Before we answer the question of how TMT bars are manufactured, let’s first know what TMT bars are. TMT or Thermo Mechanically Treated bars are high-strength reinforcement bars having a hardened outer core and a soft inner core.They are manufactured under a process called Thermo Mechanical Treatment, after which they are named.
Steel – an alloy of iron, carbon, and other elements – is a major component used in buildings, infrastructure, tools, ships, automobiles, machines etc., because of its high tensile strength. However, steel structures are adversely affected by corrosion, fire, and other environmental and accidental factors, thus severely compromising their structural integrity, safety, and longevity. Thus, steel is subjected to various processes to increase its mechanical properties like ductility, hardness, corrosion resistance, and yield strength. Thermo Mechanical Treatment (TMT) is one of these many processes; it combines mechanical or plastic deformation processes like compression or forging, rolling etc. with thermal processes like heat-treatment, water quenching, and heating and cooling at various rates into a single process.
The steps included in Thermo Mechanical Treatment Process are as follows:
- Extraction of iron from its ore and converting to steel: In this process, the raw materials, such as iron ore, coke, or fluxes (limestone and dolomite) are used to extract iron which is kept in the molten state. Then, this molten iron is subjected to pre-treatment and converted to steel in a converter and is then subjected to ladle heating for refining the chemistry of the steel. After this step, the molten steel is poured into a casting machine to produce billets (pencil ingots) of about 130 mm2 or more.
- Heating, rolling, and forming of reinforced bar: The steel billets are heated to approximately 1200 °C (2192 F) to 1250 °C (2282 F) and then rolled to reshape the billets into the final size and shape of reinforced bar (rebar) bypassing the billets through a rolling mill stand.
- Quenching: When the hot reinforced bar leaves the final rolling mill stand, it is instantaneously quenched – a type of heat treatment where the rebars are rapidly cooled by water in a quenching box to obtain certain material properties. Quenching prevents the occurrence of undesired processes such as phase transformations. It accomplishes this by reducing the time frame during which these undesired reactions have a higher chance of occurring. Also, the sudden drastic change in temperature toughens the outer layer of the steel bar, thus enhancing its tensile strength and durability. This is because quenching converts the outer surface of the reinforced bar to Martensite, a hard form of steel and causes it to shrink, which in turn pressurizes the core, thus helping to form the correct crystal structures. As a result of this process, the surface of the quenched bar becomes cold and hardened, while the core still remains hot.
- Self-tempering: After leaving the quenching box, a temperature gradient is formed through the cross-section of the quenched bar. As a result, heat flows from the core, as it is at a relatively higher temperature to the outer surface. This causes the correct tempering of the outer martensitic layer into a structure called Tempered Martensite and the formation of an intermediate ring of Martensite and Bainite (a plate-like microstructure). The core still stays in the austenitic (a typical cubical crystalline structure, commonly called as gamma-phase iron) state at this stage.
- Atmospheric Cooling: After the self-tempering process, the bars are subjected to atmospheric cooling to equalize the temperature difference between the soft inner core and the hardened exterior. Once the bars are completely cooled down, the austenitic core gets transformed into a ductile ferrite-pearlite structure.
Therefore, the cross-section of the final product demonstrates a variation in its crystal microstructure having a tough, strong, tempered martensite in its outermost layer, an intermediate layer of Martensite and Bainite, and a refined, tough and ductile ferrite and pearlite core.
On the other hand, lower grades of rebar are twisted when cold to harden them in order to increase their strength. However, TMT bars do not need hardening explicitly as the quenching process accomplishes this. Since TMT does not involve any twisting, no torsional stress occurs, which does remove the chances of surface defects forming in TMT bars. Hence, TMT bars are less susceptible to corrosion as opposed to cold, twisted, and deformed (CTD) bars.
The production quality of TMT bar depends on three major factors:
- Quality of raw materials.
- A properly designed and automated rolling mill stand.
- A well-designed quenching and tempering technology.
TMT bars, having a uniform and concentrated hardened periphery and a considerably softer core, will have the desired tensile strength coupled with high elongation as required in the construction of buildings located in areas with regular seismic activity.