The iron-carbon phase diagram is a graph that illustrates the stable phases of iron-carbon alloys (including steel and cast iron) based on temperature and carbon content. It enables predictions of what will happen to the alloys during warming and cooling.

Key features:

• Illustrates Critical Transformations: Shows the critical phase transformations—the eutectoid, eutectic, and peritectic reactions.
• Informs Heat Treatment: It is important to explain common heat treatment processes like annealing, normalizing, hardening, and tempering.
• Includes Steels & Cast Irons: This diagram is used for steels—in which the carbon content is less than 2.0%—and cast irons—in which the carbon content is greater than 2.0%.

The iron-carbon phase diagram is essential in most material metallurgy because it provides a way to control microstructure and mechanical properties. A well-defined example is at the eutectoid point, where the carbon content is 0.76% and the temperature is approximately 727°C; this is when austenite (the high-temperature phase of steel) transforms into pearlite (a mixture of ferrite and cementite) and is critical in generating strong ductile steel from iron-carbon alloys. Engineers, by understanding the iron-carbon phase diagram, are able to specify the heat treatment cycles to generate desired hardness, toughness, and ductility. The iron-carbon phase diagram can also be used to diagnose failure modes and optimize the composition of materials and to design weld procedures.