What is Bainite and how is it formed?
By David Pye – Pye Metallurgical International Consulting
Bainite is a phase that can be created by heating the steel up to an
austenite temperate followed by a specific cooling procedure.
When heat treating a steel to make the steel hard or create alternative
structures, there are three conditions required to accomplish this,
namely;
 Carbon Content
 Temperature
 Cooling rate
So it can be seen that providing we have sufficient carbon present, be
at the appropriate temperature and most importantly, have the
appropriate cooling speed, we can create a number of different
metallurgical phase conditions.
The phases that can be created are as follows;
 Ferrite
 Ferrite plus Pearlite
 Pearlite
 Upper Bainite
 Martensite
Illustration displaying the Time Temperature Transformation
Diagram from which many specific phases can be created as a
direct result of the cooling rate.
Understanding the Bainite structures and the creation
of the same
Upper Bainite
Upper Bainite is formed as a result of creating the austenite phase in
plain carbon steel, or tool steel or a martensitic stainless steel. It is
created by cooling the steel down from the austenite range down to a
temperature (say).of 950℉ up to say 1000℉. There is no ’magic
number’ for the quench down temperature. The final temperature will
be dependent on the hardness value required and other mechanical
properties.
A photomicrograph Illustration of Upper Bainite in a steel of 0.50%
Carbon content
Upper Bainite structures in low carbon/low alloy steels, high
carbon/high alloy steels. Will have a structure of what is known as
lamellar pearlite with thin ferrite laths dispersed throughout the
observed microscopic grain.
The accomplishment of the Bainitic structure is determined once again
by;
 Austenitizing temperature selection
 Cool down to a nominal approximate intermediate temperature
of 975℉
 Holding time at the cool down nominal temperature of 975℉
The expected hardness value will be determined by the carbon content
of the steel and if the steel is an alloy steel, then there will be a
combination of the carbon and carbide forming elements which will
determine the hardness value.
The observation of Upper Bainite is observed microscopically by the
following:
 Cut a sample
 Mount the sample in a thermo setting compound
 Pre-grind using silicon carbide papers from 180 grit up to 600, or
800 grit
 Rough polish using 1 micron abrasive polishing compound
(diamond or aluminum oxide slurry)
 Final polish using a 0.5 micron abrasive polishing compound
(diamond or aluminum oxide slurry)
 Rinse in cold running water
 Spray with de-natured alcohol,
 Swab the polished face with 2% to5% Nital acid reagent for no
more than 5 to 7 seconds, followed by an immediate rinse with
water and then alcohol.
 Blow dry with warm air blower
 Observe microscopically between 400X up to 1000X
Lower Bainite
Once again,. Lower Bainite is formed as a result of;
 Austenitizing temperature selection
 Cool down to a nominal approximate intermediate temperature
of 975℉
 Holding time at the cool down nominal temperature of 975℉
An illustration of the cooling rates and the various phase that
can be formed
The resulting hardness value is once again based upon;
‘The expected hardness value will be determined by the carbon content
of the steel and if the steel is an alloy steel, then there will be a
combination of the carbon and carbide forming elements which will
determine the hardness value’
Generally the lower Bainitic strength .is generally determined by the
structural fineness, and the carbide forming elements carbide
dispersion. These properties will of course be determined by the
temperature selection of the quench medium. The hardness value of the
Lower Bainite will be higher than that which would be observed from
the formation of Upper Bainite.
The unique property of either of the bainitic structure s is, that the
material does not require a tempering procedure. This is simply because
we are not transforming the created austenite to martensite, which will
necessitate a tempering procedure.
A photomicrograph Illustration of Lower Bainite in a steel of 0.50%
Carbon content.
In order to form martensite, then the cooling rate should be such as to
pass slightly to the LH side of the nose of the curve on the TTT diagram
and to pass through the Ms (Martensite Start line) on the TTT diagram.
Please be aware, that once martensite has been form, it is a mandatory
requirement to temper ASAP after the formation of the martensite.