TECHNICAL DETAILS:

TMT is produced by thermo-mechanical treatment and not by cold twisting. Therefore, there are no torsional residual stresses in the bar, which result in superior corrosion resistance characteristics compared to traditional cold twisted bars

TMT due to its low carbon equivalent has a weldability which is superior to conventional cold twisted bars. It can be butt-welded or lap-welded using ordinary rutile coated electrodes of matching strength. In manual arc welding no prewarming or post heat treatment is necessary

TMT bars are supplied with section weight lower than the nominal and within permissible parameters

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, zero flaws on the surface and many more.

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.
Until around 2005-2006, HYSD was widely used as rebars in building structures. Since the mass commercialization of TMT steel, HYSD is mostly not prefered today. Credit goes to the pros TMT has over HYSD. For a heavy structure , on an average, TMT can help optimize the steel requirement to an extent of 7 to 10 percent.

On the other hand HYSD is manufactured by a process of twisting the rod after casting. This by nature creates surface flaws and the yield strength of the rod might get affected.

http://orccng.com/2016/05/06/insist-tmt-labeled-iron-rods-building-construction-works/