What is TMT?
Thermo-Mechanical Treatment.
TMT is a metallurgical process in which high-temperature rolled steel bars are rapidly quenched on the surface and then allowed to self-temper using their own core heat. The result is a bar with three distinct microstructural zones — tempered martensite at the rim, an intermediate transition layer, and a soft, ductile ferrite-pearlite core.
This unique structure gives TMT bars their signature combination of high tensile strength and excellent elongation — the two properties that matter most in seismic zones and high-rise construction.
The Process
Four stages, one superior steel.
Step 01
First Stage (Hot Rolling Process)
The lab-tested raw material billets from iron ore are used for producing TMT bars and are heated in a pusher furnace up to an optimum temperature of 1,100°C to 1,200°C. The heated raw material passes through various rolling stands, obtaining the desired size and length.
Step 02
Second Stage (Quenching)
The hot-rolled bars, after leaving the final mill stand at about 900°C to 950°C, pass through a quenching box and are rapidly quenched by a special water-spray system. The outer surface temperature is reduced from 950°C to 300°C, hardening the surface to a depth optimised for each section through formation of a martensite ring while the core remains hot austenitic.
Step 03
Third Stage (Self-Tempering)
The core of the bar, after leaving the quenching box, still remains hot compared to the surface and allows heat to flow from the core to the surface — converting the outer martensitic layer into a solid structure called tempered martensite around the core.
Step 04
Fourth Stage (Atmospheric Cooling)
In the final stage, the bar moves to the cooling bed, transforming the core into a ductile ferrite-pearlite structure. The bar now consists of an optimum combination of a strong outer layer (tempered martensite) and a ductile core (ferrite-pearlite), giving Prime TMT bars higher strength — enabling structures to face even earthquakes successfully.
Specifications
Mechanical Properties & Chemical Composition
| Mech. Parameter | Fe 500 | Fe 500D | Fe 550 | Fe 550D |
|---|---|---|---|---|
| Grade | YSUTS 500 - 545 | YSUTS 500 - 565 | YSUTS 565 - 600 | YSUTS 565 - 600 |
| Elongation % (Min) | 12.00 | 16.00 | 12.00 | 14.50 |
| Chemical Comp. % | ||||
| Carbon | 0.30 Max | 0.25 Max | 0.30 Max | 0.25 Max |
| Sulphur | 0.055 Max | 0.040 Max | 0.055 Max | 0.040 Max |
| Phosphorous | 0.055 Max | 0.040 Max | 0.050 Max | 0.040 Max |
| S+P | 0.105 Max | 0.075 Max | 0.100 Max | 0.075 Max |
*(Typical Value for 90% of Heats)
Reference
Section Weight Chart as per BIS
| Nominal Size | Approx. Weight (Kg.) | Approx. Lengths Per MT (Pcs.) | Approx. Lengths Per MT (Mtr.) | Approx. Lengths Per Quintal (Mtr.) | Approx. Lengths Per Quintal (Feet) | No. of Lengths Per Bundle | Approx. Weight Per Bundle (Kgs.) | Permitted Tolerance | ||
|---|---|---|---|---|---|---|---|---|---|---|
| Soot | (mm) | Per Meter | Per Length | |||||||
| 2.5 | 8 | .395 | 4.736 | 211.2 | 2,534 | 253 | 831 | 18 | 85.241 | 7% |
| 3.0 | 10 | .617 | 7.399 | 135.1 | 1,622 | 162 | 532 | 13 | 96.192 | 7% |
| 4.0 | 12 | .888 | 10.655 | 93.9 | 1,126 | 113 | 369 | 9 | 95.896 | 5% |
| 5.0 | 16 | 1.579 | 18.942 | 52.8 | 633 | 63 | 208 | 5 | 94.712 | 5% |
| 6.0 | 20 | 2.466 | 29.598 | 33.8 | 405 | 41 | 133 | 3 | 88.793 | 3% |
| 8.0 | 25 | 3.854 | 46.246 | 21.6 | 259 | 26 | 85 | 1 | 46.246 | 3% |
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