Cold stretching has been in use for more than 30 years, but not under ASME code rules until 2008(Code Case 2596). Cold stretching (or cold working) increases the yield limit of a stainless steel material by work hardening it after all forming and welded steps are complete. A vessel can be cold stretched by over pressurizing it 1.5 to 1.6 times design pressure under careful control of the deformation vs pressure as outlined in VIII-1 App 44. The resulting permanent plastic deformation increases the diameter up to 7% and the volume by 14%.

1. ASME Sec. VIII Div. 1 2015ED.
Rules for Construction of Pressure Vessels
MANDATORYAPPENDIX 44
What does
“COLD STRETCHING 冷拉伸”mean?
1

2. Cold Stretching background(1/2)
 Cold stretching has been in use for more than 30 years, but not
under ASME code rules until 2008(Code Case 2596). Cold
stretching (or cold working) increases the yield limit of a stainless
steel material by work hardening it after all forming and welded
steps are complete. A vessel can be cold stretched by over
pressurizing it 1.5 to 1.6 times design pressure under careful
control of the deformation vs pressure as outlined in VIII-1 App 44.
The resulting permanent plastic deformation increases the diameter
up to 7% and the volume by 14%.
 冷拉伸已經使用超過30年，但直到2008年才符合ASME規範。
冷拉伸（或冷加工）通過在所有成型和焊接步驟完成後加工硬
化來提高不銹鋼材料的屈服極限。 如VIII-1 App 44所述，在小
心控制變形與壓力的情況下，可以通過將容器過度加壓至設計
壓力的1.5至1.6倍來對容器進行冷拉伸。所得永久塑性變形將直
徑增加至7 ％和14％的容積。
2

3. Cold Stretching background(2/2)
Stainless steel stress strain curve showing an increase in yield strength after cold working.
The ASME allowed stress in a vessel made of SA-240 304 stainless is 138MPa for cryogenic
applications. After cold stretching, the allowable stress is increased to 270MPa (Table 44-1-1
in ASME VIII-1). The higher allowable is a combination of both an increase in the yield
stress and a decrease in the factor of safety.
Circumferential Stress
515MPa Safety Factor
3 270MPa 1.9 double
2 138MPa 3.7 double
3
≤ 0.1%/hr
Guidelines such as:
a) AS 1210 Supplement 2 (1999),
b) EN 13458-2 (2002),
c) EN 13530-2 (2002),
d) ISO 20421-1 (2006),
e) ISO 21009-1 (2008),
f) ASME Boiler & Pressure Vessel
Code (BPVC) VIII-1 Appendix 44
(2013).
Elastic Plastic
P

4. 44-1 SCOPE
 designing and constructing cold-stretched austenitic stainless steel(ASS) vessels
Materials Allowable stress of ASS in different standards [MPa]
UNS
No.
Min.
Tensile
Strength
Min.
Yield
Strength
ASME BPVC
VIII-I Code
Case 2596
ASME BPVC II
Part D
GB150
S30408 515 205 270 138 137
S30403 485 170 247 115 120
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5. 44-2 GENERAL REQUIREMENTS
User or
user’s designated agent
Approval( APP. KK)
t ≤ 30 mm
MDMT ≥ −196°C
MDT ≤ 50°C
limited to single diameter, and nozzles without a reinforcing
pad. uniform nominal thickness.
UTS of the weld filler metal used in construction ≥ the
UTS for the base metals of the weld joint.
Nozzles and opening reinforcement components shall be
designed using this Division and applying the allowable stress
values specified in Section II, Part D.
designed for internal pressure only
Nozzle size ≤ NPS 6 (DN 150)
Nozzle size ≤ NPS 8 (DN 200) at the center of heads
All Cat. A weld joints Type No. 1.
All Cat. B weld joints Type No. 1 or Type No. 2.
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6. 44-5 DESIGN
 (a) Wall thicknesses shall be calculated before cold stretching.
 (b) Nominal diameters may be used in the design calculations
 (c) Pc shall be between 1.5 and 1.6 times P
 (d) The inside knuckle radius of formed torispherical heads
shall not be less than 10% of the inside crown radius of the
head.
 (e) Butt welded joints full RT or by UT.
 (f) All nozzles shall be attached with full-penetration welds.
 (g) The thickness of a support pad shall not exceed the
thickness of the shell at the attachment location.
 (h) Full-thickness fillet welds shall be provided at support-pad
attachment locations.
Note :Pc = cold-stretching pressure；P = design pressure 6

7. 44-6 FABRICATION PROCESS
 44-6.1 WELDING AND EXAMINATION
a) For all Category A joints, the reinforcement ≤ 10% of the
plate thickness or 3 mm, whichever is less.
b) Welder and welding operator identification shall be marked
with a paint pen or maintained by weld maps.
c) Butt welded joints full RT or by UT.
d) All Cat. A or B weld joints and all attachment welds shall
be examined externally for their entire lengths using the PT.
e) Renewed cold stretching shall be performed if cold stretched
parts of the vessel have been welded after cold stretching,
except for attachment or tack welds(點焊) less than 6 mm in
length.
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8. 44-6.2 COLD-STRETCHING OPERATION
a) Fill the vessel with water, the vents open for at least 15 min.
b) the circumference of all shell courses shall be measured, All
measurements shall be made with instruments that have been
calibrated to an accuracy of ±0.05%. Additional measurements of
the vessel while under Pc shall be taken as necessary to calculate
the strain rate. The strain rate during the cold-stretching operation
shall be calculated over the full circumference.
c) cold-stretching operation (witnessed by AI):
1. The pressure shall be increased until Pc is reached. Pc shall be
maintained until the calculated strain rate has dropped to ≤ 0.1%/hr.
2. The minimum holding time ≥ 1 hr.
3. The calculated strain rate shall be determined by repeated or
continuous measurements of the circumference while the vessel is
under Pc. The required maximum strain rate of 0.1%/hr shall be met
during the last half hour. 8

9. 44-6.3 COLD-STRETCHING PROCEDURE (CSP)
 A written record shall be prepared containing at least the
following information:
(a) pressurizing sequence specifying pressure readings and time
(b) circumference measurement before, during, and after
pressurization; or alternatively, time, pressure, and the elongation
of circumference
(c) strain-rate calculations
(d) notes of any significant changes of shape and size relevant to
the functioning of the vessel
9

10. 44-7 STAMPING AND CERTIFICATION
 (a) The Certification Mark on the nameplate shall include
marking with “CS” under the Certification Mark, indicating
the vessel was constructed using cold-stretching methods.
 (b) The Manufacturer shall indicate in the Remarks section of
the Manufacturer’s Data Report: “This vessel has been
constructed using cold-stretching processes in accordance
with Mandatory Appendix 44.”
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