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Steel Designers' Manual - 6th Edition (2003)
228 Applied metallurgy of steel
boundary structure. The lattice crystal structure of the ferrite material which forms
the basic matrix is essentially a body-centred cubic structure. Thus in cooling from
the liquid condition, complex changes in both lattice crystal structure and
microstructure take place dependent on the chemical composition. For the equilibrium
diagram conditions to be observed, cooling must be sufficiently slow to allow
time for the transformations in crystal structure and for the diffusion/migration of
carbon to take place to form the appropriate microstructures.
If a steel is cooled from a high temperature and held at a lower constant temperature
for sufficient time, different conditions may result; these are represented
on a diagram known as the isothermal transformation diagram. The form of the
diagram depends on the chemical analysis and in particular on the carbon or related
element content. In plain carbon steels the isothermal transformation diagram typically
has the shape of two letters C each with a horizontal bottom line as shown in
Fig. 6.4. The left-hand/upper curve on the diagram of temperature against time represents
the start of transformation, and the right-hand/lower curve represents the
800
700
600
:::
300
200
100
0 1 10
— — _feit+ pearlite
time (s)
bainite
Fig. 6.4 Isothermal transformation diagram for 0.2% C, 0.9% Mn steel
•stort
106