8 . 3 .

2 A u s t e n i t e - p e a r l i t e transformation

8.3.2.1 Nucleation and growth of pearlite

If a homogeneous austenitic specimen of eutectoid
composition were to be transferred quickly to a
bath held at some temperature between 720~ and
550~ decomposition curves of the form shown in
Figure 8.19a would be obtained. These curves, typical
of a nucleation and growth process, indicate that
the transformation undergoes an incubation period, an
accelerating stage and a decelerating stage; the volume
transformed into pearlite has the time-dependence
described by the Avrami equation (7.44). When the
transformation is in its initial stage the austenite
contains a few small pearlite nodules each of which
grow during the period A to B (see curve obtained
at 690~ and, at the same time, further nuclei form.
The percentage of austenite transformed is quite small,
since the nuclei are small and their total volume
represents only a fraction of the original austenite.
During the B to C stage the transformation rate
accelerates, since as each nodule increases in size the
area of contact between austenite and pearlite regions
also increases: the larger the pearlite volumes, the
greater is the surface area upon which to deposit further
transformation products. At C, the growing nodules
begin to impinge on each other, so that the area of
contact between pearlite and austenite decreases and
from this stage onwards, the larger the nodules the
lower is the rate of transformation. Clearly, the rate of
transformation depends on (1) the rate of nucleation of
pearlite nodules, N (i.e. the number of nuclei formed in
unit volume in unit time), and (2) the rate of growth
of these nodules, G (i.e. the rate that the radius of
the nodule increases with time). The variation of N
and G with temperature for a eutectoid steel is shown
in Figure 8.19b.

The rate of nucleation increases with decreasing

temperature down to the knee of the curve and in
this respect is analogous to other processes of phase
precipitation where hysteresis occurs (see Chapter 3).
In addition, the nucleation rate is very structure sensitive
so that nucleation occurs readily in regions of
high energy where the structure is distorted. In homogeneous
austenite the nucleation of pearlite occurs
almost exclusively at grain boundaries and, for this
reason, the size of the austenite grains, prior to quenching,
has an important effect on hardenability (a term
which denotes the depth in a steel to which a fully
martensitic structure can be obtained). Coarse-grained
steels can be hardened more easily than fine-grained
steels because to obtain maximum hardening in a steel,
the decomposition of austenite to pearlite should be
avoided, and this is more easily accomplished if the
grain boundary area, or the number of potential peadite
nucleation sites, is small. Thus, an increase in austenite
grain size effectively pushes the upper part of the
TTT curve to longer times, so that, with a given cooling
rate, the knee can be avoided more easily. The
structure-sensitivity of the rate of nucleation is also
reflected in other ways. For example, if the austenite
grain is heterogeneous, pearlite nucleation is observed
at inclusions as well as at grain boundaries. Moreover,
plastic deformation during transformation increases the
rate of transformation, since the introduction of dislocations
provides extra sites for nucleation, while the
vacancies produced by plastic deformation enhance the
diffusion process.
The rate of growth of pearlite, like the rate of nucleation,
also increases with decreasing temperature down
to the knee of the curve, even though it is governed
by the diffusion of carbon, which, of course, decreases
with decreasing temperature. The reason for this is that
the interlamellar spacing of the pearlite also decreases
rapidly with decreasing temperature, and because the
carbon atoms do not have to travel so far, the carbon
supply is easily maintained. In contrast to the rate of
nucleation, however, the rate of growth of pearlite is
quite structure-insensitive and, therefore, is indifferent
to the presence of grain boundaries or inclusions.
These two factors are important in governing the size
of the pearlite nodules produced. If, for instance, the
steel is transformed just below A~, where the rate of
nucleation is very low in comparison with the rate
of growth (i.e. the ratio N/G is small), very large
nodules are developed. Then, owing to the structureinsensitivity
of the growth process, the few nodules
formed are able to grow across grain boundaries, with
the result that pearlite nodules larger than the original
austenite grain size are often observed. By comparison,
if the steel is transformed at a lower temperature,
just above the knee of the TTT curve where N/G is
large, the rate of nucleation is high and the pearlite
nodule size is correspondingly small.
8.3.2.2 Mechanism and morphology of pearlite
formation
The growth of pearlite from austenite clearly involves
two distinct processes: (1) a redistribution of carbon
(since the carbon concentrates in the cementite and
avoids the ferrite) and (2)a crystallographic change
(since the structure of both ferrite and cementite differs
from that of austenite). Of these two processes it is
generally agreed that the rate of growth is governed by
the diffusion of carbon atoms, and the crystallographic
change occurs as readily as the redistribution of carbon
will allow. The active nucleus of the pearlite nodule
may be either a ferrite or cementite platelet, depending
on the conditions of temperature and composition
which prevail during the transformation, but usually it
is assumed to be cementite. The nucleus may form at a
grain boundary as shown in Figure 8.20a, and after its
formation the surrounding matrix is depleted of carbon,
so that conditions favour the nucleation of ferrite plates
adjacent to the cementite nucleus (Figure 8.20b). The
ferrite plates in turn reject carbon atoms into the
surrounding austenite and this favours the formation
of cementite nuclei, which then continue to grow. At
the same time as the pearlite nodule grows sideways,
the ferrite and cementite lamellae advance into the
austenite, since the carbon atoms rejected ahead of the
advancing ferrite diffuse into the path of the growing
cementite (Figure 8.20c). Eventually, a cementite plate
of different orientation forms and this acts as a new
nucleus as shown in Figures 8.20d and 8.20e.
Homogeneous austenite, when held at a constant
temperature, produces pearlite at a constant rate
and with a constant interlamellar spacing. However,
the interlamellar spacing decreases with decreasing
temperature, and becomes irresolvable in the optical
microscope as the temperature approaches that
corresponding to the knee of the curve. An increase
in hardness occurs as the spacing decreases. Zener
explains the dependence of interlamellar spacing

on temperature in the following way. If the

interlamellar spacing is large, the diffusion distance
of the carbon atoms in order to concentrate in
the cementite is also large, and the rate of carbon
redistribution is correspondingly slow. Conversely, if
the spacing is small the area, and hence energy,
of the ferrite-cementite interfaces become large.
In consequence, such a high proportion of the
free energy released in the austenite to pearlite
transformation is needed to provide the interfacial
energy that little will remain to provide the 'driving
force' for the change. Thus, a balance between
these two opposing conditions is necessary to allow
the formation of pearlite to proceed, and at a
constant temperature the interlamellar spacing remains
constant. However, because the free energy change,
A G, accompanying the transformation increases with
increasing degree of undercooling, larger interfacial
areas can be tolerated as the temperature of
transformation is lowered, with the result that
the interlamellar spacing decreases with decreasing
temperature.
The majority of commercial steels are not usually
of the euctectoid composition (0.8% carbon), but
hypo-eutectoid (i.e. <0.8% carbon). In such steels,
pro-eutectoid ferrite is first formed before the peadite
reaction begins and this is shown in the TTT curve
by a third decomposition line. From Figure 8.18b it
can be seen that the amount of pro-eutectoid ferrite
decreases as the isothermal transformation temperature
is lowered. The morphology of the precipitated ferrite
depends on the usual precipitation variables (i.e.
temperature, time, carbon content and grain size) and
growth occurs preferentially at grain boundaries and
on certain crystallographic planes. The Widmanst~itten
pattern with ferrite growing along {1 1 1} planes of the
parent austenite is a familiar structure of these steels.
8.3.2.3 Influence of alloying elements on
pearlite formation
With the exception of cobalt, all alloying elements in
small amounts retard the transformation of austenite
to pearlite. These elements decrease both the rate of
nucleation, N, and the rate of growth, G, so that the
top part of the TTT curve is displaced towards longer
times. This has considerable technological importance
since in the absence of such alloying elements, a steel
can only transform into the harder constituents of bainite
or martensite if it is in the form of very thin
sections so that the cooling rate will be fast enough to
avoid crossing the knee of the TTT curve during the
cooling process and hence avoid pearlite transformation.
For this reason, most commercially heat-treatable
steels contain one or more of the elements chromium,
nickel, manganese, vanadium, molybdenum or tungsten.
Cobalt increases both N and G, and its effect
on the pearlite interlamellar spacing is contrary to the
other elements in that it decreases the spacing.
With large additions of alloying elements, the simple
form of TTT curve often becomes complex, as shown
in Figure 8.18c. Thus to obtain any desired structure
by heat treatment a detailed knowledge of the TTT
curve is essential.