ERW steel pipe and seamless steel pipe are the two most common types of steel pipe.
Many people face the question when purchasing or designing projects: What are the differences between these two types of steel pipe? They each have their own unique characteristics in terms of manufacturing process, pressure resistance, application areas, and cost.
Understanding these differences can help engineers and purchasers select the most appropriate product based on their specific needs.
I. Difference Between ERW and Seamless Steel Pipe
| Dimension | ERW Steel Pipe | Seamless Steel Pipe |
|---|---|---|
| 1. Raw Material | Hot-rolled steel coil or strip | Solid round billet |
| 2. Forming Process | Rolled into round shape and welded by high-frequency resistance | Billet is pierced into hollow tube, then hot-rolled or cold-drawn |
| 3. Structural Feature | Has a longitudinal weld seam | No weld seam, structure more uniform |
| 4. Pressure Capacity | Adequate for medium and low pressure; failure may start from weld | Higher pressure resistance; fracture usually occurs in base metal |
| 5. Corrosion Resistance | Weld zone more vulnerable; corrosion rate higher in H₂S or harsh media | More uniform microstructure; lower corrosion rate, longer service in harsh media |
| 6. Production Efficiency & Cost | High production efficiency, low cost, suitable for mass production | More complex process, higher cost, usually for critical applications |
| 7. Application Scope | Construction, low/medium pressure pipelines, water supply, structural uses | Oil & gas, boilers, petrochemicals, offshore, high-pressure and high-temperature systems |
| 8. Service Life & Maintenance Cost | Suitable for mid-to-low pressure environments, but weld seam may be weak point requiring monitoring and anti-corrosion treatment | Longer overall service life in high-pressure or corrosive environments; lower long-term maintenance and replacement costs |
II. ERW steel pipe and seamless steel pipe manufacturing process
i. The manufacturing process flow of ERW steel pipes
(1) Raw material preparation
Select steel strips or coil plates and cut them into appropriate widths and lengths as required.
(2) Unrolling and leveling
The steel strip is unrolled from the coil and its flatness is corrected by a screed to remove bending.
(3) Forming
The steel strip is rolled into a round pipe blank by the forming roller conveyor.
(4) Welding
High-frequency resistance welding (HF-ERW) is used to weld the pipe seams to form continuous steel pipes.
(5) Weld seam finishing and straightening
Remove burrs from the weld seam, smooth the surface, and correct the straightness of the pipe through a straightening machine.
(6) Cutting and inspection
Cut to the required length and conduct quality inspections such as dimensions, welds and surface defects.
(7) Surface treatment and packaging
After anti-corrosion treatment (such as painting, 3PE coating, epoxy coating), it is packed and sent out of the factory.
ii. Seamless steel pipe manufacturing process
Method A: Hot rolled seamless steel pipe
(1) Raw material preparation
Select round steel billet (solid steel bar) and remove surface oxide scale and defects.
(2) Heating
Heat the steel billet to a high temperature (about 1200°C) to improve plasticity.
(3) Perforation
Use a perforator to punch a hole in the center of the steel billet to form a hollow tube billet.
(4) Rolling
The hot-rolled tube billet is stretched and expanded by the rolling mill to reach the required diameter and wall thickness.
(5) Sizing and straightening
Adjust the size and straightness by the sizing mill and straightening mill.
(6) Heat treatment (optional)
Normalizing or annealing is performed as required to improve mechanical properties.
(7) Inspection and surface treatment
Inspect the size, appearance, and mechanical properties, and perform anti-corrosion treatment if necessary.
Method B: Cold-drawn seamless pipe
(1) Hot rolling or hot expansion to form a blank tube
The hot-rolled blank tube after perforation is used as the raw tube.
(2) Cold Drawing
Drawing is performed in a cold state using a drawing machine to precisely control dimensions and surface finish.
(3) Straightening and Annealing
After cold drawing, annealing may be performed to eliminate internal stresses, followed by straightening.
(4) Inspection and Packaging
Dimensions, wall thickness, mechanical properties, and surface quality are inspected, and the product is packaged for shipment.
III. Material comparison between ERW steel pipe and seamless steel pipe
i. Microstructural Differences Affect Performance
The weld zone of ERW steel pipes presents columnar grains, which are slightly larger than those of the base material. This results in the impact toughness of the weld zone being approximately 15 to 25% lower than that of the overall material.
Seamless steel pipes have uniform overall grains and a stable ferrite + pearlite structure, which keeps their impact toughness between 60 and 100J, making the material more reliable as a whole.
Summarize: If the pipes are used in low-temperature or shock environments, seamless pipes can offer higher safety.
ii. Mechanical Property Comparison
ERW X60 steel pipe has a yield strength of up to 415 MPa, but stress concentration in the weld limits fatigue resistance.
Although seamless pipe has a slightly lower nominal yield strength (e.g., ASTM A106 Grade B, 240 MPa), its cycle life is 3-5 times that of ERW pipe and it lacks weld residual stress.
Summarize: In high-pressure or frequently loaded environments, seamless pipe offers better fatigue resistance, resulting in longer pipe life and lower maintenance costs.
IV. ERW Steel Pipe vs. Seamless Steel Pipe - Performance Comparison
i. Pressure-bearing capacity
In the standard hydraulic test (DNV OS-F101), the burst pressure of the Φ323.9×12.7mm ERW pipe was 38.7MPa, while the seamless pipe of the same specification could withstand 42.5MPa.
ERW pipe rupture usually starts from the weld seam, and the rupture of seamless pipes is a 45° shear fracture.
The ASME B31.8 standard stipulates that the welding coefficient of ERW pipes is 0.8 and that of seamless pipes is 1.0.
Summary: If the pipeline needs to withstand high pressure, seamless pipes offer a higher safety margin and are suitable for critical transportation and high-pressure environments.
ii. Corrosion resistance
The corrosion rate of the weld zone of ERW pipes in the medium containing H₂S is 20 to 40% higher than that of the base material, making it prone to become a weak point.
The annual corrosion rate of seamless pipes can be controlled within 0.05mm/a under the same environment.
Summary: In offshore platforms, oil and gas risers or corrosive environments, seamless pipes can achieve a design life of over 30 years, reducing maintenance and replacement costs.
V. Application areas of ERW steel pipes and seamless steel pipes
i. ERW Steel Pipe Applications
Municipal Engineering: Water supply and drainage pipelines, gas pipelines
Low-pressure fluid transportation: low-pressure oil and natural gas transmission
Building Structures: steel structures, scaffolding, support pipes
Machinery Manufacturing: automotive parts, bicycle racks, furniture pipes
Advantages: High production efficiency, low cost, suitable for high-volume, low- and medium-pressure applications
ii. Applications of Seamless Steel Pipes
High-pressure transportation: long-distance oil and natural gas pipelines, high-pressure boiler tubes
Energy industry: nuclear power and thermal power plant pipelines
Petrochemical industry: cracking units, refineries, and chemical equipment
Military and aerospace: components requiring extremely high strength and safety
Mechanical equipment: hydraulic cylinders, bearing sleeves, and precision machinery parts
Advantages: No welds, uniform strength, resistance to high pressure, high temperature, and corrosion, and extended life
Summary:
ERW steel pipe: Suitable for low-pressure fluids and general structural applications, offering excellent economical performance.
Seamless steel pipe: Suitable for high-pressure, high-temperature, and critical safety projects, offering high reliability.