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Reg. 360

Joints

a) These Regulations provide for ordinary bolted flange joints. Special types of joints may be used, subject to the approval of the Chief Inspector.
b

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 60020oC. 

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 -  750o                                        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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