TMT bars are used by many construction firms on a daily basis. Many builders and developers now buy TMT bars online, which is making their job easier. In this article, you will get fundamental truths regarding the manufacturing of TMT bars, what functions do each element in the composition serve and some of the current researches that are going regarding TMT and cost-cutting.
The Thermo-Mechanical Treatment
A low-cost low-carbon steel bar is heated to 1200 degree, then it is kept in a quenching (rapid cooling) box. The upper layers start crystallising and shrinking. The shrinking of the upper layer creates optimum pressure in the hot inner layers for formation of correct crystals. A microprocessor maintains the temperature gradient across layers by controlling the water supply to the quenching box.
It is then taken out of the box and allowed to cool slowly. This creates a boundary between hard and tensile outer layer and tough, refined & ductile inner layers. At the boundary, there is an intermediate layer which contains the properties of both the inner and outer layer. If the cooling and the pressure changes are not regulated it leads to the formation of weak areas.
As there is no need for twisting to further harden the rebar, torsional defects are absent in TMT bars.
Purpose that each element serves
The International Standardisation Organisation(ISO) has limited the maximum weight percentage of carbon, sulphur, phosphorus and if sulphur and phosphorus both are present for gradation of TMT bars under Fe415, Fe415D etc. i.e. if a bar meets these percentages and the yield strength and elongation requirements, it is allowed to be called the respective grade. The presence of each element has advantages and disadvantages.
Maximum concentration in percentage of total weight allowed in TMT bars by ISO for gradation
|Sulphur and Phosphorus||0.110||0.085||0.105||0.075||0.100||0.075||0.075|
Carbon- The tensile strength increases, but ductility reduces. Hence, ISO has selected an optimum level above which the strength is not required for most cases.
Sulphur- Its presence is considered as an impurity as it increases the brittleness of the rebar.
Phosphorus- It increases the strength but decreases the ductility faster than the carbon, hence it is also considered an impurity.
Chromium- It increases the weldability and corrosion resistance. ISO doesn’t limit its concentration, it being costly does.
Copper- Increases the strength and the corrosion resistance. Its limit is not specified by ISO.
Though ISO hasn’t limited copper, manganese and chromium concentrations but as their effect on ductility is similar to carbon their carbon equivalents are calculated. The value carbon equivalent is limited by the ISO.
Taking inspiration from the Iron Pillar of Delhi, which is standing from 1600 years without any corrosion, a study from IIT Kanpur has concluded that it is due to the presence of phosphorus which leads to the formation of a protective passive film on the surface. This layer has exceptional corrosion resistance abilities. However, phosphorus leads to brittleness and thus, is limited by current standards. But the study concludes that phosphorus can be blended in such ways to at least match the corrosion resistance offered by chromium. An added advantage of using phosphorus would be the reduction in the price of rebars.
Another study in International Journal of Engineering Research & Technology (IJERT) related to cost savings during the bending process of TMT bars, concludes that using the machine while bending TMT bars can save costs 28% less than bending manually. For an average sized company, the machine and cost of installation can be recovered within four to five months. The bending is also more uniform and quicker.