Generally speaking, the welded steel pipes are divided into general welded pipe, galvanized welded pipe, oxygen welding tube, metric system welded tube, transformer tube, welding thin wall pipe and spiral welded pipe etc.
General welded pipe adopts Q195A, Q215A and Q235A or mild steel as raw materials. It is used to transfer the low pressure fluid. In addition, the pipes need water pressure, bending and flattening test etc. Usually, it has some certain requirements on the surface quality.
The length of delivery is often from 4 to 10mm. The specifications of welded pipe in nominal diameter are different from actual size. Depending on the wall thickness, welded pipe are also divided into common steel pipe and thicken steel pipe.
In order to enhance the corrosion resistance, the general welded pipe is galvanized zinc as protective layer.
Galvanized steel pipe has two types: hot dip galvanized pipe with thicken zinc layer and electrical-galvanized pipe with low cost.
Metric system welded tube:
The metric system welded tube is made from the common carbon steel, quality carbon steel or low alloy steel with the hot and cold welding or after hot welded by the cold again. The specification adopts the seamless steel pipe form with outside diameter*wall thickness (mm). The common metric system welded tube is usually used as structural elements like axis of rotation or transporting the fluid etc. The thin-walled tube is utilized for processing furniture, lamps and lanterns. But the manufacturers need to guarantee the strength and bending of steel pipe in the processing. 
The spiral welded tube uses low carbon structural steel or low alloy structural steel strip according to certain helix angle (forming angle) rolled into a tube billet, then welded the pipe joints to be made. The spiral welded tube can use narrow strip steel to produce large diameter steel pipe. It is broadly applied as oil or natural gas pipeline. Moreover, spiral welded pipe has one side welding and double sided welding.
