The Thermo-Mechanically Treated TMT bars are high strength reinforcement bars.
They have a hardened outer surface and a softer core that results in superior flexibility and higher ductility in the bars making them ideal for construction.
Steel is used widely in the construction & automobile industry because of its higher tensile strength. However, steel structures are highly affected by corrosion, fire and other environmental factors.
To ensure high standards of integrity, safety and longevity, steel has to go through various processes to increase its ductility, hardness, corrosion resistance.
Thermo-Mechanical Treatment is one of these processes.
Here are the 5 steps involved in the manufacturing process of TMT bars:
- Extraction & Conversion of STEEL from Iron Ore
In this stage, the iron is extracted with the help of raw materials such as coke or fluxes
(limestone or dolomite) in the molten state.
The iron is then subjected to pre-heating and converted to steel in a converter. It is again subjected to ladle heating for refining the chemistry of the steel.
The molten steel, thus produced, is poured into the casting machine to produce billets of about 130 mm2 or more.
- The Formation of Reinforced Bars
The steel billets are heated approximately 1200-1250 degree Celsius (2192 F-2282 F) and then rolled to reshape the billets into the final required size and shape by passing the billets through a rolling mill stand.
As the hot reinforced bar leaves the final rolling mill stand, it is instantaneously quenched. This is a type of heat treatment where the rebars are rapidly cooled by jets of water to obtain certain material properties.
This process helps prevent the occurrence of undesired processes such as phase transformations.
As a result of this process, the outer side of the quenched bar becomes hardened and cold while the core still remains soft.
- Self Tempering
After leaving the quenching box, a temperature gradient is formed through the cross-section of the quenched bar.
This results in the heat flow from the core of the bar as it is at a relatively higher temperature.
The core still stays in the austenitic state (a typical cubical crystalline structure, commonly called as gamma-phase iron) at this stage.
- Atmospheric Cooling
The bars are subjected to atmospheric cooling to equalize the temperature difference between the soft inner core and the hardened exterior.
Once the bars are completely cooled down, the austenitic core gets transformed into a ductile ferrite-pearlite structure.
The lower grade TMT bars are twisted when cold. This hardening helps increase their strength.
The production quality of TMT bars depends on three major factors:
- Quality of raw materials
- Properly designed and automated rolling mill stand
- A well designed quenching and tempering technology