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Steel Designers' Manual - 6th Edition (2003)
232 Applied metallurgy of steel
00
a)
800
700
600
500
400
E
.3 300
200
100
0
l\ \
Ii \
ii "
j \aus'tenite
\
\ \( a)
\\
start
• finish
ferrite
+pearlite
1 10 102 103 10 10 106
time (s)
Fig. 6.7 Continuous cooling transformation diagram for 02% C, 0.9% Mn steel
it transforms back to austenite, this temperature being normally in the range 850°C
to 950°C. It is important to ensure that the temperature is sufficient for full transformation
to austenite, otherwise a very coarse-grained ferritic structure may result.
It is also important that the austenitizing temperature is not too high, and that the
time at this temperature is not too long, otherwise a coarse-grained austenite structure
will form, making subsequent transformation to fine grains more difficult. A
heat treatment in which cooling is slow and essentially carried out in a furnace is
known as annealing. This tends to lead to a relatively coarse-grained final structure,
as predicted by the basic equilibrium phase diagram, and is used to put materials
into their softest condition. If the steel is allowed to cool freely in air from the
austenitizing temperature, the heat treatment is known as normalizing, which gives
a finer grain size and hence tends to higher yield strength and better toughness for
a given composition of steel. Normalizing may be combined with rolling of a particular
product form over a relatively narrow band of temperatures, followed by
natural cooling in air, in which case it is known as controlled rolling. When the steel
product form is cooled more rapidly by immersing it directly into oil or water, the
heat treatment is known as quenching. Quenching into a water bath is generally
more severe than quenching into an oil bath.