Callister - An introduction - 8th edition

Summary • 443
Forming Operations
• Forming operations are those in which a metal piece is shaped by plastic
deformation.
• When deformation is carried out above the recrystallization temperature, it is
termed hot working; otherwise, it is cold working.
• Forging, rolling, extrusion, and drawing are four of the more common forming
techniques (Figure 11.8).
Casting
• Depending on the properties and shape of the finished piece, casting may be the
most desirable and economical fabrication process.
• The most common casting techniques are sand, die, investment, lost foam, and
continuous.
Miscellaneous Techniques
• Powder metallurgy involves compacting powder metal particles into a desired
shape, which is then densified by heat treatment. P/M is used primarily for metals
that have low ductilities and/or high melting temperatures.
• Welding is used to join together two or more workpieces; a fusion bond forms
by melting portions of the workpieces and, in some instances, a filler material.
Annealing Processes
• Exposure of a material to an elevated temperature for an extended time period followed
by cooling to room temperature at a relatively slow rate is termed annealing.
• During process annealing, 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 stressrelief
anneal.
• For ferrous alloys, normalizing is used to refine and improve the grain structure.
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 treatment.
• 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. Martensite content is determined using hardness
measurements.
• Determination of hardenability is accomplished by the standard Jominy endquench
test (Figure 11.11), from which hardenability curves are generated.
• A hardenability curve plots hardness versus distance from the quenched end of
a Jominy specimen. Hardness decreases with distance from the quenched end
(Figure 11.12) because the quenching rate decreases with this distance, as does
also the martensite content. Each steel alloy has its own distinctive hardenability
curve.
• The quenching medium will also influence the extent to which martensite forms.
Of the common quenching media, water is the most efficient, followed by aqueous
polymers, oil, and air, in that order. Increasing degree of medium agitation
will also enhance the quenching efficiency.