Steel.--Steel may be alloyed with almost any of the metals or elements, the combinations that have proven valuable numbering more than a score. The principal ones are given in alphabetical order, as follows:
Aluminum is added to steel in very small amounts for the purpose of preventing blow holes in castings.
Boron increases the density and toughness of the metal.
Bronze, added by alloying copper, tin and iron, is used for gun metal.
Carbon has already been considered under the head of steel in the section devoted to the metals. Carbon, while increasing the strength and hardness, decreases the ease of forging and bending and decreases the magnetism and electrical conductivity. High carbon steel can be welded only with difficulty. When the percentage of carbon is low, the steel is called "low carbon" or "mild" steel. This is used for rods and shafts, and called "machine" steel. When the carbon percentage is high, the steel is called "high carbon" steel, and it is used in the shop as tool steel. One-tenth per cent of carbon gives steel a tensile strength of 50,000 to 65,000 pounds per square inch; two-tenths per cent gives from 60,000 to 80,000; four-tenths per cent gives 70,000 to 100,000, and six-tenths per cent gives 90,000 to 120,000.
Chromium forms chrome steel, and with the further addition of nickel is called chrome nickel steel. This increases the hardness to a high degree and adds strength without much decrease in ductility. Chrome steels are used for high-speed cutting tools, armor plate, files, springs, safes, dies, etc.
Manganese has been mentioned under Steel. Its alloy is much used for high-speed cutting tools, the steel hardening when cooled in the air and being called self-hardening.
Molybdenum is used to increase the hardness to a high degree and makes the steel suitable for high-speed cutting and gives it self-hardening properties.
Nickel, with which is often combined chromium, increases the strength, springiness and toughness and helps to prevent corrosion.
Silicon has already been described. It suits the metal for use in high-speed tools.