Also known as thermo-mechanically treated bars, TMT bars are high-strength reinforcement bars with tough outer core and soft inner core. Not only are these bars economically viable but also offer unmatched durability and ductility, when compared to conventional steel bars.
Thanks to the special thermo mechanical processing that produces steel with a special ferrite pearlite core enclosed in tempered martensite, TMT bars possess amazing strength and anti-corrosive properties among others.
The quality of TMT bars depend on many different factors such as:
- The machinery and equipment being used for self-tempering and quenching process
- The quality of the material used
- The rolling mill which is responsible for shaping the material
The key processes involved in the manufacturing of TMT bars include:
- Extraction of material from iron ore
- Primary steel making
- Atmospheric cooling
- Extraction of Iron from its Ore
Iron ore plays the most important role in the manufacturing of the TMT bar, which makes it critical to select the right source of iron ore. Under the process, raw materials such as coke, iron ore, dolomite and limestone are put inside a blast furnace where they are piled, recovered and mixed in the right proportions. While still in molten stage, all these materials are treated with hot gases. With the help of beneficiation process the iron ore is raised to iron substance. The molten iron is then poured into a casting machine to be shaped into the desired shape. This is followed by conversion of coal to coke for future purposes.
- Primary Steel Making
After the pretreatment has been implemented successfully in the TMT manufacturing process, the steel is made to pass through the EAF (Electric Arc Furnace) for refining the composition of TMT steel bars. When steel melts, it is transferred to a ladle from where it is transferred to a continuous casting machine. The solidification begins when the liquid steel flows out of the ladle into a casting ladle and later into a water-cooled mold. The billets that come out of the casting machine are sized depending on the required length.
Quenching forms the most crucial stage in the TMT bars manufacturing process . Once the steel has been processed and converted into reinforced bars, they become susceptible to acquiring unwanted properties. This makes the quenching or cooling of heated steel bars extremely important which prevents the material from obtaining those unwanted properties. The time window during which the material is thermodynamically favorable for acquiring unwanted properties such as crystallization is reduced via rapid cooling.
After the rebar is out of the quenching box, the core remains hot when compared to the surface. The residual heat from the core gets transferred to the outer surface of the bar, which causes the desired tempering of the outer martensitic layer into a structure called ‘tempered martensitic’. This process makes the bars highly ductile thereby preserving its high yield strength.
- Atmospheric Cooling
This is the final step involved in the TMT manufacturing process. It involves laying down the bars on a cooling bed at normal temperature. This levels the difference between the soft inner core and the hardened surface and leads to the conversion of the austenitic core into a ferrite-pearlite structure. The resulting final structure has a strong outer layer with a ductile core. For producing TMT bars, cooling rods are put in rolling mills. Atmospheric cooling improves the tensile strength which makes the bars highly ductile and weldable.
TMT bars are a highly preferred material in the world of construction and rightly so. They possess many unique qualities that make them an indispensable construction material. Innovations in the TMT manufacturing process have further added to the quality of TMT bars. Therefore it is important to buy from well-known and reputed TMT bar manufacturers who adhere to the latest processes and technology while manufacturing TMT bars.