How gas cylinders are made

Stamp the discs

Discs are stamped from annealed plate or coil, with chromium-molybdenum grades such as 4130 a common choice. Each size needs its own blank.

Draw into a cup

The lubricated disc is pushed cold through dies by a hydraulic press to form a cup. A rival route skips this, backward-extruding a heated billet and then hot drawing it.

Anneal and redraw

Work hardening is relieved in a furnace, scale is blasted off, and the cup goes through another die. It repeats for two or three more stages, with the cup getting narrower and longer each time.

Spin the shoulder

The base is formed and the top trimmed to length. The top is heated to bright red and spun while a hardened block folds the metal inward, like a potter closing a vase. This thickens the shoulder and closes the neck.

Quench and temper

The formed shell is quenched and then tempered to give the best mix of strength and toughness. A study of 34CrMo4 steel cylinders used 870 °C and 620 °C.

Clean and thread

Shot blasting or chemical cleaning removes the scale, inside and out. The neck is machined with its thread and any o-ring seat.

Test with water

After inspection each shell is filled with water for a hydrostatic test. For weld-free high-pressure types the test pressure is usually 1.5 times working pressure, or 1.67 times in the United States.

1.5 to 1.67 times working pressure

Stamp the markings

Cylinders that pass get a serial number and other permanent markings stamped on, along with a neck ring in one plant's sequence.

Fit the valve

Into the threaded neck goes a valve, then a cap.

Paint the outside

A corrosion barrier paint covers the outside, or the shell is hot dip galvanized instead.