Specific Metallurgy for Carbon Steel

The best way to understand the metallurgy of carbon steel is to study.  Slow cooling will reduce the transformation temperatures; 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.

Cooling of steel below 0.8% carbon- When 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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