Welding stainless steel tubing and piping often requires back-purging with argon when using traditional processes such as gas tungsten arc welding (GTAW) and shielded metal arc welding (SMAW).But the cost of gas and the setup time of the purge process can be important, especially as pipe diameters and lengths increase.
When welding 300 series stainless steel, contractors can eliminate blowback in open root canal welds by switching from traditional GTAW or SMAW to an improved welding process, while still achieving high weld quality, maintaining the material’s corrosion resistance, and meeting The Welding Procedure Specification (WPS) requires a short-circuit gas metal arc welding (GMAW) process.The improved short-circuit GMAW process also brings additional benefits in productivity, efficiency and ease of use, helping to improve profits.
Favored for their corrosion resistance and strength, stainless steel alloys are used in many pipe and tubing applications, including oil and gas, petrochemical, and biofuels.While GTAW has traditionally been used in many stainless steel applications, it has some disadvantages that can be addressed by improved short-circuit GMAW.
First, as the shortage of skilled welders continues, finding workers familiar with GTAW is an ongoing challenge.Second, GTAW is not the fastest welding process, which hinders companies looking to increase productivity to meet customer demands.Third, it requires time-consuming and expensive backflushing of stainless steel tubing.
What is blowback?Purge is the introduction of gas during the welding process to remove contaminants and provide support.The backside purge protects the backside of the weld from forming heavy oxides in the presence of oxygen.
If the backside is not protected during open root canal welding, damage to the substrate may result.This breakdown is called saccharification, so named because it results in a surface that looks like sugar inside the weld.To prevent mashing, the welder inserts a gas hose into one end of the pipe and plugs the end of the pipe with a purge dam.They also created a vent on the other end of the pipe.They also usually put tape around the joint opening.After cleaning the pipe, they peeled off a section of tape around the joint and began welding, repeating the process of stripping and welding until the root bead was complete.
Eliminate blowback.Retraces can cost a lot of time and money, in some cases adding thousands of dollars to the project.The transition to an improved short-circuit GMAW process has enabled the company to complete root passes without backflushing in many stainless steel applications.Welding applications for 300 series stainless steels are well suited for this, while welding applications for high-purity duplex stainless steels currently require GTAW for the root pass.
Keeping the heat input as low as possible helps maintain the corrosion resistance of the workpiece.Reducing the number of weld passes is one way to reduce heat input.Improved short-circuit GMAW processes, such as Regulated Metal Deposition (RMD®), use precisely controlled metal transfer to provide uniform droplet deposition.This makes it easier for the welder to control the weld puddle, which in turn controls heat input and welding speed.The lower heat input allows the weld puddle to freeze faster.
With controlled metal transfer and faster weld pool freezing, the weld pool is less turbulent and shielding gas leaves the GMAW gun relatively undisturbed.This allows shielding gas to pass through the open root, displacing the atmosphere and preventing saccharification or oxidation on the back of the weld.This gas coverage only takes a short time because puddles freeze very quickly.
Testing has shown that the modified short-circuit GMAW process meets weld quality standards while maintaining the corrosion resistance of the stainless steel as when the root bead was welded with GTAW.
A change in welding process does require a company to recertify its WPS, but such a switch can yield huge time returns and cost savings for new manufacturing and repair work.
Open root canal welding using an improved short-circuit GMAW process offers additional benefits in productivity, efficiency and welder training.These include:
Eliminates the potential for hot channels as a result of being able to deposit more metal to increase the thickness of the root channel.
Excellent tolerance for high and low misalignment between pipe sections.Due to smooth metal transfer, the process can easily bridge gaps up to 3⁄16 inches.
Arc length is consistent regardless of electrode extension, which compensates for operators who struggle to maintain consistent extension.A more easily controlled weld puddle and consistent metal transfer can reduce training time for new welders.
Reduce downtime for process changeovers.The same wire and shielding gas can be used for root, fill and cap channels.A pulsed GMAW process can be used provided the channels are filled and capped with at least 80% argon shielding gas.
For operations looking to eliminate backflush in stainless steel applications, it is important to follow five key tips for success when switching to a modified short-circuit GMAW process.
Clean the inside and outside of the pipes to remove any contaminants.Use a wire brush designed for stainless steel to clean the back of the joint at least 1 inch from the edge.
Use a stainless steel filler metal with a high silicon content, such as 316LSi or 308LSi.The higher silicon content aids the weld pool wetting and acts as a deoxidizer.
For best performance, use a shielding gas mixture specially formulated for the process, such as 90% helium, 7.5% argon, and 2.5% carbon dioxide.Another option is 98% argon and 2% carbon dioxide.The welding gas supplier may have other recommendations.
For best results, use a tapered tip and nozzle for root channeling to locate gas coverage.Conical nozzle with built-in gas diffuser provides excellent coverage.
Note that using the modified short-circuit GMAW process without backing gas does produce a small amount of scale on the backside of the weld.This typically flakes off as the weld cools and meets quality standards for petroleum, power plant and petrochemical applications.
Jim Byrne is Sales and Applications Manager for Miller Electric Mfg. LLC, 1635 W. Spencer St., Appleton, WI 54912, 920-734-9821, www.millerwelds.com.
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