What is the difference between a steel pipe and a steel tube?

What is a steel tube?

Steel tube can be divided into seamless steel tube and welded steel tube. Generally speaking, the outer diameter is less than 89 mm but the steel tube larger than 4 mm is referred to as the small diameter seamless steel tube.

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Seamless steel tube is a kind of hollow steel, with round, square and seamless rectangular shape. Seamless steel tube consists of steel ingot or solid tube, and then hot rolled, cold rolled or cold drawn seamless steel tube. Cross section, mainly used for conveying fluids. Compared with solid steel such as round steel, the bending and torsional strength of seamless steel tube are lighter. Seamless steel tube is widely used in manufacturing structural parts and mechanical parts, such as oil drill pipe, automobile transmission shaft, bicycle bracket and steel structure scaffold.

According to different production process, seamless steel tube can be divided into hot rolled steel tube, cold rolled steel tube, cold drawn steel tube and extruded steel pipe. Usually, the hot rolled seamless tube is produced by the automatic rolling mill.

Check the solid tube and process its surface defect, and cut the tube into the required length to make the center point on the perforated surface of the solid tube; The solid tube is fed into a heating furnace. Drilling holes in the solid tube with a perforating machine; The solid tube of the solid tube is formed by grinding roller and perforating plug. The solid tube with empty cavity will be machined by automatic tube grinder. Use the rewire machine to make the pipe wall thick and uniform; The pipe is processed by the sizing machine or reducer to meet the specifications. It is an advanced method to manufacture hot rolled seamless steel tube with continuous tube rolling mill.

If you want to get a smaller, more high-quality seamless steel tube, you must use cold rolling process, cold drawing or combination of two processes. The cold rolling process is carried out in the double – roll rolling mill. The cold drawing process is carried out in single – chain or double – chain cold drawing machine. Extrusion can produce smaller diameter steel tubes.

There are three main methods of manufacturing tubing that lend their names to tubing classifications. 
 
·     Seamless. Seamless steel tubes are produced through extrusion. Extruded tubes can be formed in a hot or cold process. Long sections of steel bar are forced through a die that blocks out the intended shape of the tube. 
·     As-welded or electric resistant welded (ERW). This method involves passing a rolled sheet of steel through two weld rollers. The weld rollers have a groove around their circumference, through which the steel roll passes. There is a contact at the roll seam that transmits electricity at a high enough current to weld the seam closed. The resulting weld is very small.
·     Drawn-over-mandrel (DOM). A mandrel is a small piece of metal inserted into the tube to define a shape. It gives the tube extra support to prevent unwanted wrinkling during drawing. The tube is passed through a die that has a smaller diameter than the current tube size. As the tube is drawn, it shrinks to match the size of the die’s diameter. This process allows for tight tolerances and specifications. 
 
Each of these manufacturing processes allows manufacturers the ability to form varying tube shapes and sizes. Tubes are not always cylindrical, and can be made in triangle, square or other polygonal shapes. Steel tubes destined for certain applications require extra processes as well. Applications involving hydrogen must be factory pre-cleaned or certified as instrument grade due to hydrogen’s reactive characteristics that can cause metal embrittlement or even explosions. 
 
Generally, a steel tube manufacturing process is chosen for its interaction with the type of steel to be used. Certain types of steel react poorly to heat because of carbon content, so they can’t be easily welded. Steel types also play a role in decisions regarding use in volatile applications.
 
Steel Tube Applications

Steel tubing is used in high numbers in plumbing applications. The reasons are threefold. One, stainless steel is very sturdy, composed of 80-90 percent steel and 10-20 percent chromium. Steel tubing can handle the types of pressures exerted by water upon plumbing structures. Additionally, the surface of stainless steel does not allow for much adherence by particles or bacteria, so purification processes will not pass on unintended detritus. Finally, steel does not contaminate drinking water.

Steel tubes are also used in a wide variety of structural applications, such as industrial and residential construction. Examples include fences, gates, railings, playground and athletic equipment. Steel is often used for construction tubes over other metals like aluminum when extra stress resistance is necessary. Steel tubes can also be used in automotive applications and even as parts of furniture.

What is a steel pipe?

Steel pipe is long that are used for a variety of purposes. They are produced by two distinct methods which result in either a welded or seamless pipe. In both methods, raw steel is first cast into a more workable starting form. It is then made into a pipe by stretching the steel out into a seamless tube or forcing the edges together and sealing them with a weld. The first methods for producing steel pipe were introduced in the early 1800s, and they have steadily evolved into the modern processes we use today. Each year, millions of tons of steel pipe are produced. Its versatility makes it the most often used product produced by the steel industry.

Steel pipes are found in a variety of places. Since they are strong, they are used underground for transporting water and gas throughout cities and towns. They are also employed in construction to protect electrical wires. While steel pipes are strong, they can also be lightweight. This makes them perfect for use in bicycle frame manufacture. Other places they find utility is in automobiles, refrigeration units, heating and plumbing systems, flagpoles, street lamps, and medicine to name a few.

The Manufacturing Process Of Steel pipes

Steel pipes are made by two different processes. The overall production method for both processes involves three steps. First, raw steel is converted into a more workable form. Next, the pipe is formed on a continuous or semicontinuous production line. Finally, the pipe is cut and modified to meet the customer’s needs.

Ingot production

  • 1 Molten steel is made by melting iron ore and coke (a carbon-rich substance that results when coal is heated in the absence of air) in a furnace, then removing most of the carbon by blasting oxygen into the liquid. The molten steel is then poured into large, thick-walled iron molds, where it cools into ingots.
  • 2 In order to form flat products such as plates and sheets, or long products such as bars and rods, ingots are shaped between large rollers under enormous pressure.

Producing blooms and slabs

  • 3 To produce a bloom, the ingot is passed through a pair of grooved steel rollers that are stacked. These types of rollers are called “two-high mills.” In some cases, three rollers are used. The rollers are mounted so that their grooves coincide, and they move in opposite directions. This action causes the steel to be squeezed and stretched into thinner, longer pieces. When the rollers are reversed by the human operator, the steel is pulled back through making it thinner and longer. This process is repeated until the steel achieves the desired shape. During this process, machines called manipulators flip the steel so that each side is processed evenly.
  • 4 Ingots may also be rolled into slabs in a process that is similar to the bloom making process. The steel is passed through a pair of stacked rollers which stretch it. However, there are also rollers mounted on the side to control the width of the slabs. When the steel acquires the desired shape, the uneven ends are cut off and the slabs or blooms are cut into shorter pieces.

Further processing

  • 5 Blooms are typically processed further before they are made into pipes. Blooms are converted into billets by putting them through more rolling devices which make them longer and more narrow. The billets are cut by devices known as flying shears. These are a pair of synchronized shears that race along with the moving billet and cut it. This allows efficient cuts without stopping the manufacturing process. These billets are stacked and will eventually become seamless pipe.
  • 6 Slabs are also reworked. To make them malleable, they are first heated to 2,200° F (1,204° C). This causes an oxide coating to form on the surface of the slab. This coating is broken off with a scale breaker and high pressure water spray. The slabs are then sent through a series of rollers on a hot mill and made into thin narrow strips of steel called skelp. This mill can be as long as a half mile. As the slabs pass through the rollers, they become thinner and longer. In the course of about three minutes a single slab can be converted from a 6 in (15.2 cm) thick piece of steel to a thin steel ribbon that can be a quarter mile long.
  • 7 After stretching, the steel is pickled. This process involves running it through a series of tanks that contain sulfuric acid to clean the metal. To finish, it is rinsed with cold and hot water, dried and then rolled up on large spools and packaged for transport to a pipe making facility.

Pipe making

  • 8 Both skelp and billets are used to make pipes. Skelp is made into welded pipe. It is first placed on an unwinding machine. As the spool of steel is unwound, it is heated. The steel is then passed through a series of grooved rollers. As it passes by, the rollers cause the edges of the skelp to curl together. This forms an unwelded pipe.
  • 9 The steel next passes by welding electrodes. These devices seal the two ends of the pipe together. The welded seam is then passed through a high pressure roller which helps create a tight weld. The pipe is then cut to a desired length and stacked for further processing. Welded steel pipe is a continuous process and depending on the size of the pipe, it can be made as fast as 1,100 ft (335.3 m) per minute.
  • 10 When seamless pipe is needed, square billets are used for production. They are heated and molded to form a cylinder shape, also called a round. The round is then put in a furnace where it is heated white-hot. The heated round is then rolled with great pressure. This high pressure rolling causes the billet to stretch out and a hole to form in the center. Since this hole is irregularly shaped, a bullet shaped piercer point is pushed through the middle of the billet as it is being rolled. After the piercing stage, the pipe may still be of irregular thickness and shape. To correct this it is passed through another series of rolling mills.

Final processing

  • 11 After either type of pipe is made, they may be put through a straightening machine. They may also be fitted with joints so two or more pieces of pipe can be connected. The most common type of joint for pipes with smaller diameters is threading—tight grooves that are cut into the end of the pipe. The pipes are also sent through a measuring machine. This information along with other quality control data is automatically stenciled on the pipe. The pipe is then sprayed with a light coating of protective oil. Most pipe is typically treated to prevent it from rusting. This is done by galvanizing it or giving it a coating of zinc. Depending on the use of the pipe, other paints or coatings may be used.

Quality Control

A variety of measures are taken to ensure that the finished steel pipe meets specifications. For example, x-ray gauges are used to regulate the thickness of the steel. The gauges work by utilizing two x rays. One ray is directed at a steel of known thickness. The other is directed at the passing steel on the production line. If there is any variance between the two rays, the gauge will automatically trigger a resizing of the rollers to compensate.

Pipes are also inspected for defects at the end of the process. One method of testing a pipe is by using a special machine. This machine fills the pipe with water and then increases the pressure to see if it holds. Defective pipes are returned for scrap.

What is the difference between a steel pipe and a steel tube?

One of the most frequent questions we get from customers is “What’s the difference between pipe and tube”? Although pipes and tubes may look similar, they are in fact quite different in nomenclature and sizing. Remember that pipes and tubes are rarely interchangeable. Here are some of the main differences between the two products:
(Source: 
DIFFERENCE BETWEEN PIPE AND TUBE)

Shape
Pipes are always round in shape. Tubes can be square, rectangular or round, as shown in the image to the right.

Strength
Tube is stronger than pipe.

Metal Types
Tube is available in hot rolled steel and cold rolled steel. Pipe is typically black steel (hot rolled). Both items can be galvanized.

Rigidity
Although copper and brass tubes can be shaped relatively easily, tubes are are typically rigid. Pipes, on the other hand, are invariably rigid and cannot be shaped without special equipment.

Size


Tubing
Tubing is generally used for structural purposes and the OD is an important and exact number. Tubing size is specified by OD and the wall thickness (WT); and the measured OD and stated OD are generally within very close tolerances of each other. Tubing is usually more expensive than pipe due to tighter manufacturing tolerances.

Interestingly, while the stated and measured OD’s of tubing are almost exact most of the time, copper tubing generally has a measured OD that is 1/8” larger than stated OD. As such, maybe it should be called copper pipe. However, stainless steel, aluminum, and steel tubing all have measured and stated OD’s that are exact or very close.

Pipe
Pipes are categorized as tubular vessels used in pipeline and piping systems, and commonly transport gases or fluids. They are specified by “Nominal Pipe Size” (NPS) and Schedule (wall thickness). NPS is a size standard established by the American National Standards Institute (ANSI), and should NOT be confused with the various thread standards such as NPT and NPSC.

The manufacturing of Nominal Pipe Sizes from 1/8” to 12” is based on a standardized nominal outside diameter (OD) that is different from the measured OD. NPS pipe 14” and up have measured OD’s that correspond to the nominal size.

The reason many people, including plumbers, engineers, and others, mistakenly believe NPS refers to the ID on smaller pipes is because of how the standard was originally defined. The standardized OD was originally defined so that a pipe with a standardized OD and wall thickness typical of the period, would have a pipe ID that was approximately equal to the nominal size of the pipe. For example, 3” Schedule 40 NPS has an outside diameter and wall thickness that very, very roughly gives it an inside diameter of 3”. Regardless of the wall thickness the nominal OD of a pipe will not change.

There you have the differences explained as simply as possible. Of course if something isn’t clear certainly let us know.

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Img from CommerceMetals.com

Pipe is typically available in larger sizes than tube.


Applications

Only steel pipes are pressure rated and intended to be used for the transference of fluids or gas. Tubes, on the other hand, are used in structural applications.

Telescoping abilities
Tube can be telescoped. Remember to account for the flash weld inside the tube. Pipe, on the other hand, does not have a flash weld. DOM (Drawn over Mandrel) Tube is the best material for telescoping because the inside flash weld has been removed.

Measurement
Pipes are only provided with an inside (nominal) diameter and a “schedule” (which means wall thickness). There’s an easy way to remember this: since pipe is used to transfer fluids or gas, the size of the opening through which the fluids or gas can pass through is probably more important to you than the outer dimensions of the pipe. Tube measurements, on the other hand, are provided as an outside diameter and set ranges of wall thickness.

Source: China steel pipes Manufacturer – Yaang pipe fitting Industry Co., Limited (www.ugsteelmill.com)


(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)


If you want to have more information about the article or you want to share your opinion with us, contact us at sales@ugsteelmill.com

Please notice that you might be interested in the other technical articles we’ve published:

  • The standard and application of stainless steel seamless pipe and welded pipe

  • Manufacturing process of cold rolled steel pipe

  • Knowledge of the purge pipes

  • The Introduction of Cold Rolled Seamless Pipes

  • Cold Working Processes of Steel Pipes

  • Introductions of Cast Iron Pipes


References:

  1. http://www.encyclopedia.com/manufacturing/news-wires-white-papers-and-books/steel-pipe
  2. https://www.thomasnet.com/articles/metals-metal-products/stainless-steel-tubes/
  3. https://www.commercemetals.com/tube-vs-pipe-the-differences-explained-in-plain-english/
  4. https://www.yaang.com

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