Thermomechanical processing

Thermomechanical processing, also known as thermo-mechanical treatment (TMT), is a metallurgical process that integrates work hardening and heat-treatment into a single process.

The quenching process produces a high strength bar from inexpensive low carbon steel. The process quenches the surface layer of the bar, and simultaneously tempers the quenched layer using the heat in the bar's core.

Steel billets 125mm² ("pencil ingots") are heated to approximately 1100ºC in a reheat furnace. Then, they are progressively rolled to reduce the billets to the final size and shape of reinforcing bar. After the last rolling stand, the billet moves through a Quench box. The quenching converts the billet's surface layer to martensite. The core remains hot, and austenitic. A microprocessor controls the water flow to the quench box, to manage the temperature differences in the bars. The temperature differences assure that both processes occur, and bars have the necessary mechanical properties.

The bar leaves the quench box with a temperature gradient through its cross section. Heat flows from the bar's centre to its surface so that the bar's heat correctly tempers an intermediate ring of martensite and bainite.

Finally this quenching automatically tempers the austenitic core to ferrite and pearlite during the slow cooling of the bar on the cooling bed.

These bars therefore exhibit a variation in microstructure in their cross section, having strong, tough tempered martensite in the surface layer of the bar and a refined, tough & ductile ferrite and pearlite core. When the cut ends of bars are etched in Nital (a mixture of nitric acid and methanol), three distinct rings appear: 1. A tempered outer ring of martensite, 2. A semi-tempered middle ring of martensite and bainite], and 3. a mild circular core of bainite, ferrite and pearlite. This is the desired micro structure for quality construction rebar.

Lower grades of rebar are twisted when cold to increase their strength. However, after thermo mechanical treatment (TMT), bars do not need twisting to increase their strength. As there is no twisting, no torsional stress occurs, and so the torsional stress cannot form surface defects in TMT bars. Therefore the bars made with TMT resist corrosion better than cold twisted and deformed (CTD) bars.