by Dr. Dmitri Kopeliovich
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 diagram is presented in the picture:
The following phases are involved in the transformation, occurring with iron-carbon alloys:
Maximum concentration of carbon in δ-ferrite is 0.09% at 2719 ºF (1493ºC) - temperature of the peritectic transformation.
The crystal structure of δ-ferrite is BCC (cubic body centered).
Austenite has FCC (cubic face centered) crystal structure, permitting high solubility of carbon - up to 2.06% at 2097 ºF (1147 ºC).
Austenite does not exist below 1333 ºF (733ºC) and maximum carbon concentration at this temperature is 0.83%.
α-ferrite has BCC crystal structure and low solubility of carbon - up to 0.25% at 1333 ºF (733ºC).
α-ferrite exists at room temperature.
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 δ-ferrite. Carbon content in δ-ferrite increases up to 0.09% in course solidification, and at 2719 ºF (1493ºC) remaining liquid phase and δ-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 2097 ºF (1147 ºC). 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 2097 ºF (1147 ºC), 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 1333 ºF (733ºC). The eutectoid concentration of carbon is 0.83%.
When the temperature of an alloy reaches 1333 ºF (733ºC), austenite transforms to pearlite (fine ferrite-cementite structure, forming as a result of decomposition of austenite at slow cooling conditions).
Phase compositions of the iron-carbon alloys at room temperature:
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