TMT steel bars undergo a quenching and self-tempering (QST) process that gives them improved properties compared to other steel bars. The bars are rapidly cooled from rolling temperature to partially transform the surface layer to martensite while leaving the core unaffected. On the cooling bed, heat from the core migrates to the surface and causes self-tempering, relieving stresses in the martensite and improving ductility. As a result of the QST process, TMT bars have higher strength and ductility with better corrosion and fire resistance than cold-worked steel bars.

1. TMT BAR is better than other steel bars
How TMT BAR gets its properties
The TMT bar it leaves the rolling mill is guided through specially designed proprietary Thermex
Pipes wherein the surface temperature of 850-1000 C is brought down drastically in a relatively
short period of time on account of the intense and uniform water cooling. The temperature of the
core is largely unaffected.The entire Thermex manufacturing process is operated through
Computerised Programmed Logic Control (PLC) to ensure consistency in quality. The tmt bars are
quenched (rapidly cooled) in water from a final rolling temperature of about 950 C. The quenching
is partial - only until a surface layer has been transformed from austenite to martensite. This
controlled quenching is achieved in one or more specially designed on-line water cooling devices
through which the tmt steel bar passes at a very high speed before reaching the cooling bed.
Because the quenching is only partial, a significant part of the original heat remains in the core of
the bar and, on the cooling bed, this heat migrates towards the surface. This results in an automatic
self-tempering process where the surface layer of martensite is tempered.
This "tempering temperature" (or equalisation temperature) refers to the maximum temperature
attained by the surface of the tmt bar after quenching. Tempering enables a partial diffusion of
carbon out of the extremely brittle but strong martensite, thus relieving the inherent stresses locked
in during the sudden quenching of the red-hot steel in cold water. The resulting tempered-martensite
shows improved ductility and toughness compared to the as-quenched martensite. Due to inherent
characteristic of the quenching and self-tempering (QST) process, the product retains its properties
to temperatures lower than this self-tempering temperature. The core of the heat-treated rebars
consists of ferrite and pearlite - which is not cold worked and is therefore quite tough. In general,
when the TMT process is carried out properly the resulting product has a much higher level of both
strength and ductility with a much lower carbon equivalent than is normally attained by the cold
working processes like CTD. The fine martensitic microstructure at the surface also ensures
improved resistance to corrosion, which has been measured to be about 30% higher than the cold
twisted material. While the cold twisted material looses it tensile strength rapidly on heating about
350 C because the annealing action sets in, the TMT bars are stable up to a temperature of at least
550 C, a temperature at which is tempered. This accounts for a much better fire resistance to the
structures made with TMT bars.