Four Methods for TIG Welding Stainless Steel Pipe Root Welding

Stainless steel pipe welding typically consists of root welding, filler welding, and cap welding. Root welding is the most critical step in stainless steel pipe welding, impacting both project quality and progress. Currently, stainless steel root welding is performed using either back-filling or non-filling.

Back-filling with argon shielding is further divided into solid wire + TIG and solid wire + TIG + water-soluble paper. Non-filling with argon shielding is further divided into flux-cored wire root welding and stick (coated wire) root TIG welding.
Stainless steel root welding typically uses the TIG process. Depending on the actual site conditions, the following four methods can be used for root welding.

01. Using a plugging plate to block ventilation protection on the back of the stainless steel pipe (i.e., solid wire + TIG welding)
During stainless steel pipe prefabrication, the weld joint can usually be rotated for welding, making ventilation very easy. In this case, plugging plates are usually used to block ventilation protection on both sides of the weld joint inside the pipe, while adhesive tape is used to block the outside.
During welding, a pre-ventilation and post-ventilation process should be used. The adhesive tape on the outside should be removed as welding proceeds. Because the plugging plates are made of rubber and tinplate and are not easily damaged, this welding method effectively ensures that the inside of the weld is filled with argon gas and its purity is maintained, effectively preventing oxidation of the metal inside the weld and ensuring the quality of the root weld.

02. Using only soluble paper or a combination of soluble paper and a plugging plate for sealing and ventilation protection of stainless steel pipes (i.e., solid welding wire + TIG + water-soluble paper).
When installing and welding stainless steel pipe fixings, ventilation from the inside is difficult, and some sides are easier to seal. In this case, water-soluble paper and a plugging plate can be used for sealing. Specifically, the side that is easy to ventilate and remove is sealed with a plugging plate, while the side that is difficult to ventilate and remove is sealed with water-soluble paper. At the same time, adhesive tape is used to seal the weld seam on the outside.
When welding stainless steel fixings, ventilation often occurs on both sides of the weld. This creates a challenge in ensuring argon protection inside the weld. In actual on-site construction, we successfully resolved this problem by sealing the weld seam with water-soluble paper on both sides, ventilating from the center of the weld, and sealing the weld seam on the outside with adhesive tape. When using water-soluble paper to seal ventilation, since ventilation occurs from the center of the weld, the ventilation tube should be quickly removed during the final sealing step. The remaining argon gas inside provides protection, allowing for quick primer application and sealing.
When using this method, it's important to use a double layer of water-soluble paper, ensuring it's applied securely. Otherwise, the paper can easily become damaged or fall off, depriving the inner weld of argon protection and causing oxidation. This can lead to the weld needing to be cut open and re-welded, compromising weld quality and significantly impacting the project schedule. Therefore, prior to welding, a thorough inspection and proper application of the water-soluble paper are essential.
This method of welding primer is used on many construction sites, effectively ensuring quality. However, it also presents a certain level of difficulty, so careful and skilled welders should be employed for this task.

03. The back of the stainless steel pipe is not shielded with argon gas, and a flux-cored wire + TIG process is used.
This method has been used in my country for several years. Flux-cored wires such as E308T1-1, E308LT1-1, E309T1-1, E309LT1-1, 347T1-1, E316T1-1, and E316LT1-1 have been produced and used in on-site welding, achieving good economic benefits.
Because the back of the pipe is not filled with argon, its advantages are obvious, mainly in terms of efficiency, simplicity, and low cost, making it suitable for on-site installation. However, due to its structural characteristics, flux-cored wire requires high welder operation. The wire feed speed is fast, and the accuracy required is high, making it difficult to master. Therefore, welders should receive specialized training and become proficient in the technique before participating in welding.

04. The back of the stainless steel pipe is not protected by argon gas, and a coated wire (self-shielded flux-cored wire) + TIG process is used.

In recent years, my country has also developed stainless steel rooting wire (i.e., coated wires such as TGF308, TGF308L, TGF309, TGF316L, and TGF347) and has applied them in actual construction with good results. The protection mechanism of the stainless steel rooting wire + TIG process is that the back weld is protected by the metallurgical reaction between the slag produced by the melting wire and its alloying elements, while the front weld is protected by argon gas, slag, and alloying elements.

When using this process, the following key points should be observed: During welding, maintain a correct angle between the welding torch, welding wire, and workpiece. The ideal welding torch nozzle tilt angle is 70°-80°, and the welding wire angle to the workpiece surface is 15°-20°. Properly control the molten pool temperature by varying the welding torch angle and welding speed to ensure a well-defined weld (consistent width and no defects such as concavity or convexity).

During operation, the current should be slightly higher than when welding solid wire, and the welding torch should be slightly swung to ensure a smooth, even weld. This accelerates the separation of molten iron and molten flux, making it easier to observe the weld pool and control penetration. When filling the filler wire, it's best to feed it halfway into the weld pool and slightly press it inward. This technique ensures root penetration and prevents concavity.
During the welding process, the wire should be regularly fed in and removed, and always protected by argon gas to prevent oxidation of the wire tip, which can affect weld quality. Pay attention to the weld quality at both arc start and arc end. At arc start, the spot weld should be ground to a gentle 45° slope. During arc end, be mindful of defects such as craters and shrinkage cavities.
Using coated wire for root welding eliminates the need for argon gas flow within the weld, making it simple, quick, efficient, and cost-effective. It also ensures excellent weld quality. (In the Wushihua Petrochemical expansion and renovation project, we used this method to weld 28 passes of joints and repairs, achieving a 100% pass rate on the first pass.) This method is worthy of widespread adoption.

The above four stainless steel base welding methods each have their own advantages and disadvantages. In actual construction, we should reasonably select the construction process based on the specific conditions of the site, taking into account both the construction cost and the welding quality and construction progress.