What is TMT Bar and Manufacturing Process.
TMT bars or Thermo-Mechanically Treated bars are high-strength reinforcement bars having a tough outer core and a soft inner core. The very first step of the manufacturing process involves passing the steel wires through a rolling mill stand. Thereafter, these rolled steel wires are again passed through the Tempcore water cooling system. While passing the wires through the water cooling system, the water pressure is optimised. The sudden quenching and drastic change in temperature toughen the outer layer of the steel bar, thus making it super tough and durable. Once this process is over, the TMT bars are subject to atmospheric cooling. This is done in order to equalise the temperature difference between the soft inner core and the tough exterior. Once the TMT bar cools down, it slowly turns into a ferrite-pearlite mass. The inner core remains soft giving the TMT bar great tensile strength and elongation point. This design is unique to the TMT bars and gives superior ductility to the bars. Also, this unique manufacturing technique and the absence of Cold stress make this bar corrosion-resistant and boost its weldability.
Grades of TMT bars
There are four distinct grades of TMT Steel available in India; Fe-415, Fe-500, Fe-550, and Fe-600. The grade number, yield strength, and tensility are directly proportional. The higher the grade number, the higher the values are.
Below are the areas of uses for different grades of TMT
- Fe 415: RCC Constructions in areas with great corrosion and earthquake incidence.
- Fe 500: RCC constructions, in building, bridges and other concrete structures.
- Fe 550: RCC construction that is exposed to humid coastal, marine or underground areas.
- Fe 600: used for large RCC construction purposes.
In accordance to the latest revision of IS 1786(2008), Fe 500D is a new introduction of deformed steel bar that has same specified minimum 0.2 percent proof stress/yield stress but with enhanced specified minimum percentage elongation.
To elaborate it further, let’s take a look at the mechanical properties of different High Strength Deformed Bars and Wires
As can be seen from the table above, Fe 500 and Fe 500D ( D implies ductility which means that the steel bars have a higher percentage of elongation )have the same 0.2 percent proof stress/yield stress(Min) where as the tensile strength is increased by 2 percent.
Now let’s take a look at the chemical properties of the same bars and wires:
It shows relatively low amount of carbon and sulphur percentage in Fe500D as compared to Fe500 which enhances its resistance to to outside attacks.
1. MELTING & REFINING
The raw material for our steel bars is processed in our in-house induction furnaces. Once the raw material has been melted inside the furnace, the molten metal is separated from the slag.
At this point, we purge the molten product inside a ladle furnace to achieve the desired chemical composition. During this process, we add micro-alloying materials to fix the inherent chemistry of the molten metal. At this stage, excessive non-metallic inclusions are removed from the mix. It is crucial to maintain the chemical composition in order to achieve weldability, ductility, strength and elongation during the rolling process. We keep an eye on the chemical composition with the help of our high-end spectrometer.
2. CONTINUOUS CASTING
Molten metal from the ladle refining furnace is poured into a nearby tundish, which then pours the liquid into a mold located underneath. This process is used to cast a predetermined length of metal, transforming the molten metal mix into a solidified form. New molten metal is continuously supplied to the mold at a precise rate to keep up with the solidifying process.
Our advanced continuous casting machine helps in achieving improved yield, cost efficiency, productivity, and quality.
3. SECONDARY COOLING & HOT CHARGING
Once the solidifying billets exit the continuous casting machine, they enter a secondary cooling chamber. Here, the semi-finished billets are sprayed with water to achieve complete solidification. This process is important for maintaining the shape of the solidified billets, improving their quality in the long run.
Next, the solidified metal strands are reheated again in our reheating furnace. This is done to recrystallize the metal, improving quality and reducing material losses. The recrystallization process is very important because the metal strands can then be processed in the rolling mill without destroying the bonding.
4. ROLLING MILL
The recrystallized metal billets are transported to a cutting-edge rolling mill using conveyors. Here, they are processed using the internationally accepted roll pass design, which makes for higher strength and elongation. This fully automatic process ensures a greater degree of uniformity throughout the length of each bar.
Our state-of-the-art rolling mill technology comprises a roughing mill of two 18” stands, four intermediate mills, four pre-finishing mills, and a finishing mill of six 12” stands. All the stands have individual DC motors to maintain total automation.
5. QUENCHING & SELF-TEMPERING
After the steel rebars leave the finishing mill with the desired size, they enter a water cooling chamber where they’re subjected to a quenching process. We have an Evcon Turbo machine which employs a unique method for quenching and self-tempering the ribbed steel bars. This treatment is completed in 3 successive stages:
● As soon as the steel rebars leave the final mill stand, they are rapidly cooled to ensure surface hardening.
● Once the quenching operation is completed, the surface layer of each bar undergoes a tempering process with the help of the residual heat left in its core.
● The third stage takes place when the steel bars lie on a cooling bed, where they are are subjected to rapid air cooling.
Due to this extensive process, each of our steel bars have greater strength, ductility, bendability and weldability.
The good news is that there are tests which are performed to identify the proposed strength of the reinforced bar. These tests are:
- Tensile Test
- Yield Stress Test
- Percentage Elongation Test
- Bend & Rebend Test
- Chemical Analysis Test
- Ring Test
Tensile Test is the most common test performed on a reinforced steel bar. The TMT rebar is strained by the tensile force an elongated, generally to the point of failure, for the purpose of determining tensile strength.
Yield Stress is defined as the stress at which the TMT rebar will deform plastically i.e cannot be reshaped to the original.
Percentage elongation is the measure of reinforced bar’s ability to deform prior to breaking. Elongation is an important factor similar to yield strength in choosing the grade of TMT bars. Fe 415 has a minimum elongation of 14.5% whereas Fe 500 has a minimum elongation of 12%. Each grade TMT has its own purpose and is specifically designed for that purpose.
Bend & Rebend tests are mainly done to evaluate the ductility of the reinforced bar. The bend is performed by bending the steel bar at the midpoint resulting in bending without fracture. The Rebend test is performed to measure the effect of strain ageing on steel.
In Chemical Analysis, the TMT reinforced bar is tested for various elements or compositions using Spectrometer. Spectromax using arc and spark excitation is the preferred method to determine the chemical composition of the samples. Chemical analysis results in the percentages of carbon, sulphur, and phosphorus which are important as per BIS. As per IS:1786, the maximum allowed percentages of elements are:
|Properties||As per IS 1786 Fe500 standard.|
|Chemistry||As per IS 1786 Fe500 standards|
|carbon||0.30 maximum %|
|Sulphur||0.055 maximum %|
|Phosphorous||0.055 maximum %|
In India Bend & Rebend Tests are mainly carried out as per Indian 1786 (and British 4449) standard.
|Tests||IS 1786 FE 500|
|Rebend||5d (7d for>10dai)|
WEIGHT TOLERANCE Unit Weight ( Kg/Meter)
|As per IS 1786|
|Size of Bar (mm)||Lower Limit||Upper Limit||Market Average|
These are the tests used for evaluating the accurate strength of the reinforced TMT steel bar. Most of the tests can only be done in laboratories. So the point is do not fall for false marketing by spurious sellers. Always buy the right TMT reinforced steel bar for construction and never hesitate to do the tests if needed.