Page 74 - Shigley's Mechanical Engineering Design

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Materials 49
Figure 2–12 100
Cold-drawn
Stress-strain diagram for
80
hot-rolled and cold-drawn Yield point Hot-rolled
UNS G10350 steel. 60
Strength, kpsi 40 Yield point

20

0
0 0.2 0.4 0.6
Elongation, in

strength and hardness, and a decrease in ductility. In Fig. 2–12 the properties of a cold-
drawn bar are compared with those of a hot-rolled bar of the same material.
Heading is a cold-working process in which the metal is gathered, or upset. This
operation is commonly used to make screw and rivet heads and is capable of producing a
wide variety of shapes. Roll threading is the process of rolling threads by squeezing and
rolling a blank between two serrated dies. Spinning is the operation of working sheet mate-
rial around a rotating form into a circular shape. Stamping is the term used to describe
punch-press operations such as blanking, coining, forming, and shallow drawing.

2–14 The Heat Treatment of Steel

Heat treatment of steel refers to time- and temperature-controlled processes that relieve
residual stresses and/or modifies material properties such as hardness (strength), duc-
tility, and toughness. Other mechanical or chemical operations are sometimes grouped
under the heading of heat treatment. The common heat-treating operations are anneal-
ing, quenching, tempering, and case hardening.
Annealing
When a material is cold- or hot-worked, residual stresses are built in, and, in addition, the
material usually has a higher hardness as a result of these working operations. These oper-
ations change the structure of the material so that it is no longer represented by the equi-
librium diagram. Full annealing and normalizing is a heating operation that permits the
material to transform according to the equilibrium diagram. The material to be annealed
is heated to a temperature that is approximately 100°F above the critical temperature. It is
held at this temperature for a time that is sufficient for the carbon to become dissolved and
diffused through the material. The object being treated is then allowed to cool slowly, usu-
ally in the furnace in which it was treated. If the transformation is complete, then it is said
to have a full anneal. Annealing is used to soften a material and make it more ductile, to
relieve residual stresses, and to refine the grain structure.
The term annealing includes the process called normalizing. Parts to be normalized
may be heated to a slightly higher temperature than in full annealing. This produces a
coarser grain structure, which is more easily machined if the material is a low-carbon steel.
In the normalizing process the part is cooled in still air at room temperature. Since this
cooling is more rapid than the slow cooling used in full annealing, less time is available for
equilibrium, and the material is harder than fully annealed steel. Normalizing is often used
as the final treating operation for steel. The cooling in still air amounts to a slow quench.