How to weld aluminum-coated steel pipes with carbon steel plates, and what welding materials should be used (electrodes or TIG welding wires)?

Reposted from the internet: Key points of welding technology for aluminum-impregnated steel pipes. I. Overview: Aluminum-impregnated steel is a new type of composite steel material formed by treating carbon steel or low-alloy steel with aluminum impregnation methods such as hot-dip aluminum treatment, solid powder method, or spray impregnation, thereby depositing an aluminum-iron alloy layer 0.2–0.5 mm thick on the surface of the steel. Compared to steel that has not been aluminum-impregnated, aluminum-impregnated steel can raise the critical temperature for oxidation resistance by over 200°C, and its corrosion resistance in high-temperature hydrogen sulfide environments can increase by many times. It can replace expensive stainless steel and heat-resistant steel, offering significant economic benefits. There are two prominent problems in welding aluminized steel: first, cracks tend to form in the weld metal or fusion zone. To address this, elements with high crack resistance, such as Cr, Mo, and Mn, are added to the electrodes designed specifically for aluminized steel. Secondly, the corrosion resistance of the fusion zone decreases significantly; therefore, during welding, it is necessary to use as little welding heat input as possible or take appropriate process measures to reduce the decrease in aluminum content in the fusion zone. There are generally two methods for welding aluminized steel nowadays: one is to remove the aluminized layer at the joint, create a bevel, weld using ordinary electrodes, and then spray another layer of aluminum over the welded area. Secondly, stainless steel electrodes (such as Cr25Ni13, etc.) or electrodes specifically designed for aluminum-doped steel should be used for welding. II. Pre-welding preparations 1. Clean the welded parts; when using mechanical cleaning methods, be careful not to damage the carburized layer on the surfaces of areas that are not to be welded ； Appropriate grooves should also be made; generally, the groove angle is 60–65°, the root gap is 0.5–1 mm, the gap between the edges is about 3 mm, and the misalignment between the edges is less than 1 mm. Welding machine and welding materials; both AC and DC welding machines are suitable, generally following the instructions provided for the welding rods ； Welding materials usually need to be dried and placed in insulated containers for easy access when needed. 2. Before welding, a welding coating is applied to both sides of the inner wall at the butt joint of the aluminized steel pipe; this coating serves to support the molten pool during welding, preventing penetration and ensuring good fusion in the fusion zone on the back side of the weld ； Under welding conditions, the chemical penetrant in the coating is rapidly decomposed to generate active aluminum atoms, which then penetrate into the welded joint zone. This helps to compensate for the loss of aluminum in the fusion layer on the back side of the weld joint, thereby enhancing the high-temperature oxygen resistance and corrosion resistance of the welded joint zone. The welding coating consists of a chemical penetrant layer and a protective layer; the function of the chemical penetrant layer is to provide a source of active aluminum atoms necessary for compensating for aluminum diffusion in the welded joint area, as well as to create a higher aluminum potential ； The function of the protective layer is to prevent the oxidizing atmosphere in the welding area from oxidizing the active aluminum atoms released by the chemical aluminizing agent, thereby ensuring the proper progress of the aluminizing process. III. Welding operations: 1. Since the inner surface of the weld joint in aluminum-coated steel pipes cannot be treated using spraying or other methods after welding, the welding process must ensure one-sided welding with double-sided formation; care must be taken during the welding process. The welding area of the aluminized steel pipe should be covered with asbestos cloth or coated with chalk powder to prevent welding spatter and protect the aluminized layer. 2. For tack welding, it is best to use special fixtures for alignment; the misalignment should be less than 1 mm. The tack weld spots should be as small as possible, and once they are in place, they should not be struck or moved around casually. 3. Root welding: Root welding is crucial for single-sided welding of aluminum-coated steel pipes to achieve double-sided formation. During welding, it is necessary to closely monitor the movement of the molten pool, and strictly control the size and shape of the molten pool, ensuring that the welding arc remains aligned with the inner corners of the groove and at 90° to both sides of the workpiece. The electrode replacement must be done quickly, and it should be carried out while the weld is still hot, in order to prevent poor back-side fusion at the electrode joint. When sealing the circular joint, the electrode should be pressed slightly downward to ensure good root fusion. Filler cap welding; the procedure for filler welding is similar to that for ordinary carbon steel or low-alloy steel ； For the cover welding, the weld bead surface is required to be smooth and aesthetically pleasing, with no undercuts on either side. 4. During the entire welding process, arcing should not be initiated arbitrarily on the surface of the aluminized steel pipe to prevent damage to the aluminized layer ； The welded area should be covered immediately after welding to prevent the formation of microcracks due to hardening caused by rapid cooling. In particular, chromium-molybdenum aluminum-alloyed steel must be cooled slowly after welding! 5. Examples of welding parameters (AC power supply, electrode E-23-13-16, 3.2): ① Carbon-aluminized steel pipe (6×114mm): Root pass, current 85–95A, voltage 25–28V ； Cover layer: current 90–105A, voltage 26–30V. ②Aluminized Cr5Mo steel pipe (10×114mm): Undercoating layer, current 85–95A, voltage 26–30V ； Cover layer: current 90–110A, voltage 26–32V.