In plain carbon steels, it is often difficult to separate the bainite reaction from the ferrite and pearlite reactions, because these phases can form under similar continuous to bainitic. For example, the TTT diagram for a 0.8% C steel is a continuous curve although there is both a pearlite and bainite reaction occurring, but it is difficult to disentangle the reactions sufficiently to study their kinetics.
There are two important features of bainite kinetics which can be shown by a variety of techniques, e.g. dilatometry, electrical resistivity, magnetic measurements and by metallography.
Reaction Kinetics of Bainite Formation
In plain carbon steels, it is often difficult to separate the bainite reaction from the
ferrite and pearlite reactions, because these phases can form under similar continuous
to bainitic. For example, the TTT diagram for a 0.8% C
steel is a
continuous curve although there is both a pearlite and bainite reaction occurring, but it
is difficult to disentangle the reactions sufficiently to study their kinetics.
However, the addition of certain alloying elements separates the reactions to the extent
that they can be represented as individual curves on the TTT diagram, which then takes on
a more complex form than the familiar C-curve.
There are two important features of bainite kinetics which can be shown by a variety of
techniques, e.g. dilatometry, electrical resistivity, magnetic measurements and by
metallography. First, there is a well defined temperature Bs, above which no bainitic
will form, which has been confirmed for a wide range of alloy steels and has been
correlated with the structural transition from Widmanstätten ferrite to upper bainitic
laths. Second, below Bs temperature and time dependent process take place over a wide
temperature range (up to 150°C), which does not go to completion.
The bainitic reaction has several basic features of a nucleation and growth process. It
takes place isothermally, starting with an incubation period during which no transformation
occurs, followed by an increasing rate of transformation to a maximum and then a gradual
Using techniques such as thermionic emission microscopy it has been possible to study
directly the progress of the bainite reaction. It has been found that upper bainitic
plates lengthen and thicken during transformation by the movement along the plate
boundaries of small steps which appear to be diffusion-controlled.
The plates grow at a constant rate edgewise, which leads to a model for the reaction
in which the driving force is provided by partition of the carbon from the ferrite to
the austenite, the actual growth rate being determined by the rate of diffusion of
carbon in austenite away from the γ/α interface.
Role of Alloying Elements
has a large effect on the range of temperature over which upper
and lower bainite occur. The Bs temperature is depressed by many alloying elements but
carbon has the greatest influence, as indicated by the following empirical equation:
Bs(°C) = 830 - 270(%C) - 90(%Mn) - 37(%Ni) - 70(%Cr) - 83(%Mo)
Other alloying elements. In plain carbon steels, the bainitic reaction
is kinetically shielded by the ferrite and pearlite reactions which commence at higher
temperatures and shorter times, so that in continuously cooled samples bainitic
structures are difficult to obtain. Even using isothermal transformation difficulties
arise if, for example, the ferrite reaction is particularly rapid the addition of
metallic alloying elements usually results in retardation of the ferrite and pearlite
reactions. In addition, the bainite reaction is depressed to lower temperatures.
This often leads to greater separation of the reactions, and the TTT curves for many
alloy steels show much more clearly separate C-curves for the pearlite and bainitic
reactions. However, it is still difficult to obtain a fully bainitic structure because
of its proximity to the martensite reaction.
A very effective means of isolating the bainite reaction in low carbon steels has been
found by adding about 0.002% soluble boron to a 0.5% Mo steel. While
the straight molybdenum steel encourages the bainite reaction, the boron markedly
retards the ferrite reaction, probably by preferential segregation to the prior austenite
boundaries. This permits the bainite reaction to occur at shorter times. Consequently,
by use of a range of cooling rates, fully bainitic steels can be obtained.
Use of Bainitic Steels
Bainite frequently occurs in alloy steels during quenching to form martensite. The
cooling rate towards the centre of a steel bar is lower than the outside, so in large
sections bainite can form in the inner regions with martensite predominating towards
However, low carbon fully bainitic steels have been developed, as described, using 0.5%
Mo and very small concentrations of boron, which allow bainite to form
over a wide range of cooling rates. Further control of the reaction is obtained by use
of metallic alloying elements such as Ni, Cr,
Mn which depress the temperature of maximum rate of formation of bainite.
As the transformation temperature is lowered, for a constant cooling rate, the strength
of the steel increases substantially. For a series of steels with 0.2% C
the tensile strength can be varied between 600 and 1200 MPa. However, this increase in
strength is accompanied by a loss of ductility.
The practical advantage of bainitic steels is that relatively high strength levels
together with adequate ductility can be obtained without further heat treatment, after
the bainite reaction has taken place. The steels are readily weldable, because bainite
rather than martensite, will form in the heat-affected zone adjacent to the weld metal,
and so the incidence of cracking will be reduced. Furthermore, the steels have a low
carbon content, which improves the weldability and reduces stresses arising from