EVALUATION OF SEAMLESS vs WELDED A825 CONTROL LINE

Sandvik white paper | Evaluation of seamless and welded tubes for Sub Surface Safety Valve control line application 1
EVALUATION OF SEAMLESS
AND WELDED TUBES FOR
SUB SURFACE SAFETY VALVE
CONTROL LINE APPLICATION
Sandvik Materials Technology AB
Kukuh W. Soerowidjojo, Sandvik SEA Technical Marketing and Sales Area Manager
Wenle He , R&D Sandvik Materials Technology AB – PhD, Principal Engineer

2 Sandvik white paper | Evaluation of seamless and welded tubes for Sub Surface Safety Valve control line application Sandvik white paper | Evaluation of seamless and welded tubes for Sub Surface Safety Valve control line application 3
ABSTRACT
Today’s welding technology has been developed
to enable the negative effects of welding to be
minimized. Further processes on the longitudinally
seam welded tubing, like sink or plug drawing, will
improve the geometry of the tube in both outer
and inner diameter to smooth the weld bead and
increase the wall thickness and outer diameter
tolerances of the tube. Annealing process on the
entire welded tubing eliminates residual stress from
welding and cold forming. However, the weld defects
are difficult to remove entirely by these processes
and therefore the seam welded tubes carry more
failure risks to be used as sub surface safety valve
(SSSV) control lines and chemical injection lines.
The selected seam welded tubes and seamless tubes
of Alloy 825 (UNS N08825) and super-duplex (UNS
S32750) stainless steels in similar dimensions and
of similar chemical compositions have been closely
studied. This was carried out using electrochemical
potentio-dynamic polarization in a chloride solution,
followed by detailed surface analysis with a scanning
electron microscope (SEM) and glow discharge optical
emission spectrometry (GD-OES). This study shows
that the seam welded tubes have similar hardness
compared to the seamless tubes. However the weld
defects, such as undercut, micro-cracks and oxides
along the fusion line, could be responsible for the
reduced corrosion properties of the tube. A large
amount chromium nitride precipitates have been
observed in the outer tube surface of the Alloy 825
seam welded tubes, which could be formed during the
annealing process after being redrawn. Lack of fusion
has also been observed in a seam welded Alloy 825
tube, which could not be detected by non-destructive
testing (NDT) methods.
This study concludes that the seamless tubing is a
better option for SSSV control lines than seam welded
tube to maintain well integrity.
INTRODUCTION
Seamless stainless steel tubing is widely used in energy generation and utilization, such as heat exchanger, sub
surface safety valve (SSSV) control lines and chemical injection lines, due to its reliable mechanical properties and
corrosion resistance. With modern welding technology and additional processes, the longitudinally seam welded
tubing has been developed and produced from strip. The geometry of the tube can be improved by sink or plug
drawing to diminish the weld bead in outer and inner tube surfaces, hence improving the wall thickness and outer
diameter tolerances of the tube. Annealing process on the entire welded tubing eliminates residual stress from
welding and forming.[1]
Many people get confused by terms like welded-and-drawn, seam-integrated and seam-
free tubing when selecting tube, but closer examination reveals big difference.[2]
However, weld defects are difficult
to be removed by these processes and could be the cause of failures when the seam welded tubes are used in oil
and gas applications.[3]
The control line is a small diameter tube line, usually attached to the outside of production tubing, which controls
the SSSV or other downhole tools. Its reliability is the most important factor for engineers to consider during the
selection of materials and even types of tubing in this application. The seamless tubes were recommended to be
used as control lines.[4, 2]
This study aims to make a comparison between seamless and seam welded tube of Alloy
825 and super-duplex stainless steel 2507 in similar dimensions.

PRE - A GOOD BENCHMARK
One key benchmark for assessing localized corrosion
resistance in chloride environment and checking weld
quality is the pitting corrosion equivalent number (PRE),
as defined below:
PRE=%Cr + 3.3 (%Mo) + 16 (%N)
Exact testing procedures to determine the PRE number
are specified in the ASTM G48 standard.
Super-duplex grades samples meet the specification
outlined by ASTM A789 UNS S32750 for both
seamless and welded tubes and acquire a PRE Number
above 42. Alloy 825 of high nickel content has good
corrosion resistance against stress corrosion cracking
in downhole applications. Alloy 825 seamless tube
samples meet ASTM B423 specification for solution
annealed material and alloy 825 welded tube samples
meet ASTM B704 specification. All super-duplex
seamless and welded tubes are solution annealed.
Welded tubes are plug drawn or sink drawn to improve
external surface of the weldment and to remove weld
bead before final annealing. Normally, welded tubes are
slightly more alloyed to compensate the deterioration
of welding effects and also increase their weldability.
Cleanliness testing of Alloy 316L (UNS S31600/
S31603) tubes in dimension OD9.5 mm × WT1.25 mm
after reel-to-reel was used for discussion, however no
detailed chemical composition was available.
EXPERIMENTAL
Microstructure characterization
The microstructures of the tube surfaces and cross-
sections of the materials were examined by using
optical microscope and scanning electron microscope
(SEM). A FIB-SEM (focused ion beam integrated in a
scanning electron microscope) instrument (ZEISS
Crossbeam 1540 EsB) was used for secondary
electron (SE)- and backscattering electron (BSE)
images, and energy dispersive spectroscopy (EDS)
for elemental analysis. Electron channeling contrast
imaging (ECCI) was also used for comparison of internal
residual stress or strain between the tubes, on the
cross-section samples of the tubes. For this purpose,
the surface was prepared with a final polishing by
colloidal silica oxide suspension (OP-S, 0.04 µm).
GD-OES analysis
The chemical composition depth profiles on inner-
and outer tube surfaces were determined by Glow
Discharge Optical Emission Spectrometry (GD-OES)
analysis using a Spectruma GDA 750 Analyzer. The
analysis was run in DC mode with an anode of 2.5
mm in diameter. During the analysis the surface was
sputtered down to the depth of 10 μm. Because of
the question of oxide film on the surface and also the
finding of a nitride rich surface layer on the welded
tube, the analysis results were presented with the
focus of the nitrogen (N), chromium (Cr) and oxygen (O)
compositional profiles.
Surface roughness
The surface roughness was measured by using an
interferometer (Veeco Wyko 9100NT) on the inner
diameter (ID) tube surfaces and the outer diameter (OD)
tube surfaces. Using the Veeco Vision program and its
“Stylus” function, the interferometer simultaneously
measures a large number of lines, analogous to stylus
traces and provides statistical data over a sampling
length conforming with ISO standard 4288.[5]
The
topography of the tube surfaces was also analyzed
using this instrument.
Hardness
Since hardness of the material has an influence on the
defects, and hence the corrosion behavior, Vickers
hardness of the two tubes was measured on the inner –
and outer tube surfaces (ID and OD) and cross-sections
using a hardness tester (LECO M-400 T) with a 50 gram
indenter.
Cyclic potentiodynamic polarization
In order to study the corrosion behavior of the materials,
such as passivity break-down and pitting resistance,
cyclic potentio-dynamic polarizations were performed
for the tube materials in 1 M NaCl solution (pH 7.7)
at room temperature, using a potentiostat, VersaStat
(AMETEK). A three-electrode system was used, with a
tube cylinder specimen as working electrode, a Pt net
as counter electrode, and a reference electrode of Ag/
AgCl (3M KCl). The tube specimens were 20 mm long. In
order to study the tubes in the delivered conditions, the
inner- and outer surface were tested in the as-received
condition, while the cut edges were polished by #120
SiC paper. The specimens were cleaned in acetone
before the measurements. A solution volume of 150
ml was used for each measurement, and it was purged
with N2
before and during the measurements. After
immersion for 1 hour, measurement was started with
upward (anodic) potential scan from the open circuit
potential (OCP), with potential scan rate of 10 mV/min.
The potential scan direction was reversed at 1000 mV
vs Ag/AgCl, and the potential was scanned back to the
original OCP. The measurements were performed on
triplicate specimens for each type of tube.
Critical pitting temperature (CPT)
The corrosion resistance has been evaluated by CPT
per ASTM G 150. The tube sections each about 20
mm long were used with existing OD and ID surface
condition. Cutting edges were polished by 240 # SiC
paper. The specimens were cleaned in acetone using
ultrasonic bath, air dry before the measurements in
1 M NaCl. The measurements started at 10 ºC and
increased by 1 ºC/min. The critical pitting temperature
(CPT) has been determined by the temperature at
current density 200 µA/cm2
.
TABLE 1: CHEMICAL COMPOSITION OF THE SEAMLESS AND SEAM WELDED TUBES
Tube samples UNS number C P S Si Ni Cr Mo Cu N Mn Fe
Seam welded tube,
super-duplex
UNS S32750 0.014 0.024 0.001 0.27 7.10 25.30 3.90 0.27 0.30 0.79 bal.
Seamless tube,
super-duplex
UNS S32750 0.012 0.021 0.001 0.36 6.39 25.23 3.87 0.132 0.29 0.45 bal.
Seam welded tube,
Alloy 825
UNS N08825 0.01 0.0003 0.25 39.0 22.0 3.2 1.98 0.007 0.47 31.97
Seamless tube,
Alloy 825
UNS N08825 0.021 0.001 0.21 38.2 19.9 2.5 1.59 0.68 35.7
TABLE 2: MECHANICAL PROPERTIES OF THE SEAMLESS AND SEAM WELDED TUBES
Tube samples UNS number Dimension, mm Hardness
Yield strength
0.2%, MPa
Tensile strength
MPa
Elongation %
in 2”
Seam welded tube,
super-duplex
UNS S32750 OD19.05 × WT1.65 26 HRC 739 938 35
Seamless tube,
super-duplex
UNS S32750 OD22.09 × WT1.52 29 HRC 780 965 29
Seam welded tube,
Alloy 825
UNS N08825 OD9.53 × WT1.24 85 HRB 314 662 44
Seamless tube,
Alloy 825
UNS N08825 OD9.53 × WT1.24 73 HRB 290 662 44
MATERIALS
Commercial seamless and seam welded tubes of super-duplex (UNS S32750),
Alloy 825 (UNS N08825) have been used for the study. Chemical compositions and
mechanical properties are shown in Table 1 and Table 2 respectively.

a) welded tube Alloy 825, OD
c) seamless tube Alloy 825, OD
b) welded tube Alloy 825, ID
d) seamless tube Alloy 825, ID
Figure 2 Optical microscope images of the cross-section sample Alloy 825 (electrolytic etched) in
transversal direction. The welding defects were seen at the fusion line on the seam welded tube
surfaces, marked by red circles.
By SEM-EDS and ECCI analysis on the as-received
seam welded tube samples, a surface layer (ca. 7 µm
thick) with a large number of precipitates rich in Cr-N
was found on the outer tube surface of the welded tube
(Figure 3a), which is a unique observation for the welded
Figure 3 ECCI images of cross-sections of the seam welded tube samples (a-b) and the
seamless tube samples (c-d) of Alloy 825.The samples were cut in transversal direction, and
final polished by OP-S.
tube. In contrast, only very thin oxide layers can be seen
on the inner surface of the welded tube (Figure 3b), as
well as on the outer and inner surfaces of the seamless
tube (Figure 3c-d).
a) undercut on the ID and defect on the OD b) defect at the fusion line on the OD
Figure 1 Optical micrographs of the open seamless tube section and the seam welded tube section
of Alloy 825. Inner diameter (ID) tube surfaces and outer diameter (OD) tube surfaces are shown in
the left column (pictures a, c) and right column (pictures b, d) respectively.
a) seamless tube, ID b) seamless tube, OD
c) welded tube, ID, a weld line in the lower tube part ( ) d) welded tube, OD, defects along the fusion line ( )
SEAMLESS AND SEAM WELDED ALLOY 825 (UNS
N08825) TUBES
Microstructure observation of Alloy 825 tubes
At low magnifications, the as-received tube sample
surfaces appear similar between the seamless and
seam welded Alloy 825 tubes, see Figure 1.
The samples were cleaned in ethanol to remove the ink
marks. The sample surfaces were of normal metallic
gloss and no visible discoloration observed. A weld line
can be seen on the welded inner tube surface which
means that the seam welded tube was sink drawn that
only outer diameter had been reshaped to smoothen
the surface. However, the welding defects, undercut
with a deep slit on the outer welded tube surface
has been observed on the outer tube surface along
the fusion line. At higher magnifications, the welding
defects (undercut and mechanical damage) can be
clearly seen on the cross-section pictures in Figure 2,
marked by red circles.
RESULTS AND
DISCUSSIONS
5 mm5 mm
5 mm 5 mm
500 µm 100 µm

SURFACE ROUGHNESS OF ALLOY 825 TUBES
The topography images and surface roughness data
obtained by an interferometer are shown in Figure 6.
The inner tube surface of the welded tube (especially
the weld line) was rougher compared to the seamless
a) Seamless tube, ID, Ra=0.59 ±0.03 µm b) Seamless tube, OD, Ra=0.46 ±0.1 µm
c) Welded tube, ID, Ra=1.21 ±0.17 µm
d) Welded tube, OD, Ra=0.57 ±0.26 µm,
Defect slit along the fusion line ( )
Figure 6 Surface topography images (5x) obtained by an interferometer on the inner and outer
surfaces of the seamless tube and seam welded tube, respectively. Ra is surface roughness.
tube, while the outer tube surfaces were similar for the
two tube types. However, the welding defects, undercut
with deep slit on the outer welded tube surface has
been observed along the fusion line.
a) overview of seam weld with lack of fusion b) observation in a higher magnification than picture a)
a) welded tube Alloy 825, OD, crack and mechanical defect
along the fusion line ( )
b) welded tube Alloy 825, ID, micro crack in the undercut ( )
The ECCI images (Figure 4) show that micro-cracks were observed in the area close
to the fusion line on the outer and inner surfaces of the welded tube.
Figure 4 ECCI images of cross-sections of the welded tube sample, defects found close to the
undercut at OD (a) and ID (b) tube surfaces. The samples were cut in transversal direction, and final
polished by OP-S.
The micro-cracks observed on the welded tubes could
be additional corrosion initiation sites of this tube type.
The surface defects, including micro-cracks, are often
the weak sites for localized corrosion where initiation of
corrosion takes place.[6]
Metastable pits may nucleate
only on surfaces where grooves with certain openness
are present.[7]
Furthermore, lack of fusion and a crack formed from
the lack of fusion has been observed on a seam
welded Alloy 825 tube. The defect could be 2 cm long
along the seam welds because it could be seen in two
cross-sections on the tube in transversal direction, see
images in Figure 5. The remaining part of the tube was
sent for X-ray examination. However, no more lack of
fusion could be detected. This was probably because
of local defects and the porosity was too small to be
detected.
Figure 5 Cross-section 1 of a seam welded tube Alloy 825, lack of fusion has been observed, an
overview picture a) and a close look of the defect in picture b). The sample was etched in V2A
solution at 50 °C.
500 µm 200 µm

a) OD surface before the polarization b) OD surface after the polarization
Figure 9 BSE-SEM image of the welded sample before and after the cyclic polarization measurement in 1M NaCl solution. The
samples are cross-sections in transversal direction of the seam welded tube of Alloy 825. The large precipitate particles were
observed in the outmost tube surface OD (picture a) which disappeared after the polarization measurement (picture b).
Figure 8a Depth profiles of chromium and nitrogen in the
welded tube surface obtained by GD-OES (as-received
sample in black; POL is the sample after the polarization
measurement, in red).
Figure 8b As a comparison, the profile in
seamless tube surface is shown in blue.
IDENTIFICATION OF CHROMIUM NITRIDE
PRECIPITATES IN THE SURFACE LAYER ON
WELDED ALLOY 825
The GD-OES profiles show that chromium is enriched
in the outmost surface (chromium peak) of the
welded tube, and this chromium peak disappeared
after polarization, see Figure 8a. In contrast, no such
chromium peak was observed on the seamless tube.
Usually, a chromium-rich passive film of alloys is
beneficial for corrosion resistance. However, in this
case, the co-enrichment of chromium and nitrogen
Figure 8b, suggests the presence of chromium nitrides
in the surface layer of the welded tube, which was
confirmed by the SEM-EDS elemental mapping. The
passive film on the seamless tube is too thin, normally a
few nanometers, to be noticed in the chromium profile.
A high magnification SEM image of the outer surface
of the welded tube reveals presence of large chromium
nitride particles in the outmost surface (Figure 9a, and
Figure 10). The element mapping for chromium and
nitrogen shows the distribution of large chromium
nitride particles, and also likely chromium depletion
around these particles in the outmost surface layer
within 1 µm, which is consistent to the GD-OES analysis
shown in Figure 8 (black curve). Moreover, in the SEM
50
40
30
20
10
0
0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0
Depth from tube surfaces [µm]
MassConc.[%]
Cr-Overlay Graph
Welded tube OD
Welded tube POL
Seamless tube OD
0,50
0,45
0,40
0,35
0,30
0,25
0,20
0,15
0,10
0,05
0
0 1 2 3 4 5 6 7 8 9 10
Depth from tube surfaces [µm]
MassConc.[%]
N-Overlay Graph
Welded tube OD
Welded tube POL
Seamless tube OD
image (Figure 9a), large amount of sub-micron particles
are observed in the surface layer, which are probably
also chromium nitrides, however, they are too small to
be identified by this method.
The chromium nitrides, especially the large ones, in the
surface layer on the outer surface of the welded tube,
may cause local chromium depletion in the boundary
region adjoining them. This could be responsible for
the extensive corrosion observed after the polarization
measurement, because the chromium peak
disappeared from the depth profile on the sample after
the polarization measurement (Figure 8a, red curve).
In the present study on the seam welded tube Alloy
825, the chromium nitrides are probably formed during
the annealing heat treatment following the welding
and re-drawing process. In corrosive conditions, these
precipitates may lead to localized corrosion around the
particles because the inhomogeneous structure makes
the passive film prone to pitting and breakdown.[9]
This
interpretation is supported by the SEM image of the
welded sample after the polarization measurement in
Figure 9b. The large chromium nitride particles are not
in the outmost surface layer anymore, shallow pits are
left in the surface after the polarization measurements.
Figure 7 Cyclic polarization curves
of Alloy 825, the seamless tube
specimens (s-1, s-2, s-3) and seam
welded tube specimens (w-1, w-2,
w-3) measured in 1M NaCl solution.
The arrows (1, 2, 3, 4) show the current
changes during forward potential
scan while the arrow 5 shows the
current changes during the backward
potential scan.
CYCLIC POTENTIODYNAMIC POLARIZATION
MEASUREMENTS ON ALLOY 825 TUBES
The measurements of triplicate samples show a good
reproducibility of the results for both of the seamless and
seam welded tube materials, see the polarization curves in
Figure 7. However, there are big differences between the
polarization curves for the two tube types. The seamless
tube samples exhibited a passive behavior, with a low
current density (< 50 µA/cm2
) during the whole polarization,
in both forward and backward potential scan directions.
The results indicate that no passivity breakdown or
pitting corrosion occurred on the seamless tube under
the experimental condition, which has been confirmed
by the post polarization examination of the samples (see
sections below). Thus the seamless tube has a high pitting
resistance in the NaCl solutions.
In contrast, the welded tube samples exhibited an active
current peak during upward (anodic) polarization scan, see
Figure 7. The current density started to increase at ca. 0.50
V/Ag/AgCl, and reached a peak around the anodic potential
0.75 V /Ag/AgCl, with a high current density of 1.00 mA/
cm2
(arrow 2). Upon further increase in the potential,
the current density decreased first (arrow 3) down to
ca. 0.25 mA/cm2
, but increased again after that (arrow
4). During the backward potential scan, the current
density decreased to a low level (arrow 5), but was still
higher than that for the seamless tube. The current
peak around 0.75 V /Ag/AgCl is most likely due to some
passivity breakdown and localized corrosion taking place
on the surface, which is evidenced by the extensive
corrosion observed on the outer surface of the welded
tube after the polarization. In-situ electrochemical AFM
study would be necessary in order to clarify the details
of the localized corrosion at specific potentials.[8]
The
further increase in the current density at the potential
above ca. 0.85 V /Ag/AgCl probably results from oxygen
evolution on the surface. The oxygen generated on the
surface may have contributed to the repassivation of
the surface during the backward potential scan (arrow 5).
Again more detailed study would be needed to verify the
interpretation, see sections below.
1.00
0.75
0.50
0.25
0
0 0 0.0005 0.0010 0.0015
E(V,vsAg/AgCI)
I (A/cm2
)
s-1.cor
s-2.cor
s-3.cor
w-1.cor
w-2.cor
w-3.cor
5
1
2
3
4

a) overview of seam weld, OD d) overview of seam weld, ID
b) left side of weld, OD, fusion e) left side of weld, ID, fusion
c) right side of weld, OD, fusion f) right side of weld, ID, fusion
Figure 12 Microstructure of the seam welded super-duplex tube in transversal direction, pictures at
OD in left column (pictures a-c), ID in the right column (pictures d-f), etched in 25 vol% HNO3
Figure 12 shows that the weld profile is quite smooth
merging into base material. However some weld
defects have been observed in the fusion line at
the outer tube surface, while good welding in inner
tube surface because of a plug-drawn process.
Micro cracks have been found in the heat affected zone
adjacent to the fusion zone in both sides of the weld at
the outer tube surface. Oxides were found in the weld
area and inside micro cracks on the outer tube surface,
see Figure 13
SEAMLESS AND SEAM WELDED SUPER-DUPLEX
(UNS S32750) TUBES
Microstructure observation of super-duplex tubes
The tubes show a regular duplex structure, 50:50
ferrite and austenite, without detrimental phases or
Figure 11 Microstructure of the seamless tube (a) and seam welded tube (b) in longitudinal
direction, etched in 25 vol% HNO3
a) Seamless super-duplex tube b) Seam welded super-dupex tube
precipitates, see Figure 11. The seam welded tube
shows slightly finer microstructure compared to the
seamless tube.
a) BSE-SEM image
b) element mapping of Cr c) element mapping of N
Figure 10 Identification of chromium
nitride precipitates observed in the
outmost OD tube surface of the
welded tube Alloy 825 (a cross-
section in transversal direction): BSE-
SEM image (a), element mapping for
chromium (b) and nitrogen (c).
50 µm 50 µm
500 µm
50 µm
50 µm 50 µm
50 µm
500 µm

Observations on the samples after the CPT
measurements are shown in Figure 15, Figure 16, and
Figure 17. It seems that crevice corrosion and pitting
corrosion preferentially occurred along the fusion line
on the outer tube surface of the seam welded super-
duplex tube, and also mechanical defect lines on the
outer tube surface. Pits were observed on the outer
tube surface of seamless super-duplex tube, Figure
Figure 15 Observation of seam welded super-duplex tube samples before and after the critical
pitting temperature (CPT) measurements by ASTM G150. The corrosion occurred along the fusion
line (picture a) and on the mechanical defect lines (picture b) indicated by red arrows).
a) An original sample (left) is compared to a sample after CPT
measurement (right)
b) An original sample (left) is compared to a sample after CPT
measurement (right)
CRITICAL PITTING TEMPERATURE MEASUREMENTS
ON SUPER-DUPLEX TUBES
The duplicate tube samples have been used for the
measurements on the seamless and seam welded
tube samples by ASTM G150 in 1 M NaCl, started at
10 ºC and increased by 1 ºC/min. The critical pitting
temperature (CPT) has been determined by the
temperature at current density 200 µA/cm2
. It can be
seen that the seamless tube has CPT about 86 °C while
the seam welded tube has CPT about 78 °C, Figure
14. The seamless tube showed a better corrosion
resistance than the seam welded tube although the
seam welded tube had slightly higher Cr, Ni, and Mo.
17. The results indicate that the fusion line is still the
weak point for corrosion resistance of the seam welded
super-duplex tube. Redrawing and annealing processes
could not remove the weld defects. Micro cracks or
undercuts may lead to crevice and pitting corrosion.
In addition, mechanical damage probably from the
welding and redrawing process also have a negative
effect on the corrosion properties of the welded tube.
b) seam weld, OD, oxide in top of weld
c) seam weld, OD, fusion d) seam weld, OD, oxides nearby the fusion
Figure 13 Oxides have been found in the weld area and weld defects in the seam welded super-
duplex tube. SEM analysis was performed in a cross section of the tube in transversal direction.
Figure 14 CPT
measurements
by ASTM G150 in
M NaCl, duplicate
samples from the
tubes in complete
tube form. The
seamless tube
samples in blue, the
seam welded tube
samples in green.
1.2
1
0.8
0.6
0.4
0.2
0
CurrentDensity(mA/cm2
)
Temperature (ºC)
0 20 40 60 80 100
Seamless super-duplex -1
Seamless super-duplex -2
Seamweld super-duplex -1
Seamweld super-duplex -2
86 0C
78 0C
a) overview seam weld
5 mm 5 mm

HARDNESS OF SEAMLESS AND WELDED ALLOY 825
AND SUPER-DUPLEX TUBES
Vickers hardness has been measured on the outer tube
surface (OD) and inner tube surface (ID) of the super-
duplex and Alloy 825 tubes, both at the base material
(BM) and weld line. See the comparison in Figure 18.
Outer tube surfaces are generally harder compared to
inner tube surfaces. There are no significant differences
CLEANLINESS OF ALLOY 316L
The welding defects like micro cracks and deep slits
have been observed along the fusion line on both
outer and inner tube surfaces for the seam welded
Alloy 825 and seam welded super-duplex tubes.
Besides the corrosion properties, the cleanliness could
also be affected by these defects. The metallurgical
contamination as residue from the manufacturing
process may be trapped in the crevices. In some
applications, where a medium is cyclically pumped
through the tube, the weld seam could flake off and
affect the performance of the overall system, and that
might be detrimental.[2]
In an earlier study, cleanliness
testing had been performed on Alloy 316L seamless
and seam welded tubes.[10]
The testing was performed
on a single tube sample of 300-400 meters long each.
The tubes were flushed with hydraulic fluid until the
cleanliness reached class 6 or less by measuring the
particles in size and quantity using a Spectrex laser
particle counter and categorized based on NAS 1638
(National Aerospace Standard 1638) cleanliness test
specification. After re-spooling, reel to reel 10 times,
the tubes were checked again by filling with clean
hydraulic fluid at a lower pressure. The result showed
that significant particles increased in the hydraulic
fluid from the seam welded tubes from class 6 to class
10. Conversely, the cleanliness of the seamless tube
had not changed remaining at class 6 even after re-
spooling.
Therefore the seam welded tubes possess a higher risk
of plugging the hydraulic line in the valve unit to be used
as control lines. This is due to the defects and reduced
corrosion properties observed, which could lead to
operational risks for the SSSV system preventing it
from working properly.
Figure 18
Comparison
of Vickers
hardness
between the
super-duplex
and Alloy 825
seam welded
and seamless
tube materials.
Super-duplex,
seam weld
Super-duplex,
seamless
Alloy 825,
seam weld
Alloy 825,
Seamless
OD, BM 368 411 378 220
ID, BM 327 336 279 193
DD, weld line 368
ID, weld line 369 404
500
400
300
200
100
0
Vickershardness,Hv0.05kg
Figure 16 Pitting observed on the fusion line of a seam welded super-duplex tube after the critical
pitting temperature (CPT) measurements by ASTM G150.
Figure 17 Pitting observed on the outer tube surfaces of a seamless super-dupex tube after the
critical pitting temperature (CPT) measurements by ASTM G150.
a) OD with fusion line and pits b) Pits observed on the weld line, OD
between the hardness of the weld line and base
material. The higher hardness of the outer tube surface
of seam welded Alloy 825 compared to the inner tube
surface could be affected by the Cr-N precipitate layer
formed during the annealing process.
5 mm
5 mm
2 mm

Modern welding technology, redrawn and annealing
processes have made the seam welded tube much
more homogenous in geometry and improved
corrosion resistance of the welds. This study has
been carried out on the selected seamless and seam
welded tubes of Alloy 825 (UNS N08825) and super-
duplex (UNS S32750) stainless steels. These were in
similar dimensions and similar chemical compositions,
in order to have a detailed examination by means of
microstructure observation, surface analysis and
corrosion measurements.
• The weld defects, e.g. undercut, micro cracks and
oxides, have been observed along the fusion line on
the seam welded tubes. These could not be removed
by redrawing and annealing.
• Lack of fusion and a related crack have also been
observed in a seam welded Alloy 825 tube. The length
could be 2 cm, however, the crack would not have
been detected by non-destructive testing (NDT)
methods.
• Chromium nitride precipitate layer had been observed
and identified on the outer tube surface of the seam
welded Alloy 825, which could be formed during
annealing process after redrawing.
• The fusion line was still the weak point where pitting
and crevice corrosion tended to occur.
• By critical pitting corrosion (CPT) measurements
per ASTM G 150, the seamless super-duplex tube
showed a better corrosion resistance than the seam
welded tube even though the seam welded tube had
slightly higher Cr, Ni, and Mo. The CPT was 86 ºC for
the seamless tube while 78 ºC for the seam welded
tube. Pits have been observed along the fusion line
on the seam welded super-duplex tube after the CPT
measurements.
CONCLUSION
[1] Outokumu, acom 2-2011, A corrosion management and
applications engineering magazine from Outokumu, (2011),
“Welded stainless steel tubes & pipes vs. seamless”
[2] HandyTube Corporation, “THE DIFFERENCE IN SEAMLESS
TUBING” Wednesday, March 25, (2015), http://info.handytube.
com/blog/the-difference-in-seamless-tubing
[3] D.N. Adnyana, NACE International East Asia & Pacific RIM
Area Conference (2014), paper presentation “Corrosion fatigue
and stress corrosion cracking of heat exchanger tubes”
[4] Ellis ST, Siappas G, Colyer A, Mitchell RF. “A System Level
Approach to Subsea Hydraulic Control Line Reliability Issues”.
Offshore Technology Conference; (2008). Paper OTC-19170-
MS
[5] M. Boström, Sandvik R&D report 111746TEA (2011)
[6] C.B. In, S.P. Kim, J.S. Chun, “Corrosion Behaviour of TiN Films
Obtained by Plasma-Assisted Chemical Vapour Deposition,”
Journal of Materials Science, Volume: 29, Issue 7 (1994): p.
1818
[7] Y. Zuo, H. Wang, J. Xiong, “The Aspect Ratio of Surface
Grooves and Metastable Pitting of Stainless Steel,” Corrosion
Science, Volume 44, Issue 1 (2002): p. 25
[8] E. Bettini, T. Eriksson, M. Boström, C. Leygraf, J. Pan,
“Influence of Metal Carbides on Dissolution Behavior of
Biomedical CoCrMo Alloy: SEM, TEM and AFM Studies,”
Electrochimica Acta, Volume 56, Issue 25 (2011): p. 9413
[9] C.K. Lee and H.C. Shih, “Structure and Corrosive Wear
Resistance of Plasma-Nitrided Alloy Steels in 3% Sodium
Chloride Solutions,” CORROSION, Vol. 50 No. 11 (November
1994): p. 848
[10] Leandro Finzetto, testing report “Cleanliness test report
on 316L seamless and seamwelded tube under re-reeling
conditions”, Sandvik Materials Technology NAFTA.
ACKNOWLEDGMENT
The authors would like to acknowledge the team work within Sandvik Materials Technology, especially Pär Hedqvist
for the sample preparations, Daniel Högström for the electrochemical measurements; Christer Johansson for the
optical microscope images; Jerry Lindqvist for all SEM images and analysis; Lennart Eriksson and Jan Andersson
for GD-OES analysis; Rob McIntyre, Leandro Finzetto, Zhiliang Zhou, Guocai Chai, Jerry Lindqvist, Ulf Kivisäkk and
Anna Iversen for all valuable discussions.
REFERENCES
• The seamless Alloy 825 tube showed good
passivation during the whole potentiodynamic
polarization, with lower passive current compared to
the seam welded Alloy 825 despite current caused by
the chromium nitride.
• The micro cracks, oxide, chromium nitrides and
mechanical damage on the outer tube surfaces could
be responsible for the reduced corrosion resistance
of the seam welded Alloy 825 tubes compared to the
seamless tubes.
• Outer tube surfaces are generally harder compared
to inner tube surfaces. There were no significant
differences between the hardness of the weld line and
base material.
• More field studies were required in order to determine
if cleanliness issues were a serious threat to well
integrity where seam welded tube control lines are
used. This could be conducted by reviewing the
statistic of failures caused by hydraulic line plugging.
• Vibration testing is also suggested for further study
if hardness disparity of the weld and tube body
could affect the ‘biting’ action of the ferrule in the
compression fitting to form a perfect metal-to-metal
seal.
Based on this study, seamless tubing is the
recommendation for use in SSSV control lines due to
consideration of safety and cleanness.

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