TMT Steel stands for Thermo Mechanically Treated Steel. Hard outside and softer inner core are the two surfaces of the TMT bar. It is mostly used in concrete reinforcement structures, flyovers, dams, bridges, thermal plants, industrial structures and hydel power plants.
The manufacturing process of Thermo Mechanical Treatment includes five steps:-
- Extraction of iron from its ore.
- Heating, rolling and forming of a reinforced bar.
- Quenching.
- Self-tempering.
- Atmospheric cooling.
Step 1- Extraction of iron from its ore:
Raw materials like iron ore, coke or flukes are utilized to extract iron that is kept in the molten state. After this, molten iron is subjected to pre-treatment and converted to steel in a converter and is then subjected to ladle heating for refining the chemistry of the steel. Then, the molten steel is poured into a casting machine to produce billets of about 130 mm2 or more.
Step 2 - Heating, Rolling and formation of reinforced bar:
The steel billets are made by heating to 1200°C [2192 F] to 1250°C [2282 F] and are rolled to reshape the billets into the final size and shape of the reinforced bar before passing the billets through a rolling mill stand.
Step 3 - Quenching:
The hot reinforced bar leaving the final rolling mill stand is instantaneously quenched. Quenching process stops the occurrence of undesired processes such as phase transformations. It completes this by minimizing the time frame during which these undesired reactions have a higher chance of occurring. A sudden drastic change in temperature toughens the outer layer of the steel bar improving its tensile strength and durability. It is because, quenching converts the outer surface of the reinforced bar to Martensite,a hard form of steel and causes it to shrink that in turn pressurizes the core and helps to form the correct crystal structures.Therefore,the surface of the quenched bar becomes cold and hardened whereas the core still remains hot.
Step 4 - Self- tempering:
A temperature gradient is formed across the cross-section of the quenched bar after leaving the quenching box. As an outcome heat flows from the core at a relatively higher temperature to the outer surface.This causes the correct tempering of the outer martensitic layer into a structure known Tempered Martensite and the formation of an intermediate ring of Martensite and Bainite. The core still stays in the austenitic state at this stage.
Step 5 - Atmospheric Cooling:
The bars are subjected to atmospheric cooling to equalize the temperature difference between the soft inner core and the hardened exterior after the self-tempering process. When the bars are completely cooled down, the austenitic core gets transformed into a ductile ferrite- pearlite structure.
Hence,TMT bars are having the desired tensile strength paired with high elongation as required in the construction of building. To buy superiority quality of TMT Steel bar Click here.
