What is a TMT steel rod and what is the difference between HYSD and TMT bars? What are the pros of TMT bars over HYSD and mild steel?

TMT stands for Thermo Mechanically Treated. The TMT bars have a hard outer surface and a softer core. Their manufacturing process includes hot rolled steel wires passed through water. This makes the surface hard and keeps the core warmer and softer. This helps in making the steel corrosion resistance and also increase its weldability. Technically TMT means a type of quenching and tempering process and it would not be precise to call it a Thermal+Mechanical treatment. Because, even the HYSD bars are treated at high temperatures and then rolled and twisted for shaping and hence it is also thermally + mechanically treated. So the difference lies in the exact process of making these two types of bars and not just in their names.

On the other hand, HYSD stands for High Yielding Strength Deformed bars. These bars have more yield strength than Mild Steel. Unlike TMT bars, HYSD bars are both hot or cold worked. They are graded as Fe 415 or Fe500.

HYSD = Torsed/twisted steel (TOR Steel Bars)
HYSD = Cold Twisted Deformed Steel Bars (CTD Steel Bars)                 
HYSD = High Yield Strength Deformed Bars (Of course!)