What are the common manufacturing processes for austenitic stainless steel tubes

Austenitic stainless steel tubes, with their excellent corrosion resistance, good ductility, and toughness, are widely used in various fields, including petrochemicals, food and medical, and aerospace. The manufacturing process directly determines the mechanical properties, surface quality, and dimensional accuracy of the finished pipe. Common manufacturing processes include hot rolling, cold drawing, and welding. These processes are not used in isolation but are often combined to meet the stringent requirements of different applications.

Seamless Pipe Manufacturing Processes: Hot Rolling and Cold Drawing

Hot rolling is a common method for producing large-diameter, thick-walled seamless stainless steel tubes. The basic process involves perforating a heated solid billet to form a hollow shell, which is then rolled and stretched through a rolling mill to obtain the desired pipe size. The main advantages of hot rolling are high production efficiency and the ability to produce a wide range of pipe sizes. However, due to metal oxidation at high temperatures, the surface of hot-rolled tubes is typically rough, and dimensional accuracy is relatively low. After hot rolling, pickling and passivation treatment is usually required to remove the surface oxide scale and restore the stainless steel's corrosion resistance.

The cold drawing process is a core technology for manufacturing precision, small-diameter, thin-walled austenitic stainless steel seamless tubes. This process, performed at room temperature, involves drawing the tube blank through a die using a drawing machine. This reduces the outer diameter and wall thickness of the pipe while improving surface finish and dimensional accuracy. The cold drawing process causes work hardening of the stainless steel, significantly increasing the strength and hardness of the pipe, but also reducing its ductility. To eliminate this work hardening and restore ductility, the cold-drawn pipe must undergo a solution treatment. Solution treatment involves heating the pipe to a specific temperature (typically 1050-1150°C) followed by rapid cooling (either water quenching or air cooling) to ensure uniform dissolution of alloying elements and eliminate internal stresses. The cold drawing process produces high-precision, high-quality seamless tubes, making it particularly suitable for applications with stringent dimensional requirements in instrumentation and high-pressure fluid transportation.

Welded Pipe Manufacturing Processes: High-Frequency Welding and TIG Welding

High-Frequency Welding (HF-Welding) is an efficient and economical method for manufacturing welded tubes. High-frequency welding utilizes high-frequency current to generate induction heating at the edges of the steel strip, causing them to melt. The edges are then directly fused together by the squeezing action of squeeze rollers, forming a weld. High-frequency welding boasts extremely high production speeds, making it suitable for large-scale, industrialized production. However, due to its fusion mechanism, the weld seam may not be dense enough and presents a potential risk of intergranular corrosion. Therefore, high-frequency welded tubes are typically used for general fluid transportation or decorative applications, where corrosion resistance requirements are less stringent.

Argon arc welding (TIG welding), particularly automated TIG welding, is the preferred process for manufacturing high-quality austenitic stainless steel welded tubes. This process uses an inert gas (typically argon) as a shield and a non-consumable electrode to generate an arc, melting and joining the edges of the steel strip. Unlike high-frequency welding, TIG welding allows for precise penetration control, resulting in high-quality welds with dense structures and a minimal heat-affected zone. This effectively reduces the risk of intergranular corrosion and ensures the corrosion resistance of the welded pipe. TIG welded tubes, especially those that undergo internal weld seam treatment (such as internal weld bead smoothing) and solution treatment, offer performance nearly comparable to seamless tubes. They are widely used in industries such as food, pharmaceuticals, semiconductors, and nuclear power, where purity and corrosion resistance are paramount.

Combination and Post-Processing

In actual production, different processes are often combined. For example, large-diameter seamless tubes may be produced first through hot rolling and then cold drawn to further reduce the size or improve precision. Welded tubes often require in-line solution treatment after welding to relieve welding stress and restore corrosion resistance in the weld area. Finishing treatments such as polishing, pickling, and passivation are also key steps in the austenitic stainless steel pipe manufacturing process. Polishing significantly improves the surface finish of the pipe, meeting the hygienic requirements of the food and pharmaceutical industries. Pickling and passivation chemically remove surface impurities and oxides and form a dense passive film on the surface, maximizing the stainless steel's inherent corrosion resistance.