CHALLENGES IN QUALIFIYING WELDING PROCEDURES FOR SUPERDUPLEX
STEEL S32760
Most of the Middle East clients specify ASTM A
923 method C corrosion requirements for the welding procedure
qualification of SDSS S32760 grade. However, ASTM A 923 specifies the
corrosion requirement for base materials (Solution annealed condition) and not
for S32760 grade. Many organizations failed to meet the
corrosion requirement by accepting the material of construction as S32760
without knowing its metallurgical and corrosion properties of this specific
grade. The challenges to
overcome and the selection of appropriate MOC based on
their application are narrated in detail in this case study.
During the engineering
stage, the organization had selected SDSS S32760 material as MOC
(Vessel thickness is 10mm) based on their design basis (Sour
service). The organization has qualified welding procedures
with various parameters but could not pass the corrosion testing
of the ASTM A 923 method C. Upon investigation, it
was found that a lack of experience
personnel w.r.t. selected material was the prime reason for
the failures of PQR corrosion testing.
Based on general
practises of exotic materials, initially trials were carried out with shielding
gas of 99.999% Argon, interpass of 90°C and heat input 1.55KJ/mm. Corrosion
properties inline with A 923 method were not met and not even closer to
acceptance range (10mdd at test temp 40° c).Procured materials like pipe, flange & cap end were supplied in solution
annealed condition and were corrosion requirements.International consumable
manufactures have been consulted to guarantee the corrosion properties for
S32760, unfortunately none of them provided the guarantee.
Testing and Analysis
Girth welds must be of the welding process GTAW manual and are one
of the bottlenecks of fabrication.About 10 trails were carried
and tested for mechanical, chemical (including PREN) and corrosion
properties. Ferrite count (35-60%) were tested in accordance with ASTM E
562 along with metallography study for any intermetallic precipitates. Below
various combinations were part of the trials and modified to stringent
requirements to meet the corrosion criteria.
·
Increase of travel speed especially for root & coldpass
·
Change of shielding & backing gas from Argon to Ar+2% N2
·
Interpass temperature from 90° C to 50°c
·
Restricting heat input for Root and coldpass
·
Highest cleaning practices in handling base material,
consumable and fabrication tools
At last, best
practices of super duplex were implemented in order to meet the
corrosion criteria. Even after that, organization has found that the
corrosion results were not consistent with the trails. One of the NACE
article (paper 8838) has clearly states that it is difficult for SDSS to pass
corrosion test of A 923 method C and is not repeatable.
All purchased welded
pipes of S32760 were meeting the corrosion requirements of A 923 method
C. On comparing,we found that that the rootcause for the failures.
• Process used for welded pipes were mechanized process, whereas
organziation had utilized manual GTAW process.Travel speed was higher in case
of welded pipes than the manual welding process
• The welded pipes were undergone solution annealing heat
treatment, whereas girth weld had not undergone any heat treatment.
The root pass is difficult to be welded with
travel speed of 65 mm minimum by a manual
craftmen. Compared
to SS 316L, fluidity of weld pool is less for SDSS materials. Base material is
difficult to fuse. More over minimum 65 mm travel
speed, the welders were not able to fuse base materials
properly, resulted in lack of fusion and undercut
defects. Organization has put tremendous effort to train to achieve the
travel speed during vessel fabrication. However, organization has selected
S32750 Grade and were able to meet all chemical, mechanical, ferrite and
corrosion requirements, while following general practices of exotic
requirements as shown below
·
Travel speed of 45mm/min for rootpass
·
Shielding & backing gas Ar+2% N2
·
Interpass temperature 100° C
·
Heat input for Root (0.6-1.2 kj/mm) and coldpass (0.5-0.9
Kj/mm)
·
Filler metal of 1.6,2.0 & 2.4 mm
Conclusion:
S32760 may not
be recommended for the fabrication to suit this corrosion requirement, whereas
S32750 would be ideal to meet both fabrication and technical
requirements. Many recommend accepting S32760 grade
with a corrosion requirement inline with an international
standard ISO 17781. Acceptance criteria is 4.0g/m2 at test temperature of
35°c which is less stringent than A 923 method C.
Recommendations
Corrosion criteria
can be achieved ,if Grade S32760 meets below recommendations during welding
procedure qualification.
1.
Travel speed to be maintained 65 mm/minute minimum in root pass welding
2.
Travel speed cold pass should be higher than
the root pass. The value may range from 100-125mm
3.
Shielding and backing gas Ar+2%N2 would be the best
4.
Interpass to 50°C is highly recommended due to low thickness
5.
Oxygen content limit to 500 ppm during back purging
6.
Heat input shall be limited to 0.6-0.9 Kj/mm and cold pass to
0.52-0.86 Kj/mm
7.
Filler metal shall be 2.0 mm dia for Root and 2.4 for cold,
fill & cap passes
8.
Competent craftmen shall be utilized
9.
Control of repairs such as Lack of fusion and undercut due to
higher travel speed of 65mm/Min minimum
References
ASTM A 923:
Standard Test Methods for Detecting Detrimental Intermetallic Phase in Duplex
Austenitic/Ferritic Stainless Steels
ISO 17741 :
Petroleum, petrochemical and natural gas industries — Test methods for quality
control of microstructure of ferritic/austenitic (duplex) stainless steels
NACE Article: The Validity of Using the ASTM A923 Test Method
C Corrosion Test for Weld Procedure Qualification of 25% Chrome Duplex
Stainless Steels
ASTM E 562: Standard Test Method for Determining Volume
Fraction by Systematic Manual Point Count.
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