How steel pipe is made

Steel pipe is either a flat strip curled and welded shut, or a solid billet pushed over a spike until it is hollow.

stations: 7
things upstream: 28
raw sources: 13
steps from the ground: 6

The short version

Welded steel pipe starts as a coil of flat steel. Rollers curl it into a tube and an electric current welds the seam as it passes. Weld-free pipe is different: a hot solid billet is drawn over a piercing rod to hollow it out. Either way the pipe is cut to length and tested.

Uncoil the strip

A coil of flat steel is unrolled and its edges are trimmed square. The coil is the feedstock for the continuous welded route.

Goes in
Sheet steel
Curl it into a tube

Set after set of rollers bend the flat strip, a little further each time, until the two edges meet in a straight seam.
Weld the seam

Electric current runs through the edges of the strip, often by an induction coil around the tube. The steel resists the current, heats and fuses along the seam without any filler metal.
Size and cut

Final rollers true up the diameter, and a flying saw cuts the endless tube to length without stopping the line.
Pierce a billet instead

For weld-free pipe, a solid billet is heated and drawn over a piercing rod in a process called rotary piercing, which opens a hole down its center with no welding.

Goes in
Steel
Test under pressure

Each heat of pipe is checked for chemical composition and strength, and finished lengths are pressure tested.
Dip in zinc

Pipe for water or outdoor use may go through a galvanizing bath that coats it in zinc.

Goes in
Zinc

End of the lineSteel pipe Next it goes into container ship, treadmill, wheelbarrow.

Worth repeating

Early steel pipes were riveted and later clamped, and by the early 1930s welding had replaced both methods.
The schedule numbers on pipe, such as Sch 40, were set long ago for steel pipe and carried over to PVC.