Orbital Welding of Small Diameter Tubing
Orbital Welding of Small Diameter Tubing
Small diameter tubing, defined as round tubing with an outside diameter of 38mm or less, is used in a variety of industrial applications. For joining of small diameter tubing, particularly for more critical piping systems where weld quality or product purity is a concern, orbital welding has become the preferred joining method. Orbital tube welding is generally a fusion process in which the edges of the joint are melted and fused together without the addition of filler material.
The size range for orbital fusion welding is 3.2mm-177.8mm OD but the most common size is 6.3mm OD, 0.9mm wall which is used extensively for semi-conductor process gas lines. Orbital weld heads are of the enclosed type and form a chamber which is filled with inert gas during welding to prevent oxidation of the tube OD during welding. Each weld head accommodates a range of sizes, which is accomplished by changing the tube clamp inserts to fit the tube diameter on both sides of the head, or with the smaller heads, by changing the clamp assembly.
Figure 1. Tubing suppliers for high-purity semi-conductor applications subject the 316L electropolished stainless steel tubing to rigorous cleaning procedures. A cleanroom environment ensures that cleanliness will meet the most stringent standards. Photo courtesy of Valex Corporation
Orbital welding first became practical in the early 1980s with the development of power supplies which operated on 110 VAC and were small enough to be moved around a construction site. These power supplies provided precise control of weld parameters, making it possible to achieve a high degree of repeatability from weld to weld. The newer power supplies are microprocessor--based and the weld program listing weld parameters for each size of tubing, pipe or fitting are stored in the power supply memory and called up as needed for a particular size. Weld parameters include pulsed welding current, travel speed (RPM), level time, pulse times, etc. The weld is typically done as a single pass weld with four or more levels of welding current, but welds of two or more passes can be programmed into the power supply.
Two-day operator training is recommended to maintain consistent high quality orbital welds. The operator must be able to identify an acceptable weld for the application and to make adjustments in weld parameters or procedures in case of marginal or unacceptable welds. Most high purity applications specify 316L stainless steel, but orbital welding of other materials, including duplex stainless steel tubing for offshore applications, titanium tubing for aerospace applications and AL-6XN and Hastelloy for corrosive environments, have been successfully orbitally welded.
High-purity applications of orbital tube welding
Figure 2. Miles of 316L stainless steel tubing have been orbitally welded in place for semi-conductor process gas lines. Photo taken at Matsushita Semiconductor of America's facility by Pilchuk Mechanical Contractors
Figure 3. Orbital weld head installed on 12.7mm OD instrumentation tubing supplying vital gases to a fermentor.
The growth of the semi-conductor industry over the past decade has witnessed the building of numerous fabrication plants for the manufacture of semi-conductor devices. Miles of stainless steel process gas lines, mostly electropolished 316L tubing, have been installed by orbital welding. During this period, the line width of devices has gone from micron to submicron to tenths of microns. As the line width has dropped, concern for microcontamination has increased, as smaller and smaller particles have the potential to reduce the yield of devices to unacceptable levels.
The interior surface finish of the tubing is critical in the control of contamination, and the weld bead must also be smooth to prevent entrapment of particulates. Process gas purity has improved from the parts per million level of contaminants to the parts per billion level and must be able to pass through the process gas lines to the point of use without picking up moisture, oxygen, particulates, etc.
A very smooth surface finish on the tubing ID keeps water adsorption to a minimum and reduces the time to "dry down" the lines before they are placed in service. The tubing ID is first mechanically polished followed by electropolishing to achieve a surface finish of 10 Ra (average roughness) micro inches or less. Finishes of 7, 5, or even 2 Ra microinches are achievable.
Weld acceptance criteria for the semi-conductor industry are very stringent. A thin weld bead is preferred since the weld surface is rougher than the tubing surface and the thinner the weld bead, the less surface area will be contributed by the weld. However, the weld must be uniform in width, fully penetrated on the ID and free of discolouration due to oxidation which will occur during welding unless the purge gas is purified to the 1-2 parts per million level of oxygen or less.
A smooth weld bead is desirable, but the smoothness is limited by the chemical composition of the base metal. Very low sulphur stainless steel with low impurities, purged on the OD and ID during welding with argon containing 2-5% hydrogen, produces a very smooth weld bead. However, the addition of hydrogen to the purge gas results in a less stable arc with a tendency to arc wander, and tungsten electrode life is greatly reduced. Arc wander can also result from slag inclusions in the metal which appear as globs on the OD or ID of the weld bead during welding. This condition is the result of impurities in the base metal. Calcium and aluminum, which are added to the melt during the refining process to remove impurities, are principal components of the slag deposits.
Figure 4. Closeup of weld head which was designed to optimise axial clearance for welding in hard-to-access locations. Photo courtesy of B. Braun Biotech
|Figure 5. Special mandrel used by Northrop Corp. Aircraft Division, to weld titanium environmental control system (ECS) fittings to a titanium elbow in an orbital weld head.|
Orbital welding QC for small diameter tubing depends heavily upon the use of test coupons. Prior to the start of any orbital welding project, sample welds are made on the actual materials to be joined to assure that the welds meet the specified criteria. Test coupons are also submitted at specified intervals during the job, which would normally include the start of each shift and following procedural changes, eg a change of tubing heat number or a tungsten change. While welds done in a shop environment can usually be inspected on the ID with a borescope, field welds on piping systems may be inaccessible. The repeatability of the orbital welding process makes it likely that if the sample weld is good, subsequent welds on the same material will continue to be good.
The bioprocess and biopharmaceutical industries have become heavy users of orbital welding technology during the past 15 years. Applications in these industries include product piping, and piping for water for injection, which is used in the manufacture of pharmaceutical products which are injected into the human body, DI (deionised) water and clean steam piping.
The major issues for piping systems in the biopharmaceutical industries are cleanability and sterilisability, so equipment and piping systems must be designed with these concepts in mind. Modern pharmaceutical, food and dairy piping systems are designed to be cleaned-in-place (CIP/SIP) without being taken apart as opposed to systems which are taken apart for cleaning or cleaned-out-of-place (COP).
Although never as smooth as the tubing ID, the surface of the weld must be as smooth as possible for CIP to be effective. Indeed, orbital welds are routinely clean and smooth without the crevices and irregularities which were common with manual welding. Tubing for bioprocess applications is type 316L to ASTM A269 or ASTM A270. Electropolished tubing is typically used for more critical piping applications.
Purge gas purity and purging techniques
Orbital welding is a mechanised version of the gas tungsten arc welding (GTAW) process. Orbital welding is done in an inert gas atmosphere which is provided by argon gas in the weld head which protects the outside of the weld from oxidation during welding. An argon purge delivered to the tubing ID is used to protect the ID of the weld joint from oxidation. Purging is important for semiconductor welds because any oxides remaining on the tubing ID after welding may react with gases passed through the tubing or may form particulates which could greatly reduce product yield.
In pharmaceutical piping a form of corrosion known as rouge has been associated with the (HAZ) of welds (Coleman and Evans, 1991). Rouge, which contains the products of corrosion, circulates through the piping system and forms a film on the interior surface. In some systems it may be of little consequence while in other systems it can lead to further corrosion and invalidate the process. Loss of corrosion resistance as a result of welding has been correlated with heat tint discolouration of the weld HAZ which is proportional to the amount of oxygen in the ID purge gas.
Orbital welds on 316L stainless steel tubing show little or no loss of corrosion resistance as a result of welding when the ID argon purge gas has oxygen levels in the low ppm range or better (Grant et al, 1997). Pharmaceutical piping systems are generally passivated with a nitric acid or mixed chelant solution before being put into service to reverse some of the changes produced by welding and fabrication. Passivation removes only the lightest heat tint oxidation, so in order to get the maximum benefits of passivation and the minimum loss of corrosion resistance, welding must be done with highly purified purge gas.
Hansen et al, of the FORCE Institute in Denmark (1994) studied the correlation between loss of corrosion resistance after welding as measured by the critical pitting temperature and purge gas oxygen levels in both duplex and AISI 316L stainless steel They suggested an upper limit of 25-50 ppm of oxygen producing a straw colour heat tint to retain an acceptable level of corrosion resistance in 316L. For duplex stainless steel an oxygen level even below 5-10 ppm in Formier gas is needed to retain acceptable corrosion resistance after welding. At this level the heat tint is barely visible. Formier gas, which contains 90% nitrogen and 10% hydrogen, provides better protection for duplex than argon at the same levels of oxygen contamination. Several types of gas purifiers are available which can reduce oxygen and moisture levels in welding gas to the low ppb range. In addition, purging hoses, purge plugs, and purge techniques must provide for a leak-free purge system to deliver gas of the required purity to the weld joint.
While purging is important for stainless steel it is even more important for titanium. Titanium is not difficult to weld, but it is a refractory metal that reacts easily with oxygen or hydrogen and becomes embrittled if not protected from oxidation at temperatures above 426?C. A well-shielded weld on titanium will have a bright shiny appearance, while oxidation appears as straw, blue, grey or white discolouration.
While a slight blue oxidation is not particularly detrimental to stainless steel, it may be fatal to the mechanical properties of titanium. A trailing shield is often used for titanium welding to maintain the purge on the weld and HAZ until sufficiently cooled to prevent oxidation. For orbital welding the enclosed weld head provides a shield, and extra shielding may be added, if needed, as extensions of the head to increase the area of tubing protected by the purge.
Figure 6. Tubing for the air-to-air refueling system of the C-17 is of orbitally welded titanium. Titanium is frequently used in aircraft because of its light weight and strength.
Orbital welding was developed in the late 1950s by North American Aviation because of leaks developed during flights of the X-15 on lines joined by fittings. The Rocketdyne Division of Rockwell which, having bought NAA, recognised the need for a more reliable joining technology and adapted the GTAW process to a mechanised system in which the tungsten electrode, surrounded by an inert gas atmosphere, was rotated around the tube to complete the joint.
Today, most aerospace uses of orbital welding are for the joining of titanium tubing and fittings, with each Boeing 747 having miles of orbitally welded tubing. The C-17 has orbitally welded titanium air-to-air refueling lines. Weight is a vital consideration for aircraft design and titanium is light and strong enough to permit the use of thinner wall thicknesses than steel for the same diameters, resulting in a significant weight reduction. Orbital butt welds are also much lighter than the welded or brazed fittings they replace.
GTAW is the most widely used welding process for titanium and titanium alloys and excellent results are obtained, providing extra care is exercised in cleaning and purging the material. Weld procedure qualification of titanium fittings requires a flexure test in which the soundness of the weld joint is established by placing the tubes and fittings under pressure and rotating for 10 million cycles (10 X 106) without failure. This is a very stringent test, and improper cleaning of components, weld oxidation, or even the shape of the weld bead can result in failure.
Small diameter duplex stainless steel tubing
The use of duplex stainless steel has increased in recent years, especially for offshore applications because of its excellent corrosion resistance in the marine environment. Filler wire is usually added during welding to ensure that the material will retain a favourable phase balance in the weld and HAZ after welding. The superior mechanical properties and corrosion resistance of duplex stainless steels depend upon a balanced phase ratio' of approximately 50% ferrite and 50% austenite which may be altered during the welding thermal cycle unless filler material overalloyed in nickel is added to the weld.
Enserch obtained good results with orbital fusion welding of Sandvik's SAF 2507 superduplex stainless steel for a hydraulic control system used to regulate the flow of gas from subsea wells in the Gulf of Mexico (Henon and Hayes, 1993). Duplex was selected for this application over 316L stainless steel because of the corrosive environment and because the superior strength of duplex would stand up to the 10,000 lb/in2 operating pressure.
Figure 7. Orbital welding of super duplex stainless steel for subsea hydraulic lines. An alignment clamp is used to hold the tubes in position during orbital welding for optimal arrangement of the closely-spaced tubes. Photo courtesy of Acute Technological Services
Weight was an important consideration for this installation due to the load-bearing limitations of the construction cranes used to position the sleds. If 316L had been selected it would have required a larger diameter and wall thickness than the 12.7mm diameter, 1.7mm walled superduplex tubing that was used, and would have been much heavier. Orbital fusion butt welding was selected over manual socket welds which would have required twice as many welds and considerably more weight.
The orbital welds were subjected to intensive testing to ensure that the mechanical strength and other favourable properties of the superduplex was not compromised by welding. Sample welds were qualified to ASME Section IX of the Boiler and Pressure Vessel Code. Bend and tensile tests, radiography, dye penetrant tests and hydraulic burst tests were performed. With manual welding, ferrite counts are apt to be inconsistent from weld to weld because of the variable heat input inherent in the process. With orbital welding, once acceptable procedures have been developed, the repeatability of weld parameters provides consistent acceptable ferrite counts and repeatable weld quality. In addition to the tube-to-tube welds done on this installation, superduplex tubes were joined to Nitronic 50 fittings in an orbital butt joint.
A lip on the fitting provided a comparable metallurgical effect to the addition of filler metal but with a fusion welding process. Altogether a total of 1,000 superduplex welds were completed on this project. This successful installation clearly demonstrated the feasibility of orbital fusion welding of thin-walled superduplex tubing. Orbital welding of superduplex tubing offers significant economic advantages to conventional joining methods. For high-purity applications, better control of the welding and fabrication process is provided when assemblies are prefabricated in a shop or cleanroom rather than welded in the field.
For the semi-conductor industry, gas panels, manifolds, gas cabinets, gas sticks, etc are typically welded in a cleanroom environment. Pharmaceutical skids and equipment are generally assembled and welded in a clean fabrication area devoted to this purpose. For the semi-conductor industry, this approach provides control over all aspects of welding and inspection.
Figure 8. Much fabrication of gas distribution system components are pre-fabricated using orbital welding in a cleanroom environment. Welding operators are attired in cleanroom gowns and gloves to prevent contamination. Photo courtesy of Cambridge Fluid Systems Limited, UK
Purging: For purging, gas of certified high quality is delivered to the cleanroom with instrumentation to determine the oxygen and moisture to the ppb levels. Magnehelic pressure gauges are used routinely to monitor and control the purge gas pressure on the ID of small tubing during cleanroom welding.
A slight pressure of about 25.4mm of water may be used to achieve a flat inner weld bead on 6.3mm tubing, however, excessive pressure may result in a weld that is concave on the ID and convex on the OD or may even blow out the weld.
Fixturing: Good alignment of components being welded is fundamental for successful welding of assemblies for process gas applications. These assemblies are typically quite complex, consisting of numerous valves, regulators, fittings, tubing and other components. With simple assemblies the alignment provided by the weld head may be sufficient, but for complex assemblies with heavy valves and regulators, alternative support systems for parts being welding are essential.
One of the major reasons for rejecting an orbital weld is misalignment or mismatch (high-low) when welding small tubing (6.3mm, 3.2mm or 12.7mm OD) to microfit fittings. If the fitting is misaligned to the tube, the resulting weld will be crooked and parts will not be aligned properly, causing problems when trying to fit up the remaining components of the assembly. Since these fittings are quite expensive, some fabricators have resorted to cutting out the welds past the HAZ and refacing the end of the fitting so that it can be welded again to a new tube. Clearly, it would be more practical and less expensive to line up the parts correctly in the first place. Various tools are available that are useful to hold assemblies in place for alignment.
Engineering height gauges, or laboratory stands and clamps can be used to support and align components. One of the more practical systems has been developed by ICF Inc. It is called a MicroAlign Plate and consists of an aluminum plate with tapped holes threaded to accept a variety of stand-offs to support any number of different type components. This removes the strain from the weld head which mounts in the centre of the plate so that parts being welded are not pulled apart by the weight of the rest of the assembly.
Figure 9. Regulator assembly weld sequence.
Welding procedures (SOPs): When fabricating large numbers of similar small parts and assemblies it is important for both productivity and quality control to work from a written in-house list of Standard Operating Procedures. Assemblies to be orbitally welded are typically put together in a kit containing all the components.
The welding operators must check the kit against the bill of materials and drawings and observe the allowances for shrinkage during welding. The shrinkage for orbitally welded parts is much more predictable than for manually welded parts, making it easier to conform to the specified measurements.
The SOPs would specify when test welds must be done, usually at the start and end of each day, and for procedural changes such as a change of tubing size or after a change of tungsten electrode. The SOPs would also indicate required purge times, outlet restrictions used to increase the ID purge pressure, if used, and the proper fittings to use for purging. It may be necessary to do the welds in a particular order or sequence: eg, welds on valves or regulators must always be done with the ID purge gas passing through the component and exiting downstream of the weld.
There are several reasons for this: if the valve is left assembled, heat from the purge gas leaving the weld area may cause damage; a valve or regulator on the exit side of a weld may present a flow restriction which would pressurise the weld resulting in a blowout; contaminants released from the tubing during welding may become entrapped in the valve or regulator.
Inspection of small parts and assemblies: Biopharmaceutical and semi-conductor welds are typically inspected on the ID to verify that they meet specified weld criteria for the application. While test coupons can be cut open for inspection, actual production welds are typically inspected on the ID with a borescope. The image of the weld may be projected on a video monitor or a videotape of the weld circumference may be stored on tape for future reference.
A written specification detailing appropriate weld criteria for the particular industry or application must be in effect. It is strongly recommended that actual weld samples showing acceptable welds and welds with defects that would cause them to be rejected be available to welding personnel as a reference. Orbital tube welds for any high purity application must be fully penetrated on the ID. The weld bead must be uniform in width and fairly flat, ie, neither concave nor convex on the OD or ID with little or no discolouration due to oxidation.
Some cleanroom fabricators use a comparator to check the alignment of their orbital welds. This device projects an image of the weldment on a screen and alignment can be determined within a degree of offset and this is held to very strict tolerances.
Orbital welding has become the preferred method of joining small diameter tubing in a variety of industries. To people who are not directly involved with these industries the procedural requirements may seem esoteric and not generally applicable. However, in the semi-conductor industry, both in field applications and in cleanroom fabrication, this type of welding is done on a daily basis with a very high level of productivity. Gradual improvements in their SOPs by one mechanical contractor in biopharmaceutical installations resulted in a reduction of their orbital weld reject rate from 1.8% in 1991 to 0.2% in 1994, over a sample of 100,000 orbital welds. Orbital welding has become both practical and economical, and in many cases is the only technology by which the necessary quality and productivity can be achieved.
By Barbara K. Henon, Ph.D., Arc Machines, Inc.
The author gratefully acknowledges the contributions of the following companies who have contributed their expertise to this article: B. Braun Biotech, Allentown, Pennsylvania; Acute Technological Services, Houston, Texas; Cambridge Fluid Systems Limited, UK; ICF, Longmont, Colorado, and Wolfe Engineering, Campbell, California.
Gas tungsten arc welding: It's built to handle titanium. Titanium Institute. Welding Journal, November 1991.
Coleman, D. and R. Evans. Fundamentals of passivation and passivity in the pharmaceutical industry. Pharmaceutical Engineering, 1990.
Henon, B.K. and M.D. Hayes. Orbital GTA welding handles pressure of undersea application. Welding Journal, November 1993.