Thermo Mechanically Treated (TMT) steel bars have a huge role to play at the structural level in the construction industry. The quality of the bars has a significant bearing on the durability of the completed structure. The attributes of raw materials used as well as the finesse in the manufacturing techniques employed are key factors in the steel production industry.

It is important for steel bars to have maximum tensile strength without compromising on its flexibility and ductility. While withstanding high pressure and weight, steel bars should have the quality to be made into any size or shape matching the requirements. Corrosion, fire and earthquake resistance and ductility are the other desirable qualities of TMT bars from steel bar manufacturers. It is the responsibility of the TMT bar company and TMT bar supplier to ensure these qualities if they hope to belong to the league of top steel manufacturers of the country.

When being used in construction the bars have to be bent or resized according to design needs. Flexibility of the bar is a salient feature that helps in ensuring that  no wastage of time, money or effort happens and timelines aligned with construction contracts are adhered to.

Similarly after the construction is over, the bars should be able to elongate and regain original shape without cracks in the structure or undue strain. This is important for the structure to withstand the vagaries of weathers or any other loads

There are broadly 5 main stages in Thermo Mechanical Treatment which are:

  •         Extraction of material(iron) from raw iron ore
  •         Heating, Rolling and formation of Reinforced bar
  •         Quenching
  •         Self-tempering
  •         Atmospheric Cooling

The 3rd stage of quenching is the most important for the TMT bars to acquire the tensile strength and flexibility.

TMT Bars undergo thermal processing where compression and rolling of the bars is done through heat treatment, quenched with water and eventually heated and cooled at varying temperatures. In the quenching process, the TMT bars are quenched in quenching boxes where only the surface of the bar is cooled and the core of it retains heat and stays soft. Heat is supplied to the surface for tempering and the outer layer hardens into forming a material called Martensite. Martensite is known for its hardness and resistance to pressure and stress. The core or the inner part of the bar is soft and stays austenitic. This process enables the bars to have a high level of flexibility combined with superior tensile strength.

In the next stage, the TMT bars are passed through cooling beds to make the core have a ferrite and pearlite structure. This gives the bar the necessary ductility. The fine balance between tensile strength and ductility is thus ensured by the strong pressure resistant Martensite exterior and the ductile, moldable and flexible ferrite pearlite interior.

How is the flexibility of TMT bars tested?

A specific test called Bending and Rebending test is undertaken by TMT bar manufacturers to determine the flexibility levels of TMT bars. In this the TMT bars are bent to 135 degrees and kept in boiling water for approximately 30 minutes. In the next stage it is re-bent to approximately 157.5 degrees. This is done to ensure that the TMT bars are able to bend without any cracks appearing on the surface along the bends.

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