GTBL042-14 GTBL042-Callister-v2 August 29, 2007 8:59
Questions and Problems • 615Annealing Processes
14.4In your own words describe the following
heat treatment procedures for steels and, for
each, the intended final microstructure: full
annealing, normalizing, quenching, and tem-
pering.
14.5Give the approximate minimum temperature
at which it is possible to austenitize each
of the following iron–carbon alloys during a
normalizing heat treatment:(a)0.15 wt% C,
(b)0.50 wt% C, and(c)1.10 wt% C.
14.6What is the purpose of a spheroidizing heat
treatment? On what classes of alloys is it nor-
mally used?
Heat Treatment of Steels
14.7What influence does the presence of alloy-
ing elements (other than carbon) have on the
shape of a hardenability curve? Briefly ex-
plain this effect.
14.8Name two thermal properties of a liquid
medium that will influence its quenching ef-
fectiveness.
14.9Construct radial hardness profiles for the fol-
lowing:
(a)A 75-mm (3-in.) diameter cylindrical
specimen of an 8640 steel alloy that has
been quenched in moderately agitated
oil
(b)A 90-mm (3^12 -in.) diameter cylindrical
specimen of an 8630 steel alloy that has
been quenched in moderately agitated
water
Fabrication and Processing of Glasses
and Glass–Ceramics
14.10Soda and lime are added to a glass batch in
the form of soda ash (Na 2 CO 3 ) and limestone
(CaCO 3 ). During heating, these two ingredi-
ents decompose to give off carbon dioxide
(CO 2 ), the resulting products being soda and
lime. Compute the weight of soda ash and
limestone that must be added to 125 lbmof
quartz (SiO 2 ) to yield a glass of composition
78 wt% SiO 2 , 17 wt% Na 2 O, and 5 wt% CaO.
14.11Compare the temperatures at which soda–
lime, borosilicate, 96% silica, and fused silica
may be annealed.14.12The viscosityηof a glass varies with temper-
ature according to the relationshipη=Aexp(
Qvis
RT)
whereQvisis the energy of activation for vis-
cous flow,Ais a temperature-independent
constant, andRandTare, respectively, the
gas constant and the absolute temperature.
A plot of lnηversus l/Tshould be nearly lin-
ear, and with a slope ofQvis/R. Using the data
in Figure 14.17,(a)make such a plot for the
soda-lime glass, and(b)determine the acti-
vation energy between temperatures of 900
and 1600◦C.
14.13 (a)Explain why residual thermal stresses are
introduced into a glass piece when it is
cooled.
(b)Are thermal stresses introduced upon
heating? Why or why not?
14.14Borosilicate glasses and fused silica are resis-
tant to thermal shock. Why is this so?
14.15Glass pieces may also be strengthened by
chemical tempering. With this procedure, the
glass surface is put in a state of compression
by exchanging some of the cations near the
surface with other cations having a larger di-
ameter. Suggest one type of cation that, by
replacing Na+, will induce chemical temper-
ing in a soda–lime glass.
Fabrication and Processing of Clay Products
14.16From a molecular perspective, briefly explain
the mechanism by which clay minerals be-
come hydroplastic when water is added.
14.17 (a)Why is it so important to control the rate
of drying of a ceramic body that has been
hydroplastically formed or slip cast?
(b)Cite three factors that influence the rate
of drying, and explain how each affects
the rate.
14.18 (a)Name three factors that influence the de-
gree to which vitrification occurs in clay-
based ceramic wares.
(b)Explain how density, firing distortion,
strength, corrosion resistance, and ther-
mal conductivity are affected by the ex-
tent of vitrification.