Studies on Properties of TMT Steels for Structural Applications

Studies on Properties of TMT Steels for Structural Applications

K Priyesh

Department of Mechanical Engineering (Manufacturing Engineering), KMCT College of Engineering,

University of Calicut, Kerala, India

Abstract India is among the fastest developing nation in the world with major constructions like bridges, dams, airports, residential buildings etc. In a developing economy where infrastructure is getting boom, strength of structural members is of great importance. This is where Thermo Mechanically Treated (TMT) bars scores over Cold twisted bars (or CTD bars). For understanding the proper behaviour of structural members, physical properties, chemical composition, mean projected rib area and macrostructure are very essential. Nowadays there are many companies which supply untreated and twisted deformed bars as TMT bars which would do much harm for structural stability. There is an urgent need to use the phrase Quenching and Tempering to label the TMT bars. This project is an effort to showcase the ways to identify good quality TMT bars.

Keywords TMT bars; strength; properties; quenching and tempering

Fe 500, Fe 500D. The following chemistry of steel is used for the production of TMT bars.

1. CHEMICAL COMPOSITION AND MECHANICAL PROPERTIES OF STEEL BARS

   1. INTRODUCTION

      Under Thermo Mechanical treatment, the steel bars are passed through a specially designed water cooling system where they are kept till the outer surface becomes colder while the core remains hot. This creates temperature gradient in the bars. When the bars come out of the TMT box to the cooling bay, the heat flows from the core to the outer surface, further tempering the bars, which helps them attain higher yield strength. The resulting concentric martenstic grain structure at the surface imparts superior strength and toughness to the bars. The microstructure of the core is a very fined-grained ferrite and pearlite. TMT bars are also known as quenched and tempered rebars, because of the quenching and tempering process involved in making the rebars, and thus TMT bars produced by the TMT process scores over Cold Twisted Bars(CTD) and TOR steels. The production of quality TMT bar depends on three major factors:

      1. Quality raw materials

      2. A properly designed and automated mill.

      3. A well designed quenching and tempering technology. Deciding the percentage of carbon content in steel has been a major challenge for engineers. While a minimum level of carbon content in steel is essential to achieve the required strength, excess carbon threatens its weld ability. In TMT bars, this problem has been eliminated by restricting the carbon content to 0.23% to attain weld ability and ensuring that its strength is not lost. The various grades of TMT bars are Fe 415, Fe 415D,

      CHEMICAL COMPOSITION OF STEEL BILLETS AND INGOTS AS PER IS 2831
      Constituent	Percent
      	Min	Max
      Carbon	0.15	0.25
      Manganese	0.32	0.6
      Sulphur	—-	0.055
      Phosphorous	—-	0.055
      Carbon Equivalent= C+Mn/6	—-	0.42

      CHEMICAL COMPOSITION OF STEEL BILLETS AND INGOTS AS PER IS 2831
      Constituent	Percent
      	Min	Max
      Carbon	0.15	0.25
      Manganese	0.32	0.6
      Sulphur	—-	0.055
      Phosphorous	—-	0.055
      Carbon Equivalent= C+Mn/6	—-	0.42

      Table 1: Chemical composition of steel billets

      CHEMICAL COMPOSITION OF TMT BARS AS PER IS 1786:2008
      Constituent	Percent, Maximum
      	Fe 415	Fe 415 D	Fe 500	Fe 500 D
      Carbon	0.3	0.25	0.3	0.25
      Manganese	0.6	0.45	0.5	0.4
      Sulphur	0.06	0.045	0.05	0.04
      Phosphorous	0.06	0.045	0.05	0.04
      Sulphur and Phosphorous	0.11	0.085	0.10	0.07

      Table 2: Chemical composition of TMT steel bars.

      And the table 3 shows the mechanical properties of steel bars as IS 1786:2008.

      MECHANICAL PROPERTIES OF HIGH STRENGTH DEFORMED BARS AS PER IS 1786:2008
      SR. NO.	PROPERTY	Fe 415	Fe 415D	Fe 500	Fe 500D
      1	Yield Stress, Min, N/mm2	415	415	500	500
      2	Elongation, Percent, Min	14.5	18	12	16
      3	Tensile Strength, Min, N/mm2	485	500	545	565

      MECHANICAL PROPERTIES OF HIGH STRENGTH DEFORMED BARS AS PER IS 1786:2008
      SR. NO.	PROPERTY	Fe 415	Fe 415D	Fe 500	Fe 500D
      1	Yield Stress, Min, N/mm2	415	415	500	500
      2	Elongation, Percent, Min	14.5	18	12	16
      3	Tensile Strength, Min, N/mm2	485	500	545	565

      Number of rows of transverse rib= 2 (two sides)

      Area of transverse Ribs, Atr = x Ltr x Dtr mm2 ——– (1) Mean projected rib area, Ar = Ntr x Atr x sin mm2 —–(2)

      Str

      Values of Depth of transverse rib(Dtr), Length of transverse rib(Ltr), Spacing of transverse rib(Str), Angle of inclination() are shown in tabulated form.

      Mill1 (Sample1)	Rib 1(mm)	Rib 2(mm)	Average(mm)
      Dtr	Dtr1=0.88	Dtr2=0.92	Dtr=0.90
      Ltr	Ltr1=19.30	Ltr2=19.43	Ltr=19.365
      Str	Str1=7.38	Str2=7.35	Str=7.365
      	=75	=76	=75.5

      Table 3: Mechanical properties of TMT bars

2. EXPERIMENTAL METHOD

   1. Selection of Steel Saples

      Samples of steel bars (Fe415) were obtained from three steel manufacturing companies randomly referred to as F1, F2 and F3. Samples of reinforcing steel bars 12mm in diameter were obtained from these three sources. From each source two samples (i.e. 6 samples) measuring 1m long were randomly selected from stock piled batches.

   2. Testing of Physical Properties

      Universal testing machine is used to perform mechanical tests such as Yield strength, Ultimate Tensile Strength and Percentage Elongation. The six samples were tested in the UTM and results were plotted on the stress-strain graph. The results so obtained were then compared with the IS 1786:2008. From each source and each sample mean value is taken by testing three samples. Obtained values of physical properties are shown in Table 4.

   3. Testing of Chemical Properties

      The six samples were then tested for the chemical properties of the steel using a Spectrometer. The results obtained were then compared with IS1786:2008. From each source and each sample mean value is taken by testing three samples. Values of chemical composition is shown in Table 5.

   4. Testing of Mean Projected Rib Area

      Third test involves one of the most neglected parts of structural engineering i.e. Mean Projected Rib Area. In order to determine the Mean Projected rib area (MPRA) we have to find out Depth of transverse Ribs (Dtr), Length of transverse rib (Ltr), spacing of transverse rib(Str) and Angle of inclination (). This is done manually, and using Standard formulas we can determine the Mean Projected Rib area. The results were then compared with the Indian standard. In the standard procedure for the calculation of Mean Projected Rib area, the

      Table 6: Values of sample 1 from Mill 1 Putting the values in (1) and (2) we get, Area of transverse rib (Atr) = 11.619mm2

      Mean projected rib area (Ar) = 3.05mm2/mm

      Mill1 (Sample 2)	Rib 1(mm)	Rib 2(mm)	Average(mm)
      Dtr	Dtr1=1.11	Dtr2=1.08	Dtr=1.095
      Ltr	Ltr1=19.45	Ltr2=19.49	Ltr=19.47
      Str	Str1=7.39	Str2=7.39	Str=7.39
      	=74	=74	=74

      Table 7: Values of sample 2 from Mill 1

      Putting the values in (1) and (2) we get, Area of transverse rib (Atr)= 14.21mm2

      Mean projected rib area (Atr)= 3.69mm2/mm

      Mill2 (Sample 1)	Rib 1(mm)	Rib 2(mm)	Average(mm)
      Dtr	Dtr1=1.20	Dtr2=1.21	Dtr=1.205
      Ltr	Ltr1=20.54	Ltr2=22.57	Ltr=21.55
      Str	Str1=6.642	Str2=6.10	Str=6.371
      	=54	=56	=55

      Table 8: Values of sample 1 from Mill 2

      Putting the values in (1) and (2) we get, Area of transverse rib (Atr)= 17.311mm2 Mean Projected rib area= 4.45 mm2/mm

      Properties	Source F1		Source F2		Source F3
      Name	Sample 1	Sample 2	Sample 1	Sample 2	Sample 1	Sample 2
      Yield Strength	461.5 N/sqmm	481.5 N/sqmm	429.03 N/sqmm	426.3 N/sqmm	377.5 N/sqmm	383.1 N/sqmm
      Ultimate Tensile Strength	569.3 N/sqmm	568.6 N/sqmm	555.9 N/sqmm	550.5 N/sqmm	542.6 N/sqmm	540.8 N/sqmm
      Percentage Elongation	20.60%	20.80%	22.03%	22.10%	20.18%	20.92%

      Table 4: Obtained values of physical properties

      Table 5: Chemical composition obtained by testing TMT from various sources

      CHEMICAL COMPOSITION OF TMT(sample 1) FROM SOURCE F1
      NAME	C	Si	Mn	P	S	Cr	Mo	Ni	Al	Co	Cu
      	0.2062	0.0737	0.539	0.0562	0.033	0.1065	0.0084	0.0593	0.0010	0.0065	0.2492

      CHEMICAL COMPOSITION OF TMT(sample 2) FROM SOURCE F1
      NAME	C	Si	Mn	P	S	Cr	Mo	Ni	Al	Co	Cu
      	0.2374	0.0231	0.0126	0.0524	0.0351	0.1424	0.013	0.0732	0.0088	0.0085	0.3073

      Values obtained by testing TMT steel bars from Source F1

      CHEMICAL COMPOSITION OF TMT(sample 1) FROM SOURCE F2
      NAME	C	Si	Mn	P	S	Cr	Mo	Ni	Al	Co	Cu
      	0.2199	0.3061	0.573	0.0502	0.0631	0.1004	0.0179	0.0758	0.0084	0.0092	0.1404

      CHEMICAL COMPOSITION OF TMT(sample 1) FROM SOURCE F2
      NAME	C	Si	Mn	P	S	Cr	Mo	Ni	Al	Co	Cu
      	0.2203	0.2897	0.576	0.0489	0.0635	0.1001	0.0171	0.0763	0.0092	0.0094	0.1417

      Values obtained by testing TMT steel bars from Source F2

      CHEMICAL COMPOSITION OF TMT(sample 1) FROM SOURCE F3
      NAME	C	Si	Mn	P	S	Cr	Mo	Ni	Al	Co	Cu
      	0.2224	0.2247	0.4919	0.0696	0.0442	0.0652	<0.006	0.0588	0.0071	0.0061	0.1738

      CHEMICAL COMPOSITION OF TMT(sample 1) FROM SOURCE F3
      NAME	C	Si	Mn	P	S	Cr	Mo	Ni	Al	Co	Cu
      	0.2224	0.2247	0.4919	0.0696	0.0442	0.0652	<0.006	0.0588	0.0071	0.0061	0.1738

      Values obtained by testing TMT steel bars from Source F3

      Mill2 (Sample 2)	Rib 1(mm)	Rib 2(mm)	Average(mm)
      Dtr	Dtr1=1.13	Dtr2=1.45	Dtr=1.29
      Ltr	Ltr1=22.25	Ltr2=23.20	Ltr=22.725
      Str	Str1=6.532	Str2=6.731	Str=6.631
      	=56	=58	=57

      Mill2 (Sample 2)	Rib 1(mm)	Rib 2(mm)	Average(mm)
      Dtr	Dtr1=1.13	Dtr2=1.45	Dtr=1.29
      Ltr	Ltr1=22.25	Ltr2=23.20	Ltr=22.725
      Str	Str1=6.532	Str2=6.731	Str=6.631
      	=56	=58	=57

      Area of transverse rib = 9.294 mm2

      Table 9: Values of sample 2 from Mill 2

      Putting the values in (1) and (2)

      Mill3 (Sample 1)	Rib 1(mm)	Rib 2(mm)	Average(mm)
      Dtr	Dtr1=0.66	Dtr2=0.95	Dtr=0.805
      Ltr	Ltr1=19.71	Ltr2=18.88	Ltr=19.295
      Str	Str1=8.10	Str2=7.98	Str=8.04
      	=76	=78	=77

      Mill3 (Sample 1)	Rib 1(mm)	Rib 2(mm)	Average(mm)
      Dtr	Dtr1=0.66	Dtr2=0.95	Dtr=0.805
      Ltr	Ltr1=19.71	Ltr2=18.88	Ltr=19.295
      Str	Str1=8.10	Str2=7.98	Str=8.04
      	=76	=78	=77

      Area of transverse rib (Atr) = 19.5 mm2 Mean Projected Rib area= 4.93mm2/mm

      Table 10: Values of sample 1 from Mill1 3

      Mill3 (Sample 2)	Rib 1(mm)	Rib 2(mm)	Average(mm)
      Dtr	Dtr1=0.60	Dtr2=0.75	Dtr=0.675
      Ltr	Ltr1=20.19	Ltr2=21.12	Ltr=20.655
      Str	Str1=8.30	Str2=8.28	Str=8.29
      	=77	=78	=77.5

      Mill3 (Sample 2)	Rib 1(mm)	Rib 2(mm)	Average(mm)
      Dtr	Dtr1=0.60	Dtr2=0.75	Dtr=0.675
      Ltr	Ltr1=20.19	Ltr2=21.12	Ltr=20.655
      Str	Str1=8.30	Str2=8.28	Str=8.29
      	=77	=78	=77.5

      Putting the values in (1) and (2), we get Area of transverse rib(Atr) = 10.35mm2 Mean projected rib area = 2.507mm2/mm

      Table 11: Values of sample 2 from Mill1 3

      Putting the values in (1) and (2), we get

      Mean projected rib area = 2.188mm2/mm

   5. Testing of Macrostructure

   D Now the last test is one of the most important i.e Macrostructure. Study of macrostructure is done by preparing a nitrol solution which contains 5 to 10% nitric acid with balanced ethyl alcohol. Before starting the test each sample should be grinded and polished properly otherwise the Macrostructure obtained will be of bad nature. After preparing the solution, smooth end of each sample is pickled in it for not less than 2 to 3 minutes, once the sample is taken out from the beaker we can clearly see the Macrostructure with the naked eye i.e uniform and concentric hardened Martensitic periphery on outer surface and soft fine grained ferrite pearlite at the core. Such bars will have desired tensile strengths coupled with high elongation as required in seismic zones. Depending on the size and grade, the hardened periphery will be about 20-30% of the bar cross-sectional area for good Quenching and tempering bars. Hence the Macrostructure thus obtained is then compared with the Standard one and the differences are noted down.

3. TEST RESULTS

   1. Physical properties

      The results obtained by testing the samples from three millers F1, F2 and F3 shows those samples from F1 and F2 have acceptable yield strength, ultimate tensile strength and percentage elongation. But the sample manufactured by source F3 shows yield strength of 380.35 N/mm2 which is far below the standard and hence cannot be accepted for constructional purpose.

   2. Chemical Properties

   The chemical analysis of steel bars from source F1, F2 and F3 shows that samples from F1 have average carbon content of 0.2%, manganese content of 0.525%, phosphorous content of 0.0549% and sulphur content of 0.039% which is well within the Indian standards and is acceptable for structural purposes in constructions. The samples of steel bars from source F2 have average carbon content of 0.22%, manganese content of 0.5745%, phosphorous content of 0.049% and sulphur content of 0.0633%. Here the sulphur content is beyond 0.06% which is totally unacceptable and is a case of rejection because increase in sulphur will increase the strength and hardness of the steel, and the same time will decrease the ductility and thus making it brittle. More over the increase in sulphur also decreases the weldability of steel. The samples of steel bars collected from source F3 have average carbon content of

   1. %, manganese content of 0.484%, phosphorous content of 0.069% and sulphur content of 0.045%. We see that phosphorous content has exceeded the permissible limit of 0.06% which is accepted maximum up to the range of 0.063%, and this is also a case of rejection because increase in phosphorous too increases the strength and hardness of steel but decreases the ductility (thus making it brittle) and notch impact toughness of steel.

      Mean projected rib area

      As per IS 1786:2008 mean projected rib area for 12mm bar should not be less than 1.8 mm2/mm. the samples being tested from three sources F1, F2 and F3 have rib area above the standard. Thus we can say all the samples are acceptable as far as Mean projected area is considered.

      Macrostructure

      From the test results, the samples when subjected to macrostructure from three millers F1, F2 and F3 shows those samples from F1 and F2 are having uniform and concentric hardened martenistic grain structure at the surface and fine grained ferrite pearlite at the core as shown in the figures. And the samples from source F3 cannot be considered at all because it is not showing any macrostructure when subjected to nitrol solution. Hence these types of TMT bar should never be used for structural applications.

4. CONCLUSIONS

Various physical and chemical tests, test for mean projected rib area and macrostructure conducted on TMT steel bars shows that out of the three TMT steel manufacturers only one miller exhibits all

properties as per IS 1786:2008. While TMT steels from other two millers failed due toeither lower yield strength or increase in sulphur and phosphorous content. Based on the experiments conducted to test the various properties of TMT steel bars we suggest the following ways to identify and detect good quality TMT bars for structural applications.

1. We should check that the steel manufacturer who has supplied the TMT bar has proper and authentic quenching and tempering technology.

2. Depending on the brand name TMT one must always check for the properties.

3. We must ensure that the quenching and tempering technology given to the steel manufacturer is through genuine and authorised firm.

4. By selecting the bars randomly test can be done at the field itself. This will require filing the surface of one end of the TMT bar using hand file. The worker engaged in the job everyday can easily recognise that the surface is hard or soft. Quenched bars have surface harder than the bars which are not properly treated.

5. We can ask for the license and check whether steel manufacturer has the same for making TMT bars.

6. A license from Bureau of Indian standard will be more advantageous.

7. Finally, we can make random selection of the samples. First thing to do is to grind and polish the cross section of each sample using grinder and then dip it in a solution of nitric acid and ethyl alcohol (4- 9% nitric acid). After 2-3 minutes sample is taken out of the beaker and thus one can clearly see the macrostructure i.e tempered martensite at the periphery and fine grained ferrite-pearlite at the core. This is an effective method to identify good quality TMT bars.

REFERENCES

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1. Owens GW, Knowles PR. Steel designers manual.

   Blackwell Scientific Publications, 1992

2. DGS 527Specification for hot rolled steel bars for concrete reinforcement.

3. Pee kay steels and castings pvt limited.

4. Your guide to civil engineering basics TMT bars.mht