Stainless Steel Pipe Manufacturing Process Explained

Stainless steel pipes are manufactured through two primary methods: seamless and welded. Each method produces pipes with different characteristics, cost profiles, and suitability for various applications.

Understanding the manufacturing process helps engineers specify the right product and appreciate the testing and quality controls that ensure pipe integrity. This guide walks through both processes step by step.

Seamless pipes have no weld seam, making them stronger and more suitable for high-pressure applications.

Process Steps:

1. Billet Preparation: Round stainless steel billets (typically 150-300mm diameter) are cut to length and heated to 1200-1300°C in a rotary hearth furnace.

2. Piercing: The heated billet is pierced through its center using a piercing mill (Mannesmann process). A piercing plug and two barrel-shaped rolls create a hollow shell.

3. Elongation: The hollow shell passes through a mandrel mill (or plug mill) where the wall thickness is reduced and the length is increased.

4. Sizing: The pipe passes through a stretch-reducing mill or sizing mill to achieve the final outside diameter.

5. Heat Treatment: Solution annealing at 1050-1100°C followed by rapid water quenching to dissolve carbides and restore corrosion resistance.

6. Straightening: Rotary straightening to meet straightness tolerances.

7. Cutting & Finishing: Cut to ordered length, end-faced, and beveled if required.

Welded pipes are more cost-effective and available in larger diameters.

ERW (Electric Resistance Welded) Process:

1. Stainless steel coil/strip is uncoiled and leveled

2. Strip edges are trimmed to precise width

3. Strip is progressively formed into a round shape using forming rolls

4. Edges are heated by high-frequency induction and pressed together (no filler metal)

5. Weld flash is removed (ID and OD scarfing)

6. Heat treatment of weld zone

7. Sizing and straightening

TIG Welded (GTAW) Process:

Used for high-quality tubing (ASTM A269/A270):

1. Strip is formed into tube shape

2. Edges are TIG welded using argon shielding gas

3. Produces smooth, high-integrity weld with excellent corrosion resistance

4. Preferred for pharmaceutical, sanitary, and instrumentation tubing

SAW (Submerged Arc Welded) Process:

For large diameter pipes (typically >16" OD):

1. Plate is formed into pipe by UOE or JCOE process

2. Double submerged arc welded (inside + outside seams)

3. Expanded to final diameter

4. Used for ASTM A358 / ASTM A312 large-bore applications

Heat treatment is critical for stainless steel pipes:

Solution Annealing:

Why it matters: During manufacturing and welding, chromium can combine with carbon to form chromium carbides at grain boundaries (sensitization). This depletes the surrounding matrix of chromium, creating zones susceptible to intergranular corrosion. Solution annealing reverses this.

Low-Carbon Grades (304L, 316L): Contain ≤0.03% carbon, significantly reducing sensitization risk. This is why 316L is preferred over 316 for welded applications.

Every stainless steel pipe undergoes rigorous testing:

Mandatory Tests:

Common Supplementary Tests:

Dimensional Inspection:

Starlight Tubes performs all required tests and provides EN 10204 Type 3.1 Mill Test Certificates with every delivery.