After
the intensive cooling, the bar is exposed
to air and the core re-heats the
quenched surface layer by conduction, therefore tempering the
external
martensite. When the bars are taken out of the cooling system, the heat
flows
from the core to the outer surface, further
tempering of the bars, which helps
them attain a higher yield strength. The resulting heat-treated
structure
imparts superior strength and toughness to the bars. Cooling process is
illustrated below:
The
pre-determined cooling of the bar periphery transforms the peripheral
structure
to martensite and then annealed through the heat available at the core.
The
peripheral and core temperature difference finally equalises at around
600 0C
and the resultant bar structure is of tempered martensite at the
periphery and
of fine-grained ferrite-pearlite at the core.
Generally speaking, the resultant
soft core forms about 65-75 per cent of the area (depending upon the
desired
minimum yield strength) and the rest is the hardened periphery. The equalizing
temperature together with the final rolling temperature is the most
important
parameter to achieve the required mechanical properties.
Finally,
when the bar is discharged on to the Cooling
Beds, the
remaining austenite
transforms into a very fine-grained pearlite structure.
The
figure below illustrates a typical TMT bar manufacturing process:
After
this process of thermo mechanical treatment, a dark etched peripheral rim of
tempered martensite and a grey core of ferrite pearlite get formed.
The
tempered martensite surface layer is very hard while the microstructure
of the
core is a very fine-grained ferrite and pearlite which is quite soft. The
result is a structure with a high yield strength combined with high
ductility.
 |
 |
 |
| Microstructure
of TMT bar showing peripheral rim of tempered martensite & core
of ferrite
pearlite get formed |
Tempered
Martensite Rim |
 |
| Ferrite-Pearlite
core |
TMT
bars are also known as 'Quenched and
Tempered rebars', because of the quenching
and tempering processes involved in making the bars.
The
production of quality TMT bar depends on three major factors - quality
of raw
materials, a properly designed and automated mill, and a well-designed
quenching and tempering technology.
TMT
bars having uniform and concentrated hardened periphery and the softer
core
will have the desired tensile strengths coupled with high elongation as
required in seismic zones. Depending on the size and grade, rebars with
hardened periphery of about 15 to 30 per cent of the cross sectional
area of
the bar are ideal for civil constructions (constructions
of houses, offices,
etc.)
