TMT Steel

TMT

TMT or Thermo-Mechanically Treated bars are commonly used as essential input materials for home construction, building, and infrastructure projects to strengthen all cement and concrete structures, enhancing the overall longevity and safety of the projects. In the first step, the iron ore is processed into iron through a furnace or by direct reduction iron-making processes. In the blast furnace route, the output is liquid iron. Iron ore and scrap steel are melted in electric furnaces, such as electric arc furnaces (EAF) or induction furnaces (IF).  Further, the molten steel is cast into solid billets. The billets are directly charged to the mill in hot conditions at approximately 1200 degrees Celsius for a prolonged period, leading to appropriate heat distribution across the material. The heated billets are subjected to a rolling mill process. Multiple sets of rolling stands exert pressure on the hot charge and gradually reduce its size as it moves through at high speeds. The last roll pass contains grooves that produce the ribbed structure, characteristic of TMT bars.  

After the rolling process, the bars undergo a rapid cooling process using high-pressure water jets known as quenching. This involves quickly cooling the outer layer of the bar while the core remains hot. This creates a hardened outer (martensitic) layer with a softer core (ferrite-pearlite structure). This procedure results in steel bars with superior strength, flexibility, and resistance to various stresses. 

Categorisation based on different IS standards:

IS 1786:2008  

IS 1786:2008 is a standard set by the Bureau of Indian Standards (BIS) that specifies the requirements for high-strength steel bars, in the following strength grades: Fe415, Fe415D, Fe500, Fe500D, Fe550, Fe550D, and Fe600. 

Fe500D TMT     

 Fe550 TMT   

 Fe550D TMT  

 Fe550D CRS TMT  

TMT Applications 

Residential and commercial buildings:   

Taller buildings must bear significantly more load due to the cumulative weight of all the floors above. Fe500 has a minimum yield strength of 500 N/mm², while Fe550 has a minimum yield strength of 550 N/mm². This makes Fe550 stronger and more suitable for structures that withstand higher loads. Fe500D and Fe550D can undergo more deformation before breaking in earthquake-prone areas, which is particularly beneficial in seismic zones. Fe500D has an elongation of more than 16% (typically 20%), while Fe550D has more than 14.5% (normally 18%). Fe500D and Fe550D have tensile strengths (a material's resistance to break under tension) of over 565 N/mm² and 600 N/mm², respectively. Fe550 and Fe550D (more prevalent in seismic zones) are also used in commercial skyscrapers for higher strength-to-weight ratios (meaning they can take stress and pressure without deforming, even when thinly spread).

Bridges & flyovers: 

Fe550D TMT bars have an ultimate tensile strength of over 600 N/mm². This ensures that the material can handle stretching (greater ductility) forces without failing, which is crucial for the stability and safety of large structures subjected to dynamic loads. Fe550D bars typically have an elongation of around 18%, making them capable of absorbing and dissipating energy during stress events such as earthquakes or heavy traffic loads on bridges.  

Infrastructure projects:    

Dams must withstand a lot of hydrostatic pressure, which is the force that a fluid exerts due to its weight at rest. The high strength of Fe550D bars helps them withstand water pressure, and their ductility helps absorb and dissipate energy. These bars are also used in nuclear power plants and are designed to withstand seismic events and exhibit excellent fatigue resistance, making them essential for structures that are subjected to cyclic loading conditions (repeated or fluctuating stresses, strains, or stress intensities).  

Industrial plants and marine environments: 

Industrial buildings often house heavy machinery that generates significant vibrations. They must also handle must handle variable loads, including heavy equipment, stored materials, and dynamic loads from moving machinery. Industrial environments are also prone to corrosion attacks. Fe550D Corrosion Resistant Steel (CRS) TMT rebars can absorb and dissipate these vibrations, reducing the risk of structural fatigue and failure and protects from corrosive chemicals. Fe550D TMT rebars are treated to resist thermal stresses, while Fe550D CRS can protect the steel from rusting. 

FAQs:  

Q: What is the difference between the TMT bar and the MS bar? 

A: TMT bars are thermo-mechanically treated, while MS bars are mild steel bars. TMT bars have higher strength and better corrosion resistance.   

Q: What is the difference between the TMT bar and the High Yielding Strength Deformed (HYSD) steel bar? 

A: TMT bars, undergoing thermo mechanical treatment, exhibit superior elastic properties compared to HYSD bars which are manufactured by cold working process, enhancing concrete reinforcement in construction. Their advanced manufacturing confers higher residual stress and elasticity to TMT bars over HYSD bars.  

Q: What are the sizes available in the TMT bars?  

A: TMT bars come in various diameters, commonly 8 mm, 10 mm, 12 mm, 16 mm, 20 mm, 25 mm, 32 mm, 36 mm, and 40 mm.

Q: What is the UTS/YS ratio?   

A: The ultimate tensile strength (UTS) is the point at which a material fails under an applied load. It measures the material's ability to resist force. This strength is calculated using the maximum force applied and the actual cross-sectional area of the TMT bar. Yield stress (YS) is the stress level at which the TMT Steel Bar begins to deformation permanently.   The UTS/YS ratio indicates a TMT bar's ductility and ability to absorb energy before failure. A higher UTS/YS ratio indicates greater ductility, meaning the steel can undergo more deformation before breaking. This is crucial in construction, particularly in earthquake-prone areas, as it allows the structure to absorb more energy during seismic events, reducing the risk of catastrophic failure.   

Q: What does 'CRS' stand for in the Fe550D CRS TMT Rebar?  

A: CRS stands for Corrosion Resistant Steel. This variant is designed to resist corrosion, making it ideal for construction in coastal areas or environments with high humidity. CRS grade contains a minor percentage of alloying elements such as Chromium (Cr), Copper (Cu), Molybdenum (Mo), and Phosphorus (P), which inhibit corrosion reactions.  

Q: How does the performance of the CRS variant compare to standard TMT rebars in terms of strength?   

A: The CRS variant maintains the high strength and ductility characteristic of TMT rebars due to the presence of strength-enhancing alloying elements such as Chromium (Cr), Copper (Cu), or Molybdenum (Mo). It also has the added advantage of enhanced corrosion resistance, making it an excellent choice for challenging environments.    

Q:Which is better: 550D or 500D?   

A: The choice between 550D and 500D depends on the specific requirements of the construction project. 550D has higher yield and tensile strength, making it more suitable for heavy-duty constructions specifically in seismic zones. 500D is slightly more ductile though with lower yield and tensile strength, ideal for mid-scale construction in areas with seismic activity.     

Q: What is the importance of rib patterns on TMT bars? 

A: Rib patterns provide better bonding with concrete, preventing slippage and enhancing load-bearing capacity. They increase the strength and stability of the structure.

Q: What are the key factors to check when purchasing TMT bars?  

A: Ensure bars comply with national or international standards (like IS 1786:2008 in India), examine the bars for surface defects, assess the manufacturer's reputation for quality, confirm the grade and dimensions and ask for test certificates.