In the United States the grades of iron used for boilers are C H No. 1 S, or charcoal hammered No. 1 shell iron, for the shell, and C H No. 1 F, or charcoal No. 1 flange iron, which is used for the furnaces and such parts as require flanging.
In both countries steel is also used for boilers, except for the tubes, for which it is not entirely reliable if very high pressures are to be used.
Both the iron and steel plates are tested for tensile strength and ductility.
The breaking strain is that which is sufficient to cause rupture, while the proof strain is that which the metal is required to withstand with safety.
The safe working strain, or working pressure, W P, is the strain under which it is considered safe to work the boiler.
The strength of a boiler of a given diameter and thickness of plate varies according to the construction of the riveted seams or joints.
Boiler stays or braces are rods, ribs, or plates for supporting the weaker parts of the boiler. Thus the tube plates may be stayed by rods passing through both plates and screwed into them, or nuts and washers may be used on the stays one on each side of each tube plate.
Gusset stays are iron plates which are riveted to T irons or in some cases to L irons, which are riveted on the surfaces to be stayed.
Stay tubes are thick tubes (usually about 3⁄16 inch thick), which screw into the tube sheets and are riveted over at the ends. A superior construction, however, is to provide nuts and washers to the ends of the stay tubes, one on each side of each tube plate.
Boiler stays are usually made of such diameters that when new they will sustain a tensile strain of not more than 5,000 lbs. per square inch of cross section, this being the rule of the Board of Trade.