Iron-Carbon-Phase-Diagram explained - BorTec

Carbon is the most crucial alloying element in iron, and even minor variations in its concentration can lead to significant alterations in the material's properties. The relevance of the iron-carbon phase diagram diminishes considerably under conditions of rapid heating or cooling. Moreover, its significance becomes less pronounced if the quantity of other alloying elements increases.

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Carbon is present in two distinct forms: one is in a bound state, and the other as elementary carbon in the form of graphite. This accounts for the two representations of the iron-carbon phase diagram. There exists a stable system represented by the Fe-Graphite diagram and a metastable one known as the Fe-Fe3C diagram. Although both systems can appear in a single diagram, the metastable Fe-Fe3C system is predominantly utilized in practice.

Understanding Phase Representation in the Iron-Carbon Phase Diagram

AG Caesar, CC BY-SA 4.0, via Wikimedia Commons

The x-axis of the diagram illustrates the mass percentage of carbon, while the y-axis represents the temperature. For clarity, only the carbon content that is technically noteworthy, ranging from 0 to 6.67%, is depicted. Alloys with more than 6.67% carbon transition into a phase of 100% cementite.

The boundaries of the phase fields are defined by lines that indicate shifting breakpoints at various temperatures. Relevant points in the diagram are marked with letters. It is important to note that some diagrams may denote point I as J. Among the most critical lines is the liquidus line, characterized by the ABCD polyline. When above this line, the alloy exists in liquid form. The solidus line, known as the AHIECF polyline, indicates the region where the alloy is entirely solid. Between these two lines, the alloy possesses a semi-solid consistency, comprising residual melt, δ-iron, γ-iron, and cementite (Fe3C), with proportions that dynamically vary with temperature. During the alloy’s cooling, primary crystallization from the melt initiates once the temperature drops below the liquidus line.

Iron exhibits various allotropic modifications, leading to the formation of different phases based on carbon content and temperature. The intercalated mixed crystals created by iron δ-, γ- and α-solid solutions have varying carbon solubility due to differences in their spatial lattices and lattice constants.

Metallographic Designation Explained

In metallography, the mixed crystals are classified as δ-ferrite, while γ-mixed crystals are called austenite, and α-mixed crystals are referred to as ferrite. Below is a summary detailing the carbon content of individual phases:

Designation

Max C-content

Metallographic designation

δ-solid solution

0.10 % at ° C

δ-Ferrite

γ-solid solution

2.06 % at ° C

Austenite

α-solid solution

0.02 % at 723° C

Ferrite

Cementite (Fe3C) is an iron-carbon compound that represents an intermediate phase distinct from iron mixed crystals. The chemical composition of cementite remains constant, though it appears in three varied forms:

• Primary cementite: crystallizes from the melt (associated with the CD line).
• Secondary cementite: precipitated from the austenite (linked to the ES line).
• Tertiary cementite: formed from the ferrite (connected to the PQ line).

The secondary cementite manifests at carbon content between 2.06 and 4.3 % C, but is not represented in the diagram as it is undetectable metallographically.

In addition to distinct phases, phase mixtures are also present:

Designation

Consists of

Area of existence

Perlite: 88 % Ferrite / 12 % cementite
0.02 % ' 6.67 % at T ' 723° C

Ledeburite I: 51.4 % Austenite / 48.6 % cementite
2.06 % ' 6.67 % at 723° C ' ° C

Ledeburite II: 51.4 % Perlite / 48.6 % cementite
2.06 % ' 6.67 % at T ' 723° C

Exploring Isothermal Reactions in the Iron-Carbon Phase Diagram

The iron-carbon phase diagram illustrates three isothermal reactions. The line HIB indicates a peritectic reaction, the line ECF delineates a eutectic reaction, and the left PSK line shows an eutectoid reaction.

During the heating or cooling of steel, transformations occur along these lines, marked by distinct breakpoints. The most significant ones are:

• On the P-S-K line, austenite transforms to pearlite when the carbon content is under 0.02 % (A1).
• The M-O line indicates where ferrite loses its ferromagnetism upon heating past 769° C (A2).
• If low-carbon ferrite occurs below the G-O-S line during cooling, austenite forms with released carbon until reaching a eutectoid concentration at 723 °C (A3).

Application of the Iron-Carbon Diagram

The iron-carbon phase diagram serves as a valuable tool for comprehending the behavior of cast iron and steel. Steel, known for its malleability, can be easily shaped within the austenite range. In contrast, the higher carbon content in cast iron, often existing as graphite and ledeburite, considerably restricts its malleability.
Due to this difference, the iron-carbon diagram is essential for analyzing steel and cast iron.

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