Differences and Applications of Seamless Pipe and Welded Pipes

Based on different manufacturing processes, steel pipes are mainly divided into two categories: seamless steel pipes and welded pipes. Although they both appear to be cylindrical metal tubes, their internal "genes" and suitable "battlegrounds" are quite different.

Understanding them: Different "origins" determine different "constitutions" To understand the differences between seamless steel pipes and welded pipes, we must start with their "birth." This manufacturing process is like a unique mark imprinted on them.

Seamless Steel Pipes:

Imagine you holding a complete metal cylinder (steel billet). Manufacturing seamless steel pipes is like playing a magical "piercing" game. Technicians will take a solid steel billet heated to a high temperature and use a sharp punch (professionally called a piercing head) to "poke" it through the center, just like removing the core from an apple.

After this step, the solid steel billet becomes a thick-walled hollow tube with a hole in the center. But that's not all. This hollow tube still needs to be processed while it's still hot, undergoing a series of rolling, drawing, or extrusion processes to gradually shape its outer diameter and wall thickness to the required dimensions.

The characteristic of seamless steel pipes is that the entire pipe body is continuous, without any welded joints. It's a solid pipe from the start, then it's "hollowed out" and "shaped." This "one-step" manufacturing method gives seamless pipes their unique "constitution."

Welded Pipes:

Welded pipes are produced in a completely different way. They are usually made from steel plates or strips. Imagine you're slowly rolling up a long steel strip like a paper roll, bringing its two edges together. After they're together, these two edges need to be "stitched" together; this "stitching" process is welding.

Depending on the method of rolling into the tube blank and the welding method, welded pipes can be further subdivided into many types, such as:

a. Straight seam welded pipe: The steel strip is rolled into a cylinder, and then the two opposite edges are welded together along a direction parallel to the pipe's axis. Welding methods include high-frequency welding (ERW pipe) and submerged arc welding (LSAW pipe). The length of straight seam welded pipe is usually limited by the length of the steel strip.

b. Spiral welded pipe (SSAW): The steel strip is spirally rolled up and welded simultaneously. The weld seam extends spirally along the pipe body. This method is particularly suitable for manufacturing large-diameter steel pipes, and theoretically, the length is unlimited (as long as the steel strip is long enough).

The characteristic of welded pipe is: there is a single or spiral welded seam on the pipe body. It is a flexible range, formed by rolling and welding flat material. This "stitch" is the most fundamental difference between it and seamless pipe.

Essential Difference Revealed: Seamless vs. Welded – A Performance Showdown

It is the presence or absence of this weld seam that leads to significant differences in performance between seamless steel pipes and welded pipes. Understanding these differences is crucial to understanding their respective advantages and limitations.

1. Pressure Bearing Capacity: Seamless pipes are generally superior

Even with high-quality welding, the weld seam is theoretically still a potential weak point in the pipe body. Under high internal pressure, the pipe wall experiences circumferential stress, and the stress distribution and material properties in the weld area may differ from those of the base material.

In contrast, seamless steel pipes, due to the uniform and continuous material throughout the pipe body and the absence of the uncertainties introduced by welds, typically have a higher capacity to withstand internal pressure than welded pipes of the same material, wall thickness, and diameter. Therefore, seamless steel pipes are often the preferred choice for applications transporting high-pressure fluids (such as high-pressure natural gas, steam, and hydraulic oil) because they offer a higher safety margin.

2. Wall Thickness Uniformity and Eccentricity: Different Emphases

During the piercing and rolling process of seamless steel pipes, it is sometimes difficult to completely guarantee the uniformity of the wall thickness along the circumference, easily resulting in eccentricity (i.e., the difference between the thickest and thinnest points of the pipe wall). Although modern processes have greatly improved this, eccentricity remains a concern for seamless pipes.

The wall thickness of welded pipes mainly depends on the thickness of the raw material steel plate or strip. Steel strips typically have higher thickness uniformity. Therefore, aside from potential localized thickening or thinning in the weld area, the wall thickness uniformity of the base material in welded pipes is likely better than that of seamless pipes. However, the quality and geometry of the weld itself (the degree of internal and external burr removal) will affect the local wall thickness.

3. Size Range and Flexibility: Welded Pipes Have an Advantage

Manufacturing large-diameter, thin-walled seamless steel pipes is technically and costly. Seamless pipe production is typically concentrated in the small-to-medium diameter and medium-to-thick-walled range.

Welded pipes, on the other hand, are very flexible and particularly suitable for manufacturing large-diameter steel pipes. Whether using straight seam welding or spiral welding, relatively thin steel plates can be rolled into large-diameter pipes. Furthermore, the wall thickness selection for welded pipes is more flexible, easily achievable by selecting steel plates of different thicknesses.

4. Cost Considerations: Welded Pipes Generally Offer Better Cost-Effectiveness

Overall, for the same material and outer diameter, the manufacturing process for welded pipes is relatively simpler and more efficient, and the cost of raw materials (steel plates/strips) is usually lower than that of steel billets used for seamless pipes. Therefore, welded pipes are generally cheaper to manufacture than seamless steel pipes, especially in large-diameter and ordinary pressure applications, where welded pipes have a significant cost advantage.

Seamless Pipes vs. Welded Pipes: Application Differences

Seamless Steel Pipes' "Home Ground": High Pressure, Critical Applications, and Precision

Seamless steel pipes, with their seamless construction and high overall strength, are the preferred choice for the following applications:

1. High-Pressure Fluid Transportation: Oil, natural gas, high-pressure steam, boiler tubes, hydraulic system pipelines, etc.—in applications requiring extremely high pressure resistance, seamless pipes are the first choice for ensuring safety.
2. Critical Structural Components: Mechanical structural components, shafts, cylinder barrels, etc., requiring uniform materials and reliable performance.
3. Extreme Environment Applications: In fields such as nuclear power and aerospace, where material reliability is paramount.
4. Small Precision Tubes: Small tubing used in some precision instruments and medical equipment is often also seamless pipe.

Seamless steel pipes can be understood as a reliable "stabilizing force" in any application with stringent requirements for safety and reliability, needing to withstand high internal stress, or serving as critical mechanical components.

Welded Pipes' "Home Ground": Versatility, Economy, and Large-Scale Applications

1. Welded pipes, with their flexible dimensions, lower costs, and continuously improving quality, shine in the following applications:
2. Medium and Low Pressure Fluid Transportation: Water pipes, drainage pipes, low-pressure gas pipes, heating pipes, etc.

3. General Structural Applications: Beams, columns, scaffolding, railings, greenhouse frames, etc. in building structures.
4. Large-Diameter Pipeline Projects: Long-distance oil, gas, and water pipelines (especially submerged arc welded (LSAW/SSAW) pipes). These projects have significant requirements for pipe diameter, and welded pipes offer substantial advantages in terms of economy and manufacturing convenience.

5. General Machinery and Equipment Manufacturing: General equipment frames, exhaust pipes, etc., where pressure resistance requirements are not high.
6. Section Steel or Special-Shaped Pipes: Various square, rectangular, or other shaped steel pipes can be easily manufactured through rolling and welding.

In short, welded pipes are an efficient and economical "all-rounder" in situations requiring large quantities of steel pipes, where cost is a concern, or where large-diameter pipes are needed, and where pressure resistance requirements are low to medium.

Read more: ERW pipe vs Seamless pipe Cost or Seamless vs. Welded Tubing