Steel Pipes and Tubes
Steel Pipes and Tubes
A steel pipe is a round tubular section or a hollow cylinder which is used mainly to convey fluids (liquids, and gases), and pneumatic conveying of solids. It can also be used for structural applications. Pipes are mainly used in the process industry. Pipe is generally manufactured to several long-standing and broadly applicable industrial standards. The word ‘tube’ refers to round, square, rectangular, and oval hollow sections used generally for pressure equipment, for mechanical applications, and for instrumentation systems. While similar standards exist for specific industry application tubing, tube is often made to custom sizes and a broader range of diameters and tolerances. Several standards exist for the pipes and tubes. In general terms the pipes and tubes are being considered as almost interchangeable, but in the pipe industry and engineering discipline, these terms are distinctively defined.
There are many important factors (Fig 1) which are to be considered for the selection of steel pipes for a service. These factors include (i) functional requirements, (ii) economic efficiency, (iii) repairability / maintainability, (iv) occupational safety and health, (v) corrosion resistance and long term durability, (vi) environment, (vii) weight, (viii) quality assurance, (ix) assemblability, and (x) manufacturability.
Fig 1 Important factors in selection of pipes
Classification of steel pipes and tubes
The two broad classifications of steel pipe and tube products are subdivided into several named use groups. As an example, the term tube covers three such groups namely (i) pressure tubes, (ii) structural tubing, and (iii) mechanical tubing. Similarly the term pipe covers five such groups, namely (i) standard pipe, (ii) line pipe, (iii) OCTG (oil country tubular goods), (iv) water well pipe, and (v) pressure pipe. There are also pipes for special applications, such as conduit pipe and tubular piling which do not fit any of these classifications. These use groups have in turn a number of uses or named use subdivisions. Examples are oil and gas transmission pipes, water main pipe, drill pipe, and casing pipe etc. These names have been developed without regard to method of manufacture, size range, wall thickness, or degree of finish.
Steel pipes are also often classified by many more methods. They are sometimes being designated by the property of the media being carried such as gravity pipe, low pressure pipe, high pressure pipe and heat pipe etc. Other way of designating the steel pipes is by the type of finish such as plain pipe or corrugated pipe. Steel pipes are also differentiated by the method of their joining which can be welding, threading, mechanical coupling, flanged joining, soldering, brazing, compression fitting, push on gasket fitting, flaring and crimping. They are sometimes being identified by the type of lining used for corrosion and heat protection such as refractory lined, insulation lined, cement mortar lined, epoxy coated, rubber lined, bitumen mastic lined, poly-urethane lined, and poly-ethylene lined pipe etc.
Dimension and specifications of steel pipes
The dimensions of the steel pipes and tubes belong among their basic characteristics. Steel pipes and tubes are normally specified by (i) pipe or tube sizes and their tolerances, (ii) steel used for pipes and for tubes, (iii) technical delivery conditions (TDC), and (iv) the testing of pipes and tubes such as test types, types of document control, and individual tests. The testing of steel pipes proves that properties of the pipes meet the requirements of the appropriate standards.
In steel pipe, the word ‘grade’ designates divisions within different types based on the carbon (C) content or the mechanical properties (tensile and yield strengths). Grade A steel pipes have lower tensile and yield strengths than grade B steel pipes. This is since they have lower content of C. Grade A is more ductile and is better for cold bending and close coiling applications. Grade B steel pipes are better for applications where pressure, structural strength and collapse are the requirements. They are also easier to machine because of their higher content of C.
Pipe sizes can be confusing because the terminology normally relates to the historical dimensions. For example, a half-inch pipe does not have any dimension which is a half inch. Initially, a half inch pipe did have an internal dimension of 0.5 inch but it also had thick walls. As technology improved, the wall thickness got thinner (saving material costs), but the outside diameter stayed the same so it could mate with existing older pipe. The outside diameter is an important dimension for mating with fittings.
In the steel pipes and tubes with circular cross section, except the length, there are three main dimensions which are (i) outside diameter (OD), inside diameter (ID), and wall thickness. Thus the two parameters important for pipe designation are namely (i) diameter, and (ii) thickness. In USA, the pipe size is designated by two non-dimensional numbers basically (i) nominal pipe size (NPS), and (ii) schedule (Sch.). Schedule (40, 60, 80, 120, etc.) refers to the pipe wall thickness. As the schedule number increases, the wall thickness increases, and the ID reduces. The European designation equivalent to NPS is DN (Diamètre Nominal/nominal diameter), in which sizes are measured in millimeters (mm). The term NB (nominal bore) is also frequently used interchangeably with NPS. Designating the OD allows pipes of the same size to be fit together, no matter what the wall thickness. This method also achieves the desired strength necessary for pipe to perform its intended function while operating under various temperatures and pressures.
Tubes are supplied in sizes up to 100 mm in diameter but have a wall thickness less than that of either large bore or small bore piping. The essential difference between pipe and tube is that pipe is specified by nominal bore and schedule. Tube is specified by the OD and a wall thickness. These parameters are expressed in inches or millimeters and express the true dimensional value of the hollow section. The wall thickness of a steel tube is expressed in inches or millimeters. It is also measured with a gauge nomenclature. Important dimensions of pipes and tubes are shown in Fig 2.
Fig 2 Important dimensions of pipes and tubes
Steel materials used for pipes and tubes
The material specifications of the pipes are governed by various national and international standards. Pipes and tubes are produces in various grades of steel. The most common grades of steels used for steel pipe production are given below.
Carbon steel – The majority of pipe are produced from C steels. C steels used for the pipe manufacture can be (i) mild steel (upto 0.3 % C), (ii) medium C steels (0.3 % to 0.6 C), and (iii) high C steels (C content greater than 0.6 %). The C content of the steel influences the mechanical characteristics of the material. Steels containing C content higher than 0.35 % is generally brittle. Further, as the C equivalent content of the steel increases, the steel becomes not weldable and pipes made of such steels cannot be joined by welding. Mild steel is the most common industrial piping material.
Alloy steels – There are several alloy steel grades used for the production of pipes for specialized applications. The different types of alloy steels used for pipe production are described here.
Alloy steel pipes made of nickel (Ni) steels contain from 3.5 % to 5 % of Ni. Ni strengthens low C steels. It increases steel strength, impact strength and toughness. It also improves toughness at low temperatures when added in small amounts. Ni increases elastic limit of steel. Ni when added to the steel increases its density and hardness. It improves steel’s resistance to oxidation and corrosion as well as scale formation. It also improves abrasive resistance of steel. Ni is heat resistant, and when combined with chromium (Cr) in the steel, it increases the heat resistance of that steel.
In molybdenum (Mo) containing steels Mo provides strength to the pipes at elevated temperatures. It is frequently used in combination with Ni and Cr. Mo is used efficiently and economically in the steels for the improvement of hardenability, reduction in the temper embrittlement, resisting the hydrogen (H2) attack and sulphide stress cracking, increasing elevated temperature strength, and improving weldability, especially in HSLA (high strength low ally) steels. Mo is used extensively because its inclusion (alone or with other alloying metals) results in a more economical way of getting improved properties in steels. Mo makes a unique contribution to hot strength, corrosion resistance and toughness. It improves high temperature creep strength, hardenability and the wear resistance. In OCTG pipes, Mo content ranges from 0.3 % to 1.0 %.
Chromium steels are used for the production of alloy steel pipes. Cr increases the hardenability of steel while there is a minimal effect on the ductility. Cr by forming carbides increases the edge holding quality of steel. The tensile strength of steel increases by 80 N/sq mm to 100 N/sq mm for every 1 % of Cr added. The yield strength also increases but the notch impact value reduces. Cr is normally added to steel for increasing oxidation resistance, and for improving high temperature strength. Corrosion resistance of Cr steels increases sharply at a Cr level of greater than 12 %. Cr forms a very coherent oxide layer on the steel surface which prevents further oxidation and thus provides resistance to corrosion in the steels. An addition of upto 9 % Cr opposes the tendency to oxidize at high temperatures and resists corrosion from sulphur (S) compounds. As a hardening element, Cr is frequently added with a toughening element such as Ni to produce superior mechanical properties.
Chrome vanadium (Cr-V) steel provides the steel pipes maximum amount of strength with the least amount of weight. Steels of this type contain from 0.15 % to 0.25 % V, 0.6 % to 1.5 % Cr, and 0.1 % to 0.6 % of C. Both Cr and V make the steel more hardenable. Cr also helps resist abrasion, oxidation, and corrosion. Cr and C can both improve elasticity.
Tungsten (W) steel is a special alloy steel used for pipes. W addition has remained a synonym for improving the high temperature properties of steel, in particular hot hardness, resistance to plastic deformation and wear at high temperatures. W produces stable carbides and refines grain size so as to increase hardness, particularly at high temperatures. A good grade of this steel contains from 13 % to 19 % W, 1 % to 2 % V, 3 % to 5 % Cr, and 0.6 % to 0.8 % C.
In manganese (Mn) steels, Mn improves hot workability by preventing the formation of low melting iron sulphide (FeS). Steels with a Mn/S ratio of at least 8/1 do not show hot shortness. Manganese sulphide (MnS), which forms preferentially to FeS, has a high melting point and appears as discrete and randomly distributed globules. Although solid at hot working temperatures, the MnS inclusions are soft enough to deform into elongated stringers during rolling or forging. The important property of Mn is its ability to stabilize the austenite in steel, as is done by Ni. Since Mn is not as powerful as Ni in its ability to stabilize austenite, more Mn is required to achieve the same effect. However, Mn has the advantage of being much less expensive. The effect of Mn in forming austenite can be reinforced by combining it with nitrogen (N2), which is also an austenite forming element. Mn also increases hardenability rate, used to significant advantage, depending on the steel type and the end product, to improve mechanical properties.
Stainless steels – Stainless steel pipe and tubing are used for a variety of reasons. These reasons include corrosion resistance, aesthetic appeal, heat resistance, low life cycle cost, full recyclability, biological neutrality, ease of fabrication, cleanability and good strength to weight ratio. Stainless steel is a family of alloys of iron that contains at least 10.5 % Cr and a maximum of 1.2 % C which is essential of ensuring formation of a self healing surface passive layer. This passive layer provides the corrosion resistance. The family of stainless steels can be grouped into four types namely (i) austenitic, (ii) martensitic, (iii) ferritic, and (iv) duplex stainless steels. Each of these types has specific properties and a basic grade. Stainless steels are also classified into 200 series, 300 series, and 400 series.
Austenitic steels are iron based alloys with Ni content ranging 3.5 % to 32 %, Cr content ranging 16 % to 28 % and C content not more than 0.1 %. The microstructure of these types of steels is austenite. These steels are non magnetic and cannot be hardened by heat treatment but can be hardened by cold working. These steels have better corrosion resistance and can be welded. These steels have good ductility and toughness. These steels are having good hygienic properties and cleanability. Stainless steels of this type are having good resistance to low (cryogenic) and high (melting point of the alloy) temperatures since they retain the austenitic structure. The most common grades are 304 (most commonly used), 310 (mainly for high temperature), 316 (for better corrosion resistance), and 317 (for better corrosion resistance). The limitations of these steels include (i) maximum temperature under oxidizing conditions is 925 deg C, (ii) suitable only for low concentrations of reducing acid, (iii) In cervices and shielded areas, there might not be enough oxygen (O2) to maintain the passive oxide film and crevice corrosion might occur, and (iv) presence of very high levels of halide ions, especially the chloride ions, can result into the breakdown of the passive surface film. These steel belong to 200 series and 300 series. Extra light wall thickness (schedule 5S) and light wall thickness (schedule 10S) stainless steel pipes are normally made from this steel).
Martensitic stainless steels belong to 400 series and have Cr content ranging 11.5 % to 18 % and C content 0.15 % to 1.2 %. Mo can also be used in type of stainless steels. These stainless steels are magnetic in nature, can be hardened by heat treatment for strength and hardness. These stainless steels have poor welding characteristics. . The common grades are 410, 420, and 440C (for very high hardness).
Ferritic stainless steels also belong to 400 series. These steels have low content of C (less than 0.08 %) and Cr content in the range of 10.5 % to 30 %. Some ferritic grades of stainless steels contain Mo upto 4 %. Cr is the main alloying element in these grades. Because of low C content these grades have a different metallurgical structure. These steels are magnetic in nature and cannot be hardened by heat treatment. They are always used in the annealed or softened condition. The weldability of these steels is poor. These steels are chosen when toughness is not the primary need but corrosion resistance especially to chloride stress corrosion cracking is important. The most common grades of these steels are 409 (high temperature), and 430 (major uses).
Duplex stainless steels have high Cr content (19 % to 32 %) and Mo content (upto 6 %) and lower Ni content (3.5 % to 8 %) than austenitic stainless steels. They are identified by a dual phase microstructure. They have a well balanced two phase structure consisting of both ferritic structure and austenitic structure. These steels have physical properties which reflect this structure. These steels have high resistance to stress corrosion cracking, increased resistance to chloride ion attack, good weldability, and have higher tensile and yield strengths as compared to austenitic and ferritic stainless steels besides good weldability and formability. The typical grade is 2205.
Galvanized pipe – Galvanized iron (GI) pipe is a regular steel pipe which is coated with a thin layer of zinc (Zn). Zn coating greatly enhances the life of the pipe by protecting it from rust and corrosion so that the pipe may be exposed to the outdoor environmental elements. GI pipes are normally joined together by threaded connections. The pipes are used in outdoor applications wherever the strength of steel is desired, such as fence posts and rails, scaffolding and as protective railings. When used as water pipe, the zinc barrier coating tends to react to the minerals in the water, often causing plaque to build up inside of the pipe. This impedes the water flow and, in severe cases, may lead to bursting of the pipes.
Production of steel pipes
Broadly steel pipes can be manufactured by techniques which may or may not contain any seam. Pipes which do not contain any seam are termed as seamless pipes. Pipes having seams are produced by welding of steel strip. There are four methods normally used to produce steel pipe. These are (i) fusion welding, (ii) electric resistance welding, (iii) submerged arc welding, and (iv) production of seamless pipes.
Fusion welding process – Fusion welding process for producing pipe is sometimes called continuous welding process. The process starts with hot rolled strip (skelp) of the required width and thickness for the size and weight of pipe to be made. Successive coils of steel are welded end to end to form a continuous ribbon of steel. The ribbon of steel is fed into a leveler and then into a gas furnace where it is heated to the required temperature for forming and fusing. The forming rolls at the end of the furnace shape the heated strip into an oval. The edges of the strip are then ﬁrmly pressed together by rolls to obtain a forged weld. The heat of the strip, combined with the pressure exerted by the rolls, form the weld. No metal is added into the operation. Final sizing rolls bring the pipe into its required dimensions.
Electric resistance welding process – In case of electric resistance welding (ERW), the processing of ERW pipe begins as a hot rolled strip of steel with appropriate thickness and width for forming a pipe which conforms to its relevant speciﬁcation. ERW pipe is cold formed. The steel strip is pulled through a series of rollers which gradually form it into a cylindrical pipe. As the edges of the now cylindrical strip come together, an electric charge is applied at the proper points to heat the edges so they can be welded together. ERW pipe is a high speed production pipe product which can be made in continuous lengths of required dimensions. It produces uniform wall thicknesses and outside dimensions and is made in a wide range of speciﬁcation.
Submerged arc welded process – Submerged arc welded (SAW) pipe derives its name from the process wherein the welding arc is submerged in ﬂux while the welding takes place. The ﬂux protects the steel in the weld area from any impurities in the air when heated to welding temperatures. When both inside welds and outside welds are performed, the welding is accomplished in separate processes and the pipe is considered to be ‘double submerged arc welded’ (DSAW). There are three common types of pipe production methods by the DSAW process. These are (i) U&O method, (ii) rolled and welded or pyramid roll method, and (iii) spiral weld method.
The U&O Method is so called since it ﬁrst uses a ‘U’ press, then an ‘O’ press to complete cylinder forming from long plates ordered to size and grade. The cylinder is then welded inside and outside by the submerged arc process by using as many as ﬁve welding wires. Most U&O is cold expanded either mechanically or hydraulically. When it is cold expanded, there is improvement in the yield strength of the DSAW pipe. This method of pipe production produces exceptional quality with exact dimensional tolerances. The primary use of this type of pipe is gas and oil transmission. It requires large minimum tonnages for size setup.
The rolled and welded method of manufacturing DSAW pipes is also called the ‘pyramid roll method’ since it uses three rolls arranged in a pyramidal structure. The plate, ordered by grade and thickness, is rolled back and forth between the pyramid rolls until the cylinder is formed. The cylinder is then moved to the welding stations. Rolled and welded pipe has the advantage of being rolled in small quantities with short lead times. It can be produced in very large diameters, either ID or OD, and in extremely thick walls.
Spiral weld pipe is a steel pipe having a DSAW seam along the entire length of the pipe in a spiral form. The outside diameter is determined by the angle of the de-coiled steel against the forming head. The more acute is the angle, the greater is the diameter. The production of large, hot rolled coils of sufficient width and the development of dependable non-destructive testing methods has enabled this product to be placed in more demanding service. There is a minimum tonnage required for rolling. Because the manufacturing process is slow, it gives an advantage of short term changes to the order. This same slow production can also be a disadvantage when large tonnages are needed with a short lead time. Spiral weld pipe is produced to limited speciﬁcation.
Production of seamless pipes – Seamless pipes are produced from steel in a solid, round cylindrical shape, called a square or round bloom or billet. The input materials are heated and then either pushed or pulled (while being rapidly rotated) over a mandrel with a piercing point positioned in the centre to make it a hollow shell and then rolled or extruded and drawn to size. The pipe is then ﬁnished until it becomes the size and wall thickness desired. The seamless pipe manufacturing process consists of the three principal stages namely (i) making of a hollow pipe shell in the piercing or extrusion operation, (ii) elongating the hollow pipe shell by reducing its diameter and wall thickness, and (iii) making of a final pipe in the hot or cold rolling process. Since the pipe is formed in a heated manner, the pipe is normalized and has a consistent steel cellular pattern throughout its circumference. Also, as the manufacturing process does not include any welding, seamless pipe is perceived to be stronger and more reliable. Seamless pipes are regarded as withstanding pressure better than other types of pipes.
Seamless pipes are available in heavy wall thicknesses and exotic chemistries, and are suitable for coiling, ﬂanging and threading. There three popular processes for producing seamless pipes. These are (i) hot extrusion process, (ii) rotary piercing process, and (iii) cupping and drawing process.
Hot extrusion process is a hot working process for making hollows, suitable for processing into finished pipes of regular and irregular form, by forcing hot, pre-pierced billets through a suitably shaped orifice formed by an external die and internal mandrel. The outside of the hollow attains the size and contour imposed by the die while the inside conforms to the size and contour of the mandrel. Extruded hollows can be further worked into pipe and tubular products by cold finishing methods.
In the rotary piercing process, steel rounds of the required diameter and length are first heated to rolling temperature. The hot round is then fed into a set of rolls having crossed axes and surface contours which pull the round through the rolls, thus rupturing it longitudinally. The force of the rolls then causes the metal to flow around a piercing point, enlarging the axial hole, smoothing the inside surface, and forming a pipe. After being pierced, the rough pipe is usually hot rolled to final dimensions by means of plug mill or mandrel mill, which is usually followed by stretch reducing mill.
In the cupping and drawing process a circular sheet or plate is hot cupped in a press through several pairs of conical dies, each successive pair being deeper and more nearly cylindrical than the previous set. The rough pipe then is drawn to the finished size.
Steel pipes can be specified with a specific end preparation. There are three standard types of end preparations. These are (i) plain ends, (ii) bevel ends, and (iii) threaded ends. In case of plain ends the pipe is cut at 90 degrees, that is, perpendicular to the pipe run. This type of end is needed when being joined by mechanical couplings, socket weld fittings, or slip on flange. In case of bevel ends the pipe has a bevel end surface, that is, the end is not at a right angle (perpendicular). The standard angle on a pipe bevel is 37.5 degrees but pipes with other non-standard angles can be produced. Beveling of pipe is to prepare the ends for butt welding. The threaded ends on the pipes are typically used on pipes having diameters 80 mm and smaller. Threaded connections are referred to as screwed connections. With tapered grooves cut into the ends of a run of pipe, screwed pipe and screwed fittings can easily be assembled without welding or other permanent means of attachment. Standard threads are used for threaded end so that pipes can be assembled with fittings easily.
Appropriate controls during the production of steel pipes for the maintenance of different parameters as stipulated in the national and international standards are important. The controls include chemical analysis, dimensions, mass (single or for lot supply), mechanical properties such as tensile strength, yield strength, and percent elongation, leak-proofness or ensuring continuity which is ensured through hydrostatic, pneumatic and/or eddy current test. For the production of steel tubes, stricter process controls are needed. These controls are to be exercised for soundness through ultrasonic testing and eddy-current testing methods.