How MS pipes are manufactured: seamless vs welded processes

Mild steel (MS) pipes are essential across construction, industrial, and domestic applications. Two principal manufacturing routes dominate the market: seamless and welded. Each method produces pipes with different mechanical properties, tolerances, cost profiles, and best-use cases. This article explains how each type is made, their differences, and when to choose one over the other.

Seamless pipe manufacturing — a continuous metal body

Seamless pipes are produced from a solid round billet and have no welded seam. The key processes are:

1. Billet preparation

   Workers cut steel billets (solid cylindrical pieces) to length and heat them in a furnace to high temperatures to make them workable.

2. Piercing (Mannesmann or extrusion)

   A rotary piercing mill (Mannesmann process) pierces the heated billet through the center, or extrusion pushes it through a die. This creates a hollow shell.

3. Elongation and rolling
   The hollow shell is elongated and its wall thickness reduced using rolling mills or pilger mills. Pilgering (cold or warm) produces very accurate wall thickness and diameter control for smaller sizes.

4. Sizing and straightening

   Sizing mills or stretch reducers shape the pipe to its final dimensions, and straightening machines then align it.

5. Heat treatment and finishing

   Manufacturers may normalize, anneal, or quench the pipes to achieve the required mechanical properties. They then trim and test them — performing dimensional checks and ultrasonic or eddy-current inspections — and apply surface finishes such as black coating, paint, or galvanization as required.

Strengths of seamless pipes

• Uniform metallurgical structure and no seam → better for high-pressure applications.

• Better resistance to fatigue and impact because there’s no weld HAZ (heat-affected zone).

• Good concentricity and dimensional tolerances (especially pilgered pipes).

Limitations

• More expensive to produce, especially at larger diameters.

• Manufacturing large-diameter seamless pipes is technically complex and costlier than welded solutions.

Welded pipe manufacturing — joining rolled steel

Welded pipes are formed from flat steel plates or coils that are rolled into a tube and joined along a seam. Common welded processes include ERW (Electric Resistance Welding) for smaller diameters and SAW (Submerged Arc Welding) for larger ones (LSAW/SSAW variations for longitudinal or spiral seam).

Typical welded-pipe steps:

1. Raw material (coil or plate)
   Steel coils or plates are uncoiled, leveled, and cut to the required width.

2. Forming
   The strip is bent or formed into a cylindrical shape using forming rolls. For ERW, the edges meet closely for a butt weld; for spiral SAW, the plate is spiral-formed.

3. Welding

   • ERW uses electric resistance to heat and forge the tube edges together—fast and suited to small/medium diameters.

   • SAW (LSAW/SSAW) uses submerged arc welding for thicker walls and large diameters; it provides deep, strong welds but requires more post-weld work.

4. Post-weld treatment

   Manufacturers often normalize, heat-treat, or flash-trim the weld seam, and then inspect the welds using ultrasonic or radiographic testing to detect defects.

5. Sizing, testing and finishing
   Similar to seamless pipes: sizing mills, inspection (hydrostatic tests, NDT), and surface finishing.

Strengths of welded pipes

• Cost-effective, especially for large diameters and long lengths.

• Very flexible — easy to manufacture from widely available coils or plates.

• Modern welding and inspection give high, reliable quality for many applications.

Limitations

• Presence of a weld seam introduces a potential weakness or corrosion path if not properly executed and treated.

• Slightly less uniform grain structure across the seam compared to seamless pipe.

Which to choose — guidance by application

• Engineers usually prefer seamless pipes for high-pressure, critical, or rotating equipment (boilers, high-pressure fluid lines, certain mechanical shafts) because of their uniformity and absence of a seam.

• Industries commonly use welded pipes (ERW/SAW) for large-diameter pipelines, structural tubing, water mains, general plumbing, and economical bulk applications because they are cost-efficient and readily available.

• In corrosive environments or applications needing frequent inspection, weld execution quality and protective coatings determine suitability, and users can employ both types if they are properly treated.

Final thoughts

Seamless and welded MS pipes each have clear roles. Seamless pipes offer structural integrity and superior mechanical performance for demanding service; welded pipes offer economy, size flexibility, and scalability. Choosing the right pipe requires balancing mechanical needs, diameter/wall requirements, inspection capabilities, and budget. Always check applicable standards (IS, ASTM, or project specs), insist on proper NDT and mill reports, and match the pipe type to the service conditions for a safe, cost-effective outcome.