Stainless steel tubing is one of the most versatile metal alloy materials used in manufacturing and fabrication. The two common types of tubing are seamless and welded. Deciding between welded vs. seamless tubing primarily depends on the product's application requirements. In choosing between the two keep in mind that first, the tubing must be compliant with your project specifications and that secondly, it must meet the conditions for which the tubing will ultimately be used.
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Though both the words tube and pipe are often used interchangeably, mainly because both are hollow-shaped, there are important distinctions between the two when determining welded vs. seamless tubing needs. Tubes are measured by the outside diameter (OD) and wall thickness. A pipe, on the other hand, is measured by its inside diameter (ID). Regarding functionality, tubing is generally used in structural and aesthetic applications, whereas piping is used for transporting fluids, liquids, and gases.
Knowing that distinction can also help determine which tubing is best for a given application, whether welded or seamless. The method of manufacturing welded and seamless tubing is evident in their names alone. Seamless tubes are as defined ' they do not have a welded seam. The tubing is manufactured through an extrusion process where the tube is drawn from a solid stainless steel billet and extruded into a hollow form. The billets are first heated and then formed into oblong circular molds hollowed in a piercing mill. While hot, the molds are drawn through a mandrel rod and elongated. The mandrel milling process increases the mold's length by twenty times to form a seamless tube shape. Tubing is further shaped through pilfering, a cold rolling process, or cold drawing.
A welded stainless steel tube is produced through roll-forming strips or sheets of stainless steel into a tube shape, and then the seam is welded longitudinally. Hot-forming or cold-forming processes can accomplish welded tubing. Of the two, cold forming results in smoother finishes and tighter tolerances. However, each method creates a durable, strong steel tube that resists corrosion. The seam can be left beaded or worked by cold rolling and forging. The welded tube can also be drawn similarly to seamless tubing to produce a finer weld seam with better surface finishes and tighter tolerances.
There are benefits and drawbacks in choosing welded vs. seamless tubing.
By definition, seamless tubes are completely homogenous tubes, which give seamless tubing more strength, superior corrosion resistance, and the ability to withstand higher pressure than welded tubes. This makes them more suitable in critical applications in harsh environments, but they are expensive.
Welded tubing is generally less expensive than seamless tubing due to the more straightforward manufacturing process in creating welded tubing. It is also readily available, like seamless tubing, at long, continuous lengths. Standard sizes can be produced with similar lead times for welded and seamless tubing. Seamless tubing costs can be offset in smaller manufacturing runs if less quantity is required. Otherwise, though custom-sized seamless tubing can be produced and delivered more quickly, it is more costly.
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.
Several factors combine to make stainless steel tubing an excellent choice for structural applications in the automotive, aerospace, medical, and marine industries. If you're still unsure which tubing is best for your application, as a point of reference: though seamless tubing can be substituted for welded tubing, welded tubing can never be substituted for seamless tubing.
The advent of rolling mill technology and its development during the first half of the nineteenth century also heralded in the industrial manufacture of tube and pipe. Initially, rolled strips of sheet were formed into a circular cross section by funnel arrangements or rolls, and then butt or lap welded in the same heat (forge welding process).
Toward the end of the century, various processes became available for the manufacture of seamless tube and pipe, with production volumes rapidly increasing over a relatively short period. In spite of the application of other welding processes, the ongoing development and further improvement of the seamless techniques led to welded tube being almost completely pushed out of the market, with the result that seamless tube and pipe dominated until the Second World War.
During the subsequent period, the results of research into welding technology led to an upturn in the fortunes of the welded tube, with burgeoning development work ensuing and wide propagation of numerous tube welding processes. Currently, around two thirds of steel tube production in the world are accounted for by welding processes. Of this figure, however, about one quarter takes the form of so-called large-diameter line pipe in size ranges outside those which are economically viable in seamless tube and pipe manufacturing.
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The main seamless tube manufacturing processes came into being toward the end of the nineteenth century. As patent and proprietary rights expired, the various parallel developments initially pursued became less distinct and their individual forming stages were merged into new processes. Today, the state of the art has developed to the point where preference is given to the following modern high-performance processes..
The continuous mandrel rolling process and the push bench process in the size range from approx. 21 to 178 mm outside diameter.
The multi-stand plug mill (MPM) with controlled (constrained) floating mandrel bar and the plug mill process in the size range from approx. 140 to 406 mm outside diameter.
The cross roll piercing and pilger rolling process in the size range from approx. 250 to 660 mm outside diameter.
Mandrel Mill Process
In the Mandrel Mill Process, a solid round (billet) is used. It is heated in a rotary hearth heating furnace and then pierced by a piercer. The pierced billet or hollow shell is rolled by a mandrel mill to reduce the outside diameter and wall thickness which forms a multiple length mother tube. The mother tube is reheated and further reduced to specified dimensions by the stretch reducer. The tube is then cooled, cut, straightened and subjected to finishing and inspection processes befor shipment.
This process is used to make smaller sizes of seamless pipe, typically 1 to 6 inches (25 to 150 mm) diameter. The ingot of steel is heated to 2,370°F (1,300°C) and pierced. A mandrel is inserted into the tube and the assembly is passed through a rolling (mandrel) mill. Unlike the plug mill, the mandrel mill reduces wall thickness continuously with a series of pairs of curved rollers set at 90° angles to each other. After reheating, the pipe is passed through a multi-stand stretch-reducing mill to reduce the diameter to the finished diameter. The pipe is then cut to length before heat treatment, final straightening, inspection, and hydrostatic testing.
Mannesmann plug mill process
Plug Mill Process, a solid round (billet) is used. It is uniformly heated in the rotary hearth heating furnace and then pierced by a Mannesmann piercer. The pierced billet or hollow shell is rollreduced in outside diameter and wall thickness. The rolled tube simultaneously burnished inside and outside by a reeling machine. The reeled tube is then sized by a sizing mill to the specified dimensions. From this step the tube goes through the straightener. This process completes the hot working of the tube. The tube (referred to as a mother tube) after finishing and inspection, becomes a finished product.
This process is used to make larger sizes of seamless pipe, typically 6 to 16 inches (150 to 400 mm) diameter. An ingot of steel weighing up to two tons is heated to 2,370°F (1,300°C) and pierced. The hole in the hollow shell is enlarged on a rotary elongator, resulting in a short thick-walled tube known as a bloom.
An internal plug approximately the same diameter as the finished diameter of the pipe is then forced through the bloom. The bloom containing the plug is then passed between the rolls of the plug mill. Rotation of the rolls reduces the wall thickness. The tube is rotated through 90° for each pass through the plug mill to ensure roundness. The tube is then passed through a reeling mill and reducing mill to even out the wall thickness and produce the finished dimensions. The tube is then cut to length before heat treatment, final straightening, inspection, and hydrostatic testing.
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