Technically TMT steel bars and QST steel bars are the same. TMT steel bars are manufactured using Thermo Mechanical Treatment method using 3 process – quenching (Q), self tempering (ST) and annealing. So, technically actual term is TMT.
But the unethical manufactures who tries to sell their product with more profit by spreading gullible information in the market that same tmt steel bar with a more marketing friendly name like – QST steel bars. The customer without having accurate knowledge wrongly treats these both products as different. And thus the seller generates some added margin.
Manufacturing process of TMT bars for giving higher strength.
After manufacturing rolled hot steel bars are placed into waterjets that makes the outer surface of the steel bars to drop its temperature quickly. This sudden cooling results in changing the mechanical property and crystal structure of the steel bars. With the help of quenching the outer core becomes martensite (a very hard form of steel crystalline structure) and inner core remains austenite (gamma-phase iron).
After quenching steel bars are left in the normal room temparature. So heat from the inner core get dissipated to the outer core which converts the outer core of steel bar to tempered martensite layer.
This involves natural slow cooling of steel bars. As a result of annealing the steel bars attains atmospheric temperature and thus inner core turns to ferrite-pearlite. The outer tempered martensite layer gives tensile strength for the steel bars while inner core provides ductility. These two properties help the tmt steel bars to protect the building.
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