Carbon Steel and Stainless Steel Welded and Seamless Pipe

Stainless steel pipe and carbon steel pipe are the most common type of standard pipe used today. They are part of tubular goods, which are manufactured to different specifications and standards. Pipes are sold by "nominal pipe size" in sizes from 1/8" to 72". The two most common types of pipes are welded pipe and seamless pipe. Both are available in carbon steel and stainless steel. Welded pipe is also called ERW (Electric Resistance Welded) pipe. Most common specification for welded carbon steel pipe is A53. Seamless pipe is made by a process that requires no welding. Most common specification for seamless carbon steel pipe is A106B. Stainless Steel pipe is made to spec A312. Stainless steel pipe is also available in both seamless and welded configuration. Most common stainless material is 316 and 304.

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Types of Pipes and Tubular Goods

There are six types of pipes and tubular goods.
  1. Standard Pipe: There are three different types of standard metal pipes - welded (ERW Pipe), seamless pipe, and galvanized pipe. They represent 10% of all tubular products.
  2. Line Pipe: Used primarily in Oil and Gas Applications. Line pipe includes ERW, FW, SAW and DSAW Pipe. They are manufactured to API 5L Specification and are available in X42, X50, X60 etc. grades. They represent 21% of all tubular products.
  3. Oil Country Tubular Goods (OCTG): This includes drill pipes, tubing and casing. It is used in drilling and completion of Oil and Gas wells. OCTG are produced by ERW and Seamless manufacturing. OCTG represent 35% of all tubular goods.
  4. Pressure Tubing: used for industrial and pressure application. Pressure tubing are produced using seamless manufacturing. They represent 2% of all tubular goods.
  5. Mechanical tubing: used for mechanical and structural application and is produdced by ERW and seamless manufacturing. They conform to ASTM specification. Mechanical Tubing represent 17% of all tubular goods.
  6. Structural Tubing: used for support or retention purpose. This tubing can be round or square and are produced by ERW manufacturing. They are used for fences, construction and other misc. support needs. They represent 15% of all tubing goods.

How Pipe is Measured?

Pipe is generally identified by "Nominal Pipe Size" (NPS), with the wall thickness defined by "Schedule" (SCH). NPS is dimensionless designator and is an indication of a size but not an actual size. This is confusing for many people new to the piping industry. NPS 1/8 through NPS 12 has an outside diameter that is defined by the ansi pipe sizes. Generally speaking, a schedule 40 pipe dimension for up to 12 NPS is roughly equal to the ID of the pipe. Pipe sizes 14 and above are less confusing as a 14 NPS is equal to 14" OD (outer diameter).

Pipe Size Chart

ASME B 36.10 has more details of pipe sizes. The industry norm for pipe size also varies between different types of pipe. For a standard pipe, a 4.5" OD pipe will be referred to as 4" NPS however for line pipe and OCTG, it will be referred to as 4.5" only. Wall thickness is also mentioned differently. For standard pipe, pipe schedule is used to designate wall thickness. However line pipe is almost always referred by wall thickness whereas OCTG is referred by weight per foot.

Weight of Pipe

Weight of pipe is measured in pounds per foot or kilograms per meter. One should remember that weight of pipe in the ansi pipe size chart is "theoretical" weight and not an actual weight. Actual weight will vary based on the tolerance of pipe size.

Formula to calculate weight of steel pipe is;

W = [{OD-WT} x WT] x 10.69

W = Weight per lineal foot

OD = Outer Diameter

WT = Wall Thickness

Pipe Manufacturing

Steel pipe is grouped in two main categories;

  • Welded Pipe
  • Seamless Pipe

There are four main methods of producing metal pipe:

  1. Furnace Weld (FW). Also called Continuous Weld
  2. Electric Resistance Weld (ERW)
  3. Submerged Arc Weld (SAW), also DSAW
  4. Seamless (SMLS) method

seamless and welded pipe

Electric Resistance Weld Pipe (ERW)

ERW Pipe is produced in sizes from 1/8 to 24 in NPS. ERW welded pipe is produced from flat sheet metals. ERW pipe is first cold formed into a cylindrical shape and then welded longitudinally.  both continuous welded and elctric resistance welded pipe are made by shaping rolls of coiled steel into cylindrical forms and welding along the longitudinal seams. Picture below shows the first step of forming strips into cylindrical shape and then fusing it together in 2nd step.

Forming and electric resistance welding of pipe

There are many ways for the electric current to be used for welding the rolled strip. This includes revolving copper disc, fixed alloy Cu contacts and induction coils. These methods raise the temperature of the steel edge to 2600 deg F for effective welding. The rolling process exerts so much pressure that it creates a metal welded seam on the outside and inside. These weld seams are then removed during the finishing process. The welded pipe is heat treated, inspected and hydro tested.

ERW pipe is available in following common specs;

ASTM A53, A135, A252, A500, A795, API 5L and API 5CT.

Most common spec for welded pipe is A53 and API 5L.

Seamless Pipe

What is seamless definition? or seamless meaning? we get this question a lot. Seamless pipe is produced in sizes from 1/8 to 36" nominal. Seamless pipe has no longitudinal seam or weld when compared to ERW pipe. In simple terms the seamless pipe is produced by piercing a solid round billet leaving behind a hollow seamless pipe. There are two main methods of producing seamless pipe;

  1. Hot Rotary Piercing
  2. Extrusion Process Method

In Extrusion Process, hot billets of metal are formed into seamless pipe by forcing the billets through extrusion dies and over a forming mandrel. See picture below;

Extrusion process seamless manufacturing

Hot rotary piercing method is the most common process of producing seamless pipe. In this process hot billets are forced through rotating heavy rollers over a mandrel or piercing plug. The piercing plug creates the hole through the seamless cylindrical billets. Picture below illustrates the hot rotary process.

Piercing Mill seamless pipe

Rotary Mill seamless pipe

Seamless pipe goes through various finishing operations such as straightening, inspection, hydro testing and imaging. Seamless pipe is widely used in pressure piping, construction, petrochemical and oil refineries. Seamless pipe is available in the following specifications;

ASTM A53, A106, A252, A333, A335, A501, A523, A589, API 5L and API 5CT. Most common seamless pipe is A106B.

End finish of seam less pipe includes plain ends either beveled or square cut. Seamless carbon steel pipe is available in black, bare, galvanized or with epoxy coating.

Steel Pipe Lengths

Generally steel pipe is supplied in 21 ft lengths. It is also available in Single Random Length (SRL) which could be anywhere from 17 ft to 24 ft. Double Random Length (DRL) refers to pipes that is at least 37 ft long and up to 44 ft.

Steel Pipe End Finish

There are five common type of metal pipe end finishes;

  1. Threaded and Coupled
  2. Threaded without Coupling
  3. Plain End - Cut Square
  4. Beveled for Welding - 30 deg bevel
  5. Grooved End - Cut or rolled for mechanical coupling

Picture below shows different steel pipe end finishes;

threaded and coupled pipe end threaded end pipe 
Plain End Pipe Trupply
beveled end pipe
grooved pipe end straight tapped coupling pipe end
 taper tapped and recessed coupling


Coupling for extra strong and double extra strong pipe are taper threaded and recessed for all sizes. Picture above shows straight tapped coupling and straight tapped coupling with recess.

Steel Pipe Identification and Marking

Steel pipe is available in different classification, grade, weight, schedule number etc. Hence it is important to identify steel pipe markings. Steel pipe is identified by paint stencil or stamped marking on the pipe itself or by a tag attached to it.

Pipe identification as per ASTM

Picture below shows marking requirements as per ASTM specifications

ASTM pipe identification

Pipe identification as per API

Picture below shows marking required as per API specification.

api pipe identification

 Metal Pipe Finishing Processes

Most common finish on the metal pipe is lacquer coating that prevents corrosion and protects the pipe from weather. This is also called Black Iron Pipe. Other than lacquer, there are four types of metal pipe finishes;

  1. Bare Metal
  2. Pickled only
  3. Pickled and Oiled
  4. Galvanized Pipe

Galvanizing involves using zinc coating as a sacrificial element to prevent corrosion. Zinc is usually applied by a hot dipping process where pipe is submerged in a molten bath of zinc solution. Standard weight of zinc is 1.8 oz per sq ft. Galvanized pipes provide good protection to the pipe inside and out.

Steel Pipe Coating and Lined Steel Pipe

Pipe's corrosion resistance can also be enhanced with corrosion resistant pipe coating. Such coatings are usually long lasting compared to lacquer or galvanized finishes. There are three main types of Corrosion Protective Coating for Steel Pipe

Metallic Coating

Metallic Steel Pipe Coating

Non-Metallic Coating

Non-Metallic Steel Pipe Coating

Pipe Wrapping

Pipe Wrapping Steel Pipe

Steel Pipe Material

Metal Pipes are available in following material grades;

  • A106B/A53B/API 5L-X42/API 5L B
  • Stainless Steel  304
  • Stainless Steel 316
  • Super Stainless A20
  • Duplex Stainless D2205
  • Nickel Alloy A200
  • Nickel Alloy A400
  • Nickel Alloy A600

Stainless Steel Pipe

Stainless steel is iron based alloy containing a minimum of 16% chromium. Nickel is another important alloying element in stainless steel pipe. Stainless Steel pipes are highly resistant to corrosive media and to oxidation at almost any temperature.

Stainless steel welded and seamless pipes are used for variety of reason such as to resist corrosion, preform at high temperature, for clean look and low maintenance cost. The most important aspect for stainless steel pipe is its inherent material characteristics that allows much thinner walls to be created such as sch 5, sch 10 without compromising the structural integrity of the pipe. The use of fusion welding to join pipe also eliminates the need for threading.

Type 304 stainless steel pipe is mostly used for general corrosive resistant application. it is used in chemical plants, refineries, paper mills and food industries. 304SS pipe has maximum carbon content of 0.08%. It is not recommended for use in the temperature range between 800 deg F and 1650 deg F due to carbide precipitation at the grain boundaries. Type 304L is the same as 304 except that it has a 0.035% maximum carbon content.

Type 316SS pipe is much more resistant to pitting than other chromium nickel alloys due to the addition of 2-3% molybdenum. It is useful in applications with acids, brines, sulphur water, sea water and with hologen salts. 316L stainless steel pipe, like 304L,  has a max carbon content of 0.035%.

Welded Stainless Steel Pipe is made to specification ASTM A312 and A358. A312 is the most common spec for stainless steel pipe. Welded stainless pipe is made from 1/8" to 24" NPS.

Seamless Stainless Steel Pipe is made to specification ASTM A312 and A376. A312 is also the most common spec for seamless stainless steel pipe. Seamless stainless pipe is made from size 1/8 to 14" nominal

Steel Pipe Country of Origin

Steel pipe is made by mills all around the world and traded in different countries. Some of the big names in the pipe industry are US Steel, ArcelorMittal, Nippon Steel, Nucor Corp, Hyundai Steel, ThyssenKrup, Vallourec, Hebei Iron and Steel Group, Posco, Jiangsu Shagang, Ulma Piping, Shultz USA and Webco etc.

Q&A Steel Pipe

Trupply sells a large variety of welded and seamless pipe. Based on our years of experience, the following are main questions or key points customers inquire about;

  • Steel pipe is sold by nominal pipe size and schedule. It is important to mention both
  • A106 B is the most common seamless pipe specification. Most customers just know they want A106B pipe and it simply implies they want carbon steel seamless pipe as per A106B. A106 is NOT a specification for stainless steel pipe
  • Pipe sizes 10 inch and above do NOT correlate directly with schedule number. Such as SCH 40 and SCH 80 does NOT always mean Standard and Extra Heavy respectively. Refer to pipe chart to make sure you are ordering the correct wall thickness
  • A53 grade B is the most common specification of welded pipe. It just means a pipe that has been produced by ERW process.
  • Welded pipe does NOT have a weld seam inside or outside. Welded pipe is produced by electric resistance welding and the seam has been taken out during manufacturing process
  • Pipe sizes smaller than 2" are available in continuous weld pipe, also know as A53 grade A.
  • Stainless Steel pipe is available in both welded and seamless configuration and in 316 and 304 material grade. Other stainless grades are also available but are less common
  • Stainless steel pipe is also very common in schedule 10 which is a thin wall.
  • Stainless steel pipe schedules are specified by "s" after the schedule number such as Sch 10s, Sch 40s, Sch 80s.
  • Stainless steel pipe schedules do NOT always correlate with carbon steel pipe schedules. Check ansi pipe chart for proper determination of schedule number and wall thickness
  • Pipes are mostly sold by SRL (single random length) or Double Random Length (DRL). SRL means pipe will be usually between 17 ft and 24 ft. DRL usually means pipe will be 37 ft to 44 ft.
  • Majority of pipe sold have lacquer finish. If you want bare pipe, please specify at the time of ordering
  • Epoxy coating of pipe can be done upon request.
  • Most common pipe end type is plain end or beveled end. Threaded ends are also available upon request