Four welding methods for root welding of stainless steel pipes using TIG welding

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The welding of stainless steel pipes usually consists of several steps: root welding, fill welding, and cap welding. The root welding of stainless steel pipes is the most critical step in their welding process; it affects not only the quality of the project but also its progress. Currently, there are two methods for performing root welding on stainless steel pipes: one involves the use of argon gas on the back side, while the other does not. Argon back shielding is further divided into two methods: solid wire + TIG process, and solid wire + TIG + water-soluble paper process ； Backside welding without argon shielding is further divided into root welding using flux-cored wires and TIG welding with rods (coated wires) for root welding. Stainless steel root welding is usually carried out using the TIG process; depending on the actual conditions on site, we can employ the following four methods for root welding. 01. The method of using plugs to seal and protect against air leakage on the back side (i.e., solid wire + TIG). When prefabricating stainless steel pipes, the weld seams can usually be welded with rotation, which facilitates air circulation; in such cases, plugs are used to seal both sides of the weld seam inside the pipe in order to protect it during the root welding process, while the outside area is sealed with adhesive tape. During welding, a process of pre-ventilating and delayed shutdown should be employed. The outer adhesive tape should be removed while welding. Since the blocking plate is made of rubber and galvanized steel, it is not easily damaged; therefore, this welding method ensures that the interior of the weld is filled with argon and that its purity is maintained. This in turn prevents the metal on the inside of the weld from oxidizing, thus ensuring the quality of the root pass welding. 02. When welding the fixing joints of stainless steel pipes, it is difficult to ensure ventilation on the inner side; it is easier to seal one side. In such cases, a method involving water-soluble paper in combination with a sealing plate can be used for ventilation protection (i.e., solid wire + TIG + water-soluble paper). That is, the side that is easy to ventilate and easy to remove is sealed with a plug plate, while the side that is difficult to ventilate and hard to remove the plug plate from is sealed with water-soluble paper; simultaneously, the welds are sealed on the outside using adhesive tape. When welding stainless steel fixed joints, in many cases it is not possible to allow air to pass on either side of the weld. In such situations, ensuring that argon protection is maintained inside the weld becomes a challenge. In actual field construction, we resolved this issue by sealing both sides of the weld with water-soluble paper, allowing air to flow from the center of the weld, while sealing the outer sides with adhesive tape. When using water-soluble paper to seal off the ventilation, since ventilation occurs from the center of the weld, the ventilation tube should be removed promptly during the final sealing step; the remaining argon gas inside can then be used for protection to allow for rapid completion of the root pass and proper sealing. When using this method, it is important to note that the water-soluble paper should be double-layered and properly attached; otherwise, it is easy for the paper to get damaged or come loose, which will result in the internal welds losing the protection provided by argon gas and leading to oxidation. This in turn causes the welds to crack and require re-welding, which not only compromises the quality of the weld but also significantly affects the project timeline. Therefore, a thorough inspection should be carried out before welding, to ensure that the water-soluble paper is properly attached. At many construction sites, we use this welding method for the root pass; it ensures good quality, but it also presents certain challenges in terms of construction. Therefore, careful and skilled welders should be assigned to carry out this task. 03. No argon shielding is used on the back side; instead, a flux-cored wire plus TIG welding process is employed. This method has been in use in China for several years now. Flux-cored wires such as E308T1-1, E308LT1-1, E309T1-1, E309LT1-1, 347T1-1, E316T1-1, and E316LT1-1 have been produced and are already being used in field welding, yielding good economic benefits. Since no argon is filled on the back side, its advantages are obvious, mainly in terms of high efficiency, simplicity, low cost, and suitability for installation at construction sites. However, due to its structural characteristics, flux-cored wire requires high standards from welders during operation; it has a fast wire feeding speed and demands high accuracy in feeding, making it somewhat difficult to master. Welders must receive specialized training and achieve proficiency before they can carry out welding tasks. At Nanjing Yangba as well as on overseas construction sites, we have used this method to successfully solve the problem of being unable to introduce argon gas at joint areas and areas that require repair. 04. No argon shielding is used on the back side; instead, flux-coated welding wires (self-shielding flux-cored welding wires) together with the TIG process are employed. In the 1990s, companies in Japan such as Kobe Steel developed root welding wires. In recent years, China has also developed stainless steel root welding wires (i.e., flux-coated welding wires such as TGF308, TGF308L, TGF309, TGF316L, TGF347, etc.), which have been applied in actual construction projects with good results. We successfully utilized this method in the expansion and renovation project of Wuhu Petrochemical. The protection mechanism of stainless steel root welding wire + TIG process is that the back weld seam is protected by the slag generated from the melting of the welding wire and the metallurgical reactions of its alloying elements, while the front weld seam is protected by argon gas, slag, and alloying elements. When using this process, the following key operating points should be noted: during welding, maintain the correct angle between the welding torch, welding wire, and the workpiece; the ideal angle of inclination of the welding torch nozzle is 70°–80°, while the angle between the welding wire and the surface of the workpiece should be 15°–20° ； The temperature of the molten pool must be controlled properly; this is achieved by adjusting the angle between the welding torch and the workpiece, as well as the welding speed, in order to ensure that the weld has an attractive appearance (with a consistent width and without defects such as inward recesses or excessive bulges) ； During operation, the current should be slightly higher than that used for welding solid wire; the welding torch should be moved slightly to accelerate the separation between the molten iron and the melted flux, thereby facilitating observation of the molten pool and ensuring proper penetration ； When filling the welding wire, it is best to deliver it to about 1/2 of the depth of the weld pool and press it slightly inward; this method helps ensure full penetration at the root of the weld and prevents undercuts ； During the welding process, the welding wire should be fed in and pulled out regularly, and it is necessary to ensure that the weld area remains under argon protection at all times, so as to prevent oxidation of the wire’s end portion and avoid affecting the quality of the weld ； Pay attention to the welding quality at the start and end of the weld; at the start, the spot weld area should be polished into a 45° slope, and care must be taken to avoid defects such as arc pits and shrinkages at the end of the weld. Using coated welding wire for root welding eliminates the need to use argon gas inside the weld seam. This method is simple and fast for welders to use, offering high efficiency and low costs; it also ensures good welding quality (in the capacity expansion project of Wuhu Petrochemical, we used this method to weld 28 joints in total, with a 100% pass rate on the first attempt at inspection), making it worth promoting for use. The four aforementioned methods for welding stainless steel substrates each have their own advantages and disadvantages. In actual construction, we must take into account the specific conditions on site, considering both the cost of construction as well as the quality of welding and the construction schedule, in order to select the most appropriate welding process.