Backside protection methods for stainless steel welding

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With the rapid development of the petrochemical industry, stainless steel materials have been widely used, which has placed higher demands on the welding of pipes and sheets. As a result, the previous method of using stainless steel arc welding for root pass welding has been phased out in favor of TIG welding for root pass welding. TIG welding for root pass provides a higher level of cleanliness and faster progress compared to MIG welding for this purpose; however, some issues arise as well. During welding, the back side of the stainless steel material is prone to oxidation, which can lead to defects. Therefore, protective measures must be taken on the back side to ensure the mechanical properties and corrosion resistance of the weld. Hence, effective protection is necessary when welding stainless steel. Today, we will introduce several common methods for protecting the back side during stainless steel welding: 01 Back-side argon protection. The commonly used shielding gases include pure argon as well as mixed gases; a mixture of argon and nitrogen in a certain proportion 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, featuring 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 shielding method: This method is commonly used in the welding of stainless steel sheets and large-diameter pipes. The shielding device is connected to a metal tube as well as an argon hose; the argon valve is opened to fill the shielding device with argon. Another person holds the metal tube as a handle, allowing the shielding device 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 weld area of the pipe with tape to prevent gas leakage; the ends of the pipe are sealed using materials such as sponge, rubber, or cardboard. 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 approach facilitates proper welding by preventing indentations 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 effective protection for the weld. The drawbacks include waste, slow argon filling, and high costs. 3 Direct argon shielding at the weld seam: For pipelines that are too long or have a relatively large diameter, using argon for local shielding is too costly, does not guarantee quality, and increases project expenses. To save costs, the method of directly supplying argon at the weld seam can be employed. The sponge is processed into plugs with a diameter slightly larger than that of the pipe, and the two sponges are connected to each other 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 joints, the plugs are first placed 150–200 mm away from both sides of the area to be welded. The long wire at one end must be longer than the length of the pipe at that end, so that it protrudes beyond the end of the pipe. One end of the metal tube is flattened, while the other end is connected to an argon hose; the flattened end is inserted into the aligned weld area, and argon is introduced there. It is best to insert the tube from the top, so that it can be removed just before the final welding step, allowing the welding to be completed using the remaining gas in the pipe. After welding, use wire to pull out the plug. Before assembly, apply water-soluble paper as a seal at positions 150–200 mm on either side of the weld seam; after alignment, carry out inflation welding in the same manner as with the sponge plug. During the pipeline’s hydrostatic test, the water-soluble paper will dissolve and be 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 stainless steel back protection: (1) Before TIG welding, argon shielding should be applied to the back side of the workpiece in advance; use an appropriate high flow rate of argon, and gradually reduce the flow rate after the air has been expelled. Argon should be continuously injected into the tube during welding. 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 concavities at the root of the weld can occur, affecting the welding quality. (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 resulting in better protection effects. (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 Self-shielding wire shielding method: The back self-shielding wire is a type of wire with a coating. During welding, its protective flux plays a role in fully shielding the front and back of the weld 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-protecting 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 occur; therefore, certain skills and techniques are required from the welders. Self-shielded 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.