9.25*1.24mm ASTM A216 316/316L Stainless steel coiled tubing from China

When welding 300 Series stainless steel, contractors can eliminate backflushing at the root joints of open joint pipes while maintaining high weld quality.
Welding of stainless steel tubes and pipes often requires backflushing with argon using traditional processes such as gas shielded tungsten arc welding (GTAW) and shielded metal arc welding (SMAW). But the cost of gas and the time it takes to complete the purge process can be important, especially as pipe diameters and lengths increase.

When welding 300 Series stainless steel, contractors can switch to a modified short circuit metal arc welding (GMAW) process instead of traditional GTAW or SMAW. The improved short-circuit GMAW process also provides additional performance, efficiency and ease-of-use benefits to help increase profits.
Due to their corrosion resistance and strength, stainless steel alloys are used in many piping applications, including the oil and gas, petrochemical, and biofuel industries. While GTAW has traditionally been used in many stainless steel applications, it does have some disadvantages that can be addressed with a short circuit modified GMAW.
First, due to the ongoing shortage of skilled welders, finding GTAW specialists is an ongoing challenge. Secondly, GTAW is not the fastest welding process, which makes it difficult for companies that want to increase productivity to meet customer demand. Thirdly, a lengthy and costly backflushing of stainless steel pipes and piping is required.
Purge is the introduction of gas during welding to remove contaminants and provide support. Back side purge protects the back side of the weld from the formation of heavy oxides in the presence of oxygen.
If the backside is not protected during root canal welding, this may cause cracking of the base material. This defect is known as sugaring, so named because the surface inside the weld is very similar to sugar. To prevent sugaring, the welder inserts a gas hose into one end of the pipe, then plugs the ends of the pipe with purge plugs. They also created a vent at the other end of the pipe. They are also usually taped around the seams. After cleaning the pipe, they removed a piece of tape around the joint and proceeded to weld, repeating the stripping and welding process until the root weld was completed.
Blowback can cost a significant amount of time and money, in some cases adding thousands of dollars to the project. Switching to an advanced short cycle GMAW process has allowed the company to perform backflush-free root passes in many stainless steel applications. Welding 300 series stainless steels is a good candidate, while welding high purity duplex stainless steels currently requires GTAW for root passes.
Keeping heat input as low as possible helps maintain the corrosion resistance of the workpiece. One way to reduce heat input is to reduce the number of welding passes. A modified short-circuit GMAW process such as controlled metal deposition (RMD®) uses precisely controlled metal transfer to ensure uniform droplet deposition. This makes it easier for the welder to control the weld pool, thereby controlling heat input and welding speed. Less heat input allows the molten bath to freeze faster.
Due to controlled metal transfer and faster freezing of the weld pool, the weld pool becomes less turbulent and the shielding gas exits the GMAW torch relatively smoothly. This allows shielding gas to be forced through the exposed root, forcing out the atmosphere and preventing saccharification or oxidation on the underside of the weld. Because the puddle freezes so quickly, it takes very little time for the gas to cover it.
Testing has shown that the modified short circuit GMAW process meets weld quality standards while maintaining the corrosion resistance of stainless steel as when using GTAW for root pass welding.
Welding open root canals using a modified short circuit GMAW process can also bring other benefits in terms of productivity, efficiency, and welder training.
Changes in welding processes do require companies to requalify their processes, but this shift can pay off in terms of time and cost savings – for both new production and refurbishment.
Welding open root canals using a modified short circuit GMAW process can also bring other benefits in terms of productivity, efficiency, and welder training. These include:
Eliminates the possibility of hot channels due to the possibility of deposition of more metal to increase the thickness of the root canal.
Excellent resistance to high and low displacements between pipe sections. This process can easily bridge gaps up to 3⁄16 inches due to smooth metal transfer.
A constant arc length is maintained regardless of electrode extension, which compensates operators who struggle to maintain a constant extension length. A more controlled weld pool and consistent metal transition reduce training time for new welders.
Reduce downtime for process change. The same wire and shielding gas can be used for root, fill and shield passes. The pulsed GMAW process can be used provided that the channels are filled and sealed with a shielding gas containing at least 80% argon.
For operations seeking to eliminate backflushing in stainless steel applications, it is important to follow five key tips to successfully transition to the advanced short circuit GMAW process.
Clean the pipes inside and out to remove any contaminants. Clean at least 1 inch from the edge of the fitting with a wire brush designed for stainless steel.
Use high silicon stainless steel filler metal such as 316LSi or 308LSi. The higher silicon content helps wet the melt bath and acts as a deoxidizer.
For best results, use a shield 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 the cone tip and root canal nozzle for targeted gas coverage. Conical nozzle with built-in gas diffuser provides excellent coverage.
Note that using a modified short GMAW process (no reserve gas) results in a small amount of dross on the back of the weld. It typically flakes off as the weld cools and meets quality standards for the oil industry, power plants and petrochemicals.
Jim Byrne is a 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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