What is the Iron-Carbon Phase Diagram?

Iron-Carbon Phase Diagram

The iron-carbon phase diagram is an essential tool for understanding the various phases present in steel and cast iron. Both materials are fundamentally alloys composed of iron and carbon, with minor trace elements mixed in.

Although the entire diagram covers a broad range of compositions, our focus here is specifically on the segment that extends up to 6.67% carbon by weight—corresponding to the formation of iron carbide (Fe₃C). This choice helps simplify what can otherwise be a rather intricate graph.

On the diagram, carbon content by weight is represented along the horizontal (X) axis, while temperature is plotted on the vertical (Y) axis.

As illustrated in the figure, the Fe-C equilibrium diagram reveals the different phases and microstructures that emerge in steel and cast iron during both heating and cooling. It also maps out the key structural changes, highlights the role of critical lines, and identifies the important transition points that metallurgists pay close attention to.

Structures in Fe-C-diagram

The main microscopic constituents of iron and steel are as follows:

• Austenite
• Ferrite
• Cementite
• Pearlite

1. Austenite

Austenite refers to a solid solution where carbon and iron combine within gamma iron. When steel is heated past its upper critical temperature, its structure transitions fully into austenite—a phase recognized for its hardness, ductility, and non-magnetic properties.

This phase has a remarkable capacity to hold a significant amount of carbon. Austenite exists within a specific range as steel is heated and cooled, situated between the critical transformation points. Typically, austenite forms when steel has up to 1.8% carbon at around 1130°C.

As the temperature drops below 723°C, austenite gradually converts into pearlite and ferrite. It’s worth noting that austenitic steels remain non-magnetic and do not respond to conventional heat treatment methods used for hardening.

2. Ferrite

Ferrite refers to a form of iron that contains little to no carbon. Essentially, it is the term used for pure iron crystals, which are known for being both soft and ductile. When low-carbon steel is allowed to cool slowly below its critical temperature, it develops a ferrite structure. Unlike some other forms of steel, ferrite does not harden even if it is cooled quickly. It remains quite soft and is also noted for its strong magnetic properties.

3. Cementite

Cementite, also referred to as iron carbide (Fe₃C), is a compound formed between carbon and iron. In cast iron, when the carbon content reaches 6.67%, the material is composed entirely of cementite. In steels with more than 0.83% carbon, free cementite begins to appear, and its amount grows as the carbon content increases—a relationship that can be observed in the iron-carbon equilibrium diagram. Notably, cementite is recognized for its remarkable hardness.

The presence of cementite in cast iron is largely responsible for its characteristic hardness and brittleness. However, while it imparts these properties, it also tends to lower the tensile strength of the material. Cementite forms when carbon combines with iron in a specific, structured way, resulting in iron carbides that are exceptionally hard.

The overall brittleness and hardness of cast iron can be traced back to the quantity of cementite present. Interestingly, cementite exhibits magnetic properties, but only at temperatures below 200°C.

4. Pearlite

Pearlite is a eutectoid mixture composed of ferrite and cementite, typically found in medium and low carbon steels. In these steels, pearlite appears as a mechanical combination of ferrite and cementite, usually in a ratio of about 87:13. One important point to note is that as the proportion of pearlite increases in a ferrous material, so does its hardness.

When you compare pearlite and ferrite, there’s a noticeable difference in their properties. Pearlite tends to be stronger, harder, and yet still retains some ductility, while ferrite, by contrast, is softer and weaker but also ductile. If you ever look at pearlite under a microscope, you’ll notice it is made up of alternating layers—light and dark—which correspond to ferrite and cementite, respectively. The unique way these layers reflect light gives pearlite its name, as its surface can resemble mother-of-pearl.

The composition of steel influences its structure quite significantly. Hard steels are generally a mix of pearlite and cementite, while softer steels contain a blend of ferrite and pearlite. As the carbon content increases past 0.2%, the temperature at which ferrite separates from austenite drops. Eventually, once the carbon content reaches around 0.8% or higher, no free ferrite is separated from austenite. Steel with this specific composition is known as eutectoid steel, characterized by a fully pearlitic structure.

If you examine how iron with varying carbon percentages—up to about 6%—behaves during heating and cooling, the various phases indicated by the critical lines will reveal how the structure of iron changes throughout the process.

Significance of Transformations Lines

1. Line ABCD

The line labeled ABCD marks the point where melting of iron has finished during the heating process. Above this line, the metal exists entirely in its liquid form. In contrast, the region found below ABCD but above the line AHJECF represents a mixture—here, the metal is partly solid and partly liquid.

Within this partially solid region, the solid phase is known as austenite. So, the ABCD line essentially indicates the temperatures at which the melting process is considered complete. Once you cross above this boundary, the metal becomes fully molten. It’s worth noting that the ABCD line isn’t perfectly horizontal; rather, the precise melting temperature depends on the carbon content present in the metal.

2. Line AHJECF

This point marks the onset of melting for the metal. Since the line is sloped rather than horizontal, the melting temperature actually varies depending on the carbon content present. If we look at the region below this line but above the GSEC line, the metal remains solid and takes on an austenitic structure.

3. Line PSK

This line appears at approximately 723°C and is horizontal in nature. It is commonly referred to as the lower critical temperature line, as it marks the point where transformation in steel begins. Notably, the carbon content in the steel does not influence the position of this line; in other words, steels with varying percentages of carbon will all start to transform at the same temperature.

The area above this line, extending up to the GSE boundary, is known as the transformation range. Within this range, the line indicates that for steels containing up to 0.8% carbon, the transformation from ferrite and pearlite to austenite commences during the heating process.

4. Line  ECF

At a temperature of 1130°C, there exists a line that marks a significant phase boundary for cast iron containing between 2% and 4.3% carbon. In the region below this line, but above the SK line, cast iron is characterized by the presence of both austenite with ledeburite and cementite with ledeburite.

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