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 APPLICATIONSandvik 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 BENCHMARKOne 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.

EXPERIMENTALMicrostructure characterizationThe 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 analysisThe 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 roughnessThe 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.

HardnessSince 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 polarizationIn 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 HardnessYield strength

0.2%, MPaTensile strength

MPaElongation %

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

MATERIALSCommercial 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) TUBESMicrostructure observation of Alloy 825 tubesAt 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

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SURFACE ROUGHNESS OF ALLOY 825 TUBESThe 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.

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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 825The 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

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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 TUBESThe 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.

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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 tubesThe 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).

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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.

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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 316LThe 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

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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.

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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

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applications engineering magazine from Outokumu, (2011),

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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

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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

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Electrochimica Acta, Volume 56, Issue 25 (2011): p. 9413

[9] C.K. Lee and H.C. Shih, “Structure and Corrosive Wear

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[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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EVALUATION OF SEAMLESS vs WELDED A825 CONTROL LINE

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