Thermo-mechanical treatment (TMT):
is an acronym for the phrase ‘thermo-mechanical treatment’. The Bureau of Indian Standards while issuing the new
code IS: 1786-1985 was the first to use this phrase while making reference to the ‘Technological
advances during the last few years in the field of deformed bar production..... Microalloying ......... and thermomechanical
treatment process are worth mentioning in this field’ in para 0.2.1 of its FOREWORD.
technological advances referred to in the IS code are the quenching processes ‘Tempcore’ and ‘Thermex’
which received world patents and quick global acceptance amongst civil engineers. It must be stressed for the benefit of all
(and the civil engineers in particular) that neither of these two patented processes employs any mechanical treatment whatsoever.
Instead they obtain the unique properties in the rebars by “quenching and tempering” as explained earlier. After
rolling, the deformed steel bar is passed through a quenching line whereby the periphery is subjected to intense water
quenching in a short time whereas the core remains largely unaffected. On leaving the quenching system the core heat is utilised
to temper the quenched outer surface. The resulting structure is a concentric tempered martensite periphery with a
softer ferrite-pearlite core.
no mechanical treatment is involved in the technological advances referred to by B.I.S.
their part the steel majors TISCO and SAIL, which were the first to employ the newly developed processes, exploited the same
by a vigorous publicity of their “TMT” rebars. Thus the innocent use of an incorrect phrase by BIS gained popularity
in India and this is now looked upon by many civil engineers as an improvement on
the old CTD bars and is often mistaken for a brand name. This is far removed from the truth.
Every hot rolling mill for long products undertakes thermal and mechanical work in the
normal course of rolling. Thus, each one of them genuinely and legally produces “TMT bars”. Nothing stops them
from claiming this and selling their products as ‘TMT’ bars even when they do not employ any Tempcore or Thermex
process or for that matter any sort of quenching system, proven or unproven. And, there are about 2,500 rolling mills in India for long products.
discerning customer must not blindly just ask for ‘TMT’ bars merely because it is fashionable to do so today or
because it is in vogue, so to speak. He cannot and should not assume that he is buying a product superior than the old rusty
CTD rebars. All rebars must be purchased based on the properties of yield strength, tensile strength and elongation values.
To ignore these will only be putting your construction at peril. Many civil engineers, even today, assume that ‘TMT’
bars have yield strength of 415 N/mm2 but better elongation than CTD bars. He
should know that nothing in the current laws or regulations prevents the rolling mill to just sell untreated and untwisted
deformed bars as TMT bars - even though the strength can be as low as only 300 N/mm2.
damage has already been done by use of the label ‘TMT’ and there is now an urgent need to use the correct phrase
“Quenching & Tempering” as used globally – or any other suitable phrase which cannot be exploited by
persons who do not have the proper technology – if we are to limit further damage. The consequences of not doing so
are very frightening indeed in the years to come in view of the massive expenditure foreseen over the next decade in infrastructure
and rural development.