Argon backflush is often required for welding stainless steel tubes and pipes using conventional processes such as gas shielded tungsten arc welding (GTAW) and shielded metal arc welding (SMAW). But the cost of gas and the set-up time of the purge process can be important, especially as pipe diameters and lengths increase.
When welding 300 Series stainless steel, contractors can eliminate back-breakout in open root canal welds by switching from traditional GTAW or SMAW to an advanced welding process, while maintaining high quality welds, maintaining material corrosion resistance, and meeting the Welding Procedure Specification (WPS). ) requires a short circuit metal arc welding (GMAW) process. 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 pipe and piping applications, including oil and gas, petrochemicals, and biofuels. While GTAW has traditionally been used in many stainless steel applications, it does have some disadvantages that can be addressed with an improved short circuit GMAW.
First, as there is a continuing shortage of skilled welders, finding workers familiar with GTAW is an ongoing challenge. Secondly, GTAW is not the fastest welding process, which hinders companies looking to increase productivity to meet customer requirements. Thirdly, it requires lengthy and costly backflushing of stainless steel pipes.
What is feedback? Purge is the introduction of gas during the welding process 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 back side is not protected during welding of an open root canal, damage to the base may result. This breakdown is called saccharification because it results in a sugar-like surface inside the weld. To prevent chafing, the welder inserts a gas hose into one end of the pipe and plugs the end of the pipe with a purge valve. They also created a vent at the other end of the pipe. They also usually put tape around the opening of the joint. After cleaning the pipe, they removed a piece of tape around the joint and started welding, repeating the stripping and welding process until the root bead was complete.
Eliminate backlash. Retracing can cost a lot of time and money, in some cases adding thousands of dollars to a project. Switching to an advanced short cycle GMAW process allows the company to perform root passes without backflushing in many stainless steel applications. Welding 300 series stainless steels is well suited for this, while welding high purity duplex stainless steels currently requires a GTAW for the root pass.
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. Advanced short-circuit GMAW processes such as controlled metal deposition (RMD®) use precisely controlled metal transfer to ensure uniform droplet deposition. This makes it easier for the welder to control the weld pool, which in turn regulates the heat input and welding speed. Less heat input allows the weld pool to freeze faster.
Due to controlled metal transfer and faster freezing of the weld pool, the weld pool is less turbulent and the shielding gas exits the GMAW torch relatively smoothly. This allows the shielding gas to pass through the exposed root, forcing out the atmosphere and preventing sugaring or oxidation on the underside of the weld. This gas coverage takes a short time because the puddles freeze very quickly.
Testing has shown that the modified short circuit GMAW process meets welding quality standards while maintaining the stainless steel corrosion resistance of GTAW root bead welding.
Changing the welding process requires the company to recertify WPS, but such a switchover can result in significant time gains and cost savings on new manufacturing and repair work.
Welding open root canals using the advanced short circuit GMAW process provides additional benefits in productivity, efficiency and welder education. This includes:
Eliminates the possibility of hot channels due to the possibility of surfacing more metal to increase the thickness of the root canal.
Excellent resistance to high and low displacements between pipe sections. With smooth metal transfer, this process can easily bridge gaps up to 3⁄16 inches.
The arc length is constant regardless of electrode extension, which compensates for the difficulty of operators who find it difficult to maintain constant extension. A more easily controlled weld pool and uniform metal transfer can reduce training time for new welders.
Reduced downtime for process change. The same wire and shielding gas can be used for root, fill and cover canals. The pulsed GMAW process can be used provided that the channels are filled and closed at least 80% with argon shielding gas.
For stainless steel backflush operations, it is important to follow five key tips for a successful transition to a modified short circuit GMAW process.
Clean the pipes inside and out 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 high silicon stainless steel filler metal such as 316LSi or 308LSi. The higher silicon content promotes wetting of the weld pool 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 conical tip and root canal tip to locate the gas coverage. Conical nozzle with built-in gas diffuser provides excellent coverage.
Note that using a modified short circuit GMAW process with no back-up gas results in a small amount of dross on the underside 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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