TMT refers to the metallurgical process of "Thermo Mechanical Treatment" of the steel done during the manufacturing process. Diamonds Fe 500D TMT bars are hot rolled from steel billets/ingots and then subjected to a controlled thermo mechanical treatment in three successive stages.
Quenching is generally done by cooling at a sufficiently high rate to avoid undesirable internal microstructure as well as to ensure uniform mechanical properties, and to minimize residual stresses.
Tempering is then done to toughen the metal by transforming brittle Martensite into tempered Martensite. As the bar leaves the quenching box and is subjected to air-cooling, the core remaining hot compared to the outer surface allows heat to flow to the surface, causing tempering of the outer martensitic layer into a structure called Tempered Martensite.
This takes place on the cooling bed where austenitic core is transformed into a ductile ferrite-pearlite core. The final structure consists of an optimum combination of a strong outer layer (tempered martensite) with ductile core (ferrite-pearlite), giving the bars a unique combination of higher strength and ductility.