Introduction

The Fe-Fe₃C phase diagram (also known as the iron-carbon phase diagram) is a crucial tool in understanding the microstructure and properties of steels and cast irons, which are fundamental materials in engineering and manufacturing. This diagram provides insights into the various phases and transformations that occur in iron-carbon alloys as they are heated or cooled. In particular, it helps us predict the behavior of steel and cast iron during processes such as annealing, heat treatment, and casting.

This article will explore the Fe-Fe₃C phase diagram, its key phases and reactions, and the role of carbon content in determining the mechanical properties of steel and cast iron.

Overview of the Fe-Fe₃C Phase Diagram

The Fe-Fe₃C phase diagram represents the equilibrium between different phases in the iron-carbon system, where Fe (iron) is one component and Fe₃C (iron carbide or cementite) is another. The diagram focuses on the carbon content between 0% and 6.67% by weight, with 6.67% representing pure Fe₃C. In practice, most commercial steels and cast irons have carbon contents below 2.14 wt%, which is the upper limit for steel. Beyond this, we enter the domain of cast iron.

The phase diagram shows the relationship between temperature, composition (carbon content), and the resulting phases of iron and iron-carbide alloys. It is typically divided into several regions, representing different combinations of solid and liquid phases.

Phases in the Fe-Fe₃C System

The primary phases in the Fe-Fe₃C phase diagram include:

  1. Ferrite (α-Fe):
    • A body-centered cubic (BCC) form of iron.
    • It is stable at low temperatures and can dissolve only a small amount of carbon (up to 0.022 wt% at 727°C).
    • Ferrite is soft and ductile, which makes it a desirable phase in low-carbon steels.
  2. Austenite (γ-Fe):
    • A face-centered cubic (FCC) structure of iron.
    • Austenite can dissolve more carbon than ferrite (up to 2.14 wt% at 1147°C).
    • It is stable at higher temperatures and is non-magnetic. Austenite is typically found in steels during heat treatment and transforms into other phases upon cooling.
  3. Cementite (Fe₃C):
    • Cementite, or iron carbide, is a hard and brittle intermetallic compound that contains 6.67 wt% carbon.
    • It significantly increases the hardness and strength of steel but also makes it more brittle.
    • Cementite does not dissolve any carbon and is found in both steels and cast irons.
  4. Liquid (L):
    • The liquid phase represents molten iron or steel, existing at temperatures above the melting point of the alloy. The liquid region is found at high temperatures and high carbon contents in the diagram.
  5. Pearlite:
    • Pearlite is a lamellar (layered) structure of alternating layers of ferrite and cementite.
    • It forms when austenite undergoes a eutectoid transformation at 727°C (at 0.76 wt% C). Pearlite provides a balance between strength and ductility in steel.
  6. Ledeburite:
    • A mixture of austenite and cementite that forms during the eutectic reaction at 1147°C and 4.3 wt% C.
    • Ledeburite is found in cast irons and is very hard and brittle due to the presence of cementite.

(a) alpha-Ferrite, (b) austenite

Key Reactions in the Fe-Fe₃C Phase Diagram

The Fe-Fe₃C phase diagram contains several important reactions that occur at specific compositions and temperatures. These reactions dictate the transformation of phases as iron-carbon alloys are heated or cooled.

  1. Peritectic Reaction (1493°C, 0.16 wt% C):

    Liquid+δ→γ

    • In this reaction, liquid iron and the δ-ferrite phase combine to form austenite (γ).

  2. Eutectic Reaction (1147°C, 4.3 wt% C):

    Liquid→γ+Fe3C

    • At the eutectic point, liquid iron-carbon alloys solidify into austenite and cementite, forming ledeburite. This reaction is significant in cast irons.

  3. Eutectoid Reaction (727°C, 0.76 wt% C):

    γ→α+Fe3C

    • This reaction, known as the eutectoid transformation, occurs when austenite transforms into a mixture of ferrite and cementite at 727°C, forming pearlite. This reaction is crucial in steels.

eutectoid

hypoeutectoid

hypereutectoid

Regions of the Fe-Fe₃C Phase Diagram

The Fe-Fe₃C phase diagram can be divided into distinct regions that correspond to different combinations of solid and liquid phases. These regions provide insights into the microstructure of iron-carbon alloys at various temperatures and carbon contents.

  1. Ferrite Region (α):
    • This region exists at low carbon contents (below 0.022 wt% C) and low temperatures. Ferrite is a soft and ductile phase, commonly found in low-carbon steels.
  2. Austenite Region (γ):
    • The austenite region spans from 0.022 wt% to 2.14 wt% carbon at high temperatures. This region is important for heat treatments like annealing and quenching, where austenite transforms into other phases upon cooling.
  3. Cementite Region (Fe₃C):
    • The cementite region exists at the right side of the diagram (above 6.67 wt% carbon). Cementite is a hard, brittle phase and is present in both steels and cast irons.
  4. Liquid Region (L):
    • At high temperatures and carbon contents, the alloy is in the liquid phase. This region is important for casting processes.
  5. Pearlite Region:
    • Pearlite forms during the eutectoid transformation (727°C, 0.76 wt% C) when austenite decomposes into ferrite and cementite. It is a key phase in medium-carbon steels, providing a balance between strength and ductility.
  6. Ledeburite Region:
    • Ledeburite forms in alloys with high carbon content (around 4.3 wt% C) during the eutectic reaction at 1147°C. It consists of austenite and cementite and is commonly found in cast irons, making them hard and brittle.

Steel and Cast Iron in the Fe-Fe₃C Phase Diagram

The iron-carbon phase diagram is fundamental in understanding the behavior of steel and cast iron, the two most important classes of iron-carbon alloys.

  1. Steels (0–2.14 wt% C):
    • Steels are iron-carbon alloys with a carbon content less than 2.14 wt%. The mechanical properties of steel are heavily influenced by its microstructure, which is determined by the phases present.
    • Low-carbon steels (<0.3 wt% C): Primarily consist of ferrite with some pearlite. They are soft, ductile, and easily weldable.
    • Medium-carbon steels (0.3–0.6 wt% C): Contain a mixture of ferrite and pearlite, providing a balance between strength and ductility.
    • High-carbon steels (0.6–2.14 wt% C): Have a higher fraction of pearlite and cementite, making them harder and stronger but more brittle.
  2. Cast Irons (2.14–6.67 wt% C):
    • Cast irons contain a higher carbon content than steels, leading to the formation of cementite and ledeburite. They are typically brittle due to the high fraction of cementite.
    • Gray cast iron: Contains graphite flakes, which reduce brittleness.
    • White cast iron: Contains cementite, making it hard and brittle.
    • Ductile cast iron: Contains nodular graphite, which improves ductility.

Applications of the Fe-Fe₃C Phase Diagram

The Fe-Fe₃C phase diagram is a vital tool in industries that process steels and cast irons. It helps engineers and metallurgists understand the microstructural changes that occur during heat treatment, annealing, quenching, and tempering, all of which influence the final properties of the material.

  1. Heat Treatment: Understanding the phase transformations that occur during heating and cooling allows for precise control of mechanical properties in steel, such as hardness, strength, and toughness.
  2. Alloy Design: The phase diagram guides the design of alloy compositions to achieve desired properties in steels and cast irons.
  3. Manufacturing: In the casting and forging industries, knowledge of the phase diagram helps optimize processes such as solidification, casting, and thermal treatments.