Pipes are hollow cylindrical structures that humans have been using for thousands of years for various purposes. Pipes can be made from almost any material, but today, the significance of pipes extends beyond just being hollow tubes that carry liquids. Metals have become more popular in pipe production due to this expanded functionality. Steel, as an alloy, offers a wide range of mechanical and chemical properties, making it useful in extreme applications. As a result, steel pipes are widely used for many different applications in transportation, manufacturing, and structural purposes. Steel pipes can be produced in different grades of steel and through various manufacturing methods to meet specific application requirements.
What is a Pipe?
Steel pipes are long, hollow tubes used in a variety of applications. Their versatility makes them one of the most commonly used products in the steel industry. They are typically used to transport flowing liquids and small solid particles. Due to their high strength, they can also be used in construction for heating, plumbing, and other purposes, as well as for transporting water and gas in underground systems. Humans have been using and manufacturing pipes for different purposes for thousands of years. Archaeological evidence confirms that even in 2000 BC, ancient farmers or the Chinese used pipes made from materials like wood or bamboo for water transportation. Since the 1800s, significant advances have been made in steel pipe technology.
How are Pipes Used?
Pipes are used in structures, transportation, and manufacturing. Different materials, design features, and manufacturing methods are being developed for steel pipes, and these vary based on the application.
Structural Use
Structural use generally refers to buildings and construction, where the material is often steel pipes. Steel pipes are used, especially in high-rise buildings or constructions, to provide additional strength and stability. In structural applications, two types of steel pipes are used: bearing piles and friction piles, both of which transfer the load of the building. These pipes are driven into the ground before the foundation is laid, providing significant support, especially when the ground is unstable. Another structural application of steel pipes is scaffolding, which is made by connecting steel pipes in a cage-like structure around the building, allowing construction workers to reach areas of the building that would otherwise be inaccessible.
Manufacturing Use
Steel pipes are used for many different purposes in manufacturing. One common use is for railings, staircases, and balconies, or for providing a safety feature for cyclists and pedestrians on the street. Steel pipes can also be used as security posts to protect people, buildings, or infrastructure from vehicle traffic. Additionally, steel pipes are a popular option for outdoor furniture. Many commercial bike racks are made by bending steel pipes. The high toughness and strength of steel make these pipes secure against theft.
Transportation Use
The most common use of steel pipes is for the transportation of products, as the material’s properties make it ideal for long-term installations. As mentioned earlier, different applications require different characteristics. In low-pressure applications, a steel pipe is not expected to withstand significant stress, so it does not need to have ultra-high strength. In industries like oil and gas, more specific applications may require stricter specifications due to the hazardous nature of the products and the increased possibility of pressure. These specifications increase the cost and make quality control more critical.
Design Parameters
There are two types of pipes: seamless and welded, each with different uses. Seamless pipes are usually thinner and lighter, making them most commonly used in bicycle manufacturing and fluid transportation. Welded pipes are heavier and more rigid, providing better consistency and durability. They are typically used in gas transportation, electrical pipes, and plumbing. During production, various parameters must be controlled to ensure that the pipe meets the necessary specifications for the application. For example, a pipe’s diameter is directly related to how it will be used. Smaller diameter pipes may be used for subdermal needles, while larger diameter pipes may be used for citywide transportation. Wall thickness is also an important parameter as it directly affects the pipe's strength and flexibility.
Types of Steel Used in Pipes
Carbon Steels
Carbon steels account for about 90% of total steel pipe production. They are made of relatively small amounts of alloy elements and typically perform poorly when used alone. However, their mechanical properties and machinability are adequate, which can make them somewhat less expensive. They are preferred in low-stress applications but are not suitable for high-pressure applications or extreme conditions. Carbon steels are favored for their improved ductility and resistance to bending under load. They are commonly used in the automotive, maritime, oil and gas transportation industries. A500, A53, A106, A252 are examples of carbon steel grades that can be used as welded or seamless pipes.
Alloy Steels
The presence of alloy elements improves the mechanical properties of steel, making the pipes more resistant to high-stress applications and high-pressure conditions. The most common alloy elements include nickel, chromium, manganese, copper, etc., which can make up between 1-50% of the composition. The varying amounts of these elements contribute differently to the steel's mechanical and chemical properties, so the chemical composition of steel is modified according to application requirements. Alloy steel pipes are often used in high-load and unstable conditions, such as in the oil and gas industry, refineries, petrochemical and chemical plants.
Stainless Steels
Stainless steel is considered part of the alloy steel family. The primary alloying element in stainless steel is chromium, which makes up about 10-20% by weight. The main purpose of adding chromium is to prevent corrosion and give the steel its stainless properties. Stainless steel pipes are commonly used in the maritime industry, water treatment, pharmaceuticals, and the oil and gas industry, where corrosion resistance and high strength are vital in extreme conditions. 304/304L and 316/316L are examples of stainless steel grades used in pipe production. Grade 304 offers high corrosion resistance and strength, while grade 316 has lower strength due to its lower carbon content but is more weldable.
Galvanized Steels
Galvanized pipes are steel pipes coated with zinc to prevent corrosion. Zinc coating prevents abrasive substances from corroding the pipe. Galvanized pipes were once the most common type used for water supply lines, but due to the labor and time required for cutting, threading, and assembling these pipes, they are now only used for limited applications in repairs. These pipes are typically available in sizes ranging from 12 mm (0.5 inches) to 15 cm (6 inches) in diameter and in lengths of 6 meters (20 feet). However, galvanized steel pipes are still seen in larger commercial applications for water distribution. One significant drawback is that galvanized pipes have a lifespan of about 40-50 years. Despite the zinc coating protecting the steel from external corrosion, if abrasive materials are transported through the pipes, internal corrosion can begin. Therefore, galvanized steel pipes need to be periodically inspected and upgraded.
Types of Pipes
Pipes can be divided into two main categories based on the manufacturing method: seamless pipes and welded pipes. Seamless pipes are made by forming hollow tubes directly from the metal during rolling, while welded pipes require a welding process after rolling. Welded pipes can be further classified into spiral-welded and straight-welded, based on the welding method. There is an ongoing debate about whether seamless pipes are better than welded steel pipes, but both types can be manufactured to provide high quality, reliability, and corrosion resistance. When selecting a pipe type, the focus should be on the application’s characteristics and cost considerations.
Seamless Pipes
Seamless pipes are typically produced through a series of complex steps starting with the hollowing out of billets using cold drawing or cold rolling. Controlling the size of the pipe is more challenging compared to welded pipes, but the cold working process improves mechanical properties and tolerances. The primary advantage of seamless pipes is their ability to be manufactured with heavy and thick walls. The absence of a weld seam makes them more resistant to mechanical stress and corrosion compared to welded pipes. Seamless pipes are usually preferred for extreme environments such as high-load, high-pressure, and highly corrosive conditions.
Welded Pipes
Welded steel pipes are made by rolling a steel plate into a cylindrical shape and then welding the edges together. There are different ways to produce welded pipes depending on the external dimensions, wall thickness, and application. Each method starts with a steel billet or flat strip, which is then stretched and joined together with a weld. Welded pipes offer tighter tolerances but thinner wall thicknesses than seamless pipes. The shorter lead time and lower cost may make welded pipes more desirable than seamless pipes. However, the welding seam creates weak points where cracks can spread, so the external and internal surface finishes must be carefully monitored during production.
Pipe Manufacturing
In both manufacturing methods, raw steel is first cast into a more workable form (either a billet or flat strip). Then, the hot steel billet is either drawn into a seamless pipe or a flat steel strip is bent into a cylindrical shape, and the edges are welded together to form a pipe.
Seamless Pipe Manufacturing
Mandrel Değirmen Süreci
n the mandrel mill process, a solid round steel billet is loaded into a furnace and heated. Once heated, a small hole is made at one end of the billet. This indentation acts as a starting point for rotary piercing. Rotary piercing is a fast and dynamic process where the billet is rolled between two rollers at high speed to form a hollow tube. The piercing rollers are designed to guide the metal through and allow it to flow through a piercing point. Once the hollow tube is formed, it is moved to the floating mandrel mill for further processing. The mandrel mill consists of 16 rollers and several mandrel bars that shape the tube. The pipe is then further reduced to the desired size by a stretching machine.
Mannesmann Plug Mill Process
The Mannesmann plug mill process differs from the mandrel mill process primarily in the use of rollers instead of mandrels. In the Mannesmann process, a pair of conical cylinders are stacked on top of each other and operate in opposite directions to the material flow. A thick-walled hollow tube shell is directed toward the plug mill rollers. When the conical section of the passage grips the hollow tube, a small wave of material is cut from the hollow shell. This wave is forged to the desired wall thickness by the softening section of the passage on the mandrel; the hollow tube shell and the mandrel rotate in the same direction while moving backward until the rollers reach the idle transition. As the hollow tube shell is rotated, it is pushed forward again between the rollers.
Extrusion
Extrusion is a metal forming process in which a workpiece is forced into a mold with a smaller cross-section. The length of the extruded part will depend on the amount of material in the workpiece and the profile being extruded. This method is used to produce multiple sections. Steel pipes can be produced through direct extrusion using a mandrel attached to a model block. Along the workpiece, a hole parallel to the axis, where the ram applies force to create the extrusion, is formed. Once the process begins, the ram is forced forward. The extruded metal flows between the mandrel and mold surfaces to form the part. The inner profile of the metal extrusion is created by the mandrel, and the outer profile is formed by the extrusion die.
Seamless Pipe Manufacturing
Seamless pipes are produced from plates, continuous coils, or strips. To produce a seamless pipe, the first plate or coil is rolled into a circular cross-section with the help of a plate bending machine or a roller in continuous operation. When the circular section is rolled from the plate, the pipe can be welded either with or without filler material. Different welding methods can be used to weld the pipe.
Electric Resistance Welding Process (ERW)
In the electric resistance welding process, the pipe is produced by cold forming a flat steel sheet into a cylindrical geometry. Then, current is passed through the edges of the steel cylinder, heating the steel and forcing it to form a bond at a point where the edges join. Filler materials may also be used during ERW operations. There are two types of electric resistance welding: high-frequency welding and rotary contact wheel welding.
The requirement for high-frequency welding arises from the tendency of low-frequency welded products to suffer from selective seam corrosion, hook cracks, and insufficient seam bonding. Therefore, low-frequency ERW is no longer used in pipe manufacturing. The high-frequency ERW process is still used in pipe production. There are two types of high-frequency ERW processes: high-frequency induction welding and high-frequency contact welding. In high-frequency induction welding, the welding current is transmitted to the material via a coil, which does not touch the pipe. The electric current is induced into the pipe material through magnetic fields surrounding the pipe. In high-frequency contact welding, current is transmitted to the material via contacts moving along the strip. The welding power is directly applied to the pipe, making this method more efficient. This method is typically preferred for large-diameter pipes and those with high wall thickness.
Another type of electric resistance welding is the rotary contact wheel welding process. During this process, electric current is transmitted to the welding point through a contact wheel. The contact wheel also applies the necessary pressure for welding. Rotary contact welding is typically used in applications where placing a barrier inside the pipe is not possible.
Electric Fusion Welding Process (EFW)
The electric fusion welding process involves the use of a high-speed electron beam to weld a steel sheet. The high kinetic energy of the electron beam is converted into heat to heat the workpiece, creating the weld seam. The welding area can also be thermally treated to make the weld invisible. Welded pipes typically have tighter dimensional tolerances than seamless pipes, and their cost is lower when produced in similar quantities. Metal welding components, primarily used for welding different steel sheets or high-power density welding, can be quickly heated to high temperatures by melting any refractory metal and alloy.
Submerged Arc Welding Process (SAW)
Submerged arc welding involves the formation of an arc between a wire electrode and the workpiece. A flux is used to generate protective gases and slag. As the arc moves along the joint line, excess flux is removed through a feed hopper. Since the arc is completely covered by a layer of flux, it is usually invisible during the welding process, and heat loss is minimal. There are two types of submerged arc welding: longitudinal submerged arc welding and spiral submerged arc welding.
In longitudinal submerged arc welding, the longitudinal edges of steel sheets are first beveled to create a U-shape. The U-shaped edges of the sheets are then welded. Pipes produced through this process are subjected to an expansion process to reduce internal stresses and achieve excellent dimensional tolerance.
In spiral submerged arc welding, the weld seams form a spiral around the pipe. Both longitudinal and spiral welding methods use the same technology, with the only difference being the spiral shape of the seams in spiral welding. The manufacturing process ensures that the rolling direction has an angle to the pipe centerline, shaping and welding the steel strip so that the weld seam follows a spiral line. The biggest disadvantage of this process is the poor physical size of the pipes and the increased likelihood of defects or cracks due to the longer seam length.
Quality Control
Various measures are taken to ensure that the finished steel pipe meets the specifications. For example, X-ray gauges are used to adjust the thickness of the steel. The gauges operate using two X-rays. One ray is directed at a steel piece with a known thickness, and the other is directed at the steel passing through the production line. If there is any difference between the two rays, the gauge will automatically trigger the resizing of the rollers to compensate. The pipes are also inspected for defects at the end of the process. One method of testing a pipe is to use a special machine that fills the pipe with water and then increases the pressure to check if it holds. Defective pipes are sent to scrap.
Specifications
There may be confusion regarding how these materials are specified and what the exact properties of the pipe mean. The American Society for Testing and Materials (ASTM), the American Society of Mechanical Engineers (ASME), and the American Petroleum Institute (API) are the most commonly referenced organizations for pipe specifications in North America.
Nominal Pipe Size
The pipe size is specified as "Nominal Pipe Size" or NPS. The origin of the NPS numbers for smaller pipes (< NPS 12) differs from that for larger diameter pipes. However, all pipes with a specific NPS number have the same outer diameter (OD). The inner diameter will vary depending on the wall thickness of the material. This is because all pipes with the same NPS number can use the same structural supports, regardless of wall thickness.
Pipe Schedules
Pipe schedules are used to define the wall thickness of pipes. This is an important parameter as it directly affects the strength of the pipe and must be carefully controlled. A pipe schedule is a dimensionless number and is calculated based on a design formula for wall thickness, considering design pressure and permissible stress. As the schedule number increases, the wall thickness of the pipe increases. Since the OD is fixed by the NPS number, the schedule number essentially defines the pipe’s inner diameter.
Pipe Weight
The weight of the pipe can be calculated based on the schedule that determines the thickness and the external diameter (OD). The formula uses a constant value of 40.8 pounds per square foot per inch of thickness for the theoretical weight of steel. Pipe weight is represented by the formula: W = 10.69 × t (OD – t), where t is the thickness and W is the pipe weight.
Standards
Manufacturing standards for pipes typically require a chemical composition test and a series of mechanical strength tests for each heat of the pipe. Since all pipes from the same heat are forged from the same casting and therefore share the same chemical composition, mechanical tests can be related to many pipes from the same heat and thermal processing. Materials with associated test reports are referred to as traceable materials. For critical applications, third-party verification of these tests may be required. In such cases, an independent laboratory will prepare a certified material test report, and the material will be called certified.
Some commonly used pipe standards or pipe grades include:
ASME SA106 Grade B (seamless carbon steel pipe for high-temperature service)
ASTM A312 (seamless and welded austenitic stainless steel pipe)
ASTM C76 (concrete pipe)
ASTM A36 (carbon steel pipe for structural or low-pressure use)
ASTM A795 (steel pipe for fire sprinkler systems)
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