Evotech Pvt. Ltd., Sister Concern of Evcon Technologies, Providers of Evcon Turbo TMT System
Evcon TurboTM TMT System
"THERMO MECHANICAL TREATMENT PROCESS FOR REINFORCEMENT BARS OPENING UP NEW VISTAS"

In composite RCC, the re-enforcing steel is the costliest constituent (30 to 40% per cu. m. of concrete). This cost can be substantially reduced by using higher grades of steel re-enforcing bars. The higher yield strength of re-bars lowers the steel requirement, which results in reduced cost of construction. In India, high strength re-bars of yield strength up to 500 N/sq. mm. are produced either by cold twisting or micro-alloying or a combination of both which adds considerably to the cost of the re-enforcement bars.

Evcon Turbo TMT System During the last 30 years, high strength cold twisted deformed bars with yield strength of 415 MPa conforming to IS: 1786/85 are widely produced and marketed in our country. These cold twisted bars, though in extensive use at present, continue to have inherent problem of inferior ductility, weld-ability and increased rate of corrosion. Production of re-bars by the addition of micro-alloys gives the desired results of high strengths but at a cost, which is prohibitive. The need for reduction in the steel used for concrete re-enforcement has prompted most countries to switch to re-bars of higher yield strengths of 500 and 550 MPa. And the need for cutting down the cost of production of high strength re-bars has initiated the involvement of a more economical and competitive process -- the Thermo Mechanical Treatment (TMT) Process. The use of Thermo Mechanical Treatment process has not only helped produce re-bars of high yield strength but also having superior ductility, weld-ability, bend-ability, better corrosion resistance and thermal resistance creating a revolution in re-enforcement engineering.
Production of High Strength Re-enforcement Bars
Evcon Turbo TMT System Over the last three decades there has been an increase in the world wide demand for low cost re-enforcement steel bars with a maximum guaranteed yield point. The direct consequences to the customer are:
  • Reduced total weight of re-enforcement bars leading to reduced cost to the customer.
  • Reduced manpower for re-enforcement fabrication.
  • Lower transportation costs.
Up to recent times, such results were achieved through two different production processes, namely:
Cold twisting performed off-line.
High strength cold twisted deformed bars with a maximum yield strength of 415 MPa conforming to IS: 1786/85, though in extensive use at present, continue to have inherent problem of inferior ductility, weld-ability and increased rate of corrosion due to presence of residual stresses and higher carbon content. Additionally, cold twisting being labour intensive, enhances cost of production with limitation on production rates.
Micro-alloying process
High strength re-bars having yield strength of 500 and 550 MPa are being produced by the addition of alloying elements like C, Cr, Mn, V, Nb, etc in the steel. But it is obvious that this process is expensive and ductility is also low which makes this process not very acceptable to the customer.

Evcon Turbo TMT System The requirement of high strength re-bars of yield stress of 500 and 550 MPa at an acceptable cost to the customer so as to bring economy in civil construction has urged the evolvement of a third process. The requirement of good ductility and weld-ability of the re-inforcement concrete steel bars has also urged the users to go for lower carbon and manganese contents in steels. The above requirements have been fulfilled by the new third process - In-line Accelerated Cooling System. This process consists of a high-pressure water spraying system, after the last rolling stand, which provides accelerated cooling to the hot bar. The thermal treatment provided to the quenched bar consists of controlled cooling from the rolling heat in the core of the bar by which it is possible to achieve high levels of strength and yield points combined with good ductility and easy weld-ability.

The cost of the Accelerated Cooling System has been compared below with the other two processes.

Process Treatment Production Costs Ductility Weld-ability
Cold Twisting Cold work hardening by means of:
Stretching | Twisting
High (Due to Manpower and Equipment) Poor Good (deteriorating due to heat generated by welding)
Micro- Alloying Addition of alloying elements as: C, Mn, V, Nb etc. High (Due to alloying elements and equipment) Good Poor (Due to high carbon equivalent)
In-Line Cooling System Rapid cooling and controlled cooling from rolling heat Low Excellent Excellent (Due to low carbon equivalent)

The TMT bars have virtually replaced the conventional cold twisted deformed bars as re-inforcement material all over the world. With the availability of TMT bars here, our country is also set to change over to the use of TMT bars.

The Thermo Mechanically Treated bars are generally called TMT bars. These bars owe their advantage to their composite micro-structure, which is a soft and ductile ferrito-pearlitic core and a tough surface rim of tempered martensite.
The Cooling Process
In bar mills, directly after the last rolling stand, the bar passes through the Cooling System achieving a short and intensive cooling of the surface. Because of the reduction in the temperature at a rate higher than the critical rate for martensite quenching the surface layer of the re-bar is converted to a hardened structure while the core remains austenitic.

Evcon Turbo TMT System After the intensive cooling, the bar is exposed to air and the core re-heats the quenched surface layer by conduction, therefore tempering the external martensite. The thermal profiles of the Surface and Core merge at the 'so called' equalizing temperature that together with the final rolling temperature is the most important parameter to achieve the required mechanical properties.

Finally, when the bar is discharged on to the Cooling Beds, the remaining austenite transforms into a very fine-grained pearlite structure. The Process being offered is the most efficient system available. The Cooling Pipes consist of specially designed nozzles, which assure a high degree of heat exchange rate between cooling water and the traversing bars. The design ensures the elimination of the steam layer generated on the material surface. In the system water flows in the same direction as the bars inside the pipes, but the speed of the water flow is much more than the speed of the bar.
Product Characteristics Chemistry
IS:1786/85states the following chemistry of the steel used for the production of TMT bars:

Grade Carbon% Max. Sulphur% Max. Phos.% Max. S + P% Max.
Fe-415   0.30 0.060 0.060 0.11
Fe-500 0.30 0.055 0.055 0.105
Fe-550 0.30 0.055 0.050 0.10


In the production of TMT bars the carbon is restricted to below 0.20% for imparting better ductility and bend-ability and to ensure better weld-ability. The carbon equivalent of the steel is controlled by the addition of Manganese (from 0.50% to 1.0% depending on the grade of the TMT bar being produced. In case of production of corrosion resistant TMT bars, corrosion resisting elements are suitably added in the steel.
Macro-Micro Structure
The Macro and Micro structure of TMT bars is shown in the figure here. A dark etched peripheral rim of tempered martensite and a grey core of ferrite pearlite are observed. The tempered martensite surface layer is very hard while the ferrite pearlite core is soft. The result is a structure with a high yield strength combined with high ductility. As the heat removal system is very rapid, the tempered martensite surface layer is thinner but harder leading to higher elongation.

Fig 2: Microstructure of TMT bar Fig.3: Tempered Martensite Rim Fig.4: Ferrite-Pearlite core
Mechanical Properties
TMT bars possess a unique combination of strength, ductility and bend-ability which is significantly higher than the stipulated requirements of IS: 1786/85.

Re-bar Quality Grade Yield Strength Mpa Min. UTS
Mpa Min.
El.
(% Min.)
Bend up to
(Min.)
Is: 1786 Conventional Re-bars Fe-415
Fe-500
Fe-550
415
500
550
485
545
585
14.5
12.0
8.0
3d
4d
5d
TMT Bars Fe-415
Fe-500
Fe-550
415
500
550
500
580
630
22.0
20.0
18.0
2d
2d
2d


Where d is the nominal diameter of the test piece. In the above table the Elongation and Bend-ability values for TMT bars are particularly noticeable, which testify the product's toughness and ease of fabrication.
Special Festures of TMT Bars
  • Higher strengths combined with better elongation values as compared to conventional CTD bars.
  • Excellent Weld-ability because carbon is restricted to below 0.20%.
  • Resistant to fire hazards as there is no loss of strength up to 500 deg. C.
  • Excellent Ductility and higher Fatigue strength, which makes it suitable for structures and foundations subject to dynamic and seismic loading.
  • Better Ductility and Bend-ability makes it easily workable at site.
  • Fulfils Bond requirements as per IS: 456/78 and IS: 1786/85.
Advantages of using TMT bars
  • The advantages of TMT bars over conventional CTD bars are many and include: Savings in Steel. Reduction in Costs.
  • Reduced Transportation Costs.
  • Better Safety of structures because of higher Strength combined with higher Ductility.
  • Resistant to Fire hazards.
  • Easy working at site owing to better Ductility and Bend-ability. Pre-welded meshes can be made to eliminate manual binding at site.
  • Reduces construction and fabrication time.
  • Good Weld-ability and no loss of strength at weld joints.
Applications
TMT bars find wide applications in different spheres:
  • General purpose concrete re-enforcement structures
  • Bridges
  • Flyovers
  • Dams
  • High rise buildings
  • Industrial structures
  • Concrete roads
  • Underground structures
Range of TMT Products
Diameter 8, 10, 12, 16, 20, 25, 28, 32, 36, 40 & 45 mm
Grades Fe - 415, Fe - 500, Fe - 550
Be it high rise buildings, bridges or dams, the high strength TMT bars offer a panacea for all applications. Our process is capable of following product range:

Fixed Length 10 m. to 13 m.
Standard Length 5.5 m. to 13.5 m.
Special lengths can be supplied at the request of the Customer.

High Strength Corrosion Resistant TMT bars for combating marine corrosion in grades of Fe - 415, Fe - 500 and Fe - 550 Grade A and M.

Dimensional Tolerances: Conforms to tolerances as per IS: 1786/85

Economy through TMT Bars over CTD Bars
TMT bars irrespective of grades are technically superior to conventional plain and cold twisted bars. Besides, the replacement of conventional bars by TMT bars will result in savings in terms of weight and thereby cost. The percentage of saving will depend on the type of structural member and grade of steel. A brief summary of probable range of savings arising due to use of TMT bars is given below with a view to make the users feel about it's cost effectiveness.

Section Grade of Steel Saving in wt. with respect to Plain Bars (in %) Saving in wt. with respect to CTD Bars (in %)
Doubly Re-inforced Beam CTD 415 TMT Fe-500 TMT Fe-550 34 - 38 | 43 - 47 | 48 - 51 -14 - 15  | 19 - 21
Axially Loaded Columns CTD 415 TMT Fe-415 37.6 | 44.0 - 10.4
Uni-axial Bending with Compression CTD 415 TMT Fe-415 34.0 | 39.0 - 8.0


Section Grade of Steel Saving in Cost with respect to Plain Bars (in %) Saving in Cost with respect to CTD Bars (in %)
Doubly Re-inforced Beam CTD 415 TMT Fe-500 TMT Fe-550 28 - 33 | 34 - 37 | 38 - 42 - 5 - 7.5 | 12 - 14
Axially Loaded Columns CTD 415 TMT Fe-415 32.0 | 39.0 - 10.4
Uni-axial Bending with Compression CTD 415 TMT Fe-415 28.0 | 34.0 - 8.0
Some Typical Examples
Industrial Hall (Total Saving : 13.6%)
House (Total Saving : 11.9% )
Office Building (Total Saving : 12.8%)
Hotel (Total Saving : 14.1%)


Cooling Plate
Basic Equipment List
  • Cooling Pipes (with built-in high pressure water spraying and specially designed wear resistant nozzles). The cooling pipes are of varying sizes and each size is for the production of a batch of bar sizes i.e. 8 to 12/16 mm, 12/16 to 25 mm, 28 to 36 mm.
  • High-pressure cold water centrifugal pumps (pressure and capacity depending on sizes to be produced).
  • High-pressure hot water vertical turbine pumps.
  • Cooling Towers with cold water sump.
  • Hot Scale pit.
  • Various types of valves - non-return valves, gate valves, globe valves, pressure relief vales, etc.
  • Pipes ERW as per IS: 1239 - various sizes.
  • Radiation Pyrometers (range up to 800 deg. C and 1200 deg. C)
  • Pressure gauges, Temperature gauges, Orifice Plates.