Iron-Carbon Eutectic Reaction Temperature

The Iron-Carbon phase diagram is a fundamental tool for understanding the behavior of iron-based alloys, including steels and cast irons. This diagram illustrates the different phases that exist at various temperatures and compositions. One of the critical invariant reactions depicted in this diagram is the eutectic reaction.

Eutectic Reaction in Iron-Carbon System

A eutectic reaction is an invariant reaction where a single liquid phase transforms into two solid phases upon cooling, simultaneously and isothermally (at a constant temperature). In the context of the Iron-Carbon system, this specific reaction is crucial for the formation of certain microstructures found in cast irons.

• The eutectic reaction for the Iron-Carbon system occurs at a precise temperature of $\text{1147}^{\circ}\text{C}$.
• At this temperature, a liquid phase (molten iron with dissolved carbon) transforms directly into a mixture of two distinct solid phases.
• The two solid phases formed during this eutectic transformation are Austenite ($\gamma$) and Cementite ($\text{Fe}_3\text{C}$).
• Austenite ($\gamma$) is a solid solution of carbon in face-centered cubic (FCC) iron, known for its ductility at high temperatures.
• Cementite ($\text{Fe}_3\text{C}$) is iron carbide, which is a very hard and brittle intermetallic compound.
• The overall eutectic reaction can be represented as:

  \(\text{Liquid} (\text{4.3 wt\% C}) \xrightarrow{\text{cooling at } 1147^{\circ}\text{C}} \text{Austenite} (\text{2.14 wt\% C}) + \text{Cementite} (\text{6.67 wt\% C})\)

Key Temperatures and Reactions on the Iron-Carbon Diagram

Understanding the eutectic reaction is often complemented by knowledge of other significant points and reactions on the Iron-Carbon phase diagram, which relate to different alloy types and processing:

• Peritectic Reaction: This reaction occurs at approximately $\text{1493}^{\circ}\text{C}$. In this reaction, a liquid phase and a solid delta ($\delta$) ferrite phase combine to form a new solid phase, austenite. It is relevant for the solidification of very low carbon steels.
• Eutectoid Reaction: This is another critical invariant reaction occurring at about $\text{727}^{\circ}\text{C}$ (sometimes rounded to $\text{723}^{\circ}\text{C}$). Here, a single solid phase, austenite, transforms into two other solid phases: ferrite and cementite. This transformation is fundamental to the heat treatment of steels, forming the pearlite microstructure.
• Melting Point of Pure Iron: Pure iron melts and solidifies at approximately $\text{1538}^{\circ}\text{C}$.

Based on the Iron-Carbon phase diagram, the eutectic reaction, where liquid directly transforms into a mixture of austenite and cementite, specifically takes place at $\text{1147}^{\circ}\text{C}$.