550D or Fe 550D is a grade of Thermo Mechanically Treated (TMT) steelbars, as defined by IS:1786, with Fe being the symbol for the Iron. Iron is the primary metal that the barsare made up of. ‘550’denotes the minimum yield stress in MPa (the tensile stress required to produce a total elongation) and the letter ‘D’denotes ductility. Simply put, this connotes the fact that TMT steel bars of 550D grade have a higher percentage of tensile strength and elongation.
Buildings or structures that require 550D TMT steel bars that are those that bear heavy loads. Dams, railway bridges, fly-overs, stadiums, skyscrapers, airports, industrial structures, wind-mills, ports, hospitals, tunnels, concrete roads malls and multiplexes and other load bearing structures fall into this category, and are built using 550D TMT steel bars.
Why 550D TMT Steel bars?
The use of 550D TMT steel bars is more prevalent in earthquake prone areas because D grade steel is highly resistant to seismic activity. The lower carbon, sulphur and prosperous in Fe550D TMT steel bars helps in preserving the steel and resisting earthquake shocks.
This is so because the TMT manufacturing process creates a steel product which has a hard outer shield which provides the requisite strength to the TMT steel bar, and a soft inner core which provides the requisite flexibility to withstand the devastating earth-shaking movement, during an earthquake.
The Thermo Mechanical Treatment involves 3 steps:
- Atmospheric cooling
The first stage of primary steel making takes place in an oxygen furnace. Oxygen is combined with the carbon in the iron to remove the carbon dioxide. Next, it is passed through the EAF (Electric Arc Furnace) for further refinement of the composition of the metal. Molten steel from EAF is transferred to a ladle and then to a Continuous Casting Machine. Liquid steel flows out of the ladle into the casting ladle and then into a water- cooled mould. Here, the molten steel gets solidified and steel billets come out continuously from the CCM in the required length. Once the hot rolled bars are released, they are treated to a water spray system known as the Thermex System. The jets of water cool the outer layer of the bars rapidly and harden them, forming a martensitic outer rim. The core of the bars continues to be molten at this stage. This cooling technology is known as Quenching.
The hot inner core allows heat to flow from inside to the surface causing tempering of the outer Martensite layer, thereby forming a structure called ‘Tempered Martensite’. This process is called
Self-tempering. Once the self-tempering is complete, the bars are further cooled on a cooling bed under normal room temperature. During this process called atmospheric cooling, the soft, molten core turns into a ferrite-pearlite structure which is ductile and soft. This process called
Atmospheric cooling increases the tensile strength and makes the steel bars highly ductile and weldable.
550D grade TMT bars are preferred for building critical infrastructure in the country for the following properties of these bars:
Earthquake resistance: TMT bars are resistant to earthquakes due to their soft pearlite core. 550D TMT bars can bear seismic and dynamic loading. TMT bars have high fatigue resistance to seismic activity. Their ductility quality qualifies these bars to be used for constructions in earthquake prone areas
High elongation: TMT bars can elongate up to 50% more than the original length without compromising on the measurements. This is again a desirable property
Corrosion and rust resistance: TMT bars are resistant to corrosion and have moisture resistant properties. As a result, they can be used in constructions such as bridges, dams, canals and sewages, where they may be exposed to water as well as salinity in the groundwater
Flexibility and bendability: TMT bars have excellent capacity for bending, so they can be moulded into different aesthetic shapes and architectural designs
Fire resistance: Top quality TMT bars from India’s best TMT steel bar manufacturers boast of high thermal stability that can resist temperatures ranging from 400 to 600 °C. Thereby, they provide safety of the residents living in the structures built using TMT steel bars
Fatigue Resistance: A unique feature of TMT Bars are their High Fatigue Resistance on dynamic loading on account of the high strength of the surface layer.Weldability: TMT Bars with low carbon equivalent content can be used for butt/ lap and other weld joints without reduction in strength at the weld joints.
The best TMT bars in India
The best 550D TMT steel bars in India are manufactured at Integrated Steel Plants where everything from sourcing to the final product are controlled under one roof.
In the business since 1960, Radha TMT is one of India’s leading TMT steel manufacturers. Strategically located in India’s fastest growing smart city, Hyderabad Radha TMT caters to the region’s ever-growing demand for quality steel through our state-of-the-art manufacturing plants. Our GST compliant infrastructure in Hyderabad includes dedicated steel melting induction furnaces, fully automated High-speed Rolling Mill, Ladle Refining Furnace (LRF), Continuous Casting Machine (CCM), German Thermex Technology and world-class testing facilities.
Article By: RADHA TMT
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