A) Twisted bars and TMT are both high strength reinforcement steel bars wherein Twisted refers to cold treated bars and TMT refers to hot treated bars.
Both of them are rolled in the same process i.e. hot rolling in a temperature of 1200oC to 1000oC drop.
CTD (Cold twisted deformed) Bar:
1) It is entered into cooling bed from the final pass (roll) then kept for natural cooling and being twisted in the room temperature.
2) The bar is stretched to some extent during twisting by which, the inside grains are in crystle forms, are crushed and thereby, the bars attains the required strength. The cold twisting gives to the longitudinal rib a spiral shape.
3) The reinforcement bars produced by this method was popularly known as Torsteel or CTD (cold Twisted Deformed) bars. In these bars the strength of the steel is obtained through cold working and not by the adjustment in the chemistry of the steel.
4) The process of cold working of reinforcement steel is a mechanical process which involves stretching and twisting of reinforcement bars rolled from mild steel, beyond the yield plateau, and subsequently releasing the load.
5) For many years these bars were dominating the use of reinforcement steel. Because of reasonable carbon in this reinforcement steel, this steel is weldable.
However, these bars have the following disadvantages too:
1) The cold twisting process has adverse effect on the steel ductility.
2) The process breaks the protective blue oxide; it exposes the surface of the reinforcement steel to corrosion.
Thermo-mechanical treated bars’ (TMT):
This treatment takes place in three successive stages:
(a) Quenching – The hot rolled bar leaving the final mill stand is rapidly quenched by a special water spry system. This hardens the surface of the bar to a depth optimised for each section through formation of martensitic rim while the core remains hot and austenitic.
(b) Self Tempering – When the bar leaves the quenching box, the core remains hot compared to the surface allowing heat to flow from to the surface causing tempering the outer martensitic layer into a structure called Tempered Martensite. The core still remains austenitic at the stage.
(c) Atmospheric Cooling – This takes place on the cooling bed where austenitic core transformed into ductile ferrite pearlite structure. Thus the final structure consists of an optimum combination of strong outer layer (tempered martensite) with ductile core (ferrite pearlite).
As there is no twisting during TMT, no torsional stress occurs, and so torsional stress cannot form surface defects in TMT bars. Therefore TMT bars resist corrosion better than cold, twisted and deformed (CTD) bars. For further more information on TMT bars please visit the following link
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