Orbital Welding of 316L Stainless Steel Tubing
Orbital Welding of 316L Stainless Steel Tubing
Materials Characteristics and High-Purity Applications
Type 316L stainless steel tubing is used extensively for critical piping systems in industries in which maintaining the cleanliness of high-purity liquids and gases passing through these systems is fundamental to successful product yield.
Stainless steel is used by the dairy, food, and other industries because it can be polished to a smooth, hard, relatively inert surface finish that can be maintained in a sanitary condition without degradation by corrosion.
A passive chromium oxide surface film protects the stainless steel surface, resulting in a very low corrosion rate in air at room temperature and allowing it to remain "stainless" for long periods of time. If the surface film is damaged, it will reform or heal itself if exposed to air or an oxidizing environment as long as the surface is free of contaminants.
In addition, stainless steels have excellent mechanical properties. The high strength and toughness of stainless steel, even at elevated service temperatures, make it suitable for applications in the semiconductor and biopharmaceutical industries.
Hygienic and semiconductor piping systems were joined with compression fittings as recently as the 1970s, but now welding, first manual gas tungsten arc welding (GTAW) and now orbital welding, is used for joining them.
Skilled manual welders can assemble and install leak-free piping systems that meet the weld criteria for most of the accepted standards. However, orbital welding has become the accepted method of joining piping systems in high-purity industries, not only because skilled manual welders are increasingly difficult to find, but because crevices and voids frequently found in manual welds have been nearly eliminated by orbital welding.
The orbital weld bead is smoother, providing a higher-quality weld that can be repeated consistently. Orbital welds in high-purity piping systems now number in the millions.
|Figure 1. Orbital tube welding is done as a fusion GTA weld on a square end preparation, usually in a single pass. Weld current is reduced around the weld circumference in a series of steps or levels. A timed current downslope completes the weld.|
|Figure 2. For 316L heats of average sulfur content, the weld puddle has a good width-to-depth ratio as shown at right. Weld puddles on low sulfur heats tend to be wide, shallow, and concave on the OD.|
Orbital Fusion Tube Welding. Orbital tube welding is typically done as a single-pass fusion butt weld (see Figure 1). The weld sequence starts with a timed prepurge, during which the enclosed weld head fills with inert gas, usually argon. An arc is struck between the tungsten electrode and the weld joint, and arc rotation begins after a timed delay during which the weld puddle is formed and just penetrates the tube.
Rotational speed (RPM) is calculated to achieve an electrode travel speed at the weld surface of about 5 inches per minute (IPM) for most tube sizes. For a 1.000-inch outside diameter (OD) tube, a travel speed of 5 IPM would equate to 1.59 RPM, and it would take about 38 seconds to complete the single pass. Additional arc time must be added for the delay at the start of the weld and for the tie-in and downslope at the end of the weld.
Welding current is based on wall thickness, with about 1 amp of current for each 0.001 inch of wall. Thus, a weld program for a 1/4-inch OD tube with a wall of 0.035 inch would start with 35 amps in the first level.
If the amperage required to achieve initial penetration of the tubing wall were held constant for the entire weld, penetration would increase as the electrode traveled around the tube, producing overpenetration and OD concavity.
Therefore, provisions are made to gradually reduce the current through a series of steps or levels during the weld sequence. The welding current is pulsed between a primary current, during which the puddle achieves its maximum size, and a lower or background current, during which the puddle begins to partially solidify before becoming liquid again during the next high-current pulse.
Although welds on small-diameter thin-wall tubing may be done without pulsation, the pulse provides control of the weld puddle in all positions on heavier-wall material. Without a pulse, the puddle would tend to flow downhill and come into contact with the electrode, causing the arc to be extinguished.
The last level of the weld overlaps the start of the weld to ensure a complete tie-in, and then the current is gradually reduced in a timed downslope. In the absence of a downslope, a crater or hole would form in the last weld bead, which could extend through the entire wall.
The purge continues to flow in the weld head until the weld metal cools to a temperature at which excessive discoloration from oxidation does not occur.
A welding technique in which two or more passes are made at relatively high travel speeds and without the use of pulsation has been used in some specialty semiconductor applications. The first pass is essentially a preheat pass, so penetration of the weld joint is not achieved until the second or subsequent passes.
With this technique, it is possible to achieve a thinner weld bead, reducing the surface area of the welds in a piping system. Since the weld bead is naturally rougher than the tubing surface, this is considered advantageous for process gas lines. Welding without a pulse helps to eliminate the ripples that are a part of a typical weld bead.
Step Rotation. For fusion welds on small-diameter pipe or for welds of fittings, valves, and other components, a step procedure can provide better penetration and a wider, more forgiving weld bead.
In step mode, the electrode travel is stopped or slowed during the high or primary current pulse to allow for optimal penetration. Travel is at a normal speed during the low current pulse. The effect is similar to a series of overlapping spot welds.
This technique takes longer than the continuous rotation method, but it can help to overcome situations in which the fit-up between components is less than ideal or for welding tubing to fittings with unmatched wall thicknesses or sulfur contents.
Figure 3. The surface of the orbital weld bead is far smother on EBR 316L than on 316L refined by conventional AOD methods. At higher magnification, AOD material shows a typical dendritic crystallization pattern with an undesirable degree of roughness for critical semiconductor applications. The surface finish of the weld on the EBR material is much closer to that of the electropolished tubing. SEMs courtesy of Valex Corporation.
Manual Tacking. For some applications, tubing is manually tacked in place with a hand-held GTAW torch to align the tubes or fittings for orbital welding. If this is done, the inside diameter (ID) of the weld joint must be purged during the tacking process. An orbital arc may deflect around an unpurged tack and cause a lack-of-penetration defect, and oxidized tacks may become corrosion initiation sites.
High-Purity Applications of Orbital Welding
Semiconductor. By far the largest application of orbital tube welding is the semiconductor industry for use in process gas distribution systems. Since this industry has developed complex electronic devices with small line widths, preventing contamination during the construction of ultra-high-purity (UHP) piping systems is critical.
Process gas lines are typically 0.250, 0.375, or 0.500 inch in diameter, although tube sizes of up to 6 inches have been used for transporting gases such as nitrogen.
Process gas lines transport gases used in the production of semiconductor devices from the gas pad to the point of use. A typical gas line has more than 300 welds. Semiconductor equipment requires extensive networks of gas line tubing that are installed with orbital welding.
Gases at the parts-per-billion purity range must be able to pass through the piping systems without accumulating particulates, moisture, or other contaminants from the piping. To accomplish this, the tubing ID is mechanically polished and then electropolished to a surface finish measured in microns or microinches - typically 10 microinches (0.25 microns) Ra (average roughness), although smoother finishes are becoming common.
Bioprocess Applications. For bioprocess applications, the materials of construction that come in contact with the process must not affect the purity or integrity of the product. Type 316L stainless steel meets this requirement for most bioprocess applications, as long as its corrosion resistance is not compromised by improper welding and fabrication practices.
Autogenous welding causes changes in the passive surface film and localized changes in composition that affect the corrosion resistance in the weld and the heat-affected zone (HAZ). Careful material handling, control of weld parameters, and passivation of the welded system are needed if the piping system is to have corrosion resistance comparable to that of the parent material.
In biopharmaceutical plants, welding is used on tanks, vessels, and piping systems. Stainless steel sheets forming tanks and vessels may be welded together using the GTAW or plasma arc welding (PAW) process, in which the electrode moves on a track over stationary metal.
Cylindrical tank sections may be joined using a manipulator welder with the gas metal arc welding (GMAW) process, in which the tank is rotated under a stationary electrode. Many structural parts are welded, and some components are welded onto the tanks using manual GTAW or GMAW.
Orbital welding, which uses the GTAW process, is typically used to join tubing, fittings, and other piping system components in this industry and has been used in most of the major pharmaceutical plants in the U.S.
Typical biopharmaceutical applications of orbital welding include piping for water for injection (WFI) used in the preparation of injectable pharmaceutical products, USP Purified Water, USP Clean Steam, and other product or process contact surfaces.
Equipment manufacturers use orbital welding to weld tubing for equipment such as stills, for instrumentation tubing supplying vital gases to fermenters, and for connections from specialized equipment to process piping. Equipment skids are fabricated off-site and installed in the field by orbital welding.
Most pharmaceutical, food, and dairy plants have piping systems that are designed to be cleaned in place (CIP). This means that the piping system is not disassembled for cleaning, but chemicals from a separate piping system are passed through the process piping for cleaning. Computer-controlled valves and transfer panels route the cleaning solutions through the proper system for cleaning.
For CIP to be effective, the interior tubing surface and weld bead must be smooth enough to permit complete removal of product and be free of crevices and voids which might hide bacteria from cleaning or sterilization.
Tube diameters for biopharmaceutical tubing are typically from 1 to 4 inches, but 1/2-inch OD instrumentation tubing is becoming more common, and tubing up to 8 inches in diameter for a deionized (DI) water distribution system has been installed with orbital welding.
Much of the tubing is mechanically polished to grit finish with an Ra of 20 to 30 microinches (0.5 to 0.75 micron), although this can vary depending on the application. The use of electropolished 316L is increasing for the more critical biopharmaceutical applications, and this process reduces the Ra to 10 to 20 microinches (0.25 to 0.5 micron).
Working with Type 316L
Wrought stainless steels of the American Iron and Steel Institute (AISI) 300 series are essentially austenitic at room temperature, consisting of grains in which the crystal lattice is of the face centered cubic (fcc) configuration of iron. Welds on these materials, which are structurally equivalent to cast material, generally contain some ferrite grains in which the crystal structure is in the body centered cubic (bcc) configuration.
Advances in steel refining technology and analysis methods over the past decade have made it possible to precisely control the chemical composition of 300 series materials. Since small changes in the percentage of alloying elements and trace elements can markedly affect performance, weldability, machinability, corrosion resistance, and surface finish, several subgroup specifications have been developed within the AISI specification.
In the temperature range of 800 to 1,500 degrees Fahrenheit, carbon dissolved in the austenite grains comes out of solution and draws the chromium into the grain boundaries. The loss of chromium leaves the metal grains vulnerable to corrosion.
A reduced-carbon or "L" (or ELC, extra-low carbon) version of 316 has been designed with a 0.035 percent maximum carbon content instead of the 0.08 percent maximum carbon found in the standard AISI type.
The reduced amount of carbon decreases the loss of chromium from the grain boundaries in the sensitizing temperature range, which may occur either during welding or in service. Both orbital welding, which controls the heat input during welding, and the "L" grade of 316 have lessened the concern over carbide precipitation.
Type 316L is referred to in the DIN standard as alloys 1.4435 and 1.4404, which have 2.5 to 3 percent as compared to 2.0 to 2.5 percent molybdenum respectively. Type 316L, designated as S31603 in the UNS numbering system, is favored by the high-purity industries and is relatively easy to machine and weld.
The requirement for a precision fit between the components being welded is more stringent for orbital welds, which must have consistent uniform end preparations to achieve consistent high-quality welds. As long as the cutting blade on the facing equipment remains sharp, the chip from a 316L tube end comes off in a long spiral ribbon and the tube end remains square, with little or no burr.
Tubing for the high-purity industries is typically welded autogenously with no filler metal. The welding process subjects materials to a thermal cycle that affects the distribution of elements in the surface film and element segregation within the grains.
Although these changes affect the base metal's corrosion resistance and mechanical properties, if welding is done without excessive heat and with a purge to prevent excessive heat tint due to oxidation, the changes in corrosion resistance of 316L after orbital welding are minimal.
It is less expensive and simpler to install a piping system by autogenous orbital welding than by orbital welding with the addition of filler to the weld (required when welding higher-alloy material). Thus, a material such as 316L, which can be welded without the addition of filler material while retaining its mechanical properties and corrosion resistance, offers an advantage.
A variety of fittings and other piping system components is available with end preparations suitable for orbital welding and mechanically or electropolished interior surface finishes suitable for either semiconductor or biopharmaceutical applications.
While stainless steel, especially electropolished Type 316L, is expensive, it is less costly than some other corrosion-resistant alloys. If proper fabrication and welding procedures are followed, piping systems made from 316L tubing should have a long, productive service life.
Sulfur. Research has shown that elements such as sulfur and oxygen, which cause the temperature coefficient of surface tension to be positive, result in a weld puddle in which heat is transferred from the perimeter inward and downward with good penetration of the weld bead.
Removing sulfur and oxygen, either by refining or by the presence of elements that combine with sulfur or oxygen, such as aluminum, has an opposite effect on penetration. In the latter case, the temperature coefficient of surface tension becomes negative, producing a wide, shallow weld with poor penetration and a tendency toward concavity (see Figure 2).
Other elements, including manganese and silicon, have slight effects on penetration, but sulfur has by far the greatest effect.
However, sulfur in stainless steel combines with manganese to form nonmetallic inclusions called manganese sulfide (MnS) "stringers." When tubing is passivated or electropolished, the stringers leave pits in the 0.25- to 1.0-micron size range. The tiny pits show up on SEMs (scanning electron micrographs) used to screen tubing samples for surface finish qualification.
Since pitted surfaces are undesirable for high-purity applications and are typically the first places to show evidence of corrosion, tubing manufacturers and distributors have rallied to drive down the sulfur content of 316L tubing.
The American Society for Testing & Materials (ASTM) has recently added a supplement for Pharmaceutical Quality Tubing to the ASTM A270 specification for Seamless and Welded Austenitic Stainless Steel Sanitary Tubing. The supplement (S2) limits the sulfur content of this grade to 0.005 to 0.017 percent. These values allow for ease of welding with lower MnS inclusions than would be found at the higher sulfur values of 316L.
With moderate to high sulfur content, type 316L is easier to machine than the low-sulfur materials, so it is favored by some fitting manufacturers. Thus, engineers, contractors, and welding personnel must take care in ordering tubing and fittings and record and track material heat numbers during fabrication to avoid costly problems.
VIM/VAR and EBR Materials
316L bar stock made by the vacuum induction melted plus vacuum arc remelted (VIM plus VAR) processes that contains a very low level of sulfide and other inclusions is now available.
This "clean" material, called 316L-SCQ™, is intended for use for ultraclean gas supply components such as valves, regulators, fittings, glands, gaskets, pipe, and tubing. The base metal has a finer, more uniform grain structure than conventional type 316L, and orbital welds on this material have a much smoother appearance.
Stainless steel produced by the electron beam refining (EBR) process has been used experimentally for special ultra-high-purity (UHP) semiconductor applications. This EBR process uses entirely virgin materials in the melt, producing an unusually clean material. Manganese and other trace elements are reduced to very low levels, resulting in reduced stringers and improved corrosion resistance.
This material also has less of a tendency to discolor from oxidation during welding. The blue "halo" that typically appears on either side of a weld on some heats of material does not appear on properly purged EBR material.
The surface of the EBR weld bead is significantly smoother than welds on even VIM/VAR material. The dendritic crystallization structure of welds on typical argon oxygen decarburization (AOD) and VIM/VAR materials form a rough surface with minute projections seen at high SEM magnifications. These projections might result in some particulation into UHP gas lines (see Figure 3).
One recurring problem with conventional type 316 is the tendency for some heats to form slag islands or weld dross on the OD or ID weld bead. A change in shielding gas or weld parameters may reduce this problem, but it is difficult to eliminate entirely.
Slag typically contains silicon and compounds of calcium and aluminum, which are added to standard melts to remove impurities. The EBR melt is very low in impurities, so additions to remove them are unnecessary, and the slagging problem is nonexistent.
Stainless steel's lack of consistent weldability from heat to heat has kept orbital GTAW from becoming a fully automatic process.
Weld programs for each tubing diameter and wall thickness are entered into the power supply memory. A weld program or schedule will produce consistent uniform welds on a particular batch of tubing, but if tubing of a different heat number is introduced, some adjustment of amperage may be required, and parameter verification through test coupons is advisable.
Controlling the chemical composition of 316L materials with advanced refining technologies might eventually minimize heat-to-heat variations in penetration and allow orbital GTAW of tube to become a nearly automatic process. This will save the production time currently spent on optimizing weld programs for individual material heats.
Welding Type 316 for Retention of Corrosion Resistance
Welding and fabricating tubing may result in a loss of corrosion resistance relative to the unwelded base metal. The HAZ of welds has been implicated in the formation of rouge, a rust-like film containing the products of corrosion, in pharmaceutical water systems.
Contamination of stainless steel tubing, particularly with carbon, carbon steel, or chlorides, can severely affect corrosion resistance. Heat tint oxidation produced during welding also severely reduces the corrosion resistance of stainless steel, with the loss of corrosion resistance proportional to the oxygen concentration of the purge gas.
After fabrication, pharmaceutical piping systems are typically passivated with nitric acid or a mixed chelant solution before being placed into service. Passivation, however, is a relatively mild treatment and only affects the outer surface layer to a depth of 30 to 50 Å.
If the heat tint extends below this level, and it has been shown to extend to a depth of about 1,000 Å in severe cases, then passivation will not be able to remove the heat tint. For passivation to be effective, the surface must be clean.
Both welding and fabrication must be done carefully to limit damage to levels that can be corrected by passivation. Recent studies have shown that welded 316L tube samples purged at oxygen levels of 108 parts per million (ppm), 8 ppm, and less than 0.1 ppm show visible effects of corrosion in proportion to the levels of oxygen in the argon purger.
The amount of heat tint and corrosion was slightly greater on mechanically polished tubing than on electropolished tubing of the same heat number. This is most likely due to the larger surface area on mechanically polished tubing, which has a greater affinity for oxides.
Orbitally welded 316L tube samples were compared to previously studied unwelded tube samples and found to have comparable pitting potentials(2). Samples passivated with either mixed chelant solutions or with nitric acid were found to have significantly higher pitting potentials than unpassivated samples, whether or not they had been welded (see Figure 1 and Figure 2).
The exception to this was the group of mechanically polished tubes welded with the lowest-purity purge gas (108 ppm oxygen). These samples had active polarization curves, indicating a severe loss of corrosion resistance. This severe effect was not observed on electropolished tubes purged with the low-purity gas.
The most heavily corroded parts of the welds subjected to corrosion testing were the areas of overlap and downslope. Since these areas were welded twice, it suggests that the additional heat input caused a localized reduction in corrosion resistance and indicates that careful control of the heat input during welding is important for corrosion control.
Passivation was found to remove light heat tint, but not heavy heat tint, such as occurred with oxygen levels of 108 ppm. Thus, for passivation to be effective in restoring the corrosion resistance of welded tubing to about the preweld condition, the welding process must be controlled to limit heat tint.
During the past decade, advances in orbital welding, fabrication, and materials have contributed to the production of very high-quality piping systems. These advances have been driven by the need to control contamination in semiconductor process gas lines and to avoid contamination of pharmaceutical and bioengineered products, which require smooth, uniform, consistent high-quality welds.
At the same time, contractors have become sophisticated in the use of equipment such as oxygen analyzers, particle testers, and helium leak testers. Contractors have also devised standard operating procedures (SOPs) that have gradually reduced rejection rates for orbital welds to as low as 0.2 percent.
In addition, tube manufacturers, electropolishers, and fitting manufacturers have developed techniques for achieving and quantifying surface finishes. Finally, new stainless steel refining technologies offer the promise of even more satisfactory piping systems in the future that can be customized to suit the needs of a particular industry or application.
Advances in technology require change, and that can be expensive. Orbital welding equipment, electropolished tubing, highly purified argon gas, and specialty stainless steel all cost more than their counterparts.
However, without these changes, some of the products we tend to take for granted today would not have been possible. Improvements in welding technology, materials, and fabrication are combining to achieve quality piping systems with a favorable cost of ownership.
By Barbara K. Henon, Ph.D., Arc Machines, Inc.
Barbara K. Henon is Manager of Technical Publications with Arc Machines, Inc., Pacoima, California. Originally presented at the Twelfth Annual World Tube Congress, October 7-10, 1996, sponsored by the Tube and Pipe Association, International (TPA). For more information on the standards mentioned in this article, contact AISI, 1101 17th St. NW, Washington, D.C. 20036, Tel: (202) 452-7100; ASTM, 100 Barr Harbor Dr., W. Conshohocken, PA 19428, Tel: (610) 832-9500.
The author gratefully acknowledges the following people who have contributed their expertise in the preparation of this manuscript: Craig Burton, Metallurgical Engineer with Valex Corporation; Norm Schmidt, Product Application Manager with Carpenter Steel Division, Carpenter Technology Corporation; Steve Wilkinson of Dockweiler, U.K.; and Robert Snyder of Binsky & Snyder Mechanical Contractors.
1. A. Grant, B.K. Henon, and F. Mansfeld, "Effects of purge gas purity and chelant passivation on the corrosion resistance of orbitally welded 316L stainless steel tubing," Pharmaceutical Engineering, January/February 1997.
2. S.H. Lin, "Corrosion protection of high copper aluminum alloys and stainless steels by surface modification," Ph.D. thesis University of Southern California, 1995.