The Metallurgy Of Carbon Steel

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The Metallurgy Of Carbon Steel

مُساهمة من طرف Admin في الجمعة نوفمبر 02, 2007 3:05 pm

size=18] The best way to understand the metallurgy of carbon steel is to study the ‘Iron Carbon Diagram’. The diagram shown below is based on the transformation that occurs as a result of slow heating. Slow cooling will reduce the transformation temperatures; for example: the A1 point would be reduced from 723°C to 690 °C. However the fast heating and cooling rates encountered in welding will have a significant influence on these temperatures, making the accurate prediction of weld metallurgy using this diagram difficult.

Austenite This phase is only possible in carbon steel at high temperature. It has a Face Centre Cubic (F.C.C) atomic structure which can contain up to 2% carbon in solution.
Ferrite This phase has a Body Centre Cubic structure (B.C.C) which can hold very little carbon; typically 0.0001% at room temperature. It can exist as either: alpha or delta ferrite.
Carbon A very small interstitial atom that tends to fit into clusters of iron atoms. It strengthens steel and gives it the ability to harden by heat treatment. It also causes major problems for welding , particularly if it exceeds 0.25% as it creates a hard microstructure that is susceptible to hydrogen cracking. Carbon forms compounds with other elements called carbides. Iron Carbide, Chrome Carbide etc.
Cementite Unlike ferrite and austenite, cementite is a very hard intermetallic compound consisting of 6.7% carbon and the remainder iron, its chemical symbol is Fe3C. Cementite is very hard, but when mixed with soft ferrite layers its average hardness is reduced considerably. Slow cooling gives course perlite; soft easy to machine but poor toughness. Faster cooling gives very fine layers of ferrite and cementite; harder and tougher
Pearlite A mixture of alternate strips of ferrite and cementite in a single grain. The distance between the plates and their thickness is dependant on the cooling rate of the material; fast cooling creates thin plates that are close together and slow cooling creates a much coarser structure possessing less toughness. The name for this structure is derived from its mother of pearl appearance under a microscope. A fully pearlitic structure occurs at 0.8% Carbon. Further increases in carbon will create cementite at the grain boundaries, which will start to weaken the steel.

Cooling of a steel below 0.8% carbon When a steel solidifies it forms austenite. When the temperature falls below the A3 point, grains of ferrite start to form. As more grains of ferrite start to form the remaining austenite becomes richer in carbon. At about 723°C the remaining austenite, which now contains 0.8% carbon, changes to pearlite. The resulting structure is a mixture consisting of white grains of ferrite mixed with darker grains of pearlite. Heating is basically the same thing in reverse.

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