Difference between seamless and welded pipe

Seamless steel pipe (SMLS) is formed by drawing a solid billet over a piercing rod to create the hollow shell, without welding or seam.It is suitable for bending and flanging. The most advantage is increasing the ability of withstanding higher pressure.Seamless steel pipe can be cut,threaded or grooved.And the coating method includes black/ red lacquer,varnish painting,hot dip galvanization,etc.

Welded pipe (steel pipe manufactured with a weld) is a tubular product made out of flat plates, known as skelp, that are formed, bent and prepared for welding.
Welded steel pipes are always available in plenty because of their lower production cost and raw materials availability. It is, therefore, one of the most widely used metals in the world. Its application in different industries and the manufacture of numerous products. Examples of ASTM or API 5L welded steel pipes include SSAW pipe, ERW pipe, and LSAW pipe.

Many people confuse the ERW pipe with seamless steel because it does not use fusion metals, the weld seam is not always visible, and you cannot feel it. On the contrary, a Double Submerged Arc Weld (DSAW) leaves evident traces of weld beads, which can be eliminated using different processes depending on the application method.

Welded steel pipes, as opposed to seamless steel pipes, are widely used in several industries because they are cost-effective. They can also be manufactured in different lengths and welded from different tubing shapes. Application is entirely dependent on the use and other related factors.

How is a Welded Steel Pipe Made?
Most welded steel pipes are from steel plates and steel strips. These strips and plates are made by moving steel loops and coils with a proper thickness gauge to form a leveled material. The flat leveled material is then measured and cut to the desired lengths.

The process starts with bending the steel plate, and then it is welded to the desired shapes, be it a circle, rectangle, square, and many more using a progression of bending machines or rollers. The metallic pipe weld using a powerful energy source. Some pipes can also be welded together with filler material. Most welded steel pipes must pass through the heat treatment process.

The weakest piece of a welded steel pipe is always the joints. The weld joint’s quality determines the quality of a welded pipe—welding with a robot mostly in automated production lines such as in the automobile industry. Alternatively, for stronger steel pipe welds, human labor has proven to be perfect; for example, in aerospace construction.

First of all, they have many differences in manufacturing processes, which are as follows:

Concentricity: The manufacturing process of seamless pipes is to punch a hole in a stainless steel blank at a temperature of 2200°F. This high-temperature tool steel is softened by stamping and drawing and formed into a spiral shape from the hole. This kind of pipe has uneven wall thickness and high eccentricity.

Therefore, the ASTM standard allows the wall thickness difference of seamless pipes to be greater than that of welded pipes. The slotted tube is made of precision cold rolled sheet (4-5 feet) per roll width. The maximum wall thickness of these cold rolled sheets is usually 0.002 inches. The steel plate is cut to a width πd, where d is the outer diameter of the tube. The wall thickness tolerance of the slit tube is small, and the wall thickness is very uniform throughout the circumference.

Welding performance: There is a certain difference in chemical composition between general pipes and seamless pipes. The production of seamless pipe steel composition only meets the basic requirements of ASTM. The steel slotted pipe produced contains a chemical composition suitable for welding. For example, elements such as silicon, sulfur, manganese, oxygen, and triangular ferrite are mixed in a certain proportion, which can produce solder melting and easy transfer of heat during the welding process, so that the entire weld can be penetrated. Steel pipes lacking the above-mentioned chemical composition, such as seamless pipes, will produce a variety of unstable factors during the welding process, and are difficult to weld and impermeable to welding.

Grain size: Generally, the metal grain size is related to the heat treatment temperature and the time to maintain the same temperature. Therefore, the grain size of the annealed slit pipe is the same as that of the seamless pipe. If there is a seam pipe with the least cold treatment, the weld grain size is smaller than the weld metal grain size, otherwise the grain size is the same.

Strength: The strength of the pipeline depends on the alloy composition, so seamless pipes and slotted pipes containing the same alloy and the same heat treatment are essentially the same strength. After tensile test and three-dimensional vibration test, slotted pipe tearing almost only occurs far away from the welding point or heating area. This is because there are fewer impurities in the weld and a slightly higher nitrogen content, so the strength of the weld is better than other areas. However, the joint pipes of the ASME Boiler and Pressure Vessel Association can only withstand 85% of the allowable pressure, mainly because the data collected as early as today has improved the welding equipment. ASME stipulates that 100% of seamed pipes can withstand allowable pressure after ultrasonic testing. Similarly, Europe and Asia also provide seamed pipes to ensure the quality of their welding performance through eddy current testing. Eddy current testing requires the approval of legal procedures and the license held by the organization. Trent eddy current testing is approved by the Swedish Ministry of Electricity. ASME low current loss is based on the quality performance of the slotted tube.

Corrosion resistance: The corrosion resistance also depends on the composition of the alloy. The fully heat-treated seamless pipe has the same chemical composition and the corrosion resistance of the slotted pipe is consistent. ASTM provides supplementary tests to prove that the corrosion resistance of the weld is equal to or better than that of the metal being welded. In an acid chloride environment, insufficient heating at the weld will accelerate the corrosion of the slit pipe, but it is only required for corrosion testing. In fact, the environment is not that bad. Bending and extension: The weld extension specified by ASTM is verified by the following test: bend 45° to 90°, flatten along the weld; then rotate the seam pipe and repeat the above steps to make the inner diameter of the weld bend 180°. In the case of tearing or separation between crystals at 40 times magnification, the weld quality that meets the standard is not allowed. The bending radius of the pipe is controlled by the alloy composition, and the minimum bending radius is generally 2D.

The ideal welding condition is that the weld is in a neutral or compressed state. In addition, the pipeline should be annealed to reduce its hardness and improve bending performance.

The other differences are listed in the below-attached table
Parameter Seamless Pipe Welded Pipe
Strength Able to withstand more pressure and load as there is no weak seam. Due to welding they are believed to withstand 20% less pressure and load as compared to seamless pipe.
Length Relatively shorter in length due to manufacturing difficulties. Can be manufactured in long continuous lengths.
Size Manufactured for nominal size 24 inch or less. No such size restriction on production.
Corrosion Resistance Less prone to corrosion, means more corrosion resistant. Weld areas are more prone to corrosion attacks, means less corrosion resistance.
Surface Quality Rough due to extrusion process Smooth high quality surface as compared to seamless pipe.
Economy Costlier More economic
Production Process Complex, long procurement lead time Simpler, short procurement lead time.
Tests Seamless pipes does not require testing for weld integrity. Must be tested before use.
Application Suitable for high presure temperature and corrosive environment Normally used for less corrosive and low pressure environment.
Availability Less available, limited material types, longer delivery time Readily available for various different materials; shorter delivery time
Wall Thickness Inconsistent wall thickness across length, thicker so heavier Wall thickness more consistent than seamless ones, thinner
Ovality Better ovality, roundness Poor ovality and roundness as compared to seamless counterpart.
Internal surface check Checking not possible Internal surface can be checked before manufacturing

Costs of Welded vs Seamless
Costs of seamless and welded tubing are also related to such properties as strength and durability. Welded tubing’s easier manufacturing process can produce larger diameter tubing with thinner wall sizes for less. Such properties are more difficult to produce in seamless tubing. On the other hand, heavy walls can be achieved more easily with seamless tubing. Seamless tubing is often preferred for heavy wall tubing applications that require or can withstand high pressure or perform in extreme environments.

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