Callister - An introduction - 8th edition

444 • Chapter 11 / Applications and Processing of Metal Alloys
• Relationships between cooling rate and specimen size and geometry for a specific
quenching medium frequently are expressed on empirical charts (Figures
11.17a and 11.17b). These plots may be used in conjunction with hardenability
data to predict cross-sectional hardness profiles (Example Problem 11.1).
Precipitation 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 strength, is accomplished by two heat
treatments:
In the first, or solution heat treatment, all solute atoms are dissolved to form
a single-phase solid solution; quenching to a relatively low temperature
preserves this state.
During the second or precipitation treatment (at constant temperature), precipitate
particles form and grow; strength, hardness, and ductility are dependent
on heat treating time (and particle size).
• Strength and hardness increase with time to a maximum and then decrease during
overaging (Figure 11.23). This process is accelerated with rising temperature
(Figure 11.27a).
• 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.
Processing/Structure/Properties/Performance Summary
At this time, we have completed our processing/structure/properties/performance
commentary for steels. By way of summary, Figure 11.28 shows relationships of these
processing, structure, and property elements for this group of alloys. It was compiled
from summaries provided in previous chapters, and, in addition, includes topics
discussed in this chapter.
For the most part, the individual components found in the interrelationships of
Figure 11.28 are conceptual in nature—that is, they represent the scientific (as
opposed to engineering) aspects of materials. We have also generated a processing/
structure/properties/performance relational diagram (for steel alloys) taken from
the materials engineering perspective; it is shown in Figure 11.29.
Important Terms and Concepts
alloy steel
annealing
artificial aging
austenitizing
brass
bronze
cast iron
cold working
compacted graphite iron
drawing
ductile (nodular) iron
extrusion
ferrous alloy
forging
full annealing
gray cast iron
hardenability
high-strength, low-alloy
(HSLA) steel
hot working
Jominy end-quench test
lower critical temperature
malleable cast iron
natural aging
nonferrous alloy
normalizing
overaging
plain carbon steel
powder metallurgy (P/M)
precipitation hardening
precipitation heat treatment
process annealing
rolling
solution heat treatment
specific strength
spheroidizing
stainless steel
stress relief
temper designation
upper critical temperature
welding
white cast iron
wrought alloy