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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.

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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