Pipes are hollow cylindrical tubes that have been utilized by mankind for thousands of years for different purposes. Pipes can be produced from almost every material, however, since the modern-day meaning of pipes requires more than only being hollow tubes that transport fluid, metals have become more popular in pipe production. As a metal alloy, steels offer a great variety of mechanical and chemical properties that can be utilized under even extreme applications, therefore today, steel pipes have been utilized in many different applications for transportation manufactural and structural purposes. Steel pipes may be produced with different grades of steel with different production methods which vary due to application requirements.
What is a Pipe?
Steel pipes are long and hollow tubes that are used for many different applications in a variety of places. Its versatility makes pipes the most often used product that is produced by the steel industry. They are commonly used to convey fluid substances that can flow, and small solid particles. Due to their high strength, they can also be utilized in underground transportation of water and gas through cities, or in construction for purposes like heating, plumping, etc. People have been utilizing and producing pipes for different purposes for thousands of years. Archeological evidence verifies that even in 2000 BC ancient agriculturists or Chinese people have utilized pipes made from different materials like wood or bamboo, for water transportation. Since the 1800s, great strides have been accomplished in the technology of steel pipes, including improving manufacturing methods, developing applications for their use, and establishment of regulations and standards that govern their certification.
How is Pipe Used?
Pipes are utilized in structures, transportation, and manufacturing. Different materials, design characteristics, and production methods for steel pipes have been developing and varying accordingly to the application.
- Structural Usage
Structural use is commonly building and construction in which the building material is commonly referred to as steel tubes. Steel tubes are used to provide additional strength and stability for especially high buildings or constructions. Two types of steel pipes are utilized in structural usage as end-bearing piles and friction piles that both have the aim of transferring the load of the building. In those applications, steel pipes are driven deep into the earth before the foundation is laid, which constitutes great support to the building especially when the ground is not secure. Another structural application of steel pipes is scaffolding poles which allow construction workers to access all areas of the building that are out of reach. They are made by linking steel tubes into each other as a cage that surrounds the building.
- Manufacturing Usage
Steel pipes are being utilized for many different purposes in manufacturing use. Guard rails are one of the most common usages to provide a safety feature for stairs and balconies, or, in street, for cyclists and pedestrians. Steel pipes can also be utilized as security bollards which are used to cordon off an area from vehicle traffic to protect people, buildings, or infrastructures. Also, steel pipes constitute an option for outdoor site furnishings. Many commercial bike racks are formed by bending steel tubes. High toughness and strength of steel make it secure against thieves.
- Transportation Usage
The most common usage of steel pipes is the transportation of products since the characteristics of the raw material is very suitable for long-term installations. As it has mentioned before, different applications require different properties, as for low-pressure applications it is not expected for a steel pipe to exhibit ultra-high-strength since it does not expose to significant loading. More specialized applications to be used in the oil and gas industry may require more stringent specifications due to the hazardous nature of the product and the possibility of increasing pressure. These requirements bring a higher cost and quality control becomes more critical.
There are two types of pipes as seamless and welded seam, and both have different uses. Seamless pipes are generally thinner and lighter, thus they are most widely used in bicycle production and fluid transportation. Seamed pipes are more heavy and rigid to obtain better consistency and durability. Pipes that are used for gas transportation, electrical conduit, and plumbing are generally seamed. During production, several parameters should be controlled to maintain the required properties for the application. As an example, the diameter of a pipe is designed directly related to how it will be used. While pipes with a smaller diameter may be used for hypodermic needles, large diameter pipes may be used for transport through cities. Wall thickness is also an important parameter to control since it directly affects the pipe’s strength and flexibility. Length, coating, and end finish are also other controllable parameters that are all related to each other as will be explained later.
Steel Types Used in Pipes
- Carbon Steels
Carbon steels represent approximately 90% of total steel pipe production. They are consist of relatively lower amount of alloying elements and are generally perform weak as used alone. Since their mechanical properties and machinability are sufficient they may cost a little less and they may be preferred more for especially low-stress applications. Lack of alloying elements lowers the suitability of carbon steels for high-pressure applications and extreme conditions, so they become less durable under high loadings. The main reason to prefer carbon steels for pipes may be their advanced ductility and non-bending nature under loading. They are generally used in the automotive and marine industry, and oil and gas transportation. A500, A53, A106, A252 are carbon steel grades that can either be used as seamed or seamless.
- Alloyed Steels
The presence of alloying elements improves the mechanical properties of steel, thus pipes become more resistant to high-stress applications and high pressures. The most general alloying elements are nickel, chromium, manganese, copper, etc. which are present in the composition between 1-50 weight percent. Different amounts of different alloying elements contribute mechanical and chemical properties f the product in varying manners, therefore the chemical composition of steels is also modified accordingly to requirements of applications. Alloyed steel pipes are generally used under high loadings with unstable conditions as in the oil and gas industry, refineries, petro-chemistry and chemistry factories.
- Stainless Steels
Stainless steel can also be included in the alloyed steel family. The main alloying element in stainless steel is chromium whose fraction varies between 10-20 weight percent. The main purpose of chromium addition is to make steel gain stainless properties by preventing corrosion. Stainless steel pipes are generally utilized under extreme conditions where corrosion resistance and high strength are vital, as in the marine industry, water purification, medicine and, oil and gas industry. 304/304L and 316/316L are stainless steel grades that can be used in pipe production. While 304 grade has great corrosion resistance and strength; due to low carbon content, 316 series exhibit lower strength and can be welded.
- Galvanized Steels
Galvanized pipes are steel pipes that are treated with a zinc coating to prevent corrosion. Zinc coating keeps corrosive substances from corroding the pipe. It was once the most common type of pipe for water supply lines, but because of labor and time that goes into cutting, threading, and installing galvanized pipe, it no longer used much, except for limited use in repairs. These types of pipes are prepared from 12 mm (0.5 inches) to 15 cm (6 inches) in diameter. They are available in 6 meters (20 feet) length. However, galvanized pipe for water distribution is still seen in larger commercial applications. One important disadvantage of galvanized pipes is their 40-50 years of lifetime. Despite that zinc coating covers the surface and avoids external substances to react with steel and corrode it, if the transported substances are corrosive pipe may start corroding from inside. Therefore, it is crucial to control and upgrade galvanized steel pipes in certain periods.
Pipes are classified into two groups as seamless pipes and seamed pipes accordingly to the manufacturing methods. Seamless pipes are formed at one stage during rolling, however seamed pipes require a welding process after rolling. It is possible to classify seamed pipes into two due to the seam geometry as spiral welding and straight welding. Although there is a debate about whether seamless is better than seamed steel pipes, both seamless and welded pipe manufacturers can produce steel pipes providing high quality, reliability, and corrosion resistance. The main focus should be the specifications of the application and cost aspects while determining pipe type.
- Seamless Pipes
Seamless pipes are generally made in complex steps starting from drilling hollows from billets, by cold drawing and cold rolling processes. To control outside diameter and wall thickness, seamless type dimension is difficult to control compared to the welded tube, cold work improves the mechanical properties and tolerances. The most significant advantage of seamless pipes is that they can be produced in heavy and thick wall thicknesses. Due to their having no weld seam, makes they to be considered as exhibiting better mechanical properties and corrosion resistance than seamed pipes. Also, a better ovality or roundness is expected from seamless pipes. They are generally preferred to be used under extreme environmental conditions as high loading, high pressure, and high corrosivity.
- Seamed Pipes
The welded steel pipe is formed by welding a steel plate rolled into a tubular shape by a seam or a spiral seam. Depending on the outer dimension, wall thickness, and application, there are different ways of manufacturing welded pipes. Each method initiates with steel hot billet or flat strips and then made into a pipe by stretching the hot steel billet and forcing the edges together and sealing them with a weld. Seamed pipes offer tighter tolerances but thinner wall thickness rather than seamless pipes. Shorter lead time and lower cost may also be reasons why seamed pipes can be preferred over seamless pipes. However, since weld seam may constitute sensitive areas that may be suitable for any crack to propagate and lead to fracture of the pipe, surface finishing of outside and inside of the pipe should be controlled during production.
In both the manufacturing methods, raw steel is first cast into a more workable starting form (hot billet or flat strip). It is then made into a pipe by stretching the hot steel billet out into a seamless pipe or forcing the edges of the flat steel strip together and sealing them with a weld.
- Seamless Pipe Manufacturing
Mandrel Mill Process
In the Mandrel Mill Process, a solid round steel billet is used. The billet is charged into a rotary hearth furnace. After the billet is discharged from the rotary hearth furnace, a small hole is punched into its end. This indentation acts as a starting point to aid in the rotary piercing. Rotary piercing is a very fast and dynamic rolling process that cross rolls the preheated billet between two barrel-shaped rolls at a high speed. The design of the piercer rolls causes the metal to flow along with the roll and over a piercer point as it exits the process. The piercer point is a high-temperature, water-cooled alloy tool designed to allow the metal to flow over it as a pipe shell forms from the rotary process. Once the pierced pipe shell is produced, it is immediately transferred to the floating mandrel mill. The floating Mandel mill comprises eight rolling stands using 16 rolls and a set of mandrel bars. These bars are inserted into the pierced pipe shell and then conveyed into the mandrel mill, and rolled into a pipe shell. Then the mandrel mill is re-heated in order to complete the final rolling stage and to gain final dimensions. While the mill is leaving the furnace, the iron-oxide scale is removed from the surface via high-pressure water descale. The pipe shell is further reduced to specified dimensions by the stretch mill.
Mannesmann Plug Mill Process
Mannesmann plug mill process differs from mandrel milling with a great difference of rolling plug usage instead of a mandrel mill. In the Mannesmann process, a pair of conical rolls are arranged on top of each other and operate in the opposite direction to material flow. A hollow pipe shell having thick walls is guided towards the plug mill rolls. As soon as it is gripped by the tapered portion of the work pass, a small material wave is sheared off the hollow pipe shell. This wave is forged to the desired wall thickness on the mandrel by the smoothing portion of the work pass, with the hollow pipe shell plus mandrel moving backward in the same direction as the rolls are rotating until they reach the idler pass of the rolls and are released. As the hollow pipe shell is rotated it is once again pushed forward between the rolls, and a new rolling cycle begins.
Extrusion is a metal forming process in which a workpiece is forced into a die of the smaller cross-section. The length of the extruded part will vary, dependent upon the amount of material in the workpiece and the profile extruded. Numerous cross-sections are manufactured by this method. Steel pipes can be directly produced by extrusion with the usage of a mandrel attached to the dummy block. A hole is created through the workpiece parallel to the axis over which the ram applies the force to form the extrusion. When the operation begins, the ram is forced forward. The extruded metal flows between the mandrel and the die surfaces, forming the part. The interior profile of the metal extrusion is formed by the mandrel, while the exterior profile is formed by the extruding die.
- Seamed Pipe Manufacturing
Seamed pipes are manufactured from plate or continuous coil or strips. To manufacture a seamed pipe, the first plate or coil is rolled in the circular section with the help of a plate bending machine or by a roller in the case of a continuous process. When the circular section is rolled from the plate, the pipe can be welded with or without filler material. There are different welding methods 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 sheet of steel into ay cylindrical geometry. Then the current is passed through the edges of the steel cylinder to heat up the steel and form a bond between the edges at a point that they are forced to meet. During ERW processes filler materials may also be utilized. There are two types of electric resistance welding as high-frequency welding and rotary contact wheel welding.
The requirement for high-frequency welding has arisen from the tendency of low-frequency welding products to undergo selective seam corrosion, hook cracks, and inadequate bonding of seams. So, low-frequency ERW is no longer used to manufacture pipes. The high-frequency ERW process is still being used in pipe manufacturing. There are two types of high-frequency ERW processes. High-frequency induction welding and high-frequency contact welding are types of high-frequency welding. In high-frequency induction welding, the weld current is transmitted to the material through a coil. The coil does not contact the pipe. The electrical current is induced into the pipe material through magnetic fields that surround the pipe. In high-frequency contact welding, the current is transmitted to the material through contacts that ride on the strip. The welding power is applied directly to the pipe, which makes this process more effective. This method is generally preferred for large diameter and high wall thickness pipe production.
Another type of electric resistance welding is the rotary contact wheel welding process. During this process, electrical current is transmitted through a contact wheel at the weld point. The contact wheel also applies the pressure necessary for welding. Rotary contact welding is generally utilized for applications that cannot accommodate an impeder inside the pipe.
Electric Fusion Welding Process (EFW)
The electric fusion welding process refers to an electron beam welding of a steel plate by the use of the high-speed movement of the electron beam. High impact kinetic energy of the electron beam is converted into heat to heat the workpiece so that the weld seam is produced. The welding zone can also be heat treatment so that the weld is not visible. Welded tubes generally have tighter dimensional tolerances than seamless tubes, and if made in the same amount, the cost is lower. Mainly used for dissimilar steel welding sheet or high power density welding, metal welding parts can be quickly heated to high temperatures, melting any refractory metals and alloys.
Submerged Arc Welding Process (SAW)
Submerged arc welding involves arc formation 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 via a hopper. As the arc is completely covered by the flux layer, it is not normally visible during welding, and heat loss is also extremely low. There are two types of submerged arc welding processes as longitudinal submerged arc welding and spiral submerged arc welding processes.
In longitudinal submerged arc welding, longitudinal edges of steel plates are first beveled by milling to form a U shape. Edges of the U shaped-plates are then welded. Pipes manufactured by this process are subjected to expanding operation in order to relieve internal stresses and obtain a perfect dimensional tolerance.
In spiral submerged arc welding, weld seams are like a helix around the pipe. In both of the longitudinal and spiral welding methods the same technology is utilized, the only difference is the spiral shape of seams in spiral welding. The manufacturing process is rolling the steel strip, to make the rolling direction have an angle with the direction of the pipe center, forming and welding, so the welding seam is in a spiral line. The major disadvantage of this process is bad physical dimension of pipes and higher seam length which can easily lead into a defect or crack formation.
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.
There can be confusion about the way these materials are specified, and what the means to the exact characteristics of the pipe. The American Society for Testing and Materials (ASTM) along with The American Society of Mechanical Engineers (ASME) and the American Petroleum Institute (API) are the most referenced organizations for piping specifications in North America.
- Nominal Pipe Size
Pipe size is quoted as a “Nominal Pipe Size” or NPS. The origin of the NPS numbers for smaller pipes (< NPS 12) is different to the origin for larger diameter pipes. However, all pipes of a specific NPS number have the same external or outer diameter (OD). The internal diameter will vary depending on the wall thickness of the metal. The reason for this is so that the same structural supports can be used for all piping of a specific NPS number regardless of the wall thickness.
Steel pipe schedules are used to describe the wall thickness for pipes. Since it is a significant parameter that affects the strength of the pipe directly, it should be controlled properly. A pipe schedule is a dimensionless number and is calculated based on the design formula for wall thickness, given the design pressure and allowable stress. As the schedule number increases, the wall thickness of the pipe increases. The schedule number of a pipe therefore defines the internal diameter, as the OD is fixed by the NPS number.
- Pipe Weight
Pipe weight can be calculated by depending on NPS, which defies outer diameter, and the schedule which defines wall thickness of pipes. The formula uses the theoretical weight of steel of 40.8 pounds per square foot per 1 inch of thickness to determine the constant. Pipe weight is represented by the following formula where t is thickness, OD is outer diameter and W is pipe weight: W = 10.69 x t (OD – t)
Manufacturing standards for pipes commonly require a test of chemical composition and a series of mechanical strength tests for each heat of pipe. A heat of pipe is all forged from the same cast ingot, and therefore had the same chemical composition. Mechanical tests may be associated to a lot of pipe, which would be all from the same heat and have been through the same heat treatment processes. Material with these associated test reports is called traceable. For critical applications, third party verification of these tests may be required; in this case an independent lab will produce a certified material test report, and the material will be called certified.
Some widely used pipe standards or piping classes are as following:
- 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 specifically for fire sprinkler systems)