TMT bars play an important role in the construction industry. Also known as thermo-mechanically treated bars, TMT bars are high-strength reinforcement bars distinguished by their tough outer crust, beneath which sits the soft inner core. The manufacturing of TMT bars involves a series of processes that dictate its strength and flexibility. These bars are well-known for their high tensile strength and ductility. Different grades in which TMT bars are available include Fe 415, Fe 500, Fe 550, and Fe 600.
The main factors that determine the quality of TMT bars are as follows:
- Quality of the raw materials used
- The rolling mill used as that ensures uniform shape of all rebars
- Quenching and tempering system
The 5 main processes involved in the manufacturing of TMT bars are:
- Extraction of material from iron ore
- Primary steel making
- Atmospheric cooling
Extraction of Iron from its Ore
Iron ore is the most integral participant of the TMT bar and hence its extremely critical to select the right source of iron ore. Raw materials such as iron ore, coke, dolomite and limestone are put into a blast furnace where they are piled, recovered and mixed in the required proportion. All these materials are treated with hot gases while in their molten form. Iron ore undergoes the beneficiation process to raise the iron substance. The molten iron is poured into a casting machine to produce the desired shape. The coal is converted to coke for future purposes.
Primary Steel Making
Once the pretreatment has been successfully carried out in the TMT manufacturing process, the steel is passed through the EAF (Electric Arc Furnace) to refine the composition of TMT steel bars. The steel melts and is transferred to a ladle from where it is shifted to a continuous casting machine. Liquid steel flows out of the ladle into a casting ladle and later into a water-cooled mold where solidification begins. The billets coming out of the casting machine are sized according to the required length.
Quenching is the most important stage in the manufacturing of TMT bars. After the steel has been processed and converted into reinforced bars, they become prone to acquiring undesirable properties. Therefore, the heated steel bars are ‘quenched’ or cooled down via a rapid cooling process which prevents the material from obtaining those undesirable properties. Rapid cooling reduces the time window during which the material is thermodynamically favorable for obtaining undesirable characteristics such as crystallization.
Once the rebar is out of the quenching box, the core in comparison to the surface remains hot. The residual heat from the core transfers to the outer surface of the bar, which causes the desired tempering of the outer martensitic layer into a structure called ‘tempered martensitic’ The tempering process ensures the proper ductility of material, simultaneously preserving its high yield strength.
This is the final stage involved in the TMT manufacturing process. Once self-tempering is done, the bars are laid on a cooling bed at normal temperature. This is done to level the difference between the soft inner core and the hardened surface. This leads to the conversion of the austenitic core into a ferrite-pearlite structure. Therefore, the final structure has a strong outer layer with a ductile core. Cooling rods are put in rolling mills for the production of TMT bars. Atmospheric cooling improves the tensile strength making the bars highly ductile and weldable.
TMT bars are indispensable construction materials in most of the construction projects. Their excellent strength, superior bonding properties with concrete and earthquake resistance qualities are some of the reasons why they are highly preferred.
New techniques employed in the TMT manufacturing process are based on new innovations that further add to the quality of TMT bars. Failing to use them can result in low-quality TMT bars. Therefore, the procurement team or individual buyers must ensure that they analyze the manufacturing process of TMT bars carefully before finalizing a particular brand and buy only from reputed TMT bar manufacturers.