STEEL - Materials for Civil and Construction Engineers - page 95

electrical and magnetic properties. Four types of annealing can be performed,
depending on the desired results of the heat treatment:
Full annealing requires heating the steel to about 50°C above the
austenitic temperature line and holding the temperature until all the steel
transforms into either austenite or austenite-cementite, depending on the
carbon content. The steel is then cooled at a rate of about 20°C per hour in
a furnace to a temperature of about 680°C, followed by natural convection
cooling to room temperature. Due to the slow cooling rate, the grain struc-
ture is a coarse pearlite with ferrite or cementite, depending on the carbon
content. The slow cooling rate ensures uniform properties of the treated
steel. The steel is soft and ductile.
Process annealing is used to treat work-hardened parts made with low
carbon steel (i.e., less than 0.25 percent carbon). The material is heated to
about 700°C and held long enough to allow recrystallization of the ferrite
phase. By keeping the temperature below 727°C, there is not a phase shift
between ferrite and austenite, as occurs during full annealing. Hence, the
only change that occurs is refinement of the size, shape, and distribution of
the grain structure.
Stress relief annealing is used to reduce residual stresses in cast, weld-
ed, and cold-worked parts and cold-formed parts. The material is heated to
600 to 650°C, held at temperature for about one hour, and then slowly
cooled in still air.
Spheroidization is an annealing process used to improve the ability of
high carbon (i.e., more than 0.6 percent carbon) steel to be machined or cold
worked. It also improves abrasion resistance. The cementite is formed into
globules (spheroids) dispersed throughout the ferrite matrix.

■ 3.3.2 Normalizing

Normalizing is similar to annealing, with a slight difference in the tempera-
ture and the rate of cooling. Steel is normalized by heating to about 60°C
(110°F) above the austenite line and then cooling under natural convection.
The material is then air cooled. Normalizing produces a uniform, fine-grained
microstructure. However, since the rate of cooling is faster than that used for
full annealing, the rate of cooling of shapes with varying thicknesses results
in the normalized parts having less uniformity than could be achieved with
annealing. Since structural plate has a uniform thickness, normalizing is an
effective process and results in high fracture toughness of the material.

■ Hardening

Steel is hardened by heating it to a temperature above the transformation
range and holding it until austenite is formed. The steel is then quenched
(cooled rapidly) by plunging it into, or spraying it with, water, brine, or oil.
The rapid cooling “locks” the iron into a BCC structure, martensite , rather
than allowing the transformation to the ferrite FCC structure. Martensite has
a very hard and brittle structure. Since the cooling occurs more rapidly at

3.3.3