The selection of welding methods for thick-walled steel pipes should be based on the material and wall thickness of the pipe. Different welding methods have different arc heat and arc force, and therefore different characteristics. For example, tungsten inert gas (TIG) welding is characterized by lower current density, stable arc combustion, and good weld formation, making it particularly suitable for thin-plate welding, but not for thick-plate welding. Plasma arc welding is characterized by high arc column temperature, high energy density, good arc straightness, and a wide range of adjustable rigidity and flexibility, and it is stable in operation, but it is more complex to operate. Submerged arc welding has the characteristics of high penetration depth and high wire deposition rate, thus greatly increasing welding speed and lowering welding costs, but the working conditions and environment are relatively poor. Therefore, different welding methods have different capabilities and different operating costs. Appropriately selecting the welding method based on the material and wall thickness of the thick-walled steel pipe is a very important task to ensure welding quality, improve productivity, and reduce costs.
Pickling thick-walled steel pipes is a method of removing oxide scale and rust from the surface of steel using acid solutions. Acids used for pickling include sulfuric acid, hydrochloric acid, phosphoric acid, and mixed acids. The pickling process removes surface oxide scale, followed by lubrication treatment (carbon steel – phosphating, stainless steel – tallow and lime, copper and aluminum pipes – oiling, and using older processes – copper plating), and then further processing such as drawing. If thick-walled steel pipes are not pickled, their surfaces may contain oxides and oil stains, which the phosphating solution cannot remove, resulting in lower phosphating quality. Furthermore, during the manufacturing process of thick-walled steel pipes, multiple processes are involved, and even slight negligence can leave scratches on the surface, reducing the corrosion resistance of components and directly affecting their service life.
What detailed treatments are required before using thick-walled steel pipes?
1. Thick-walled steel pipe cutting: The steel pipes should be cut using a metal saw or a toothed saw according to the actual required pipeline length. Protective Measures for Raw Materials When Using Water Welding During Cutting: During cutting, fire-resistant and heat-resistant baffles should be used at both ends of the cut to catch sparks and hot molten iron, protecting the original plastic coating.
2. Thick-Walled Steel Pipe Connection: After plastic coating is applied, connect and install the pipes and fittings. During connection, place rubber gaskets between flanges and tighten bolts until a seal is achieved.
3. Plastic Coating Treatment of Thick-Walled Steel Pipes: After grinding, heat the pipe openings using oxygen and C2H2 until the internal plastic coating melts. Then, a technician will evenly apply the prepared plastic powder to the pipe openings, ensuring thorough application. For flanges, the coating should extend above the waterstop line. Strict temperature control is crucial during this process. Excessive heat will cause bubbles during coating, while insufficient heat will result in incomplete melting of the plastic powder. Both of these issues will lead to plastic coating peeling after the pipes are put into use, and subsequent corrosion and damage to the thick-walled steel pipe.
4. Grinding the ends of thick-walled steel pipes: After cutting, the plastic coating at the pipe ends should be ground using an angle grinder. This is to prevent the plastic coating from melting or even burning during flange welding, which could damage the pipe. Use an angle grinder to grind the plastic coating at the pipe ends.
To improve the corrosion resistance of thick-walled steel pipes and extend their service life, they need to undergo pickling and passivation surface treatment to form a protective film on the surface. Thick-walled steel pipes have high hardenability, good machinability, moderate cold deformation plasticity, and weldability. Furthermore, the steel’s toughness decreases only slightly during heat treatment, but it still possesses considerable strength and wear resistance, especially maintaining high toughness after water quenching. However, this steel is highly sensitive to white spots, has a tendency for temper brittleness during heat treatment, and is sensitive to overheating. It has high strength, hardenability, and toughness, with minimal deformation during quenching, and high creep strength and long-term strength at high temperatures. It is used to manufacture forgings that require higher strength and larger quenched and tempered sections than 35CrMo steel, such as large gears for locomotive traction, turbocharger drive gears, rear axles, connecting rods, and spring clips subjected to heavy loads. It can also be used for drill pipe joints and fishing tools in oil wells below 2000m, and can be used for dies in bending machines.
Post time: Dec-02-2025