Post 420 – by Gautam Shah
Ferrous alloys refer to metals, where the chief constituents are Iron and carbon. Ferrous alloys are formed with metallic and nonmetallic compounds that enter into the structure and occupy the interstices of compounds. The metallic compounds include: Chromium, Manganese, Molybdenum, Nickel, Silicon, Titanium, Tungsten, Vanadium, etc. The non-metallic compounds include elements of smaller atomic numbers like Carbon, Nitrogen, and Boron.The word ‘Ferro-alloy’ generally refers to alloys of iron with a high proportion of one or more other metal elements. Such alloys have distinctive qualities.
Metals are alloyed because these become far more suitable for various uses, than in their pure state, or without the alloying agents. Alloys are formed when a metal element and its alloying compound form a solution at certain high temperature and solidify to form a solid solution. Sometimes the intermingling is so close that dissolved substance cannot be distinguished or separated by mechanical means. This in someway, is a result of the differing softening and melting point, and mechanical processes of amalgamation.
In some alloys the metals do not show complete solubility, and separate constituents may be recognized. Capacity of one metal to accommodate another metal varies with the temperature. In an aluminium copper alloy, the aluminium at 530° C can hold 5 % of copper in solution, but at room temperature it can hold only 0.5 % of copper. So if a 5 % copper alloy is rapidly cooled from 530° C, the excess copper cannot go out of the mass, but remains in the alloy, well dispersed in the mass.
In alloys where inter-metallic compounds predominate, the alloy shows toughness of the solid solution and hardness of the inter-metallic compound. But the alloy with such inter-metallic compounds, may be hard but very brittle.
When a component of an alloy melts at a temperature lower than all other constituents, than that alloy is called eutectic. Such alloys have thin layers of the metal or small globules of one metal embedded in a matrix of another metal.
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, is a compound of iron containing about 7 % carbon. It is extremely brittle and hard.
● Pearlite is an intimate mixture of ferrite and cementite having a specific composition, characteristic structure, and physical properties 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. The process lasts till the steel has 0.8 per cent of carbon, then 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.