PHASES OF STEEL

In pure iron, austenite stabilizes between approximately 910 °C and 1400 °C. However, in steels—which are alloys of iron with carbon and other elements—this range varies considerably. The onset temperature of the austenitic phase depends directly on the carbon content: a higher concentration means a lower temperature is needed for the steel to enter this phase. Therefore, some steels begin to form austenite from 700 °C, while others require temperatures above 1200 °C.

The importance of austenite lies in its capacity to accommodate carbon atoms within its crystal lattice. These atoms can become trapped in the interstices of the structure without forming chemical bonds, which allows for the controlled modification of the steel's mechanical properties. This characteristic is fundamental in processes such as quenching, carburizing, or annealing, where phase transformation determines the material's final hardness, toughness, and strength.

If you wish, I can also help you develop the role of austenite in iron-carbon phase diagrams or in stainless steels, where its stability at room temperature is key.

Although pure cementite is metastable and has few industrial applications, its presence within high-carbon steels is fundamental. In these cases, it is distributed in the form of lamellae or networks within the ferritic or pearlitic matrix, contributing to increase the material's hardness and wear resistance. However, this mechanical improvement is accompanied by a loss of toughness, which limits its use in components subjected to impacts or dynamic stresses.

Unlike eutectic compounds, cementite is not part of a mixture with a minimum melting point between iron and carbon, but rather appears as a secondary phase in the iron-carbon diagram, especially in hypereutectoid steels. Its control during manufacturing is key to defining the final properties of the steel, as an excess can compromise workability and fracture resistance.

If you wish, I can help you link this content to the behavior of cementite in heat treatments such as quenching or tempering, where its partial transformation plays a decisive role.

This phenomenon occurs mainly in two scenarios: in cast irons, where the high carbon content is deliberate and controlled, and in poorly manufactured steels, where the presence of graphite indicates a failure in the refining process. In the first case, graphite can adopt lamellar, nodular, or compact shapes, depending on the additives used and the cooling conditions. In the second, its presence is usually undesirable, as it compromises the mechanical integrity of the steel.

Both graphite and cementite are essential constituents of cast iron, but with very different properties. While cementite provides hardness and brittleness, graphite confers internal lubricity and improves machinability, albeit at the cost of reducing structural strength. Understanding its formation and distribution is key to designing alloys with specific properties, especially in applications where a balance between strength, ductility, and ease of machining is required.

In its pure state, iron is entirely ferritic, meaning its crystalline structure remains stable in this phase until it reaches temperatures close to 910 °C, at which point the transition to austenite begins. In the context of steels, the term "ferrite" is used to describe iron free of chemical combinations, that is, the portion of the alloy where carbon has not formed compounds like cementite or pearlite. This phase is essential in low-carbon steels, where it provides toughness, malleability, and corrosion resistance, although with relatively low hardness.

The presence of ferrite in a metallic microstructure can be observed using metallographic techniques, and its proportion directly influences the steel's behavior during heat treatments, mechanical stresses, and forming processes. Therefore, understanding its role is key to designing alloys with specific properties, especially in structural applications or in components requiring good weldability and impact resistance.

The obtaining of martensite is not limited to chemical composition but requires a specific heat treatment: the steel must be heated until it reaches the austenitic phase (temperatures above 700 °C, depending on the type of steel) and then rapidly cooled by quenching, either in water or mineral oil. This rapid cooling prevents carbon diffusion and forces the transformation of austenite into martensite, a distorted tetragonal structure characterized by its very high hardness and wear resistance.

However, this extreme hardness is accompanied by considerable brittleness. Martensite in its natural state is brittle and susceptible to cracks under mechanical stress. Therefore, it undergoes a tempering process, which consists of reheating it to moderate temperatures to relieve internal stresses and improve its toughness without completely losing the acquired hardness. This balance between hardness and ductility is what makes martensite useful in demanding mechanical applications, such as cutting tools, machine components, or structural elements subjected to abrasion.

If you wish, I can help you integrate this content into a section on heat treatments or isothermal transformation diagrams, so that the chapter gains even more technical depth.