Because the bending dies for pipe are different than for tubing. Tubing is measured on the outside, and pipe is measured on the inside. IOW - you ask for 1 1/2" tubing, the outer diameter is 1 1/2". If you ask for 1 1/2" pipe, the outer diameter will be more like 1 5/8", depending on the pipe
ERW= electric resistance welded
CDEW= cold drawn electric welded
and it looks like its strengths depend on the type of steel used in the making. there are different types available from most steel specialty shops. alloys and high carbon. they all say that the welds are stronger than the steel of the pipe/tube itself. you can get the specs from most places and compare. i think most race care places use DOM as its seamless and i would think bends better and possibly the others would be better if there were no bends needed for your applications. but what do i know? i am thinking about using solid round stock for my steering rods since there isnt a local place that has DOM in stock. i will use any of the others if they are locally available. i am going to ask at the machine shop. i got 2 of my TREs today and they are lifetime warranty so i am happy about that. i will carry a spare or two in case of breakage. i will post anything else i learn for your benefit.
DOM isn't actually seamless, it's just been smoothed out in the manufacturing process to eliminate that seam on the inside. It's also more consistent in it's wall thickness than your standard HREW, which is why it's preferred for rollcages and tube chassises. DOM is also a bit harder to bend, because of it's produced from steel strip by cold forming, electric resistance welding (ERW) and cold drawing to finished dimensions. HREW has a yield strength of about 45K lbs, and DOM is about 70K lbs.
While HREW would be just fine for a rollcage, and cheaper, I feel better about using DOM
i really dont think it is worth getting cheaper material for a rollcage or something that is for safety like that. do you really want something cheaper that has a good possibility of failing? or do you want to spend a few extra dollars and not have to worry about failure? its your life you are risking just spend the extra cash
seamless pipe is truly seamless. there is a mill near my house that makes seamless up to 16" diameter. They start with a billet and run the steel into a furnace and then a piercer starts the process of opening it up in the middle. They keep increasing the size of the plug over which they run the tube until they get to a final size.
ERW is electric resistance welded as mentioned earlier. They start out with coil steel and slit it into narrow strips on a slitter. They then put the strips onto another line where they go through a device that forms the steel into a tube shape as it uncoils the strip. The seam is then welded and clean and pressure tested. before being cut to length. When you start bending ERW it will have a tendency to want to crack where the welds are. There are residual stresses in the material where the weld was done.
DOM is Drawn over mandrel - haven't worked with it alot but it is welded but it has additional strength due to the cold forming process.