Identifying Quenched & Tempered (or “TMT”as referred to in India) rebars
Project site engineers and all civil engineers should be able to distinguish
and identify good quality Q&T rebars. How does one ensure that the rebars have a concentric tempered martensite periphery
with a softer ferrite-pearlite structure? A few guidelines are presented here.
a. Ensure the
mill that has supplied the rebars has a genuine ‘quenching & tempering’ technology.
b. Check whether the
Q & T technology supplied to the mill is by an authorised and competent organisation.
c. Check the licence,
if any, issued to the mill for the specific Q & T rebars.
d. A Licence from B.I.S.
is an added asset.
e. Always test the rebars for properties instead of merely relying on the
name ‘TMT’.
f. Field
test the rebars at random. The first thing to be done is filing the surface of the rebars with the help of workers ‘rough’
hand file. A site worker will easily recognise if the surface is hard or soft. All Q & T rebars have a harder surface
than unquenched bars.
g. Finally, random
sample should be drawn. The cross-section is smoothened to a fine polished state by grinder and emery paper. The smooth end
of sample is then pickled in nitrol solution (5 to10% nitric acid with balance ethyl alcohol). The result should show a uniform
tempered martensite periphery with a softer core in case of good Q & T rebars.
One
must point out that elongation helps in identifying bad Fe415 “TMT” bars and based on data presented by Dr. C.
S. Vishwanath, the following can be used for identifying such “TMT” bars:
|
A5 Elongation % |
YS, N/mm2 |
Remark |
|
36 - 45 |
265 - 300 |
‘TMT’
rebars without any treatment |
|
32 - 40 |
360 - 390 |
‘TMT’
rebars with improper/inadequate Q&T treatment |
|
Below 6% |
600 or more |
‘TMT’
rebars quenched in water tank after rolling |
