Approaches to the Orbital Welding of Duplex Stainless Steel Tubing of Several Alloy Compositions

Approaches to the Orbital Welding of Duplex Stainless Steel Tubing of Several Alloy Compositions

ABSTRACT

Orbital GTA welding is an excellent joining method for duplex stainless steel tubing and pipe. The nature of the process and the fine control of heat input makes it possible to develop procedures that meet the highest quality specifications for phase balance, corrosion resistance, and mechanical strength. A major advantage of orbital welding over manual welding is that once a procedure has been established for a particular duplex alloy, consistent welds which meet all of the requirements of the qualified test coupons can be achieved with a high degree of repeatability throughout a project.

This presentation described orbital welding procedures and fabrication techniques for welding duplex stainless steel tubing in sizes from 1/4 inch O.D. to 1 inches O.D. in wall thickesses from 0.035 inches to 0.109 inches. The use of special gas mixtures for welding and purging, the most practical weld joint configurations, and the use of backpressure to control the inner weld bead profile was discussed. An evaluation of the appropriate use of filler wire and fusion welding techniques and the suitability of these techniques for service environments requiring high strength and/or corrosion resistance was presented.

Equipment considerations for welding the various tubing sizes such as welding amperage requirements, water-cooling of weld heads, and the use of pulsation was presented. Procedures used are not limited to a specific alloy, but rather data and test results from a number of duplex alloys including duplex alloy 2205 (UNS 31803), superduplex alloy 2507 (UNS 32750), UNS 39274, UNS 39277, and UNS 32760 were provided. Experiences and data from several end-users and applications involving orbital welding of duplex stainless steel tubing were presented. These included: Sandvik Chomutov (Czech Republic), FMC Kongsberg (UK), and others. Particular emphasis were given to the use of orbital GTA welding systems and techniques for offshore and subsea applications.


 Barbara K. Henon, PH.D., Arc Machines, Inc. and Michael D. Hayes, President, Acute Technological Services, Inc.

Stainless Steel World, America 2002 Conference, Houston, Texas, February 2002

Over the past decade orbital GTA welding of various duplex stainless steel alloys has been done in the field for a wide variety of offshore and subsea applications1. These applications ranged from small diameter thin-wall tubing2 to large diameter heavy-wall pipe and test manifolds1,3. While the use of filler material overalloyed with austenite-producing elements is essential for maintaining a favorable phase balance in welds of heavy-wall pipe, orbital welding of small diameter thin-wall duplex stainless steel tubing has been done successfully using both filler wire4 and autogenous welding techniques2. In autogenous welding applications it has been shown that the precise, consistent application of heat during orbital welding results in a more uniform phase distribution than was possible using manual welding techniques. Autogenous orbital welding of super duplex SAF 2507 stainless steel also provided improved productivity compared to manual welding and proved to be a very satisfactory and economical fabrication technology.

Three years ago FMC and Shell prohibited the autogenous welding of duplex stainless steel as they perceived a problem with high ferrite welds in subsea environments. The requirement for lower ferrite orbital welds led Acute Technological Services (ATS) to do preliminary testing of overalloyed insert rings that can be used in place of filler wire with orbital fusion welding equipment5. ATS also explored the use of purge gas supplied by Air Liquide containing helium, argon and nitrogen to provide lower ferrite counts and better penetration, and experimented with a commercially-available flux to provide lower ferrite counts and better penetration. ATS also began the use of I.D. pressure balancing to gain better control of the weld bead profile.

These new approaches to the orbital welding of small bore duplex stainless steel tubing with respect to the attainment of optimal phase balance, corrosion resistance, mechanical strength, and weld bead profile of weldments promise to make autogenous orbital welding an even more effective joining technology. The purpose of this discussion is to evaluate the advantages and disadvantages of these approaches, used alone or in combination, in order to assist customers in identifying the most economical and practical installation procedures for their specific types of duplex tube welding applications. (Results shown in Table I and Table II on pages 9 and 10.)

Weld parameters and equipment requirements for orbital welding of tubing sizes from 1/4 inch O.D. with 0.035 inch wall to one inch O.D. with 0.109 inch wall will be discussed. These considerations include welding current requirements, the use of pulsation including STEP rotation in which the weld head rotor is stopped or slowed during the primary current pulse, and the use of water cooling of the cables and weld head to permit higher duty cycle welding.

Some recent examples of orbital welding applications using several different approaches will be presented.

Ferrite Control
The primary issue in welding duplex stainless steels is to achieve welds with a balanced phase structure because the proportion of austenite to ferrite in the metal determines its mechanical properties such as strength, ductility and hardness, and its resistance to corrosion. Duplex stainless steels have a balanced phase structure of approximately 50 per cent austenite and 50 per cent ferrite. The ideal phase balance is considered to be between 45 and 55% ferrite but steel manufacturers generally produce materials with base metal ferrite counts between 35 to 45% to avoid higher ferrite counts after welding.

During welding, the liquid puddle solidifies as ferrite and during cooling there is partial transformation of the ferrite to austenite. The proportion of austenite to ferrite in the finished weld depends on the base metal chemistry and on the welding thermal cycle. Most specifications for offshore/subsea applications allow a range of 35 to 65% ferrite in the weld and HAZ, but for the more critical applications requiring superior corrosion resistance, ferrite numbers below 50 are preferred6. Control of base metal chemistry was the first step in achieving predictable ferrite counts after welding7, however, precise control of heat input during welding is also essential to maintain the optimum phase balance. Too rapid cooling can make the phase balance shift to a too-high ferrite content with a loss of corrosion resistance, ductility, and increased susceptibility to hydrogen cracking. The variable heat input from weld-to-weld and at different locations within manual welds of duplex can result in unacceptably high ferrite numbers. The amount of nitrogen added as an alloying element to SAF 2507 has been increased to restore the austenite/ferrite balance more rapidly during cooling, so this grade of duplex is somewhat more forgiving with respect to ferrite. However, the higher percentage of chromium and molybdenum in this alloy increases the propensity for sigma formation thus making welding heat input control important. Filler material overalloyed in nickel is typically added during welding to overcome the tendency towards higher ferrite numbers. For SAF 2507 the preferred wire is 25.10.4L which contains 25% chrome, 10% nickel, compared to 7% nickel in the base metal, 4% molybdenum and less than 0.02% carbon.

Ferrite determination


Production line for the manufacture of coiled tubing at Sandvik Chomotov in the Czech Republic uses a Model 227 power supply with a cooling unit and a Model 95-1500 orbital weld head from Arc Machines, Inc.  Photo courtesy of Sandvik Chomutov.

Accurate determination of the volume per cent of ferrite in welds is critical in order to correctly evaluate various welding processes. The quickest method of ferrite determination is using a handheld feritescope that is non-destructive and gives an instant readout. However, the feritscope has been found to be inaccurate for small diameter tubing because the magnetic field cannot conform to the curvature of the small tubing and becomes non-uniform at the surface. The feritescope can be calibrated to measure either volume percent or FN.

The traditional way of determining the phase balance is to perform a point count according to ASTM E-5628 in which a section of the weld is examined under a microscope and the number of ferrite grains in contact with points on a grid are counted to determine the volume fraction of ferrite in the sample. Any points that fall on the grain boundary are considered to be half a point. ASTM E 562 gives detailed instructions for sample preparation, type of grid, magnification, number of fields to count and other information needed to achieve statistically significant values at the 95% confidence level. There are many variations in technique and the reliability of the method depends to some extent on the skill of the metallographer and the preparation of the sample. Some laboratories are able to achieve very consistent point counts while others are less reliable, and even at best ± 5% ferrite accuracies are common.

Orbital Welding Techniques
Orbital welding of duplex tubing with filler wire It is difficult on thin-wall material (0.065 inches and less) to add wire uniformly by hand so the appearance of manual welds with wire may be somewhat irregular. Sandvik has successfully used an Arc Machines, Inc. Model 95-1500 open-frame type orbital weld head with wire feed capabilities to produce smooth even welds during the production of coiled 2507 super duplex tubing for subsea applications4. The AMI Model 227 power supply was programmed to adjust the wire feed rate in synchronization with the primary and background current pulses. These welds met requirements for all applicable codes and standards including bend and tensile tests for qualification to ASME Sect. IX of the Boiler and Pressure Vessel Code9, critical pitting temperature (CPT) according to ASTM G-48A10, Vickers Hardness Testing, and were qualified to the Norwegian standards, NORSOK. In production all welds are examined by radiography immediately after welding. Sandvik said that the long coil application would not have been possible without orbital welding technology.

The photo on page 4 (top) shows an orbital weld developed in the Arc Machines Weld Lab on a 3/4 inch diameter 2205 duplex tube with a wall thickness of 0.083 inches. The end preparation was a square butt and filler wire was added to the weld using a Model 79-2375 open-frame weld head. A single-pass STEP program which synchronized the welding current, travel speed and wire feed was used to achieve good penetration and an attractive weld appearance. The amperage required for Level 1 was 90 primary and 30 background at a travel speed of 4 R.P.M. during the background pulse time only. The time per weld was 1 minute, 51 seconds including pre purge and post purge times, and required approximately 15 inches of 0.035 inch wire. This technique is very practical for diameters of 5/8 inch and greater with wall thicknesses of 0.065 inches and above.


Orbital weld with wire feed on 3/4" diameter 0.083" wall 2205 duplex tubing using an Arc Machines, Inc. Model 415 Power Supply and a Model 79-2375 open-frame weld head. (Lightly brushed).

Autogenous orbital welding in an enclosed weld head of a 1/2 inch O.D. test coupon using a Magnehelic pressure gage to control the weld bead profile.

Autogenous orbital welds with argon shield gas - comparison  to manual GTA welds The addition of filler material should always be done with manual welding of duplex stainless steels. The lack of uniform heat input with manual welding, even with the addition of filler, results in uneven distribution of ferrite with higher than acceptable ferrite levels in some parts of the weld. Manual welding of duplex is difficult because the weld pool is sluggish. Manual welders may overcompensate for the excessive viscosity by over penetrating the root or by pushing through the root with wire leaving bits of wire projecting into the inside of the weld. Welds such as this would fail radiography and very likely have secondary phase precipitation and lower pitting resistance due to the overheating inherent in the manual GTA welding process. Autogenous orbital welding with argon shield gas in a square butt preparation of 0.500 inch diameter, 0.065 inch wall SAF 2507 duplex was found to have significant advantages over manual socket welds with the addition of filler2. The manual socket welds proposed as an alternative joining technology would have increased the weight of the system, doubled the number of welds in the subsea piping system, and the socket weld joint configuration was not amenable to radiographic inspection. The butt joint eliminated a crevice inherent in the socket welds and the orbital test welds passed all required tests including a hydraulic burst test at 41,200 lb./in2, well in excess of the required 3 times operating pressure or 3 X 10,000 lb./in2. ATS completed over 1,000 autogenous orbital welds using this technique. However, ferrite counts of autogenous orbital welds welded with argon shielding gas are higher than with other approaches described below. There has been some concern that autogenous welds with higher ferrite may be susceptible to hydrogen cracking in cathodically protected environments.

Another disadvantage of autogenous orbital welds with argon shield gas is that the welds on some heats of material, especially on heavier wall thicknesses (0.065 to 0.109 wall) may be unacceptably concave on the O.D. with excessive build up on the I.D. when sufficient heat for a full-penetration weld is applied. The use of pressure balancing may help to overcome weld bead O.D. concavity.11,12

Weld beads with a with a high width-to-depth (W/D) ratio are typical of welds on stainless steel heats that are very low in sulfur. This is consistent with the very low sulfur content of super duplex which is about 0.002 wt.%. Surface active elements such as sulfur and oxygen have favorable effects on penetration. Welds between tubes of 300 series stainless steel of unmatched sulfur contents may show a weld pool shift towards the low sulfur side when one of the heats is very low in sulfur. A similar but different weld pool shift appears to occur in duplex material in that weld pool shifts still occur, but the effect is seen below the surface of the weld pool. In 300 series stainless steel the effect of sulfur on the weld pool has been attributed to the Marangoni effect, in which there is a positive temperature coefficient of surface tension when surface active elements are present and a negative temperature coefficient of surface tension in materials low in surface active elements. These actions of trace elements in the melt have a profound effect on weld pool fluid dynamics with a concomitant effect on penetration.

Insert rings: shape, chemistry, availability, practicality and comparison with filler wire


SAF 2507 super duplex (UNS S32750) stainless steel tubes 5/8" diameter, 0.065 inch wall shown with T-shaped insert rings of 25.10.4L. Brush shown at left was used on some welds for the application of flux.

It has long been known that insert rings of the correct chemical composition could be used inplace of filler material during orbital welding of small diameter tubing using equipment designed for autogenous (fusion) welding of tubing5. ATS used an insert made from duplex 2205 filler to weld a socket-weld joint of Nitronic 50-to-Nitronic 50 in an autogenous orbital GTA weld head in 19922.

ATS has done preliminary work with inserts rolled from existing filler wire as well as rings machined from barstock. However, it is difficult to find a source of material with nickel content higher than the base metal in a form other than filler wire. On one attempt material was melted to the 25.10.4L formula with a nitrogen blanket and formed into rings. There was porosity in the ingot that resulted in problems with the welds. More recently ATS has experimented with insert rings of 22-8-3-L (1.000 inch O.D.) and has tested rings of 25-10-4L on 1.000 inch and 0.625 inch diameters for orbital welding of 2507 super duplex. (See Table I and Table II. on pages 9 and 10.) When ATS began work with preformed insert rings, the rings were only available in 1 inch and 5/8 inch diameters. When split insert rings are used, the split has to be on downhill side of the weld. For thin-wall tubing of one inch diameter and less, the weld appearance using an insert is superior to either orbital fusion butt welds or orbital welds with filler wire. The weld profile is slightly convex on both the I.D. and O.D. with a T-shaped ring or flat on both surfaces with a washer-shaped ring of the same diameter as the tubing.

Ferrite counts on the 1 inch 0.109 wall 2507 tube with an insert ring welded with ARCAL® gas averaged 48 with a 41.6 minimum and a maximum of 50 (Fischer Feritscope).

An insert ring used in conjunction with autogenous orbital welding equipment appears to be a practical and effective alternative to using manual welding technology or orbital welding equipment designed to add wire to the weld, although the cost difference between orbital fusion welding equipment and orbital welding equipment with filler wire capabilities is much less than in the past. However, the success of installations using an insert ring will depend on obtaining a reliable source of rings free from porosity and of the correct chemical composition. To date no manufacturer of insert rings has indicated a serious interest in producing the rings on a commercial basis. Careful cleaning of the insert rings and tubing ends must be done in order to prevent porosity in the welds. Experience with an actual installation using the insert ring technique is needed to better evaluate the practicality of this approach.

Shielding gases containing nitrogen

Autogenous welds of SAF 2507 with shielding gas of 98%argon, 2% nitrogen
As early as 1992 a procedure for autogenous orbital welding was developed by Teamtrade A.S. in Norway for 1 inch SAF 2507 pipe (0.116 inch wall) using an argon shielding gas containing 2% nitrogen. A STEP rotation travel mode was used at a maximum primary current of 107 amps. The ASTM G-48-80 Method A corrosion test was done by MTS Norge AS and was passed at 35° C with no visible signs of pitting.

Left: Control lines exiting Subsea control module base were installed by autogenous orbital welding. Duplex stainless steel tubing diamers wre 3/8 and 1/2 inch (9.5 and 12.5mm). Shielding gas containing nitrogen was used to control the ferrite levels. Right: Set up of AMI Model 227 power supply (on cart) and Model 9AF-750 weld head prior to welding. Photos courtesy of FMC Konsgerb Subsea.

Shielding gas mixture containing 10% Helium, 88% Argon, 2% Nitrogen
The use of a gas mixture containing 10% Helium, 88% Argon, 2% Nitrogen in conjunction with the controlled heat input provided by orbital welding power supplies has been shown to consistently produce balanced ferrite welds. A helium/argon mixture has a higher arc voltage than argon providing more heat input at lower current settings making full penetration easier to achieve than with pure argon. However, without the addition of nitrogen to the gas mixture, obtaining consistently acceptable ferrite levels is problematic. Nitrogen is an austenite former and its presence in the gas mixture helps to retain the nitrogen in duplex alloys such as SAF 2507 which contain it. ATS has done more than 1,000 orbital welds using the He/Ar/N2 gas and consistently achieves ferrite numbers in the 50-60% range when this gas is used.

The higher arc voltage associated with the gas mixture results in a hotter weld than with pure argon. This makes exceptional demands on enclosed type weld heads. When welding the 1 inch O.D. 0.109 wall tubing, the arc shields on the inside of the 9-1500 weld head were blackened. Others have reported that heating the gas to eliminate traces of moisture eliminates the discoloration. However, it may be advisable to allow the head to cool for a period after completing approximately 6 welds at a 100% duty cycle even though the head is water cooled. If clearances permit, it might be advisable to use a larger head size rather than weld the largest tube size that could be accommodated by the weld head.

FMC Kongsberg Subsea
FMC Kongsberg Subsea has recently used autogenous orbital welding in conjunction with a mixed gas supplied by Air Products containing 10% helium, 88% argon and 2% nitrogen on the pipework for the supply of 13 sub-sea “Christmas Tree” Well Heads for the Terra Nova Capex II Project for Petro-Canada. FMC also used pressure balancing to optimize the weld bead profile.

They used an Arc Machines, Inc. Model 227 and a Model 9AF-750 weld head and achieved acceptable ferrite levels of less than 60% with 3/8 inch diameter 0.071 inch wall duplex tubing (UNS S31803). Prior to using this approach FMC’s reject rate had been quite high due to human error and access-related problems. After changing their approach to incorporate the use of mixed gas containing nitrogen, pressure balancing, the use of special alignment clamps and better material control, their weld reject rate fell to a respectable 2% and the time to weld a tree was reduced from 3 weeks to 6 days. Similar experience has been obtained at FMC’s Houston operations; in fact, in the case of one sub-sea tree, all 150 welds were completed with zero rejects. For this particular case the different alloys, 316L, 2507, and Inco 825, were welded to themselves and to each other with autogenous orbital GTAw with a mixed gas.

The weld procedure and orbital welders for FMC Kongsberg were qualified to FMC’s own weld specification which was based on the requirements of ASME IX and NORSOK Standard M-601. Bend and tensile tests, visual inspection, ferrite counts, macro and micro examinations, and Vickers hardness testing were all found to be acceptable. No deleterious phases such as sigma were observed. The ASTM G-48 corrosion test results were not available at the time of this writing.

The weld schedule was a STEP procedure with zero R.P.M. during primary pulse time and a travel speed of about 4 inches per minute during the background pulse time. Fairly long pulse times were used for optimal penetration and to make an attractive weld bead. The time per weld was 1 minute and 13 seconds including a total pre-purge and post-purge time of 25 seconds exclusive of fit-up time.


Fittings machined from duplex stainless steel were made by ATS for joining 1/4" O.D. tubing in a bend configuration and installed using autogenous orbital welding with a mixed gas containing nitrogen. Photo courtesy of Acute Technological Services, Inc.

Material control was found to be essential for achieving repeatable weld results. Typically batches of material would be segregated by heat number and weld schedule amperage adjusted to give the correct amount of penetration for each heat. some materials require more amperage for penetration and, in this case, duplex materials were segregated into three groups according to penetration and three PQRs were qualified for the different groups. The amperage required for penetration of duplex from the low penetration group was 68 amps for level 1 primary amps compared to 52 amps for the high penetration group. Background amps were 20.5 for the duplex grade most difficult to penetrate compared to 15 for the easiest to penetrate. Parameter adjustments of +/- 15% for each schedule were permitted.

A one pipefitter/one welder team plumbed each tree. The approximately 160 welds on the Christmas trees were about 42% stainless steel-to-stainless steel, 11% stainless steel to duplex, and 47% duplex-to-duplex. On every shift each operator was required to make two “in” test coupons for each orbital weld schedule for qualification of the machine settings and operator prior to production welding. The first ‘in” coupon was cross sectioned by the Supplier and visually inspected. If the coupon failed the visual inspection a non-conformance report (NCR) was issued and corrective action taken. Then two additional coupons were made and deemed acceptable before proceeding with production welding.

Orbital weld on 1 inch O.D., 0.109 inch wall 2507 super duplex tube with a 25-10-4L insert ring using ARCAL gas. (Brushed)Orbital weld on 1 inch O.D., 0.109 inch wall 2507 super duplex tube welded with flux using argon gas. (Unbrushed)


The second “in” coupon received a radiographic test and was either found acceptable or an NCR issued against the failed coupon and the production welds completed after the failed coupon were cut out and the condition corrected before recommencing production. The accepted coupons were labelled and bagged and kept on file and results of all inspections were maintained in a “coupon log book” for future reference.

A weld map and Production Orbital Data Log were maintained for traceability. Welds were identified by weld number, weld location, the WPS (Weld Procedure Specification) number, and the welder ID number. Identification numbers were permanently recorded on the piping next to each weld.

FMC Houston, Texas carried out the initial Capex I project with the second project, Capex II, carried out by FMC in Dunfermline, Scotland. With a contract start date of January 2000, FMC are required to complete all 13 Christmas trees by the 14th August, 2002. As of September 2001, Tree 7 was being completed. A total of 842 welds of 316L stainless steel and duplex were completed on the first 6 Trees.

1/4 inch diameter 2205 duplex

On another application, more than 500 autogenous orbital welds of smaller diameter 2205duplex tubing (1/4 inch O.D.) were done by ATS for the production of long reel coiled tubing using the ARCAL® shield gas. For this application it was found that pressure balancing by inserting a tee at the weld joint prior to welding to determine the correct pressure11 was too time consuming and not practical for long lengths of tubing in the field. The tubing was purged on the I.D. with nitrogen. Just before welding, the joint was pulled apart and the I.D.s of both tubes were back-purged with nitrogen, repositioned for welding and welded in an enclosed orbital weld head without having flowing nitrogen past the joint. To keep the ferrite numbers down, the R.P.M. of the weld head rotor was kept low and low amperage was used with very short pulse times. The welds passed the G-48 corrosion test at the required temperature with acceptable ferrite levels. The weld bead appearance was very slightly concave, but there was no excessive build-up on the I.D. Perhaps those specifications demanding zero concavity of the outside of the weld bead of autogenous welds on small diameter tubing may need to be re-examined in light of the overall performance of welds having slight O.D. concavity when other weld criteria are met.

Penetration Enhancing Flux
A commercially available flux was obtained from Liburdi-Dimetrics (LFX-SS7) that, when applied as a suspension in a volatile liquid to the O.D. of the weld joint with a brush prior to welding, consistently produced ferrite numbers in 40-50% range with autogenous orbital welding techniques. The flux is similar to the A-TIG process which was originally developed by the E. O. Paton Electric Welding Institute13, in the former Soviet Union and has been investigated by The Welding Institute (TWI) and the Edison Welding Institute (EWI)14. The flux increases penetration most likely by oxygen modification of weld pool surface tension (Marangoni effect) and reduces ferrite numbers by promoting homogenous nucleation of austenite within a fine ferrite grain matrix.

Consistent ferrite numbers below 50, obtainable with the use of flux, are not achievable using pure argon or even argon-helium mixtures with autogenous orbital welding. With ferrite numbers above 50% it is difficult to avoid some pitting or weight loss at the most severe temperatures with the ASTM G-48 corrosion resistance test. For critical pitting temperatures in the 30 to 40° C range, ferrite numbers in the 50-60% range, which can be obtained with the addition of nitrogen to the shielding gas mixture, are acceptable and easily meet specifications for satisfactory strength and ductility requirements. However, when the specification calls for passing the G-48 test at the highest temperature (CPT ~ 45° C) it may be necessary to have ferrite numbers in the 40-50% range which the flux could provide. It should be noted that similar low levels of ferrite can be obtained with filler-wire techniques by adjusting the joint configuration to increase the amount of filler wire added to the weld.

Although the flux is useful for achieving low ferrite autogenous welds, there are some disadvantages. The volatility of the solvent used to mix the flux makes consistent uniform application of the flux somewhat problematic. This could result in variable penetration or unreliable ferrite levels. The presence of oxides on the O.D. surface requires extra care during cleaning operations in order to guarantee full removal of the oxides that otherwise could promote corrosion. The surface oxides are not firmly attached but can particulate from the I.D. into process equipment such as needle valves or from the O.D. surface into the weld head during welding or into the piping system in service. The oxide particles are sufficiently hard and brittle to cause damage to the weld head gears. In service the particles could plug up small pin valves connected to the subsea system making it inoperable. In welding with flux applied to the weld joint the tungsten electrode was prone to oxidation requiring more frequent tip changes. The weld bead and HAZ surfaces also appear to be rougher than welds done without the flux.

Heat Input
Since the heat input during welding can affect the austenitic-ferritic phase balance as well as the production of undesirable phases, such as sigma which can result in embrittlement, it is very important to have precise control of the heat input during the welding of duplex alloys. Several of the welding parameters controlled by orbital welding power supplies may be adjusted to regulate the heat input into the weld. The major control of heat input during welding is the primary current (amps). The background current (background amps) is generally set at a percentage of the primary current and the average current is the average of these two values assuming the pulse times remain equal. Current values are usually reduced by about 20% in a gradual series of steps called levels as the electrode rotates around the joint circumference. Four to six levels are typical for single- pass tube welding. The arc voltage which controls the power into the weld is not set directly for autogenous welding but is determined by the current settings and the arc gap. Increasing the arc gap increases the arc voltage making the weld somewhat hotter.

When the arc is pulsed, the ratio of the primary pulse time during which the primary amps is active, and the background pulse time during which the background amps are in effect also affects the heat input. The longer the primary pulse time with respect to the background pulse time, the higher the average amperage will be. The travel speed (R.P.M.) may also be used to influence the heat input. A faster travel speed results in less heat input at the same current setting. A slower travel speed allows for better penetration at reduced current settings. All of the weld parameters controlled by the welding power supply are entered on a weld schedule or program and stored in the power supply memory.

The precise control of heat input afforded by the orbital welding power supplies makes possible consistent uniform heat input from weld joint-to-weld joint that can be repeated weld after weld giving assurance that the production welds will meet the same criteria as the test welds submitted for weld procedure qualification. Since there is some variability in the base metal chemistry from heat to heat, there may be some weld parameter adjustments required when changing heats. In addition, variables other than those controlled by the power supply must be also be controlled in order to achieve the goal of weld quality and weld repeatability. These variables include joint preparation and fit-up, tungsten type and tungsten length which determines the arc gap, and thus the arc voltage, gas type and gas flow rates for the shield and back-up gases, cleaning procedures, dimensional tolerances of weld components and workmanship11,15

Power supply requirements
Up to 115 amps (primary) were required for penetration of the 1 inch diameter super duplex tubing with 0.109 inch wall. This amount of amperage requires a power supply such as the AMI Model 207 which will provide 100 amps when plugged in to 110 VAC or up to 150 amps when plugged into 220 VAC. For wire feed applications a Model 227 power supply will deliver 100 amps plugged into 110 VAC or up to 200 amps at 100% duty cycle when plugged into 220 VAC. Peak currents of 225 amps are possible if the average amperage does not exceed 200 amps. Power supplies for wire feed applications must have additional controls for wire feed and may also have controls for maintaining a constant arc gap (AVC) and for torch oscillation to weave the weld bead back and forth across the joint.

Weld head requirements
Welding of duplex and super duplex, especially when welding the larger tubing with heavier wall thickness requires a substantial amount of heat. Although successful installations of the smaller diameters have been done without water cooling, water cooling is recommended for high duty cycle or high-amperage welding. At the higher temperatures created by higher amperages weld head components expand placing stress on the free rotation of the weld head. Cooling the head can completely overcome this. When possible, it is advisable to use a head with the capability of welding a size larger than the largest size being welded to permit heat dissipation. Open frame weld heads such as the Model 95-1500 and Model 79-2375 can take more heat than enclosed type weld heads such as the 9-1500, and 9-AF-750 although these can operate at 100% duty cycle for the smaller tube sizes. A water-cooled version of the AMI Model 9-500 weld head is now available
for autogenous welding of tubing 1/2 diameter and smaller.

Orbital Welding Techniques
Pressure balancing to control weld bead profile The use of pressure balancing as well as careful control of weld parameters can help to control the weld bead profile.11,12,15 An autogenous weld should be flat on the O.D. and flat on the I.D. Excessive concavity resulting from a wide weld puddle can be overcome within limits by increasing the I.D. purge gas flow combined with a restricted exit orifice or with a Magnehelic® gage to monitor the pressure. The danger of this technique is that too high a pressure will cause the weld bead to become convex on the O.D. and concave on the I.D. Autogenous welds (without insert rings) should never be convex on the O.D. Extreme application of pressure during welding may
cause the weld to blow out. Pressure balancing is more readily controlled while welding at a bench than with long runs of tubing in the field. In field installations, a tee may be placed in the weld joint and one end of the tee connected to a Magnehelic® pressure gage. This permits the installer to adjust the flowrate of the I.D. purge gas to determine the correct amount of flow for a particular weld joint. This is time consuming and may not be appropriate for all types of installations, particularly long tubing runs or multi-port systems. Nevertheless, when used correctly, pressure balancing is an effective aid in achieving a desirable weld bead profile on small diameter duplex tubing.

Use of Pulsation and STEP Rotation
Pulsation of the welding current between a higher, primary amperage and a lower, background amperage is normally used for orbital GTA welding of tubing and pipe. An exception to this was the orbital welding of small diameter (1/4, 3/8, and 1/2 inch) tubing for semiconductor process gas lines in an attempt to produce a very smooth weld bead surface15. In pulsation mode the heat from the arc is melting the puddle during the high current pulse and slightly freezing the puddle during the low current pulse. This helps to control the molten weld puddle which would otherwise tend to run into the tungsten electrode on the downhill side of the weld. While it is possible to weld thin-wall material without pulsation, on heavier wall or on low sulfur materials the puddle becomes more difficult to control.

For heavier wall (0.083 inch and greater) a STEP rotation mode may be used in which the rotation, instead of proceeding at a constant speed (CONT), is stopped or slowed during the primary current pulse and the electrode advanced at the programmed RPM during the background current pulse. The slowing (or stopping) of rotational speed during the high current pulse during a STEP procedure provides even more puddle control and provides maximum penetration for a particular current setting. The use of a STEP procedure usually doubles the arc time per weld. A STEP welding procedure was used to weld the 1.000 inch O.D. 0.109 inch wall tubing described in Table 1. STEP mode provides excellent penetration for heavier walls and provides additional puddle control on materials with a high W/D ratio which helps to achieve an acceptable weld bead profile.

On the thin-walled 1/4 inch diameter tubing described above the primary pulse times, which can range from seconds to hundredths of a second, were maximized in order to minimize the the cooling rate to keep the ferrite count as low as possible.

Summary
Methods of welding duplex stainless steel tubing successfully include the following:
• For some applications, orbital GTA welding with the addition of filler wire may be preferred.
This technique produces a favorable weld bead profile and can be done successfully with pure
argon shield gas.
• For autogenous orbital welding, a shield gas mixture of 10% helium, 88% argon, and 2%
nitrogen can consistently produce welds with ferrite counts acceptable for all but the most
critical applications.
• Insert rings of the proper chemical composition and free of porosity used in conjunction with
autogenous orbital welding appear to be an acceptable alternative to the use of filler wire for
achieving a desirable phase balance. The weld bead profile produced with this technique is
typically favorable. Results of field installations will be needed to confirm the practicality of
this technique. Usage of this technique is currently hindered by the lack of availability of
superduplex insert rings.
• Penetration-enhancing flux may be used in conjunction with autogenous orbital welding to
increase penetration and reduce ferrite numbers to the 40 to 50% ferrite range. The potential
for contamination by particulation of oxides from the weld may outweigh the advantages of
this technique for some applications. Uniformity of flux application is also a concern.
• Manual welding of duplex tubing may be appropriate for some applications, with limitations
as discussed above. These limitations include the scarcity of manual welders with skills sufficient
for making acceptable welds on duplex.
Pressure balancing can be an effective tool for producing an acceptable weld bead profile, but may not be practical for all applications.

Orbital welding power supplies must provide sufficient amperage and duty cycle to accomplish successful welds on a particular size and wall thickness at a reasonable rate of productivity using the selected joining technology.

Orbital weld heads must be of sufficient size and construction to withstand the heat of welding with the selected joining method. Water cooling of weld heads is recommended for welding at moderate to high duty cycles and is essential for welding tubing of heavier wall thickness.

Conclusions
There have been significant advances in the orbital welding of small diameter duplex tubing during the past few years. The use of more compact power supplies with wire-feed capability and open-frame type weld heads has proven successful for the orbital welding of small diameter duplex tubing. Concurrent advances in autogenous orbital welding techniques offer the promise of more economical joining procedures that meet the stringent offshore/subsea specifications. Orbital GTA welding offers repeatability of process giving assurance that the production welds will be of the same quality as the qualification welds.

Orbital welding technology offers an abundance of choices of procedures for successful joining of small diameter duplex materials for a wide variety of offshore and subsea applications. As installation technology has improved, specifications for duplex welds have also become tighter. A better understanding of the relationship between the austenite-ferrite balance of welds and results of the ASTM G-48 corrosion resistance test combined with more accurate ferrite determinations will lead to specifications for ferrite levels more consistent with the severity of the service environments. Experience gained from the field using some of the new orbital welding techniques will provide end users with information needed to make the most practical and economic selection of welding technology that will provide a long service life and reasonable cost of ownership for their specific duplex stainless steel application.

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Acknowledgements:
The authors wish to thank the following individuals who have contributed their knowledge, skill and expertise to this work:

Mike D. Haught, Project Manager, Andy Wolfskill, Sr. Technician, Acute Technological Services and Andy Brunning, Manager UK Office, Arc Machines, Inc.

Insert rings of 25.10.4L filler metal were designed by Acute Technological Services and manufactured on commercial insert ring production equipment.

Shield gas containing 10% helium, 88% argon, and 2% nitrogen (ARCAL®) was obtained from Air Liquide.