How Seamless Stainless Steel Tubing is Made

Seamless stainless steel tubing is made using a completely different process compared to welded tubing - there is no weld seam. Instead, the tube is formed from a solid billet of stainless steel that is pierced, elongated, and finished into the final tube shape.

Here's a detailed breakdown of how seamless stainless steel tubing is made:

Raw Material: Stainless Steel Billet

A billet is a solid, cylindrical piece of stainless steel (like SUS304, SUS316, etc.).

It's usually hot-rolled from ingots or slabs and cut to the desired length for piercing.

Step 1: Heating

The billet is heated in a rotary hearth furnace or induction furnace to ~1200–1300°C.

This softens the metal for piercing and forming.

Step 2: Piercing (Mannesmann Process)

Mannesmann piercing is the key process for making seamless tube.

The billet is rotated and compressed between two angular rollers with a piercing mandrel at the center.

This creates a hollow center, forming a rough "mother tube" or hollow shell.

The piercing relies on rotational shear forces and internal stress concentration to crack the billet center and form a hollow tube without seams.

Step 3: Elongation / Tube Rolling

The pierced hollow is elongated to the desired wall thickness and diameter using one of the following:

Plug Mill

A plug (internal tool) and a series of rollers reduce wall thickness and elongate the tube.

Pilger Mill

The hollow is passed through oscillating dies while being reduced in diameter and wall thickness.

Mandrel Mill

A long mandrel bar is inserted inside the hollow.

The tube is rolled over it using several stands to stretch and thin it.

Step 4: Sizing / Reducing

The tube is passed through sizing mills or stretch-reducing mills to:

Improve dimensional precision.

Achieve the final outside diameter.

Smooth out any surface irregularities.

Step 5: Cooling

The hot tube is cooled using water spray or air before moving to finishing processes.

Step 6: Straightening and Descaling

Tube is passed through straightening machines.

Surface descaling (removal of oxide layer) via:

Acid pickling (chemical cleaning).

Shot blasting or mechanical polishing.

Step 7: Inspection and NDT (Non-Destructive Testing)

Eddy current testing, ultrasonic testing, and visual inspection check:

Surface flaws

Internal defects

Wall thickness uniformity

Straightness and length

Step 8: Cutting and End Finishing

Tubes are cut to length.

End finishing may include:

Chamfering

Threading

Beveling

Deburring

Step 9: Cold Drawing (Optional)

For small-diameter or precision seamless tubes, cold drawing is performed:

The tube is pulled through a die and over a mandrel at room temperature.

Enhances surface finish, tight tolerances, and mechanical properties.

Step 10: Final Inspection and Packaging

Tubes are pressure-tested, marked, and packed.

Testing includes:

Hydrostatic test

Dimensional checks

Surface quality

Applications