Rebars/TMT
Thermomechanical processing, also known as thermo-mechanical treatment (TMT), is a
metallurgical process that integrates work hardening and heat-treatment into a single
process.[1] A description of its application in rebar steel follows.
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.
Steel billets 125mm² ("pencil ingots") are heated to approximately 1100°C 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, 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 bar leaves the quench box with a temperature gradient through its cross section. As
the bar cools, heat flows from the bar's centre 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 microstructure in their cross section,
having strong, tough, tempered martensite in 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 TMT 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.
In contrast, lower grades of rebar are twisted when cold, work hardening them to
increase their strength. However, after thermo mechanical treatment (TMT), bars do not
need more work hardening. As there is no twisting during TMT, no torsional stress
occurs, and so torsional stress cannot form surface defects in TMT bars. Therefore TMT
bars resist corrosion better than cold, twisted and deformed (CTD) bars.