Consider the capabilities, productivity before making the choice
June 28, 2013
Matching a consumable to a 300-series (austenitic) stainless steel is fairly straightforward, but fabricators should be aware that the various options (rod for GTAW, solid or metal-cored wire for GMAW, stick electrode for SMAW, or flux-cored wire for FCAW) have huge productivity differences.
Known for its exceptional corrosion resistance, austenitic stainless steel, the 300 series, has become a staple in tube and pipe applications, with 304L and 316 grades being the most prevalent. The corrosion resistance comes mainly from their chromium content which, combined with other elements, provides ideal protection against corrosive elements in industries such as pharmaceutical, energy, and chemical processing. Austenitic stainless steel systems also are used to comply with the sanitary standards required for many food-processing applications.
In addition to corrosion resistance, austenitic stainless steel offers good mechanical properties, including yield strengths from 30,000 to 40,000 pounds per square inch (PSI) and tensile strengths from 70,000 to 80,000 PSI. The material also provides good toughness, particularly at low temperatures.
Like any other application, welding austenitic stainless steel tube and pipe presents specific challenges. It requires special attention to achieve optimal results, and it requires the right filler metal.
Hot cracking — a defect that occurs immediately after welding — is the biggest potential problem when welding austenitic stainless steel tube and pipe. This material is particularly susceptible to this defect due to its rather large grain structure.
Having the right filler metal and controlling the material’s temperature, the heat input and postweld cooling, can help prevent cracking, as well as minimize the risk of losing corrosion resistance during the welding process.
The Right Match. Like filler metals for welding other materials, those for austenitic stainless steel must match the base material’s chemical and mechanical properties. The process of making this match, however, is somewhat easier with austenitic stainless steel than with many other materials. The intended service conditions dictate the tube or pipe alloy to be used, which in turn determines the filler metal.
In the case of 304L austenitic stainless steel tube and pipe, the proper filler metal match is one with an American Welding Society (AWS) 308 or 308L designation, while a 316L filler metal is the appropriate choice for welding 316 austenitic stainless steels. It’s also not uncommon to join an austenitic stainless steel tube or pipe to a carbon steel component. In such a case, a 309L-grade filler metal is the appropriate choice.
All of these filler metals contain alloys that contribute to the weld’s integrity. For example, chromium helps ensure corrosion resistance, as does molybdenum. Molybdenum also offers good high-temperature performance, while nickel helps increase weld strength. The addition of silicon helps maintain weld pool fluidity and stabilize the amount of austenite in the weld. That stabilization, along with the introduction of ferrite into the weld by way of chromium and nickel, is important, because it helps refine the grain structure of the weld and make it more crack-resistant.
The Ferrite Number. Engineers and designers often refer to a property in stainless steels known as the ferrite number. This number quantifies the amount of ferrite versus austenite in the material’s structure, providing an indication of its ability to resist cracking, and in some cases the number indicates its corrosion resistance. The amount of ferrite present is determined largely by the chemistry of the material. In general, the goal when welding austenitic stainless steel is to reach a maximum ferrite number between 3 and 7; however, special applications may call for higher ferrite numbers to achieve the best results.
While filler metals with 308, 308L, 316, and 309L designations are appropriate for welding austenitic stainless steel tube and pipe, it’s still important to know the attributes, benefits, and limitations of the specific types of filler metals within each designation. Every type has its best uses, proper welding parameters, and shielding gas requirements. The filler metal manufacturer can recommend proper usage.
These filler metals should be stored in a clean, dry area with an ambient temperature similar to that of the weld location. A substantial temperature difference between the storage area and weld cell can lead to condensation, which damages the filler metal and can introduce weld defects.
Because it can produce high-quality results, gas tungsten arc welding (GTAW) is among the most commonly used proccesses for welding thin-walled tube and pipe. The filler metals, called cut lengths or filler rods, do not produce any slag, eliminating the opportunity for slag entrapment and the need for postweld cleaning. Cut lengths are available in diameters from 1⁄16 inch to 1⁄8 in. and in industry-standard lengths of 36 in.
On thin-walled tube and pipe, a single welding pass usually is adequate to complete a weld. A GTAW root pass is necessary when using multipass welding on thick-walled pipe (particularly for high-pressure, critical applications). GTAW’s disadvantages are its slow deposition rate and the need for backpurging with a shielding gas, which can add to costs and downtime for setup.
Shielded metal arc welding (SMAW) electrodes are readily available and familiar to many welders. A key benefit is that they aid in achieving proper ferrite numbers. However, SMAW is considered to be slow and inefficient compared to other welding processes. SMAW electrodes are only 12 in. long, so they require frequent changeover during welding. It’s impossible to use the entire electrode, and difficult to weld with a very short stub. Most welders discard the electrode when it is about 3 in. long. The downtime for changeover and the stub loss cost time and money.
Still, for austenitic stainless steel tube and pipe applications that are out of position or require access into complex joints a welding gun cannot reach, SMAW electrodes are a good choice. These filler metals also do not require an external shielding gas, so they are portable and work well for field welding.
Stainless steel stick electrodes typically are available in diameters from 3⁄32 to 5⁄32 in. Some welding operators may find them to be a bit more difficult to weld with, in part because the weld pool’s fluidity varies with the specific electrode. The slag on these filler metals can also be cumbersome, since it’s a bit more fluid than that found in stainless steel flux-cored wires. In either case, the slag must be removed, which adds to the time needed for postweld cleaning.
Solid wires are a common filler metal choice for welding austenitic stainless steel tube and pipe thicker than ½ in. They are suitable for the fill and cap passes in the flat and horizontal positions while the pipe is being rotated. Solid wires can also be used to weld out of position for root, fill, and cap passes using a pulsed or advanced short-circuit process. They are available in diameters from 0.030 to 1⁄16 in. and provide good deposition rates.
Fabricators should note that filler wire manufacturers can often formulate solid wires with specialized chemistries for unusual applications, if needed. The drawbacks are that they take time to develop and the minimum order quantity is usually large.
Metal-cored wires are becoming an increasingly popular alternative for welding thick-walled austenitic stainless steel pipe. They are available in 0.045- to 1⁄16-in. diameters.
Like solid wire, stainless steel metal-cored wire requires a pulsing or advanced short-circuit process to weld out of position and can be used for all passes, from root to cap. These wires, however, provide even greater productivity advantages. Metal-cored wires typically offer faster travel speeds and greater deposition rates than solid wires. Metal-cored wires also help to bridge gaps more so than some other consumables, so a tubular workpiece that is out-of-round presents less of a challenge during the welding process.
One disadvantage to stainless steel metal-cored wires is their cost. They tend to be more expensive than solid wires. Fabricators choosing to use metal-cored wires should consider a time study to determine whether the versatility and faster welding speed outweigh the higher price.
Note that metal-cored wires are easier to alloy than solid wires. Specific chemistries can be developed more readily and in smaller quantities, reducing the waiting period and the minimum order quantity.
Gas-shielded flux-cored arc welding (FCAW) can be an excellent option for achieving high deposition rates when welding out of position on thick-walled austenitic stainless steel pipe. Unlike solid or metal-cored wires, this filler metal does not require a special power source to weld out of position; the flux coating on the wire acts like a dam that helps hold the molten weld pool in place.
Flux-cored wires, however, produce higher levels of smoke than other filler metals and generate a slag that the welder must chip off or grind away between passes and after the cap pass. The latter activities could lead to greater downtime for cleanup.