Production and maintenance of stainless steel welded pipes

What are welded stainless steel pipes?

Welded stainless steel pipes are formed by rolling flat steel into a tubular shape and then welding the edges together. This method is less expensive than manufacturing seamless pipes. Welded pipes are well-suited for everyday use. Special welding methods, such as laser welding, can create strong and reliable welds.Welded pipes offer good flexibility and are widely used in numerous industries, such as construction, food production, and machinery manufacturing. They perform well in non-high-pressure or high-temperature environments.

Stainless steel welded pipes are suitable for general pressure and corrosive media and low-temperature piping.  Welded pipes with an outer diameter of 1/4 inch to 8 inches are generally formed using a forming machine, while stainless steel welded pipes larger than 8 inches in diameter use pressure forming or three-roll bending machines. The welding process generally uses inert gas shielded arc welding. For wall thicknesses ≤4mm, tungsten inert gas welding (TIG) is used, while for wall thicknesses greater than 4mm, metal inert gas welding (MIG) or submerged arc welding is used, with a welding speed of less than 1m/min. To increase welding speed and stabilize weld quality, new welding processes such as plasma arc welding and laser welding are used, with welding speeds reaching up to 5m/min depending on the wall thickness.

The production process flow of stainless steel welded pipes

Regardless of the welding method used, various treatments and inspections are performed after welding according to relevant standards. Generally, after cutting, the pipes undergo straightening, end processing, degreasing, heat treatment, pickling and passivation, air or water pressure testing, eddy current testing, ultrasonic testing, and various other inspections (including appearance, metallographic structure, mechanical properties, and corrosion resistance). For sanitary pipes, the inner and outer surfaces of the stainless steel welded pipes are ground and polished. For large-scale production, defects are removed using continuous belt or flat-head grinding and polishing machines. PVA grinding wheels or brush grinding are used in the fine grinding process.

To improve the surface finish, dimensional accuracy (generally including outer diameter deviation of 0.5% and wall thickness deviation of ±10%), mechanical properties, and corrosion resistance, and to produce thin-walled, small-diameter, and irregularly shaped pipes, stainless steel welded pipes require final cold drawing.

The process flow of a stainless steel welded pipe manufacturer is as follows: Cold-rolled steel coil preparation → welding → cutting → hydrostatic testing → degreasing → bright heat treatment → lubricant application → cold drawing → mandrel removal → cutting → degreasing → bright heat treatment → eddy current and ultrasonic testing → chamfering → marking → material grade analysis → surface appearance inspection → packaging → warehousing → shipping.

1. Raw Material Selection
Stainless steel welded pipes start with steel coils. Grades like 304, 316, or duplex stainless steel are chosen based on corrosion resistance, strength, and the actual working environment.

2. Forming the Pipe
The steel strip is fed through forming rollers and slowly shaped into a round tube. This cold-forming step controls the pipe size and prepares the edges for welding.

3. Welding the Seam
The pipe seam is welded using TIG, laser, or plasma welding. Inert gas protection is applied during welding to prevent oxidation and keep the weld strong and clean.

4. Sizing and Straightening
After welding, the pipe goes through sizing and straightening rollers to make sure it is round, straight, and within required tolerances.

5. Heat Treatment (If Needed)
Some pipes are heat treated to reduce internal stress and improve ductility and corrosion resistance, depending on the grade and application.

6. Cutting to Length
The pipe is cut into required lengths using precision cutting equipment to ensure consistency.

7. End Finishing
Pipe ends can be left plain or processed with beveled or threaded ends, depending on how the pipe will be installed.

8. Inspection and Testing
Each pipe is checked for appearance and dimensions, with tests such as ultrasonic or hydrostatic testing used to confirm quality.

9. Packaging and Delivery
Finished pipes are bundled, wrapped, and protected to avoid damage during transport and storage.

How to maintain stainless steel welded pipes?

Stainless steel pipes often require welding during use. To ensure the performance of stainless steel pipes, protective measures are necessary, especially in special working environments. Today, we will discuss how to implement protective measures during the welding process of stainless steel pipes:
1. Stainless steel pipes should be stored in a dedicated warehouse and should not be mixed with carbon steel pipes. They should be protected from moisture, impact, and contamination and damage from ferrous metals.
2. Welding of stainless steel pipes should be done in a dedicated workshop or on a wooden platform to avoid direct contact between stainless steel and carbon steel platforms.
3. The steel wire ropes used for lifting stainless steel pipes should have special protection to avoid direct contact between stainless steel and carbon steel wire ropes.
4. Welders are required to use stainless steel wire brushes and slag hammers. Grinding wheels used on stainless steel pipes are prohibited from being mixed with those used on carbon steel pipes.
5. Dedicated tools for processing stainless steel should not be mixed with those used for carbon steel.
6. Do not use steel needles or paint for marking on stainless steel pipes; markings can be made with ammonia-free ink. Some damage caused during the processing of stainless steel pipes, such as welding spatter, arc burns, smoke stains, and mechanical scratches, must be thoroughly removed (after rough grinding with a grinding wheel, then polishing).