Small-diameter welded pipe units (units below 8 inches) have numerous forming methods. The earliest ones were the guide plate forming method and the pair-roll forming method (including edge bending method, center bending method, and circumferential bending method). In the late 1970s, the pinch roll forming method was introduced.
In the 1980s, to improve product quality and expand the range of product specifications, numerous new forming methods were successively developed, mainly including:
The vertical roll forming method (VRF method) and linear pinch roll forming method for producing thin-walled pipes;
The assembled forming method with edge forming machines for producing thick-walled pipes;
The composite forming method for producing extremely thick-walled pipes;
The W-bending roll forming method and flexible forming method (FF method) capable of producing both thin-walled and thick-walled pipes;
The roll-free natural forming method and crownless bow roll forming method (CBR method) for producing extremely thin-walled pipes.
In recent years, except for the guide plate forming method, other forming methods are being adopted. The thinnest produced pipe has a t/D ratio of 0.3%, and the thickest welded pipe has a t/D ratio of 26%. The produced steel grades include carbon steel, alloy steel, and stainless steel.
