Editor’s Note: Pharmaceutical Online is pleased to present this four-part article on orbital welding of bioprocess piping by industry expert Barbara Henon of Arc Machines.This article is adapted from Dr. Henon’s presentation at the ASME conference late last year.
Prevent loss of corrosion resistance.High purity water such as DI or WFI is a very aggressive etchant for stainless steel.Additionally, pharmaceutical grade WFI is cycled at high temperature (80°C) to maintain sterility.There is a subtle difference between lowering the temperature enough to support living organisms lethal to the product and raising the temperature enough to promote “rouge” production.Rouge is a brown film of varying composition caused by corrosion of stainless steel piping system components.Dirt and iron oxides may be the main components, but various forms of iron, chromium and nickel may also be present.The presence of rouge is lethal to some products and its presence may lead to further corrosion, although its presence in other systems appears to be fairly benign.
Welding can adversely affect corrosion resistance.Hot color is the result of oxidizing material deposited on welds and HAZs during welding, is particularly detrimental, and is associated with the formation of rouge in pharmaceutical water systems.Chromium oxide formation can cause a hot tint, leaving behind a chromium-depleted layer that is susceptible to corrosion.Hot color can be removed by pickling and grinding, removing metal from the surface, including the underlying chromium-depleted layer, and restoring corrosion resistance to levels close to base metal levels.However, pickling and grinding are detrimental to the surface finish.Passivation of the piping system with nitric acid or chelating agent formulations is done to overcome the adverse effects of welding and fabrication before the piping system is put into service.Auger electron analysis showed that chelation passivation could restore the surface changes in the distribution of oxygen, chromium, iron, nickel and manganese that occurred in the weld and heat affected zone to the pre-weld state.However, passivation only affects the outer surface layer and does not penetrate below 50 angstroms, whereas thermal coloration can extend 1000 angstroms or more below the surface.
Therefore, in order to install corrosion-resistant piping systems close to unwelded substrates, it is important to attempt to limit welding and fabrication-induced damage to levels that can be substantially recovered by passivation.This requires the use of a purge gas with minimal oxygen content and delivery to the inside diameter of the welded joint without contamination by atmospheric oxygen or moisture.Accurate control of heat input and avoidance of overheating during welding is also important to prevent loss of corrosion resistance.Controlling the manufacturing process to achieve repeatable and consistent high-quality welds, as well as the careful handling of stainless steel pipes and components during manufacturing to prevent contamination, are essential requirements for a high-quality piping system that resists corrosion and provides long-term productive service.
Materials used in high-purity biopharmaceutical stainless steel piping systems have undergone an evolution toward improved corrosion resistance over the past decade.Most stainless steel used before 1980 was 304 stainless steel because it was relatively inexpensive and an improvement over the copper used previously.In fact, 300 series stainless steels are relatively easy to machine, can be fusion welded without undue loss of their corrosion resistance, and do not require special preheat and post heat treatments.
Recently, the use of 316 stainless steel in high-purity piping applications has been on the rise.Type 316 is similar in composition to Type 304, but in addition to the chromium and nickel alloying elements common to both, 316 contains about 2% molybdenum, which significantly improves 316′s corrosion resistance.Types 304L and 316L, referred to as “L” grades, have a lower carbon content than standard grades (0.035% vs. 0.08%).This reduction in carbon content is intended to reduce the amount of carbide precipitation that may occur due to welding.This is the formation of chromium carbide, which depletes the grain boundaries of the chromium base metal, making it susceptible to corrosion.The formation of chromium carbide, called “sensitization,” is time and temperature dependent and is a bigger problem when hand soldering.We have shown that orbital welding of super-austenitic stainless steel AL-6XN provides more corrosion resistant welds than similar welds done by hand.This is because orbital welding provides precise control of amperage, pulsation and timing, resulting in a lower and more uniform heat input than manual welding.Orbital welding in combination with “L” grades 304 and 316 virtually eliminates carbide precipitation as a factor in the development of corrosion in piping systems.
Heat-to-heat variation of stainless steel.Although welding parameters and other factors can be kept within fairly tight tolerances, there are still differences in the heat input required to weld stainless steel from heat to heat.A heat number is the lot number assigned to a specific stainless steel melt at the factory.The exact chemical composition of each batch is recorded on the Factory Test Report (MTR) along with the batch identification or heat number.Pure iron melts at 1538°C (2800°F), while alloyed metals melt within a range of temperatures, depending on the type and concentration of each alloy or trace element present.Since no two heats of stainless steel will contain exactly the same concentration of each element, welding characteristics will vary from furnace to furnace.
SEM of 316L pipe orbital welds on AOD pipe (top) and EBR material (bottom) showed a significant difference in the smoothness of the weld bead.
While a single welding procedure may work for most heats with similar OD and wall thickness, some heats require less amperage and some require higher amperage than typical.For this reason, heating of different materials on the job site must be carefully tracked to avoid potential problems.Often, new heat requires only a small change in amperage to achieve a satisfactory welding procedure.
Sulfur problem.Elemental sulfur is an iron ore-related impurity that is largely removed during the steelmaking process.AISI Type 304 and 316 stainless steels are specified with a maximum sulfur content of 0.030%.With the development of modern steel refining processes, such as Argon Oxygen Decarburization (AOD) and dual vacuum melting practices such as Vacuum Induction Melting followed by Vacuum Arc Remelting (VIM+VAR), it has become possible to produce steels that are very special in the following ways.their chemical composition.It has been noted that the properties of the weld pool change when the sulfur content of the steel is below about 0.008%.This is due to the effect of sulfur and to a lesser extent other elements on the temperature coefficient of surface tension of the weld pool, which determines the flow characteristics of the liquid pool.
At very low sulphur concentrations (0.001% – 0.003%), the penetration of the weld puddle becomes very wide compared to similar welds made on medium sulphur content materials.Welds made on low sulfur stainless steel pipe will have wider welds, while on thicker wall pipe (0.065 inches, or 1.66 mm or more) there will be a greater tendency to make welds Recess welding.When the welding current is sufficient to produce a fully penetrated weld.This makes materials with very low sulfur content more difficult to weld, especially with thicker walls.At the higher end of the sulfur concentration in 304 or 316 stainless steel, the weld bead tends to be less fluid in appearance and rougher than medium sulfur materials.Therefore, for weldability, the ideal sulfur content would be in the range of approximately 0.005% to 0.017%, as specified in ASTM A270 S2 for pharmaceutical quality tubing.
Producers of electropolished stainless steel pipe have noticed that even moderate levels of sulfur in 316 or 316L stainless steel make it difficult to meet the needs of their semiconductor and biopharmaceutical customers for smooth, pit-free interior surfaces.The use of scanning electron microscopy to verify the smoothness of the tube surface finish is increasingly common.Sulfur in base metals has been shown to form non-metallic inclusions or manganese sulfide (MnS) “stringers” that are removed during electropolishing and leave voids in the 0.25-1.0 micron range.
Manufacturers and suppliers of electropolished tubes are driving the market towards the use of ultra-low sulfur materials to meet their surface finish requirements.However, the problem is not limited to electropolished tubes, as in non-electropolished tubes the inclusions are removed during passivation of the piping system.Voids have been shown to be more prone to pitting than smooth surface areas.So there are some valid reasons for the trend towards low-sulfur, “cleaner” materials.
Arc deflection.In addition to improving the weldability of stainless steel, the presence of some sulfur also improves machinability.As a result, manufacturers and manufacturers tend to choose materials at the higher end of the specified sulfur content range.Welding tubing with very low sulfur concentrations to fittings, valves or other tubing with higher sulfur content can create welding problems because the arc will be biased towards tubing with low sulfur content.When arc deflection occurs, the penetration becomes deeper on the low-sulfur side than on the high-sulfur side, which is the opposite of what happens when welding pipes with matching sulfur concentrations.In extreme cases, the weld bead can completely penetrate the low-sulfur material and leave the interior of the weld completely unfused (Fihey and Simeneau, 1982).In order to match the sulfur content of the fittings to the sulfur content of the pipe, the Carpenter Steel Division of Car-penter Technology Corporation of Pennsylvania has introduced a low sulfur (0.005% max) 316 bar stock (Type 316L-SCQ) (VIM+VAR) ) for the manufacture of fittings and other components intended to be welded to low sulphur pipes.Welding two very low sulfur materials to each other is much easier than welding a very low sulfur material to a higher sulfur one.
The shift to the use of low-sulfur tubes is largely due to the need to obtain smooth electropolished inner tube surfaces.While surface finish and electropolishing are important to both the semiconductor industry and the biotech/pharmaceutical industry, SEMI, when writing the semiconductor industry specification, specified that 316L tubing for process gas lines must have a 0.004% sulfur cap for optimum performance Surface ends.ASTM, on the other hand, modified their ASTM 270 specification to include pharmaceutical-grade tubing that limits the sulfur content to a range of 0.005 to 0.017%.This should result in less welding difficulties compared to lower range sulfurs.However, it should be noted that even within this limited range, arc deflection may still occur when welding low-sulfur pipes to high-sulfur pipes or fittings, and installers should carefully track the heating of the material and check prior to fabrication Solder compatibility between heating.Production of welds.
other trace elements.Trace elements including sulfur, oxygen, aluminum, silicon and manganese have been found to affect penetration.Trace amounts of aluminum, silicon, calcium, titanium and chromium present in the base metal as oxide inclusions are associated with slag formation during welding.
The effects of the various elements are cumulative, so the presence of oxygen can offset some of the low sulfur effects.High levels of aluminum can counteract the positive effect on sulfur penetration.Manganese volatilizes at welding temperature and deposits in the welding heat-affected zone.These manganese deposits are associated with loss of corrosion resistance.(See Cohen, 1997).The semiconductor industry is currently experimenting with low manganese and even ultra-low manganese 316L materials to prevent this loss of corrosion resistance.
Slag formation.Slag islands occasionally appear on the stainless steel bead for some heats.This is inherently a material issue, but sometimes changes in welding parameters can minimize this, or changes in the argon/hydrogen mixture can improve the weld.Pollard found that the ratio of aluminum to silicon in the base metal affects slag formation.To prevent the formation of unwanted plaque-type slag, he recommends keeping the aluminum content at 0.010% and the silicon content at 0.5%.However, when the Al/Si ratio is above this level, spherical slag may form rather than the plaque type.This type of slag can leave pits after electropolishing, which is unacceptable for high-purity applications.Slag islands that form on the OD of the weld can cause uneven penetration of the ID pass and can result in insufficient penetration.The slag islands that form on the ID weld bead may be susceptible to corrosion.
Single-run weld with pulsation.Standard automatic orbital tube welding is a single pass weld with pulsed current and continuous constant speed rotation.This technique is suitable for pipe with outside diameters from 1/8″ to approximately 7″ and wall thicknesses of 0.083″ and below.After a timed pre-purge, arcing occurs.Penetration of the tube wall is accomplished during a timed delay in which arcing is present but no rotation occurs.After this rotational delay, the electrode rotates around the weld joint until the weld joins or overlaps the initial portion of the weld during the last layer of welding.When the connection is complete, the current tapers off in a timed drop.
Step mode (“synchronized” welding).For fusion welding of thicker walled materials, typically greater than 0.083 inch, the fusion welding power source can be used in synchronous or step mode.In synchronous or step mode, the welding current pulse is synchronized with the stroke, so the rotor is stationary for maximum penetration during high current pulses and moves during low current pulses.Synchronous techniques use longer pulse times, on the order of 0.5 to 1.5 seconds, compared to the tenth or hundredth of a second pulse time for conventional welding.This technique can effectively weld 0.154″ or 6″ thick 40 gauge 40 thin wall pipe with 0.154″ or 6″ wall thickness.The stepped technique produces a wider weld, making it fault tolerant and helpful for welding irregular parts such as pipe fittings to pipes where there may be differences in dimensional tolerances, some misalignment or Material thermal incompatibility.This type of welding requires approximately twice the arc time of conventional welding and is less suitable for ultra-high-purity (UHP) applications due to the wider, rougher seam.
Programmable variables.The current generation of welding power sources are microprocessor-based and store programs that specify numerical values ​​for welding parameters for a specific diameter (OD) and wall thickness of the pipe to be welded, including purge time, welding current, travel speed (RPM) ), number of layers and time per layer, pulse time, downhill time, etc.For orbital tube welds with filler wire added, program parameters will include wire feed speed, torch oscillation amplitude and dwell time, AVC (arc voltage control to provide constant arc gap), and upslope.To perform fusion welding, install the welding head with the appropriate electrode and pipe clamp inserts on the pipe and recall the welding schedule or program from the power source memory.The welding sequence is initiated by pressing a button or membrane panel key and welding continues without operator intervention.
Non-programmable variables.To obtain consistently good weld quality, the welding parameters must be carefully controlled.This is achieved through the accuracy of the welding power source and the welding program, which is a set of instructions entered into the power source, consisting of welding parameters, for welding a specific size of pipe or pipe.There must also be an effective set of welding standards, specifying welding acceptance criteria and some welding inspection and quality control system to ensure that welding meets the agreed standards.However, certain factors and procedures other than welding parameters must also be carefully controlled.These factors include the use of good end preparation equipment, good cleaning and handling practices, good dimensional tolerances of tubing or other parts being welded, consistent tungsten type and size, highly purified inert gases, and careful attention to material variations.- high temperature.
The preparation requirements for pipe end welding are more critical for orbital welding than manual welding.Welded joints for orbital pipe welding are usually square butt joints.To achieve the repeatability desired in orbital welding, precise, consistent, machined end preparation is required.Since the welding current depends on the wall thickness, the ends must be square with no burrs or bevels on the OD or ID (OD or ID), which would result in different wall thicknesses.
The pipe ends must fit together in the weld head so that there is no noticeable gap between the ends of the square butt joint.Although welded joints with small gaps can be accomplished, weld quality may be adversely affected.The bigger the gap, the more likely there is a problem.Poor assembly can result in a complete failure of the soldering.Pipe saws made by George Fischer and others that cut the pipe and face the pipe ends in the same operation, or portable end preparation lathes like those made by Protem, Wachs, and others, often used to make smooth end orbital welds suitable for machining .Chop saws, hacksaws, band saws and tubing cutters are not suitable for this purpose.
In addition to the welding parameters that input power to weld, there are other variables that can have a profound effect on welding, but they are not part of the actual welding procedure.This includes the type and size of tungsten, the type and purity of the gas used to shield the arc and purge the inside of the weld joint, the gas flow rate used for purging, the type of head and power source used, the configuration of the joint, and any other relevant information.We call these “non-programmable” variables and record them on the welding schedule.For example, the type of gas is considered an essential variable in the Welding Procedure Specification (WPS) for welding procedures to comply with the ASME Section IX Boiler and Pressure Vessel Code.Changes in gas type or gas mixture percentages, or elimination of ID purging require revalidation of the welding procedure.
welding gas.Stainless steel is resistant to atmospheric oxygen oxidation at room temperature.When it is heated to its melting point (1530°C or 2800°F for pure iron) it is easily oxidized.Inert argon is most commonly used as a shielding gas and for purging internal welded joints through the orbital GTAW process.The purity of the gas relative to oxygen and moisture determines the amount of oxidation-induced discoloration that occurs on or near the weld after welding.If the purge gas is not of the highest quality or if the purge system is not completely leak free such that a small amount of air leaks into the purge system, the oxidation may be light teal or bluish.Of course, no cleaning will result in the crusty black surface commonly referred to as “sweetened”.Welding grade argon supplied in cylinders is 99.996-99.997% pure, depending on the supplier, and contains 5-7 ppm of oxygen and other impurities, including H2O, O2, CO2, hydrocarbons, etc., for a total of 40 ppm a maximum.High-purity argon in a cylinder or liquid argon in a Dewar can be 99.999% pure or 10 ppm total impurities, with a maximum of 2 ppm oxygen.NOTE: Gas purifiers such as Nanochem or Gatekeeper can be used during purging to reduce contamination levels to the parts per billion (ppb) range.
mixed composition.Gas mixtures such as 75% helium/25% argon and 95% argon/5% hydrogen can be used as shielding gases for special applications.The two mixtures produced hotter welds than those done under the same program settings as argon.Helium mixtures are particularly suitable for maximum penetration by fusion welding on carbon steel.A semiconductor industry consultant advocates the use of argon/hydrogen mixtures as shielding gases for UHP applications.Hydrogen mixtures have several advantages, but also some serious disadvantages.The advantage is that it produces a wetter puddle and a smoother weld surface, which is ideal for implementing ultra-high pressure gas delivery systems with as smooth an inner surface as possible.The presence of hydrogen provides a reducing atmosphere, so if traces of oxygen are present in the gas mixture, the resulting weld will look cleaner with less discoloration than a similar oxygen concentration in pure argon.This effect is optimal at about 5% hydrogen content.Some use a 95/5% argon/hydrogen mixture as an ID purge to improve the appearance of the internal weld bead.
The weld bead using a hydrogen mixture as the shielding gas is narrower, except that the stainless steel has a very low sulfur content and generates more heat in the weld than the same current setting with unmixed argon.A significant disadvantage of argon/hydrogen mixtures is that the arc is far less stable than pure argon, and there is a tendency for the arc to drift, severe enough to cause misfusion.Arc drift may disappear when a different mixed gas source is used, suggesting that it may be caused by contamination or poor mixing.Because the heat generated by the arc varies with the hydrogen concentration, a constant concentration is essential to achieve repeatable welds, and there are differences in pre-mixed bottled gas.Another disadvantage is that the lifetime of tungsten is greatly shortened when a hydrogen mixture is used.While the reason for the deterioration of tungsten from the mixed gas has not been determined, it has been reported that the arc is more difficult and the tungsten may need to be replaced after one or two welds.Argon/hydrogen mixtures cannot be used to weld carbon steel or titanium.
A distinguishing feature of the TIG process is that it does not consume electrodes.Tungsten has the highest melting point of any metal (6098°F; 3370°C) and is a good electron emitter, making it particularly suitable for use as a non-consumable electrode.Its properties are improved by adding 2% of certain rare earth oxides such as ceria, lanthanum oxide or thorium oxide to improve arc starting and arc stability.Pure tungsten is rarely used in GTAW because of the superior properties of cerium tungsten, especially for orbital GTAW applications.Thorium tungsten is used less than in the past because they are somewhat radioactive.
Electrodes with a polished finish are more uniform in size.A smooth surface is always preferable to a rough or inconsistent surface, as consistency in electrode geometry is critical for consistent, uniform welding results.Electrons emitted from the tip (DCEN) transfer heat from the tungsten tip to the weld.A finer tip allows the current density to be kept very high, but may result in a shorter tungsten lifetime.For orbital welding, it is important to mechanically grind the electrode tip to ensure repeatability of the tungsten geometry and weld repeatability.The blunt tip forces the arc from the weld to the same spot on the tungsten.The tip diameter controls the shape of the arc and the amount of penetration at a particular current.The taper angle affects the current/voltage characteristics of the arc and must be specified and controlled.The length of the tungsten is important because a known length of tungsten can be used to set the arc gap.The arc gap for a specific current value determines the voltage and thus the power applied to the weld.
The electrode size and its tip diameter are selected according to the welding current intensity.If the current is too high for the electrode or its tip, it may lose metal from the tip, and using electrodes with a tip diameter that is too large for the current may cause arc drift.We specify electrode and tip diameters by the wall thickness of the weld joint and use 0.0625 diameter for almost everything up to 0.093″ wall thickness, unless the use is designed to be used with 0.040″ diameter electrodes for welding small precision Components.For repeatability of the welding process, tungsten type and finish, length, taper angle, diameter, tip diameter and arc gap must all be specified and controlled.For tube welding applications, cerium tungsten is always recommended because this type has a much longer service life than other types and has excellent arc ignition characteristics.Cerium tungsten is non-radioactive.
For more information, please contact Barbara Henon, Technical Publications Manager, Arc Machines, Inc., 10280 Glenoaks Blvd., Pacoima, CA 91331.Phone: 818-896-9556.Fax: 818-890-3724.