Steel is an alloy of iron and carbon, and its properties are fundamentally controlled by its internal structure. Thermal processing, involving heating and cooling, causes specific internal reorganizations known as phase transformations. These solid-state changes alter the arrangement of atoms, resulting in different microscopic phases with unique properties. Understanding these transformations is paramount, as the phases that form dictate the ultimate strength, hardness, and ductility of the finished steel product.
The Eutectoid Transformation Baseline
The foundation for understanding steel’s phase changes rests on the eutectoid transformation. This reaction occurs in the high-temperature phase called Austenite, which is a solid solution of carbon dissolved in iron with a face-centered cubic (FCC) crystal structure. Austenite is the parent phase for most steel heat treatments, typically forming above 727°C (1,341°F).
The eutectoid transformation is the simultaneous conversion of Austenite into two new solid phases upon cooling. This occurs precisely at the eutectoid temperature (approximately 727°C) and at a carbon content of around 0.76% to 0.8%. When steel with this exact composition cools through this temperature, the Austenite completely decomposes into a fine, layered mixture called Pearlite.
Pearlite is a composite microstructure consisting of alternating plates of Ferrite and Cementite. Ferrite is nearly pure iron with a body-centered cubic (BCC) structure, making it relatively soft and ductile. Cementite is a hard, brittle intermetallic compound (Fe₃C) with a much higher carbon content. This lamellar Pearlite structure is the standard benchmark for the transformation, producing a balance of strength and ductility.
Defining the Proeutectoid Phases
A proeutectoid phase forms from the parent Austenite before the steel reaches the critical eutectoid temperature of 727°C. This pre-transformation occurs because most commercial steels do not have the exact 0.76% eutectoid carbon concentration. The system attempts to adjust the carbon content of the remaining Austenite to the eutectoid composition before the final Pearlite transformation begins.
The precipitation of a proeutectoid phase (either Ferrite or Cementite) acts to deplete or enrich the carbon content of the remaining Austenite. This adjustment is driven by the fact that Austenite becomes thermodynamically unstable upon cooling. The specific phase that forms depends entirely on whether the steel has less or more carbon than the eutectoid composition.
Proeutectoid Ferrite (Hypoeutectoid Steels)
Steels containing less than 0.76% carbon are classified as hypoeutectoid steels. As these steels cool, they cross a boundary where Ferrite becomes stable above 727°C. Since Ferrite has very low carbon solubility, it precipitates out of the Austenite.
This proeutectoid Ferrite typically nucleates and grows along the Austenite grain boundaries. As Ferrite forms, it rejects carbon atoms into the surrounding Austenite matrix. This carbon enrichment continues until the remaining Austenite reaches the eutectoid composition of 0.76% carbon at 727°C. The enriched Austenite then transforms into Pearlite, resulting in a final microstructure of Pearlite islands surrounded by the proeutectoid Ferrite network.
Proeutectoid Cementite (Hypereutectoid Steels)
In contrast, hypereutectoid steels contain carbon content greater than 0.76%. As these steels cool, they enter a region where Cementite (Fe₃C) becomes the stable phase above 727°C. Cementite is a compound containing 6.67% carbon by weight.
The precipitation of proeutectoid Cementite reduces the carbon concentration of the surrounding Austenite. This Cementite typically forms a network along the Austenite grain boundaries. This process continues until the remaining Austenite is depleted to the 0.76% eutectoid composition. The final microstructure consists of the primary Cementite network surrounding the regions of Pearlite formed from the remaining Austenite.
Influence on Material Microstructure and Properties
The presence and distribution of the proeutectoid phase significantly impact the final mechanical performance of the steel. The mechanical properties are a composite effect of the proeutectoid phase and the harder Pearlite phase. Engineers control the amount of proeutectoid phase through composition and cooling rate to tailor the material for its intended use.
Proeutectoid Ferrite, which forms in hypoeutectoid steels, is soft and ductile. Its presence increases the overall toughness and ductility of the steel, making the material less prone to sudden fracture. Conversely, the addition of this soft phase reduces the overall strength and hardness compared to a pure eutectoid steel. Steels with a high percentage of proeutectoid Ferrite are commonly used in structural applications where resistance to impact and plastic deformation is prioritized.
Proeutectoid Cementite found in hypereutectoid steels has the opposite effect on properties. Since Cementite is a hard and brittle phase, its formation significantly increases the hardness and wear resistance of the final product. However, the formation of a continuous network of this brittle Cementite along the grain boundaries can severely reduce the steel’s ductility and increase its susceptibility to brittle fracture. These materials are often chosen for applications like cutting tools or bearings where extreme hardness and resistance to abrasion are the primary requirements.