What am I bending?: The Differences Between Tube, Pipe, and More

Mechanical Tubing refers to tubing that has been produced by the electric resistance welding (ERW) process that is provided in the state in which it leaves the welder, with only minimal downstream processing.

The manufacturing process for as-welded mechanical tubing begins with coils of steel, which are slit to the proper width for the desired tube size. The cut strip passes through a series of rolls that form it into a tubular shape. It then passes through an electric resistance welder which joins the edges together, under pressure, to complete the tubular shape. In-line equipment then tests the tube’s weld for integrity.

Hot-Rolled ERW (HREW) is rolled into a tube at elevated temperatures. This process produces more malleable (easier to form) tubing, which is not as strong, covered with scale, and not as uniform in dimension as cold-rolled tubing. It is, however, the most inexpensive option.

Cold-Rolled ERW (CREW) follows the same manufacturing process, except at room temperature. Compared to HREW, CREW is stronger because of the improvement in the crystal lattice structure from improved grain size, shape, and orientation. As a result of these characteristics, the finished product is straighter, has a much smoother and more uniform surface finish, and is made to tighter, more consistent dimensions.

For either type, no filler is added in the welding process, so the composition of the weld is the same as the parent material. Welding leaves a small amount of flash on the tube’s outer and inner surfaces. The outer flash is removed to provide a smooth contour and maintain OD tolerance. Depending upon customer specifications, the inner flash can be controlled to varying degrees.

Drawn-Over-Mandrel (DOM) tubing basically starts its life as CREW. After shaping, the weld is tested for integrity and cut to length for further processing. The tube gets cleaned and annealed and is then drawn through a series of dies and over mandrels. This reduces the diameter of the tube and thins its walls to the required dimension. Close dimensional accuracy is achieved through tight control of both outside and inside diameters.  Drawing improves the tube’s concentricity, tensile strength, hardness and machinability. The cold-drawing process creates a uniform, precision product with substantially improved tolerances; superior surface finish and tensile strength; increased hardness; and good machinability.

Strength: Cold-drawing gives DOM high yield and tensile strength.

Uniformity: DOM has uniform wall thickness and concentricity, as well as uniform mechanical properties.

Close Tolerances: DOM offers exceptionally close tolerances for OD, ID and wall thickness dimensions.

Surface Quality: OD and ID surfaces of DOM are free of oxide and scale and have a smooth, dense finish with no trace of the weld.

Seamless Tubing requires no welding at all and produces an end product without the seam that you would find in a welded product. It starts out as a large piece of solid rod that is heated up in a large furnace to high temperatures. As it exits the furnace, it enters a piercing mill, which draws the hot rod over a mandrel which creates the hollow center. Here, seamless tubing is broken down into two categories:

Hot-Finished Seamless Tubing then enters a mandrel piercing mill, where it is drawn over another mandrel and a series of dies to reduce the tubing’s outer and inner dimensions to specifications. After passing through a stretch-reducing mill, the tubing enters a cooling mill, which cools the tube to room temperature. Hot-Finished Seamless Tubing, which is most effective for tubing that requires thicker walls than provided by other tube manufacturing methods is a cost effective option where precise finish dimensions and surface quality are of secondary importance. Hot-Finished Seamless Tubing is manufactured to OD and wall dimensions.

Cold-Drawn Seamless Tubing follows basically the same manufacturing process, except it is cooled just after it exits the first piercing mill. Just like CREW, Cold-Drawn Seamless Tubing is a cold drawn, which offers uniform tolerances, enhanced machinability and increased strength and tolerances compared to hot-finished products. Typically, Cold-Drawn Seamless Tubing is available only in smaller sizes than Hot-Finished Seamless Tubing.

When it comes to tube versus pipe, there’s one thing you really need to know: 1-1/2” tubing is not the same as NPS 1-1/2 pipe. For 1-1/2” tubing, the actual outside diameter (OD) is 1.500”. For NPS 1-1/2 pipe, the actual outside diameter (OD) is 1.900”. This is true for all sizes of pipe less than NPS 14. That means that if you have 1-1/2” tubing and NPS 1-1/2 pipe, you will need a different die for each size. Our benders are rated for bending Schedule 40 Pipe between NPS 1/4 and 2. The chart below demonstrates some dimensions for Schedule 40 Pipe up to NPS 14. If you’re still confused, check out the description below the chart. 

Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes. Pipe size is specified with two non-dimensional numbers: a nominal pipe size (NPS) for diameter based on inches and a schedule (Sched. or Sch.) for wall thickness. NPS is often incorrectly called National Pipe Size, due to confusion with national pipe thread (NPT). Based on the NPS and schedule of a pipe, the pipe outside diameter (OD) and wall thickness can be obtained from reference tables such as those above. For NPS ⅛ to 12 inches, the NPS and OD values are different. For NPS 14 inches and up, the NPS and OD values are equal. In other words, an NPS 14 pipe is actually 14 inches OD. The reason for the discrepancy for NPS ⅛ to 12 inches is that these NPS values were originally set to give the same inside diameter (ID) based on wall thickness standards at the time. However, as the set of available wall thicknesses evolved, the ID changed and NPS became only indirectly related to ID and OD. For a given NPS, the OD stays fixed and the wall thickness increases with schedule.

Pipe is sometimes used in structural applications like handrails, but its intended use is to transport substances which can flow (i.e. liquids, gases, (fluids), masses of small solids), so the critical dimensions are inside diameter (ID) and wall thickness. The ID determines the ability of the pipe to carry materials. ID, coupled with wall thickness, determines specifications like burst pressure. Like tubing, pipe is manufactured in different ways for different needs and applications. There are three ways to manufacture pipe.

(1) In centrifugal casting, a permanent mold is rotated continuously about its axis at high speeds as the molten metal is poured. The molten metal is centrifugally thrown towards the inside mold wall, where it solidifies after cooling. (2) Welded pipe is manufactured like ERW tubing and (3) Seamless pipe is manufactured like seamless tubing.

Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes. Pipe size is specified with two non-dimensional numbers: a nominal pipe size (NPS) for diameter based on inches and a schedule (Sched. or Sch.) for wall thickness. NPS is often incorrectly called National Pipe Size, due to confusion with national pipe thread (NPT). Based on the NPS and schedule of a pipe, the pipe outside diameter (OD) and wall thickness can be obtained from reference tables such as those above. For NPS ⅛ to 12 inches, the NPS and OD values are different. For NPS 14 inches and up, the NPS and OD values are equal. In other words, an NPS 14 pipe is actually 14 inches OD. The reason for the discrepancy for NPS ⅛ to 12 inches is that these NPS values were originally set to give the same inside diameter (ID) based on wall thickness standards at the time. However, as the set of available wall thicknesses evolved, the ID changed and NPS became only indirectly related to ID and OD. For a given NPS, the OD stays fixed and the wall thickness increases with schedule.

Pipe is sometimes used in structural applications like handrails, but its intended use is to transport substances which can flow (i.e. liquids, gases, (fluids), masses of small solids), so the critical dimensions are inside diameter (ID) and wall thickness. The ID determines the ability of the pipe to carry materials. ID, coupled with wall thickness, determines specifications like burst pressure. Like tubing, pipe is manufactured in different ways for different needs and applications. There are three ways to manufacture pipe.

(1) In centrifugal casting, a permanent mold is rotated continuously about its axis at high speeds as the molten metal is poured. The molten metal is centrifugally thrown towards the inside mold wall, where it solidifies after cooling. (2) Welded pipe is manufactured like ERW tubing and (3) Seamless pipe is manufactured like seamless tubing.