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Material.-
(a) Tubes shall be manufactured from steel produced by an
open hearth, or electric process or any of the oxygen
processes.
(b) Manufacture – The tube shall be made by electric resistance
welding and heat treated as per the table given below.
In case of cold finished tubes the heat treatment shall
be done after the final cold finishing process.
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Tube
Grade
|
Heat
treatment
|
|
WC
1
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Normalized*
or sub-critically annealed #
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WC
2 &
WC 3
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Normalized*
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*
Temperature range
800 Deg.C to
940 Deg.C.
#
Temperature range
660 Deg.C to 720 Deg.C.
The
tube shall be well finished and free from harmful defects. The external weld (flash) shall be removed completely i.e.
flushed with outside surface.
The internal weld upset shall be trimmed throughout the
length of the tube so that the maximum height shall not exceed
0.25 mm.
(c) Chemical Composition.- Ladle analysis of the steel grades shall
conform to the following requirements.
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Tube
Grade
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C
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Mn
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Si
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P
|
S
|
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WC
1
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0.16
max
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0.30/0.70
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0.35
max
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0.040
max
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0.040max
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WC
2
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0.17
max
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0.40/0.80
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0.10/0.35
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0.035
max
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0.035max
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WC
3
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0.12/0.18
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0.90/1.20
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0.10/0.35
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0.035
max
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0.035max
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NOTE
:1 WC1
grade may be fully killed, rimmed or semi-killed.
NOTE:
2 For
rimmed or semi-killed steel, in case of grade WC1, the C max
is 0.19%.
Where
the temperature is below 400 Deg. C either rimming,
semi-killed or killed steel may be used.
If rimming steel is used, the strips shall be rolled in
single width, and not slit longitudinally.
The
steel used for design steam temperature above 400 Deg. C shall
be of fully killed type.
When
ladle analysis is not available the analysis of sample tube
selected at random may be used.
(d) Thickness . - The thickness of each tube excluding
the weld shall conform to the permissible variation
given below :
Permissible
variation (in percent) in wall thickness.
______________________________________________________________
Tube
Wall
thickness
_Permissible variation_
Grade
Over
Under
_______________________________________________________________
WC1
3.2 mm and under
10
10
WC1
Over 3.2 mm
7.5
7.5
WC2 &WC3 All
10
10
________________________________________________________________
The minimum
thickness in the weld area shall not be less than that
permitted in the body of the tubes.
(e) Diameter – The
outside diameter of each tube measured at any position shall
conform to the specified diameter within the limits of plus
0.75% and minus 0.75% with a minimum of 0.30 mm over and under.
(f) Length- Unless otherwise specified tube shall be supplied as
random length. Where the length is specified as exact length
or cut length, the permissible variation shall be + 6mm -0 mm
for length up to and including 6 meters.
For every 3 metre increase in length above 6 metres,
the positive tolerance shall be increased by 1.5 mm with a
maximum of 12 mm.
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