Thermo-mechanical treatment (TMT):

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.

The 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.

Thus, no mechanical treatment is involved in the technological advances referred to by B.I.S.

On 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.

The 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.

Enough 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.

Basic Objectives of developing Tempcore & Thermex Technologies:

It must be clearly comprehended that both Tempcore & Thermex technologies were developed in the mid-eighties to produce rebars that had far superior properties than that available in CTD bars. They essentially aimed at meeting the then global demand for low cost rebars that had high yield strength of about 450 to 500 N/mm² (to effect saving in steel used) combined with good ductility so that adequate safety was feasible when used in high seismic zones. This meant that these quenched and tempered rebars, as produced by Tempcore and Thermex Systems, genuinely gave civil engineers a product that was by far superior to CTD rebars.

Sub-standard & Defective “TMT” Rebars

Dr. C. S. Vishwanath of Bangalore and his colleagues have done pioneering work in study of the various “TMT” rebars available in the country today and have published many articles on the subject. His team has already warned the users about how many mills are flooding the market with sub-standard and defective “TMT” rebars. They have found many “TMT” bars of the main producers and secondary mills with a Yield Strength in the range of only 350-390 N/mm². This only strengthens my worry that some civil engineers today assume all rebars are of Grade Fe 415 and so avoid testing.

Many a ‘TMT’ rebar manufacturer has approached the author for guidance and improvement in their ‘TMT process’ because the test results revealed that the minimum YS was about 350 to 390 N/mm² only as against 415 specified in IS: 1786-1985 for grade Fe 415. To save capital costs such mills have developed their own quenching lines in their rolling mill fitted with Edenborne coilers. Now, it is a well established fact that the rolled bar should be at about a minimum temperature of 730 to 760 ^o C for coiling with such coilers. On the other hand, for proper “Quenching & Tempering” to take place the equalising temperature should be in the region of 580 to 650 ^o C depending on the size and grade to be made. Thus it is clear that these mills cannot make proper “TMT” rebars of even Fe 415 grade let alone Fe 500. This has been pointed out every time we receive requests for guidance but unfortunately the production of such ‘TMT’ rebars continues even today and one such mill has become a sort of brand name in India! Such mills (with Edenborne coilers) merely carry out inadequate quenching and the important ‘tempering’ phase is almost non-existent.