How Seamless Stainless Steel Tubing Is Made

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What is a Seamless Stainless Steel Tube? What is its specialty?

Seamless tubes and welded tubes are the two most in-demand and highly used types of tubes available in the market. While welded tubes as the name suggest are welded to form a joint, seamless pipe or tubes have no weld in the process of manufacturing them. Seamless SS tubes are forged from a single piece of billet that runs through a die. Thus, unlike welded pipes, seamless pipes do not have a visible welded joint mark on the tube. 

We will get into the detailed manufacturing process later in the blog. 

Firstly, let us have a look at a categorized chart of applications that demand seamless Stainless Steel 309 Seamless Tubes.

1. Hydrogen Fuel Cells:

Hydrogen-powered cells and battery material

2. Aerospace and defense equipment:

  • Hydraulic systems
  • High-pressure fuel lines

3. Heat Exchangers:

Coil-wound components

4. Shipbuilding and marine:

  • High-pressure fuel lines
  • Hydraulic applications

5. Oil and Gas:

  • Gas transfer
  • CNG and LPG transport
  • Control lines
  • Safety valves

6. Chemical Industry:

  • Fluid  transfer
  • Gas transfer 
  • Heat trace bundles

7. Architecture and Construction:

  • Handrails
  • Architectural tubing
  • Decorative tubing

The Manufacturing Process / Making of Seamless Stainless Steel pipes:

The traditional method of manufacturing Seamless tubes is by extruding, drilling, and piercing. However, these methods fail to provide accurate and symmetric outer diameters throughout the tube. 

Lubricants are used on the inner diameter (ID) and outer diameter (OD) of the tube while reducing it. The seamless pipes are annealed i.e., treated after the process of forging. Cold reducing has proven to be the most effective forging method. 

Let us have a look at the main types of Drawing methods. 

1. Floating Plug method:

In this process, the tube is forged by pulling it through a conical die and a conical-shaped plug is inserted into the inner diameter (ID) of the tube. The size of the conical plug inserted determines the thickness and dimensions of the tube. However, tubes forged using this method can only be used for light wall application. 

2. Sink Draw Method:

The sink Drawing method of forging a seamless pipe also involves a conical plug that determines the dimensions and thickness of the tube. The thickness of the tube is dependant on the size of the conical plug. However, this method of drawing a tube produces heavy-duty seamless pipes for large-scale applications. 

The Annealing process:

The annealing of the seamless tube takes place after the welding or in this case, the drawing process. Annealing the tubes after welding is extremely necessary as it relieves the tubes of the stress incurred during the process of drawing. Annealing also ensures retention of maximum chemical and physical properties along with maximum corrosion resistance. 

Annealing is carried out between cold working procedures to soften the material as it hardens. The tube is annealed to induce ductility and then worked on again. 

Tests conducted:

Tests performed to ensure the quality of the tubes drawn includes:

  • Material flaw test
  • Mechanical property test
  • Specific hard spot tests, etc