Nowadays TMT steels are commonly used in constructions. What is TMT? Why is it used than other types of steel? Well, before TMT was introduced Mild Steel (MS) bars, Cold Twisted Deformed (CTD) bars were primarily used with concrete for constructing buildings. Those steel were graded Fe 250 but their low strength was a problem for buildings requiring more strength.
TMT reinforcement bars
Thermo Mechanically Treated (TMT) bars were introduced in India during 2002-2005. It was an advancement from the earlier types of steel bars because of its higher tensile strength and ductility. The TMT steel bars are produced in the same way as other steels are produced but specific steps involved is what makes it different. They are:
- Self -Tempering
In Quenching process, the hot bars coming out of the last rolling mill-stand are rapidly cooled through a series of water jets. To obtain uniform cooling, an automated intelligent system controls the water jets pressure to manage the temperature difference through the cross-section of the bars. It results in the bars having a hardened outer surface known as martensite with a hot and soft core.
Self Tempering is the stage in which the heat from the core flows to the cooler surface, causing the tempering of the outer martensitic layer into a structure called “Tempered Martensite”.
In Annealing, the inner core is turned into ferrite-pearlite aggregate composition by cooling it separately on a cooling bed.
The result is a reinforced bar, providing an optimum combination of high strength, ductility, bendability & flexibility. Another notable factor is that the formation of an inner core and a hardened outer layer, which is absent in steel bars. The existence of the hardened outer tempered martensite layer leads to higher tensile strength.
The true TMT rebar grade is the Fe 415, where 415 denotes yield strength. It is the only grade where tensile strength and ductility property are practically matching together, others are special grades where we need to make compromises, but sometimes they are unavoidable. Other special TMT grades are Fe 415D, Fe 415S, Fe 500, Fe 500D, Fe 500S, Fe 550D, Fe 600 etc.
How are different grades for TMT steel bars achieved?
Higher tensile strength special grade steel rebars are achieved using the same TMT process that is used to produce Fe 415. The benefit of the TMT process is that it can produce different grades of steel rebar by making slight changes in the process. If the steel is quenched a little more, the outer martensite layer that provides the high strength to the steel rebar becomes thicker at the expense of the soft inner core that makes them brittle.
In any TMT grade, the outer core and inner core should be in proportion where neither ductility nor tensile strength is compromised. If one exceeds the other, TMT will not have sufficient properties. If the outer core is increased at the expense of inner core, TMT rebars will have more strength; but less ductility. Higher tensile strength of TMT Steel rebars is a desired quality but not at the expense of the ductile ferrite pearlite inner core.