Two Routes to the Same Strength Class
L360 in ISO 3183 and GB/T 9711, equivalent to X52 in API 5L, can be delivered either as seamless pipe or as welded pipe. The mechanical property requirements are the same on paper, but the way the pipe is made is fundamentally different, and that difference is visible in the dimensional accuracy, the surface, the weld and the range of sizes that can be supplied. Understanding the two routes makes it much easier to write a specification that matches the application instead of paying for a process the line does not need.
Raw Material
Seamless pipe starts as a solid round bloom produced by continuous casting to tube stock requirements such as YB/T 4149. Because the bloom is rolled into a tube rather than joined, the steel must be clean, free from centre segregation and properly conditioned on the surface before it enters the furnace. Welded pipe starts as flat product. Longitudinal welded pipe of the high frequency resistance welded type is made from hot rolled coil, while longitudinal submerged arc welded pipe made by the UOE or JCOE route uses plate. Spiral welded pipe is made from hot rolled strip of carbon or low alloy structural steel, coiled to the required diameter at a helix angle.
Forming Without a Weld and Forming With One
L360 seamless pipe is formed by hot rolling, cold rolling or cold drawing. In the hot route the bloom is heated to roughly 1100 to 1250 Celsius, pierced into a hollow shell, elongated on a mandrel or plug mill and then sized or reduced to the ordered outside diameter. In the cold route a processed hollow is pickled and then reduced at room temperature by cold pilger rolling or cold drawing, which produces the tightest wall tolerances and the smoothest surface. No weld exists anywhere in the product, and no filler metal is consumed.
Welded pipe is formed by shaping flat product into a cylinder and joining the edges. In high frequency resistance welding the edges are heated by the skin and proximity effects of a high frequency current and forged together without filler metal. In submerged arc welding an arc burns under a layer of flux and filler wire is added, with the weld made in one or two passes from the inside and the outside. In spiral welded pipe the strip is fed at an angle and welded inside and outside along a helical seam, which allows a narrow strip to produce a large diameter pipe.
Comparison of the Two Products
| Item | L360 seamless pipe | Welded pipe |
|---|---|---|
| Starting material | Continuously cast round bloom | Hot rolled coil or plate, or strip |
| Forming method | Piercing and rolling, hot or cold | Cold forming then welding of the edges |
| Seam | None | One longitudinal seam, or one spiral seam |
| Dimensional accuracy | High, particularly in the cold finished condition | Good, but the weld zone can show local deviation |
| Surface | Smooth, with possible rolling or finishing marks | Visible weld bead or seam on the surface unless dressed |
| Typical size range | Small to medium outside diameter | Medium to very large outside diameter |
| Roundness and straightness | Generally better in the seamless product | Ovality becomes more noticeable as diameter increases |
| Heat affected zone | Not present | Present along the seam and verified separately by test |
| Relative cost at equal size | Higher | Lower, because the forming route is shorter |
Where Each Product Fits
Seamless L360 pipe is normally selected for high pressure and high temperature duties, for sour service where hardness limits must be met across the whole wall, for refinery and boiler tubing, and for instrumentation and small diameter lines that welded mills cannot produce economically. Welded L360 pipe is selected for long distance transmission of oil, gas and water where large diameter and long lengths matter more than the absence of a seam. Spiral welded pipe in particular gives the lowest cost per tonne for large diameter, moderate pressure water and gas lines, while longitudinal submerged arc welded pipe is the usual choice for high pressure cross country transmission at large diameter.
FAQ
Q: Is L360 seamless pipe stronger than L360 welded pipe?
No. Both must meet the same minimum yield and tensile strength requirements for the grade. The difference lies in the manufacturing route, the dimensional consistency and the presence or absence of a weld, not in the nominal strength class.
Q: Why does welded pipe usually cost less?
The route is shorter and less energy intensive. Flat product is formed and joined rather than pierced and rolled from a solid bloom, and a narrow strip or plate can be turned into a large diameter pipe, which reduces the amount of hot deformation required.
Q: Can welded pipe be used at high pressure?
Yes, provided the weld is made to a qualified procedure and accepted by the inspection requirements of the standard. Longitudinal submerged arc welded pipe is widely used in high pressure transmission service.
Q: Are the weld and heat affected zone tested separately?
They are. Impact, tensile and hardness tests are taken across the weld and the heat affected zone as well as from the pipe body, and the acceptance criteria are stated in the governing standard and the purchase specification.
Q: Which product should be chosen for sour service?
Seamless pipe is often preferred because there is no weld seam or heat affected zone with a locally different hardness. Where welded pipe is used, the hardness survey across the weld and the heat affected zone becomes a critical acceptance item.
Q: Does the forming route change the heat treatment requirement?
It can. Cold finished seamless pipe normally needs annealing or normalising after the final cold pass, while welded pipe may be supplied as rolled, normalised or in a quenched and tempered condition depending on the toughness and hardness values the order requires.
