Thermo-Mechanical Treatment (TMT), is a metallurgical process that integrates work hardening and heat-treatment into a single process. A description of its application in rebar steel is as follows.
Mild steel square billets of sizes 100/110/125mm², having the appropriate chemical constituents i.e. Carbon, Manganese, Sulphur, and Phosphorus, are heated to temperatures from 1050°C – 1150°C, in the reheating furnace. The heated raw material is passed through a series of DC controlled Rolling Mill Stands, to produce the finished steel at a temperature of around 900°C – 950°C.
The bars are hot rolled from tested raw materials of a required chemical specification. This is done through a series of electronically controlled finishing stands and online PLC controlled thermo-mechanical treatment. They are progressively rolled to reduce the billets to the final size and shape of the reinforcing bar. After the last rolling stand, the billet moves through a quench box.
The quenching converts the bar surface layer to Martensite, and causes it to shrink. The shrinkage pressurizes the core, that helps 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 that the bars have the necessary mechanical properties.
The bar leaves the quench box with a temperature gradient through its cross section. As the bar cools, heat flows from the bar’s center to its surface, so that the bar’s heat and pressure correctly tempers an intermediate ring of Martensite and Bainite.
Finally, the slow cooling after quenching automatically tempers the Austenitic core to Ferrite and Pearlite, on the cooling bed.
These bars, therefore, exhibit a variation in their cross section, having strong, tough and tempered Martensite on the surface layer of the bar, an intermediate layer of Martensite and Bainite, and a refined, tough and ductile Ferrite and Pearlite core.
When the cut ends of the TMT Bars are etched in Nital (a mixture of nitric acid and methanol), three distinct rings appear:
- A tempered outer ring of Martensite,
- A semi-tempered middle ring of Martensite and Bainite, and
- A mild circular core of Bainite, Ferrite and Pearlite.
This is the desired micro structure for any quality construction Rebar.