Iron - carbon phase diagram

 Iron-carbon phase diagram:

Iron-carbon phase diagram describes the iron-carbon system of alloys containing up to 6.67% of carbon, discloses the phases compositions and their transformations occurring with the alloys during their cooling or heating. Carbon content 6.67% corresponds to the fixed composition of the iron carbide Fe3C. The following phases are involved in the transformation, occurring with iron-carbon alloys: L-Liquid solution of carbon in iron;

S-ferrite Solid solution of carbon in iron. Maximum concentration of carbon in 8-ferrite is 0.09% at 1493°C temperature of the peritectic transformation.

The crystal structure of 8-ferrite is BCC.

Austenite interstitial solid solution of carbon in y-iron.

Austenite has FCC crystal structure, permitting high solubility of carbon - up to 2.06% at 1147 °C. Austenite does not exist below 723°C and maximum carbon concentration at this temperature is 0.83%. a-ferrite solid solution of carbon in a-iron.

a-ferrite has BCC crystal structure and low solubility of carbon - up to 0.025% at 723°C. a-ferrite exists at room temperature.

Cementite iron carbide, intermetallic compound, having fixed composition Fe3C. Cementite is a hard and brittle substance, influencing on the properties of steels and cast irons.The following phase transformations occur with iron-carbon alloys:

Alloys, containing up to 0.51% of carbon, start solidification with formation of crystals of S-ferrite. Carbon content in 8-ferrite increases up to 0.09% in course solidification, and at 1493°Cremaining liquid phase and 8-ferrite perform peritectic transformation, resulting in formation of austenite. Alloys, containing carbon more than 0.51%, but less than 2.06%, form primary austenite crystals in the beginning of solidification and when the temperature reaches the curve ACM primary cementite stars to form.Iron-carbon alloys, containing up to 2.06% of carbon, are called steels.

Alloys, containing from 2.06 to 6.67% of carbon, experience eutectic transformation at 1147 °C. The eutectic concentration of carbon is 4.3%. In practice only hypoeutectic alloys are used. These alloys (carbon content from 2.06% to 4.3%) are called cast irons. When temperature of an alloy from this range reaches 1147 °C, it contains primary austenite crystals and some amount of the liquid phase. The latter decomposes by eutectic mechanism to a fine mixture of austenite and cementite, called ledeburite.

All iron-carbon alloys (steels and cast irons)

 experience eutectoid transformation at 723°C. The eutectoid concentration of carbon is 0.83%. When the temperature of an alloy reaches 733°C, austenite transforms to pearlite (fine ferrite-cementite structure, forming as a result of decomposition of austenite at slow cooling conditions).