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Arc
welded butt joints of pipes or socket welded joints for
pipes, valves and fitting shall be made by metallic
shielded arc process and post weld heat treated
effectively except in the following cases.
For Carbon steel -
(i)
The nominal wall thickness in case of butt joints
does not exceed 20 mm or carbon content does not exceed
0.25%;
(ii)
The
throat thickness of fillet weld for socket welded joints
or other types of joints does not exceed 20 mm or carbon
content does not exceed 0.25%.
Note.-
In case of carbon content for above cases lies between
0.25% < C £
0.30%, the PWHT is not required for maximum thickness of
9mm.
For
Alloy steel: -
(i)
In case of 0.5 Molybdenum steel, if the thickness
does not exceed 13mm and outside diameter does not
exceed 127mm and for any fillet welds, the throat
thickness does not exceed 13mm.
(ii)
In case of 1 Chromium and 1/2 Molybdenum steel,
if the thickness does not exceed 13mm and outside
diameter does not exceed 127mm and pre-heated to 125oC
and for any fillet welds if the throat thickness does
not exceed 13mm.
(iii)
In case of 2-1/4 Chromium 1 Molybdenum steel,
post weld heat treatment is not mandatory under the
following conditions.
(1)
a
maximum specified Chromium content of 3.0%.
(2)
a
maximum nominal outside diameter of 102mm.
(3)
a
maximum thickness of 13mm.
(4)
a
maximum specified carbon content of
0.15%.
(5)
a
minimum pre-heat temperature of 150oC.
(6)
a
maximum throat thickness of 13mm in case fillet weld.
Method
of Heat Treatment: -
(1) For carbon steel, a stress relieving
heat treatment shall be performed by heating the part to
at least 600±20oC.
When
required by the characteristics of the material,
different temperatures may be necessary to obtain proper
stress relieving. The
part to be stress relieved shall be brought slowly up to
the specified temperature and held at that temperature
for a period proportionate on the basis of at least
2-1/2 minutes per millimetre of the maximum thickness of
the part (approximately one hour per twenty five
millimetres of thickness) and shall be left to cool in
the furnace to a temperature which, for parts with
thickness greater than 20 millimetres, does not exceed
400oC. After
withdrawal from the furnace, the part shall be allowed
to cool in a still atmosphere.
A
temperature-time diagram of the stress relieving process
shall be provided when the Inspecting Authority requires
it.
(2) For alloy steel a stress relieving
heat treatment shall be carried out on the basis of the
composition of the alloy as shown in the table below: -
Table
_____________________________________________________________________________
Type of
steel
Range of temperature
Time at the temperature
per 25mm of thickness
of the plate.__________
1 Cr 1/2
Mo
620oC
- 650oC
1 hour (1 hr. min.)
1/2 Cr 1/2 Mo
----------------------------------------------------------------------------------------------------------------
1 Cr 1/2
Mo
620oC
- 660oC
1 hour (1 hr. min.)
1.1/4 Cr
1/2 Mo
----------------------------------------------------------------------------------------------------------------
2.1/4 Cr
1 Mo
625oC -
750oC
1
hour (1 hr. min.)
___________________________________________________________________________
Note.-
This wide range for the post weld heat treatment
temperature is necessary because of the marked
dependence of the mechanical properties of this steel on
the tempering temperature.
In production a definite temperature with a
tolerance of ±20oC
would be selected to ensure that the mechanical
properties upon which the design was based are in fact
achieved.
Heat
treatment shall be carried out by one of the following
methods: -
(i)
Local heating using portable muffle induction coil or
other suitable heating appliance.
Particular care shall be taken to apply heat
uniformly over the area to be treated.
The use of procedure that does not provide
adequate control for this purpose, such as manual
operation of gas torches, is not permissible.
The temperature shall be maintained symmetrically
over peripheral band of metal of a minimum width of
three times the width of the butt-welded preparation.
The temperature shall be measured by
thermocouples pinned, welded or otherwise suitably
attached to the surface of the pipe and, where
necessary, protected from flame impingement.
(ii)
Heating in stationary furnace:-
The temperature of the joint shall be measured by
the thermocouples so disposed within the furnace as to
give a true measure of the joint temperature.
In
case the materials made to the foreign codes or
standards are accepted under regulation 3(2)(i), the
method of heat treatment shall conform to that specified
in the codes or standards.
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