diagrams. Furthermore, alloying element additions retard the formation rate of
pearlite and bainite, thus rendering the martensitic transformation more competi-
tive. Mechanically, martensite is extremely hard; applicability, however, is limited
by its brittleness. A tempering heat treatment increases the ductility at some sacri-
fice of strength and hardness. During tempering, martensite transforms to tempered
martensite, which consists of the equilibrium ferrite and cementite phases. Embrit-
tlement of some steel alloys results when specific alloying and impurity elements are
present, and upon tempering within a definite temperature range.Heat Treatments (Precipitation Hardening)
Mechanism of Hardening
Some alloys are amenable to precipitation hardening—that is, to strengthening by
the formation of very small particles of a second, or precipitate, phase. Control of
particle size, and subsequently the strength, is accomplished by two heat treatments.
For the second or precipitation treatment at constant temperature, strength increases
with time to a maximum and decreases during overaging. This process is accelerated
with rising temperature. The strengthening phenomenon is explained in terms of an
increased resistance to dislocation motion by lattice strains, which are established in
the vicinity of these microscopically small precipitate particles.Crystallization (of Polymers)
Melting
The Glass Transition
Melting and Glass Transition Temperatures
Factors That Influence Melting and Glass Transition Temperatures
For polymeric materials, the molecular mechanics of crystallization, melting, and
the glass transition were discussed. The manner in which melting and glass tran-
sition temperatures are determined was outlined; these parameters are important
relative to the temperature range over which a particular polymer may be utilized
and processed. The magnitudes ofTmandTgincrease with increasing chain stiffness;
stiffness is enhanced by the presence of chain double bonds and side groups that