Question

What is the maximum concentration of carbon possible in the FCC phase of iron before causing precipitation of either Fe3C or liquid? At what temperature is the carbon solubility in FCC iron a maximum?

Answer 1

In the following Iron-Carbon phase diagram, we can see that the maximum concentration of Carbon is 6.67%. This is due to the fact that maximum solubity of carbon in iron is 6.67% as it forms iron carbide (Fe3C form).

• Atomic mass of Fe = 55.845
• Atomic mass of C = 12.01

Hence, in Fe3C -> wt% of C is 6.67%

At 6.67%, pure cementite (Fe3C) is present whereas at lower concentration other phases also occur.

C gets dissolved in Fe during the production of steels. Pure Fe melts at a temperature of 1540 deg C, and at this temperature, C readily dissolves into the liquid iron, generating a liquid solution. When this liquid solution solidifies, it generates a solid solution, in which the C atoms are dissolved into the solid iron. The individual C atoms lie in the holes between the Fe atoms of the crystalline grains of austenite (at high temperatures) or ferrite (at low temperatures). Austenite has a face centred cubic (fcc) structure while the ferrite has a body centred cubic (bcc) structure (Fig 1). If the amount of C dissolved in the liquid iron is kept below 2.1 %, the product is steel, but if it is above this value, then the product is cast iron. Although liquid iron can dissolve C at levels well above 2.1 % C, solid iron cannot. This leads to a different solid structure for cast irons (iron with total C greater 2.1 %).