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Message No 43 on TMT, Thermex and Tempcore bars

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inchak at tatasteel.com

PostPosted: Sat Jul 09, 2005 10:35 am    Post subject: Message No 43 on TMT, Thermex and Tempcore bars Reply with quote

Dear All:
What is the difference between TMT, Tempcore and Thermex reinforcement
In India we normally use the word TMT for all quenched and tempered bars
but I believe it has a different connotation than the treatment being used
in India presently.
Could someone pl expand on this.

Thanks. This is another FAQ. and very relevant

Actually the base process has a metallurgical terminology of Thermo
Mechanical Treatment (TMT). Thermex and Tempcore are both reputed trade
names registered by the two major providers of this TMT technology. When
PROPERLY OPERATED both Thermex and Tempcore can produce good quality TMT
rebars. However, the technology of imparting online heat treatment to
rapidly moving red-hot bars is quite involved and requires specialised
design of equipment as well as control devices. Computerised process
control is often required to dynamically adjust the many rapidly changing
parameters depending on the chemical composition of the steel, the desired
grade and size of the rebar etc. A makeshift arrangement, irrespective of
the quantity of water used, is highly unlikely to produce the desired

I take this opportunity to provide some insight into the TMT process.
Manufacture of high strength reinforcing steel is usually carried out
through one of three alternative process routes:
1. Hot rolling after micro-alloying
2. Hot rolling followed by cold work and
3. Hot rolling followed by heat treatment (TMT).
The TMT process is actually a heat treatment imparted to steel on-line just
after rolling. . For the larger diameters, a small addition of micro-alloys
may be resorted to.  After leaving the last stand of the rolling mill, the
bars are quenched (rapidly cooled) in water from a final rolling
temperature of about 950 Deg. C.  The quenching is partial - only until a
surface layer has been transformed from austenite (a steel phase stable
only at very high temperatures) to martensite (a steel phase stable at
ambient temperatures). This controlled quenching is achieved in one or more
specially designed on-line water cooling devices through which the steel
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 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 350deg.C because the annealing action
sets in, the TMT bars are stable up to a temperature of at least 550 Deg.
C, a temperature at which is tempered. This accounts for a much better fire
resistance to the structures made with TMT bars.

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