TMT bars or Thermo Mechanically Treated bars have captured the imagination of the steel manufacturing industry in India over the last decade. This new-generation high strength bars are much in demand in the construction Industry due to its highly ductile nature and strength. Named after the process used to manufacture, the reinforced steel bars have a soft inner core and a harder crystalline outer core.
Hot rolled steel billets are passed through water to harden the outer surface while the inner core is kept warm at a higher temperature to make it more tensile. The changes in the microstructure of the bar contribute to its immense strength and resilience. Clearly, the development of steel TMT bars has been a game changer in the area of steel manufacture. TMT bar manufacturers are further encouraged by the fact that they are able to meet international standards of quality with a cost-effective option.
Prior to the arrival of TMT bars, the construction industry was dependent on TOR or CTD Steel bars. Also known as Cold Twisted Deformed bars, the steel production technique involved elongation and twisting the bars after the heating process. The deformations thus created, helped in better bondage to the concrete; and aided to surpass the minimum strength requirements.
However, despite all its qualities, steel bar manufacturers realized that the TMT bars scored way higher in bend-ability and resisting rust/corrosion. Since the TMT bars are molded steel manufactured, they are not subject to deformations. This ensures the chances of surface defects are negligible and have nil scope for oxidation, which could lead to rust or corrosion. The outer core, because of the tempering, is also extremely heat resistant up to 600°C and is possibly the best fire resistant option the steel bar suppliers have to offer. TMT bar companies claim the bars help reduce the construction cost by 8% to 11% by way of using lesser bars for the strength needed.
Let’s take a quick look at the manufacturing process of TMT Bars. Top TMT manufacturers realize that the quality of their bars depends on 3 critical aspects:
- The source and quality of raw materials
- The rolling mill used to give it form
- The machinery & equipment used for heat treatments
Sourcing of Raw Material
The materials and processes used by TMT steel companies are governed by ISI specifications in India. The raw materials include iron ore, coal and dolomite. These are piled, recovered and mixed in definite proportions. The iron ore is subjected to what is known as a beneficiation process in order to raise the iron substance. High quality steel scrap / metal fines are sourced to shape pellets that help improve efficiency.
In order to fine-tune the steel composition, limestone is added to be mixed and processed in a furnace. These vary from an Electric Arc Furnace (EAF), Basic Arc Furnace (CAF) or an Induction furnace, depending on the desired steel composition. Sulphur, Phosphorus, Manganese and Carbon are the main alloying elements. Further addition or removal of chemicals to suit the composition can be done during this process. Degassing, Stirring, Ladle Furnace are some of the processes used for this purpose. The molten mass is collected at the base of the furnace while the impurities (slag) float up to the top.
Manufacture of Steel Billets
The molten form of steel is heated up to 1700°C and poured into casting moulds using the Continuous Casting Process to create long rectangular rods called Billets. The solidification of the molten mass is done in the casting machine. Steel bar manufacturers use the quenching system for this cooling process. These billets are still hot when they are sent to a Rolling / Reduction Mill where the thickness is further reduced and elongated across three stands – Roughing, Intermediate and the Finishing stands.
Pulkit TMT Bars produces top-rated Thermo Mechanically Treated steel or TMT bars that have high strengths not obtained in any other process of steel-making. TMT steel is manufactured by heating initially at 1200^0c, which is then austenitized. Post which, it undergoes a process called quenching that allows rapid cooling and conversion to martensite.
Thermo Mechanical Treatment
This is undoubtedly the defining stage of the manufacture of TMT bars. The billets are subjected to 3 specific heat treatments as prescribed by international quality standards to impart strength and agility to the final product. Steel bar manufacturers across India use what is known as the Thermex system for this cooling process.
Freshly casted rods can be subjected to undesirable changes in property if allowed to remain at higher temperature. In order to prevent this, the bars are led through a Rapid Cooling system where they are sprayed with cold water in a Quenching chamber. This results in the formation of a layer that is called Martensite on the outer surface of the bar.
While the steel bars are allowed to cool down, the inner core of the bar is at a much higher temperature than on the outside. On exposure to air and conduction, heat flux is transferred from the inner core to the outer surface, further tempering the Martensite. This process results in rings of Martensite and Bainite formed around the bars and is known as Tempered Martensite that aids the bar in attaining higher yield strength. The core remains austenitic (hot and malleable) at this stage.
The bars are now taken to special cooling beds where the temperature of the inner core is allowed to reduce to room temperature. Here the distinction between the outer and inner core is visible. The bar now has Ferrite –Pearlite structure in the centre making it more weld-able and tensile.
The bars are then tested, bundled and sent to various destinations for construction projects that need high reinforcement like Bridges, Dams, High rises etc. TMT bars come in 9 grades. In India, Grades Fe – 415, Fe – 500 and Fe – 550 are the most commonly used varieties, among which Fe-550 is the strongest. TMT steel companies also adhere to international standards like DIN 488, Bst 500, BS 444, to name a few.
It is not uncommon to see products of lesser quality masquerading as TMT bars. Cheap bars are water sprayed to give the impression of the exterior of TMT bars. Hence, the importance of choosing steel bar suppliers who are reputed and verified for their quality standards.