Structure of steel
Structure of Steel
The physical properties of various types of steel and of any given steel alloy at varying temperatures depend primarily on the amount of carbon present and on how it is distributed in the iron. Before heat treatment most steels are a mixture of three substances: ferrite, pearlite, and cementite. Ferrite is iron containing small amounts of carbon and other elements in solution and is soft and ductile. Cementite, a compound of iron containing about 7 percent carbon, is extremely brittle and hard. Pearlite is an intimate mixture of ferrite and cementite having a specific composition and characteristic structure, and physical characteristics intermediate between its two constituents. The toughness and hardness of a steel that is not heat treated depend on the proportions of these three ingredients. As the carbon content of a steel increases, the amount of ferrite present decreases and the amount of pearlite increases until, when the steel has 0.8 percent of carbon, it is entirely composed of pearlite. Steel with still more carbon is a mixture of pearlite and cementite. Raising the temperature of steel changes ferrite and pearlite to an allotropic form of iron-carbon alloy known as austenite, which has the property of dissolving all the free carbon present in the metal. If the steel is cooled slowly the austenite reverts to ferrite and pearlite, but if cooling is sudden, the austenite is “frozen” or changes to martensite, which is an extremely hard allotropic modification that resembles ferrite but contains carbon in solid solution.
Heat Treatment of Steel
The basic process of hardening steel by heat treatment consists of heating the metal to a temperature at which austenite is formed, usually about 760° to 870° C and then cooling, or quenching, it rapidly in water or oil. Such hardening treatments, which form martensite, set up large internal strains in the metal, and these are relieved by tempering, or annealing, which consists of reheating the steel to a lower temperature. Tempering results in a decrease in hardness and strength and an increase in ductility and toughness.
The primary purpose of the heat-treating process is to control the amount, size, shape, and distribution of the cementite particles in the ferrite, which in turn determines the physical properties of the steel.
Many variations of the basic process are practiced. Metallurgists have discovered that the change from austenite to martensite occurs during the latter part of the cooling period and that this change is accompanied by a change in volume that may crack the metal if the cooling is too swift. Three comparatively new processes have been developed to avoid cracking. In time-quenching the steel is withdrawn from the quenching bath when it has reached the temperature at which the martensite begins to form, and is then cooled slowly in air. In martempering the steel is withdrawn from the quench at the same point, and is then placed in a constant-temperature bath until it attains a uniform temperature throughout its cross section. The steel is then allowed to cool in air through the temperature range of martensite formation, which for most steels is the range from about 288° C to room temperature. In austempering the steel is quenched in a bath of metal or salt maintained at the constant temperature at which the desired structural change occurs and is held in this bath until the change is complete before being subjected to the final cooling.
Other methods of heat treating steel to harden it are used. In case hardening, a finished piece of steel is given an extremely hard surface by heating it with carbon or nitrogen compounds. These compounds react with the steel, either raising the carbon content or forming nitrides in its surface layer. In carburizing, the piece is heated in charcoal or coke, or in carbonaceous gases such as methane or carbon monoxide. Cyaniding consists of hardening in a bath of molten cyanide salt to form both carbides and nitrides. In nitriding, steels of special composition are hardened by heating them in ammonia gas to form alloy nitrides.