4.11.1 For type C independent
tanks of carbon and carbon-manganese steel, post-weld heat treatment
should be performed after welding if the design temperature is below
-10°C. Post-weld heat treatment in all other cases and for materials
other than those mentioned above should be to the satisfaction of
the Administration. The soaking temperature and holding time should
be to the satisfaction of the Administration.
4.11.2 In the case of large
cargo pressure vessels of carbon or carbon-manganese steel for which
it is difficult to perform the heat treatment, mechanical stress relieving
by pressurizing may be carried out as an alternative to the heat treatment
and subject to the following conditions:
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.1 Complicated welded pressure vessel parts such as
sumps or domes with nozzles, with adjacent shell plates should be heat treated
before they are welded to larger parts of the pressure vessel.
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.2 The mechanical stress relieving process should
preferably be carried out during the hydrostatic pressure test required by
paragraph 4.10.10.3, by applying a higher pressure than the test
pressure required by 4.10.10.3.1. The pressurizing medium should be water.
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.3 For the water temperature, paragraph 4.10.10.3.2 applies.
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.4 Stress relieving should be performed while the
tank is supported by its regular saddles or supporting structure or, when stress
relieving cannot be carried out on board, in a manner which will give the same
stresses and stress distribution as when supported by its regular saddles or
supporting structure.
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.5 The maximum stress relieving pressure should be
held for 2 h per 25 mm of thickness but in no case less than 2 h.
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.6 The upper limits placed on the calculated stress
levels during stress relieving should be the following:
- equivalent general primary membrane stress: 0.9R
e
- equivalent stress composed of primary bending stress plus membrane
stress: 1.35R
e
where R
e is the specific lower minimum yield stress or 0.2% proof stress at
test temperature of the steel used for the tank.
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.7 Strain measurements will normally be required to
prove these limits for at least the first tank of a series of identical tanks
built consecutively. The location of strain gauges should be included in the
mechanical stress relieving procedure to be submitted in accordance with 4.11.2.14.
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.8 The test procedure should demonstrate that a
linear relationship between pressure and strain is achieved at the end of the
stress relieving process when the pressure is raised again up to the design
pressure.
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.9 High stress areas in way of geometrical
discontinuities such as nozzles and other openings should be checked for cracks by
dye penetrant or magnetic particle inspection after mechanical stress relieving.
Particular attention in this respect should be given to plates exceeding 30 mm in
thickness.
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.10 Steels which have a ratio yield stress to
ultimate tensile strength greater than 0.8 should generally not be mechanically
stress relieved. If, however, the yield stress is raised by a method giving high
ductility of the steel, slightly higher rates may be accepted upon consideration
in each case.
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.11 Mechanical stress relieving cannot be substituted
for heat treatment of cold formed parts of tanks if the degree of cold forming
exceeds the limit above which heat treatment is required.
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.12 The thickness of the shell and heads of the tank
should not exceed 40 mm. Higher thicknesses may be accepted for parts which are
thermally stress relieved.
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.13 Local buckling should be guarded against
particularly when torispherical heads are used for tanks and domes.
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.14 The procedure for mechanical stress relieving
should be submitted beforehand to the Administration for approval.