Backside protection methods for stainless steel welding

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Methods for back-side protection during stainless steel welding. With the rapid development of the petrochemical industry, stainless steel materials have been widely used, which has led to higher requirements for the welding of pipes and sheets. The traditional method of using arc welding for root pass welding has been phased out in favor of TIG welding for this purpose. TIG welding provides a cleaner root pass and works faster than arc welding; however, certain problems arise during welding. Since the back side of the material is prone to oxidation during TIG welding, back-side protection measures must be taken to ensure the mechanical properties and corrosion resistance of the weld. Therefore, effective protection is necessary when welding stainless steel. Here are several common methods for back-side protection during stainless steel welding: 01 Back-side argon protection. Common shielding gases include pure argon or mixed gases; a mixture of argon and nitrogen in certain proportions is more suitable for welding austenitic stainless steel. Some inert gases are not used due to their high cost. Argon shielding is a relatively traditional method for back protection; it features good back protection, ease of use, high cleanliness, and a high pass rate. It is divided into methods such as argon shielding with a protective cover, local argon shielding, and direct argon shielding at the weld area. 1. Argon protection with a shield: This method is commonly used in the welding of stainless steel sheets and large-diameter pipes. The shield is connected to a metal tube as well as an argon hose; the argon valve is opened to fill the shield with argon. Another person holds the metal tube as a handle, allowing the shield to slide over the molten pool on the back side, in sync with the welding process on the outside of the sheet or pipe. This provides effective protection for the back side. Its advantages include concentrated protection, no need to use high pressures of argon, and reduced argon waste. 2. Local argon shielding: This method is suitable for pipes that are located in confined spaces and are of small size or short length. The procedure involves sealing the pipe joints with tape to prevent air leakage; the ends of the pipe are covered with sponge, rubber, cardboard, etc. An argon hose is inserted at one end to supply argon, and it’s advisable to make a small hole at the sealed end of the pipe (this step isn’t necessary if using sponge). This helps during the final welding process, as it prevents dents or other defects caused by excessive internal pressure. During welding, in order to avoid significant loss of argon from the weld area, the sealing tape should be removed in sections as welding progresses, thereby reducing argon loss and providing better protection for the weld. The drawbacks include waste, slow argon filling, and high costs. 3. Direct argon shielding at the weld area: For pipelines that are too long or have a relatively large diameter, using argon shielding only in specific areas is wasteful, does not guarantee quality, and results in high project costs. To save costs, the method of directly applying argon at the weld area can be used. The method of using plugs on both sides of the weld involves shaping sponge material into plugs that are slightly larger in diameter than the pipe, and connecting the two plugs together with wire at a distance of 300–400 mm, thus creating a double plug. One end of each plug is connected to a longer piece of wire. When aligning the pipes for welding, the plugs are placed 150–200 mm away from both sides of the area to be welded. The longer wire end should be longer than the length of the pipe at that end, with enough length extending outside the pipe end. One end of the metal tube is flattened, while the other end is connected to an argon gas hose; the flattened end is inserted into the aligned weld area, and argon gas is introduced. It is best to insert the tube from the top, so that it can be removed just before completing the root welding, allowing the welding to be finished using the remaining gas in the pipe. After welding, pull the plug out with wire. In the water-soluble paper protection method, water-soluble paper is applied as a seal at 150–200 mm on either side of the weld joint before assembly; after assembly, it is welded using inflation, in the same way as with sponge plugs. During the pipeline hydrostatic test, the water-soluble paper dissolves and is carried away by the water. 4. Judgment of argon shielding: The effectiveness of argon shielding can be determined by the color of the internal weld; this allows operators to adjust the amount of argon used based on that color, thereby achieving the best shielding effect. The best colors are white and golden yellow, while gray-black is the worst. 5 Precautions for back protection of stainless steel (1) Before TIG welding, argon should be used to protect the back side of the workpiece; the flow rate should be high enough, and it should be reduced gradually after the air is expelled. Argon should be continuously supplied into the tube during the welding process. The argon hose can be removed only after welding is complete, so as to provide good protection for the weld. It should also be noted that welding can only be carried out after the air has been completely removed; otherwise, it will affect the protective effect of argon filling. (2) The argon flow rate should be appropriate. If the flow rate is too low, protection is inadequate, and the back side of the weld is prone to oxidation; if the flow rate is too high, defects such as concave areas at the root of the weld can occur, affecting the quality of the weld. (3) The argon inlet should be placed as low as possible in the closed section, while the air outlet should be located slightly higher in the closed tube section. Since argon is heavier than air, filling it from a lower position ensures a higher concentration, thereby providing better argon protection. (4) To reduce the loss of argon gas inside the tube through the gap at the joint, which could affect the shielding effect and increase costs, tape can be applied along the weld gap before welding, leaving only a length sufficient for the welder to complete one continuous weld; the tape is then removed as the welding proceeds. 02 Shielded welding wire protection method: Backside self-shielding welding wire is a type of welding wire with a coating. During welding, its protective flux plays a role in fully shielding the front and back of the molten pool, forming a dense protective layer that prevents the back side of the weld seam from oxidizing. After cooling, this protective layer falls off on its own, and it is completely removed during purging and pressure testing. The method of using this welding wire is basically the same as that for ordinary TIG welding solid wires, and the properties of the weld metal meet the required standards. Self-shielding welding wires are not restricted by various welding conditions, making the welding preparation process faster and simpler. However, due to the flux coating on the surface of the welding wire, there are certain difficulties for welders when operating it. As a result of these difficulties and because the welding techniques used are not suitable for flux-coated wires, defects such as indentations can sometimes occur; therefore, certain skills and techniques are required from the welders. Self-shielding welding wire is best used for the root pass due to its high cost. Additionally, there are many brands of self-shielding welding wires available on the market, each with different levels of suitability; those imported from abroad generally offer the best quality and performance.