What is Thermo mechanical Processing?
Thermo mechanical processing is a metallurgical process that combines mechanical deformation process like compression or forging, rolling etc. with thermal processes like heat-treatment, water quenching, heating and cooling at various rates into a single process.
TMT bars or Thermo-Mechanically Treated bars are generated through this thermo mechanical process. It is this process which makes the bar having a tough outer core and a soft inner core. First step of the manufacturing process involves passing billets into a furnace through a rolling mill stand. Steel billets are heated to approximately 1200°C to 1250°C in a reheat furnace. Then, they are progressively rolled to reduce the billets to the final size and shape of reinforcing bar.
Then these rolled steel are again passed through the Quenching Box-water cooling system. While passing the steel through the water cooling system, the water pressure is optimized. The quenching converts the billet's surface layer to martensite, and causes it to shrink. The shrinkage pressurizes the core, helping to form the correct crystal structures. The core remains hot, and austenitic. A microprocessor controls the water flow to the quench box, to manage the temperature difference through the cross-section of the bars. The correct temperature difference assures that all processes occur, and bars have the necessary mechanical properties. The quenching process produces a high strength bar from inexpensive low carbon steel. The process quenches the surface layer of the bar, which pressurizes and deforms the crystal structure of intermediate layers, and simultaneously begins to temper the quenched layers using the heat from the bar's core. The sudden quenching and drastic change in temperature toughen the outer layer of the steel bar, thus making it super tough and durable.
Once this process is over, the TMT bars are subject to atmospheric cooling. This is done in order to equalize the temperature difference between the soft inner core and the tough exterior. Once the TMT bar cools down, it slowly attains Martensite and austenitic conditions. The inner core remains soft giving the TMT bar great tensile strength and elongation point. This design is unique to the TMT bars and gives superior ductility to the bars. This helps the TMT bar to become corrosion-resistant and also boost its weldability. After Thermo mechanical processing, TMT Bars become much stronger compared with conventional CTD Bars and give up to 20% more strength to concrete structure with same quantity of steel.