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Reg. 357 C
Heat treatment after welding
| c) |
Types 1, 2, 3, 4, 5 Carbon
steel types flanges welded on by the metal arc process
shall have the welds stress relieved where the design
depth of the weld recess is greater than 20 mm. |
| d) |
Where type 1 flanges are attached by oxy-acetylene
welding the welds shall be normalised.
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| e) |
The procedure to be observed
in the stress relieving and normalising of welds shall
be in accordance with the following requirements:-
(i) Stress relieving:-(a) For Carbon Steel, a stress relieving
heat treatment shall be performed by heating the part
to at least 600°C ± 20°C.
When required by the characteristics of the material,
different temperature may be necessary to obtain appropriate
stress-relieving. The part to be stress-relieved shall
be brought, slowly upto the specified temperature and
held at that temperature for a period proportionate on
the basis of at least two and half minutes per millimetre
of the maximum thickness of the part (approximately one
hour per twenty six millimetre of the thickness) and shall
be left to cool in the furnace to a temperature which,
for parts with the thickness greater than 20 millimetre
does not exceed 400°C. After withdrawal from the furnace,
the part shall be moved to a still atmosphere. A temperature
time diagram of the stress-relieving process shall be
provided when the Inspecting Authority requires it.
Note: Other stress relieving heat treatments at a temperature
as low as 550°C may be adopted, provided attention
is given to the holding time in order to ensure a specific
degree of stress relieving.
(b) 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
in degree centigrade |
Time at temperature per mm of
thickness of plate |
½Mo
½Cr ½Mo |
620-660 |
2.5 minutes (30 minutes min.) |
1Cr ½Mo
1½Cr ½Mo |
630-670 |
2.5 minutes (30 minutes min.) |
| 2¼Cr 1Mo |
650-750 |
2.5 minutes (1 hour min.) |
Note: For pipes of 2¼Cr 1Mo specification up to
and including 127 mm outside diameter and up to and 13
mm wall thickness, the time may be 30 minutes minimum.
The heat treatment of steels having Cr Mo V shall be carried
out as per the requirements of the code of manufacture
of plates, pipes
(c) Stress relieving shall be carried out by any of the
following methods:
(1) Local heating using a portable muffle furnace, induction
coils or other suitable appliances. Particular care shall
be taken to apply heat uniformly over the area to be treated.
The use of procedures that do not provide adequate control
for this purpose such as manual operation gas torches
is not permissible. The temperature shall be measured
by the thermocouples pinned, welded or otherwise suitably
attached to the surface of the pipe and where necessary,
protected from flame impingement.
(2) Heating in a stationary industrial furnace. The temperature
of the joint shall be measured by thermocouples so disposed
within the furnace as to give a true measure of the joint
temperature.
(ii) Normalising.-Welds in mild steel pipes shall be normalised
by heating the metal to a temperature between 900 and
950°C. (1650-1740°F.), the temperature then being
maintained for not less than two minutes for pipes up
to and including 102 mm (4”) bore, and for not less
than five minutes for pipes over 102mm. (4”) bore.
Welds in pipes made from the alloy steel shall be normalised
by heating the metal to a temperature between 925 and
975°C. (1700-1790°F), the temperature then being
maintained for not less than two minutes for pipes up
to and including 102 mm (4”) bore. The higher temperatures
of between 950 and 975°C (1740-1790°F) shall be
used for alloy steels having a carbon content up to and
including 0.12 per cent and the lower temperatures of
between 925 and 950°C (1700-1740°F) shall be used
for alloy steel having a carbon content of more than 0.12
per cent. After normalising the joint shall be covered
with a suitable muff or asbestos cloth to ensure slow
and even cooling. The normalising of welds in pipes up
to 102 mm (4”) bore may be carried out with welding
flames the temperature being judged with the aid of a
colour card, or measured by means of one or more thermocouples
or an optical pyrometer. The normalising of welds in pipes
over 102 mm (4”) bore shall preferably be carried
out in a portable pipe normalising furnace or muffle,
the temperature being measured by means of one or more
thermocouples or an optical pyrometer. The width of the
zone to be heat treated to the specified temperature shall
extend at least 13 mm. (1/2”) on either side of the
weld reinforcement.
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