The parts generally made of brass are those subject to abrasion or corrosion, as the connecting rod brasses, the bearings for the crank shaft, the pump plungers or pistons, and their rods, linings for the pump barrels or bores, the bores of the glands, the condenser tubes, and all cocks and valves.
White metal or babbitt metal is sometimes used in place of, or in connection with, brasses, serving as an anti-abrasion surface. It is easily renewed, as it is cast into its place, but will melt and run out at a temperature of about 600° Fahrenheit.
Muntz metal is used where iron or steel would suffer greatly from corrosion when in contact with salt water. It can be forged.
The difference in the composition of cast iron and steel has never been determined; the difference lies in the percentage of carbon they contain and the structure of the metal. Cast iron will not weld.
Cast iron is brittle, of granular structure, and always breaks short, having a very low elastic limit.
Wrought iron is tough and fibrous, will weld but will not harden, and is stronger than cast iron.
Steel is stronger than wrought iron, and will weld and harden and temper. The breaking strain of wrought iron varies from about 42,000 to 60,000 lbs. per square inch of section.
Steel is tempered by first being heated red hot and suddenly cooled (usually by plunging it into cold water), which hardens it. The surface is then brightened, and on being reheated the tempering colors appear, beginning at a pale yellow, and deepening into red, brown, purple, and blue, the latter gradually fading away as the metal is re-heated to a red heat. The higher the temperature to which the hardened steel is reheated the softer or lower it is tempered.
These colors merely indicate the temperature to which the piece is reheated, since they will appear on steel not hardened and upon iron.
Case hardening is a process that converts the surface of wrought iron into steel, which is accomplished by placing them in a box filled with bone dust, animal charcoal, or leather hoofs, etc. The box is sealed with clay, heated red hot for about 12 hours, and the pieces are quenched in water.