Iron-carbon phase diagram

12/07/2021
The iron-carbon phase diagram describes the iron-carbon system of alloys containing up to 6.67% of carbon, describe 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:

Picture14

The following phases are involved in the transformation, occurring with iron-carbon alloys:
  • L - Liquid solution of carbon in iron;
The maximum concentration of carbon in δ-ferrite is 0.09% at 1493ºC – temperature of the peritectic transformation.
The crystal structure of δ-ferrite is BCC (cubic body-centred). Austenite has FCC (cubic face centred) crystal structure, permitting high solubility of carbon – up to 2.06% at 1147 ºC.
Austenite does not exist below 723ºC and the maximum carbon concentration at this temperature is 0.83%.
  • a-ferrite – solid solution of carbon in α-iron.
a-ferrite has a BCC crystal structure and low solubility of carbon – up to 0.025% at 723ºC.
α-ferrite exists at room temperature.
  • Cementite – iron carbide, intermetallic compound, having fixed composition Fe3C.
Cementite is a hard and brittle substance, influencing the properties of steel and cast irons.
The following phase transformations occur with iron-carbon alloys:
 
Alloys, containing up to 0.51% of carbon, start solidification with the formation of crystals of δ-ferrite. Carbon content in δ-ferrite increases up to 0.09% in course solidification, and at 1493ºC remaining liquid phase and δ-ferrite perform a Peritectic transformation, resulting in the formation of austenite.
Alloys, containing carbon more than 0.51%, but less than 2.06%, form primary austenite crystals at the beginning of solidification and when the temperature reaches the curve ACM primary cementite starts 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 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 the temperature of an alloy from this range reaches 1147 ºC, it contains primary austenite crystals and some amount of the liquid phase. The latter decomposes by the eutectic mechanism to a fine mixture of austenite and cementite, called Ledeburite.
All iron-carbon alloys (steels and cast irons) experience eutectoid transformation at 723ºC. The eutectoid concentration of carbon is 0.83%.
When the temperature of an alloy reaches 733ºC, austenite transforms to pearlite (fine ferrite-cementite structure, forming as a result of decomposition of austenite at slow cooling conditions).
 
Critical temperatures
· Upper critical temperature (point) A3 is the temperature, below which ferrite starts to form as a result of ejection from austenite in the hypereutectoid alloys.
· Upper critical temperature (point) ACM is the temperature, below which cementite starts to form as a result of ejection from austenite in the hypereutectoid alloys.
· Lower critical temperature (point) A1 is the temperature of the austenite-to-pearlite eutectoid transformation. Below this temperature austenite does not exist.
· Magnetic transformation temperature A2 is the temperature below which α-ferrite is ferromagnetic.
Phase compositions of the iron-carbon alloys at room temperature
· Hypoeutectoid steels (carbon content from 0 to 0.83%) consist of primary (pro eutectoid) ferrite (according to the curve A3) and pearlite.
· Eutectoid steel (carbon content 0.83%) entirely consists of pearlite.
· Hypereutectoid steels (carbon content from 0.83 to 2.06%) consist of primary (pro eutectoid) cementite (according to the curve ACM) and pearlite.
· Cast irons (carbon content from 2.06% to 4.3%) consist of pro eutectoid cementite C2 ejected from austenite according to the curve ACM, pearlite and transformed Lefebure (ledeburite in which austenite transformed to pearlite).



 

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