Thermomechanical processing
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Thermo-mechanical treatment (TMT) is …
The quenching process produces a high strength bar from steel of low chemical composition. This consists of special heat treatment process i.e. quenching of surface layer of bar and tempering of this quenched layer by the heat in the core.
Steel Billets/Pencil Ingots 125mm² are heated to approximately 1100ºC in a reheat furnace and rolled through a sequence of rolling stands which progressively reduce the billets/pencil ingots to final size and shape of reinforcing bar. On leaving the last stand the bar passes through the quench box where the surface layer are quenched to martensite while the core remains austenitic. A micro-processor is able to control the water flow in the quench box to ensure that the desired tempering temperature and resultant mechanical properties are attained.
The bar leaves the quench box with a temperature gradient through its cross section. Heat then flows from centre to the surface resulting in self tempering of martensite.
Finally the austenitic core transforms to ferrite and pearlite during the slow cooling of the bar on the cooling bed. These bars therefore exhibit a variation in microstructure in their cross section, having a strong & tough tempered martensite in the surface layer of the bar and a refined, tough & ductile ferrite/pearlite core. When cut ends of bars are dipped in the NITRAL Solution (10% Nitric Acid + 90% methanol Acid) three distinct rims of Tempered Core of Martensite, Semi-tempered core of Martensite + Bainite, & Mild Core of Bainite + Ferrite/Pearlite, which exhibits desired micro structure for quality construction.
As after this process there is no need to twist these bars to increase their strength, twisting process is omitted here. As there is no twisting of bars, there is no stress formation in the bars and hence higher corrosion resistance as compared CTD (cold twisted & deformed) bars.