The two species of ERW tubing like this

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.
 
The Steel Tube Institute adds, "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.