Carbon Makes Steel Hard
Steel is iron that is alloyed with carbon. Other elements may be present in the alloy, but the term carbon steel is generally used when carbon and steel are the only elements that affect the properties of the alloy. The hardness of steel depends upon how much carbon is in the alloy and also on certain treatments like tempering and quenching.
Carbon Makes Steel Brittle
Carbon molecules change the way that the iron molecules fit together so that more energy is required to break the iron molecules apart, which means an alloy of carbon and steel is harder than plain iron. The more carbon in the alloy, the harder the steel will be. However, higher carbon content makes the steel less ductile, or more brittle, meaning it is harder to form into shapes, especially thin sheets and wires. High carbon also lowers the melting temperature of the steel and makes it harder to weld. Depending on the intended use, carbon steel contains from 0.3 percent to 2 percent carbon by weight.
Heat Treating Alters the Mechanical Properties of Steel
Alternately heating and cooling steel changes the arrangement of the carbon and iron molecules and consequently the way that the steel responds to mechanical stresses. The carbon content of the steel, temperature, heating time and cooling time are all factors in hardness and ductility of the final steel product. Several methods of heat treating may be used in various combinations to achieve the desired mechanical properties. The most common and useful for roughly illustrating the principles is the technique of quenching and tempering.
Quenching Causes Steel to Become Hard and Brittle
Carbon steel with a minimum 0.4 percent carbon content is heated to a normalizing temperature calculated according to the chemical composition of the steel. The steel is then quickly cooled in a bath of water or oil. The volume and temperature of the water are carefully measured so that the precise temperature of the steel can be calculated. When the steel has cooled to a certain temperature, it is removed from the bath. The cooling temperature is calculated according to the carbon content of the steel. Quenching causes the iron and carbon molecules to become arranged in a crystalline structure that is very brittle.
Tempering Causes Steel to Regain Some Ductility
Reheating the steel after quenching, to a calculated temperature lower than normalizing temperature, causes further rearrangement of the carbon and iron molecules. The resulting structure is less rigidly crystalline. Some of the properties of ductility are restored to the steel and it is less brittle. Some hardness is also lost, but the resulting product is still significantly harder than steel that has not been quenched and tempered.
Case Hardening Hardens Only the Surface of Steel
Large pieces of low carbon steel are difficult to harden. So when the properties of hardened steel are required for low carbon steel (as in guns, locks and certain machine parts), processes are used to harden only the outer surface of the steel. The idea behind case hardening is to introduce more carbon, possibly in combination with nitrogen, into the surface of the steel to a certain depth. This is done by exposing the steel surface to heat or chemicals. Once the carbon content of the steel on the surface is high enough, it can be heat treated like other higher carbon steels.
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