What are the welding methods for the root pass of stainless steel pipe argon arc welding

How is the root pass welding of stainless steel pipes performed? The welding of stainless steel pipes typically consists of several parts: root pass welding, filler weld, and cover pass welding. The root pass welding is the most critical step in stainless steel pipe welding, affecting not only the quality but also the project schedule. Currently, stainless steel root pass welding is divided into two processes: back argon purging and no argon purging. Back argon purging protection is further divided into solid wire + TIG process and solid wire + TIG + water-soluble paper process; no argon purging protection is divided into flux-cored wire root pass welding and welding rod (coated wire) TIG root pass welding. Stainless steel root pass welding usually uses the TIG process. Depending on the actual site conditions, we can use the following four methods for root pass welding.

1. Method of sealing and venting protection using a plug plate on the back (solid welding wire + TIG).

During the prefabrication of stainless steel pipes, the weld joints can usually be rotated for welding, making venting very easy. In this case, a plug plate is typically used to seal and vent the weld joints on both sides of the pipe for the root pass, while the outer side is sealed with adhesive tape. During welding, a process of venting in advance and stopping venting later should be adopted. The adhesive tape on the outer side is removed while welding. Because the plug plate is composed of rubber and galvanized iron sheet, it is not easily damaged. Therefore, this welding method can effectively ensure that the inside of the weld is filled with argon gas and maintain its purity, thereby effectively preventing the metal inside the weld from being oxidized and ensuring the quality of the root pass weld.

2. Method of sealing and venting protection using only soluble paper or a combination of soluble paper and a plug plate (solid welding wire + TIG + water-soluble paper).

When installing and welding the fixed joints of stainless steel pipes, venting the inside is more difficult. In some cases, one side is easier to seal. In this situation, water-soluble paper + a plug plate can be used for sealing. The side that is easy to ventilate and remove is sealed with a blocking plate, while the side that is difficult to ventilate and remove the blocking plate from is sealed with water-soluble paper. Simultaneously, the outer side is sealed with adhesive tape pasted onto the weld. When welding stainless steel fixed joints, it is common for both sides of the weld to be unable to ventilate. In such cases, ensuring argon purging protection inside the weld becomes a challenge. In actual on-site construction, we successfully solved this problem by sealing both sides of the weld with water-soluble paper, venting from the center of the weld, and sealing the outer side with adhesive tape. When using water-soluble paper for sealing and venting, since venting is from the center of the weld, the vent pipe should be quickly removed during the final sealing stage to utilize the remaining argon gas for protection. The initial venting should be completed quickly, and the joint sealed. When using this method, it is important to ensure that the water-soluble paper is double-layered and properly adhered. Otherwise, the paper may be damaged or fall off, leaving the inner weld unprotected by argon gas, causing oxidation, and requiring the weld to be cut open and re-welded. This not only compromises weld quality but also severely impacts the construction schedule. Therefore, a thorough inspection and proper adhesion of the water-soluble paper are essential before welding. This welding method is commonly used for the root pass in many construction sites, effectively guaranteeing quality. However, it presents certain challenges, so careful and skilled welders should be selected for this task.

3. No argon gas protection is applied to the back side; 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 applied in field welding, achieving good economic benefits. Because the back side is not argon-filled, its advantages are obvious, mainly manifested in high efficiency, simplicity, and low cost, making it suitable for on-site installation. However, due to its structural characteristics, flux-cored welding wire requires highly skilled welders. Its wire feeding speed is fast, and the accuracy of wire feeding is crucial, making it somewhat difficult to master. Welders should undergo specialized training and become technically proficient before participating in welding.

4. No argon protection is provided on the back side; flux-cored wire (self-shielded flux-cored wire) + TIG process is used. 

In recent years, my country has also developed stainless steel root pass welding wires (i.e., flux-cored wires, such as TGF308, TGF308L, TGF309, TGF316L, TGF347, etc.) and applied them in actual construction, achieving good results. The protection mechanism of the stainless steel root pass welding wire + TIG process is that the back weld is protected by the metallurgical reaction between the slag produced by the melting of the welding wire and its alloying elements, while the front weld is protected by argon, slag, and alloying elements. When using this process, the following operational points should be noted: During welding, maintain the correct angle between the welding torch, welding wire, and workpiece. The ideal back tilt angle of the welding torch nozzle is 70°–80°, and the angle between the welding wire and the workpiece surface is 15°–20°. Properly control the molten pool temperature by changing the angle between the welding torch and the workpiece, and by changing the welding speed, to ensure a beautiful weld formation (consistent width, no concave or convex defects). During operation, the current should be slightly higher than when welding solid wire, and the welding torch should be slightly oscillated to allow the iron... Water and molten flux separate quickly, facilitating observation of the weld pool and control of penetration. When filling the weld, it's best to feed the wire to half the height of the weld pool and press it slightly inward to ensure root penetration and prevent concavity. During welding, the wire should be fed in and removed regularly, ensuring it remains under argon gas protection to prevent oxidation of the wire tip, which would affect weld quality. Pay attention to the welding quality at the arc initiation and termination points. At the arc initiation point, the tack weld should be ground into a 45° gentle slope. When terminating the arc, be careful to avoid defects such as craters and shrinkage cavities. Using flux-coated wire for the root pass eliminates the need for argon gas in the weld, making it simple, quick, efficient, and low-cost for welders. It also effectively guarantees weld quality (in a petrochemical capacity expansion project, we used this method to weld 28 joints and repair welds, achieving a 100% pass rate on first-pass inspection), making it worthy of widespread adoption.

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