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The Tube Bible

quoted from billavistas article:

Steel tubing is usually supplied in one of the following forms:

Seamless Tube.
This is expensive and specialized stuff. It IS NOT the commonly used (and misreferenced) DOM tubing, as DOM tubing does indeed have a seam (albeit, almost invisible - more details below). True seamless tubing is uncommon in 4x4 and automotive use. It is seamless because it is manufactured by a process know as "extrusion" where a solid bar of steel is pierced down the center with a die, at unthinkable pressures, to form a tube. The process looks similar to how hollow pasta (macaroni etc) is made. There are 2 sub-types of seamless tube:

Cold Drawn Seamless
(CDS) Tube is normally drawn to O.D. and I.D. dimensions and produced to standard dimensional tolerances (this differs from most other types of tubing except DOM) . It is normally made from SAE 1018 and is considered good quality.
Hot Finished Seamless (HFS) Tube is lower in cost than cold drawn and most applicable where precise dimensions and surface quality are of secondary importance. It is manufactured to O.D. and wall dimensions from SAE 1026 steel and is scaly, less dependable and not as strong as cold drawn tube.

Electric Resistance Welded
(ERW) Tube
ERW is the most economical and readily available type of mechanical tuning. It is produced by taking a flat bar of steel and rolling it into a tube shape (picture rolling up a newspaper - but without any overlap) and then welding the seam - by, you guessed it - electric resistance - hence the name. Electric resistance welding is somewhat like a long, continuous spot weld. It's often computer controlled and extremely consistent. ERW is normally SAE 1010 (for wall thickness < 16 ga) or SAE 1020. ERW tube comes in 2 flavours:

Hot Rolled ERW (HREW)
HREW is rolled into a tube at elevated temperatures, usually way above room temperature. This produces a tubing that is more malleable and therefore easier to form but that is also not as strong, is supplied covered with scale, and not as uniform in dimension as cold rolled. It is also quite a bit cheaper than cold rolled.

Cold Rolled ERW (CREW)
CREW is manufactured by a process in which a steel bar is rolled into a tube and the seam welded, usually at room temperature. Compared to hot rolled, CREW is stronger - (greater yield strength) - because of the improvement in the crystal lattice structure from improved grain size, shape, and orientation imparted by being worked at cold (room) temperatures), straighter, has a much smoother and more uniform surface finish, and is made to much tighter, more consistent dimensions. It is the best economical choice for tube work, and because of the better surface finish and tighter dimensional tolerances it is much nicer to work with than HREW.

Drawn Over Mandrel (DOM)
Strong and well-finished DOM is an electric resistance welded tube tested for soundness of weld and drawn through a die and over a mandrel. This process imparts significantly improved mechanical properties to the tube, due to the cold working process. It is considered a high quality tube, and is normally constructed from SAE 1020 or 1026 steel. Note that, technically DOM refers to the process by which the tube is finished after having started as an ERW tube. Technically, DOM is not a type of steel tube, but rather a process. As so often happen though - in common use the term has become accepted to mean a specific type of tubing rather than a process. In this case, when people say "DOM" they normally mean an ERW tube drawn over a mandrel at (close to) room temperature and made from SAE 1020 steel. It is normally drawn to O.D. and I.D. dimensions. Here is what the Steel Tube Institute of North America has to say about DOM:

The DOM Manufacturing Process

The manufacturing process for DOM tubing begins with coils of steel, which are slit to the proper width for the desired tube size. The strip is cold formed and passed through an electric resistance welder which joins the edges together, under pressure, to complete the tubular shape. After testing the weld's integrity, the tubing is cut to length for further processing.

The cold-drawing process creates a uniform, precision product with substantially improved tolerances, surface finish and tensile strength, increased hardness and good machinability. In this process, the tube is cleaned and annealed, and one end of each length is squeezed to a point so it can be gripped by the drawing mechanism. The tube is then drawn through one or more dies and over mandrels. This reduces the diameter of the tube and thins its walls to the required dimensions in a controlled fashion to provide the qualities desired in the finished product. Metallurgically, drawing improves the tube's concentricity, tensile strength, hardness and machinability. Close dimensional accuracy is achieved through tight control of both outside and inside diameters.[10]

Alloy Steel Tubing
Is not really a different type of tubing, as it will be manufactured by one of the above described methods, usually by extrusion, but from alloy steel instead of mild steel. It is generally available in either the normalized or the annealed condition. Commonly referred to as chrom-moly tube - it has very strict welding process and post-welding heat treatment and stress relieving requirements. It is my opinion that it can (should) only be TIG or Oxy-Acetylene welded, and then only if proper stress relieving will be done post welding. Sure people MIG weld it all the time, and you can safely do so - BUT - what you have in the end is a superior tube with an inferior weld joint which reduces the overall strength of whatever you fabricated to the weakest link (the weld in this case) and so you have a very expensive structure that is no better overall than one made from 1020 DOM.

Exact chemical content, heat treatment, physical properties, production method and therefore mechanical properties will vary from one supplier to the next, even for seemingly similarly named products - the wise fabricator double-checks all assumptions carefully before building anything. Different suppliers will also have available different products. For example, Ryerson-Tull, one of North America's largest suppliers list in their catalogue of steel tubing between 1" and 2' OD, the following types (not all available in all sizes):

CDS, HFS, DOM, ERW, and a proprietary product they call "Ry-Star 512 Extra".

Many suppliers will have such special proprietary products - you will have to check with your supplier for its proprietary product properties and specs.

One thing that has to be watched out for is that the industry bends a lot of carbon steel tubing to make lots of things and so most carbon steel tubing is available in the annealed condition—woe to him who does not detect it before he builds the part. I have a very good friend who once got an entire roll cage cut, bent, fitted and tacked before he realized that his merry men were working with annealed boiler tube. The other thing that we don't want is "free machining tubing." I currently use round carbon steel DOM mechanical tubing for most things other than suspension links (there I use E4130N and stress relieve and heat treat after welding). For roll cages I use either 4130 or DOM 1020. I do not want to know about hot-finished tubing because I do not want to clean it. I am old enough to remember the days when English ERW tubing was liable to split along the weld seam. As a matter of principle (or, possibly, stubbornness) I do not use ERW or butt welded tube on the race car; although, since it is a lot cheaper, I use it all over the trailer and the shop.[11]

My personal mantra on the subject of steel tubing choice for 4x4s and rock crawler's is:

"You can't go wrong with 1020 DOM!"
Poo Pipe or schedule 40 is totally different from tube. In numerous ways. Material. How it's measured. Etc. It is made like HREW but not of the same quality steel (i'm pretty sure atleast). There was actually a thread on Pirate one time that tested the strength of Sch. 40
Pipe is measured by I.D. while tube is measured by O.D. They're are completely different in most every aspect and I wouldnt use poop pipe on anything except to flush my poop out