How is the extrusion processing of seamless stainless steel pipes implemented?

How Are Seamless Stainless Steel Pipes Produced by Extrusion?

Seamless stainless steel pipes can be manufactured using the hot extrusion process, where a heated stainless steel billet is pierced and forced through an extrusion die under high pressure to form a hollow tube. This method is especially suitable for thick-wall pipes, special alloys, and complex cross-section tubes, offering flexible production and stable product quality.

What Is the Extrusion Process for Seamless Stainless Steel Pipes?

Extrusion is an important manufacturing method for seamless stainless steel tubes, particularly for small-batch, multi-specification, and special-shaped pipe production.

During the extrusion process, a heated billet is subjected to extremely high pressure inside an extrusion press, forcing the metal through a die to form a pipe with the required diameter and wall thickness.

Modern improvements in vertical hydraulic piercing equipment allow the wall thickness deviation of extruded pipes to be controlled within 5%–7%, significantly improving dimensional accuracy.

Typical Extrusion Production Process of Seamless Stainless Steel Pipes

The production of extruded seamless stainless steel pipes generally involves the following steps.

1. Billet Preparation

The raw materials are typically stainless steel billets, including:

Continuous casting billets

Forged billets

Using continuous casting billets allows manufacturers to maintain stable chemical composition and consistent mechanical properties.

2. Billet Heating

The billet is heated in a furnace to improve metal plasticity and ensure proper deformation during extrusion.

Proper heating temperature is critical for:

preventing cracking

improving forming efficiency

ensuring uniform microstructure

3. Hydraulic Piercing

A vertical hydraulic piercing mill is used to create a hollow center in the billet.

This process prepares the billet for extrusion and helps control wall thickness uniformity.

4. Extrusion Forming

The hollow billet is then pushed through a high-strength extrusion die under extremely high pressure.

For example:

A 50 MN (5000-ton) extrusion press can produce stainless steel pipes with diameters ranging from 25 mm to 245 mm.

Extrusion dies are relatively easy to design and replace, which makes this process ideal for multi-specification pipe production.

5. Finishing and Inspection

After extrusion, pipes undergo several finishing processes:

straightening

dimensional inspection

surface inspection

heat treatment (if required)

These steps ensure compliance with industrial quality standards.

Advantages of Extrusion for Stainless Steel Pipes

Extrusion offers several technical advantages compared with conventional rolling processes.

1. Flexible Production

Extrusion dies can be quickly replaced, making the process suitable for:

small-batch orders

customized pipe sizes

multi-specification production

2. Ability to Produce Special-Shaped Pipes

Extrusion can manufacture irregular cross-section pipes, including:

square tubes

rectangular tubes

complex alloy pipes

These shapes are difficult to produce using traditional rolling mills.

3. Direct Use of Continuous Casting Billets

Extrusion allows the direct use of continuous casting billets, reducing intermediate processing steps and ensuring stable product quality.

4. Suitable for High-Alloy Materials

Extrusion is particularly suitable for stainless steel and high-alloy steel pipes, which are more difficult to process using conventional rolling methods.

Main Limitation of the Extrusion Process

The primary drawback of extrusion-produced seamless stainless steel pipes is relatively low material yield.

Compared with some rolling processes, extrusion may result in higher material loss during billet preparation and forming.

To address this issue, many manufacturers have introduced improved processing techniques.

Process Optimization Methods of Seamless Stainless Steel Pipes

To improve production efficiency and yield, manufacturers commonly use optimized production routes.

- Small-Diameter Pipes

For small pipes:

solid billets are directly pierced using a vertical hydraulic piercing machine

then extruded into pipes.

- Medium-Diameter Pipes

For medium-sized pipes:

billets are pre-drilled with small holes

then extruded.

This reduces forming resistance and improves material utilization.

- Large-Diameter Pipes

For large pipes:

billets are pre-drilled with large holes

then directly extruded by the extrusion press.

This approach improves forming stability and reduces production defects.

Extrusion vs Rolling: Key Differences

Extrusion is therefore widely used for specialized stainless steel pipes and high-performance alloy tubes.

Surface Quality and Post-Processing of Seamless Stainless Steel Pipes

Extruded pipes, as well as pipes produced through cold drawing or cold rolling, may initially be semi-finished products.

Common surface issues include:

uneven wall thickness

surface dents

scratches

pits or spots

Additional processing such as:

grinding

polishing

dimensional correction

may be required to meet customer specifications.

Common Standards for Seamless Stainless Steel Pipes

Extruded stainless steel pipes are often manufactured according to international standards, including:

ASTM A312 – Seamless stainless steel pipes for high-temperature service

ASTM A213 – Seamless alloy and stainless steel boiler tubes

ASTM A269 – Seamless stainless steel tubes for general service

EN 10216-5 – Seamless stainless steel tubes for pressure purposes

Compliance with these standards ensures quality, safety, and international compatibility.

FAQ

1. What is the extrusion process for seamless stainless steel pipes?

Extrusion is a manufacturing method in which a heated stainless steel billet is forced through a die under high pressure, forming a seamless pipe with the required dimensions.

2. What sizes of stainless steel pipes can be produced by extrusion?

Large extrusion presses (such as 50 MN units) can produce pipes with diameters ranging from 25 mm to 245 mm, depending on equipment capacity.

3. Why is extrusion used for stainless steel pipe manufacturing?

Extrusion is ideal for thick-wall pipes, high-alloy materials, and complex cross-section tubes, offering flexible production and high product quality.

4. What is the main disadvantage of extrusion?

The main limitation is lower material yield compared with some rolling processes, which may increase production costs.

5. What industries use extruded stainless steel pipes?

Extruded stainless steel pipes are widely used in:

petrochemical industry

power plants

aerospace applications

high-pressure equipment manufacturing