Low-carbon steels, commonly known as "mild steels," are characterized by their low carbon content, typically ranging between 0.02% and 0.15% by mass. This low concentration gives them high ductility and ease of forming, but severely limits their heat-treatability. Because they do not reach the carbon threshold required to form structures like martensite, these steels cannot be hardened by quenching, making them soft and malleable materials, ideal for applications that do not require high mechanical strength.

Their use is widespread in everyday products where toughness is not a priority: paper clips, DIY nails, piano wire, decorative components, or coins such as the 1, 2, and 5 euro cent coins. In these cases, the goal is ease of manufacture, moderate corrosion resistance, and low cost, rather than structural performance. From a metallurgical perspective, these steels are very close to pure iron, and their microstructure is typically dominated by ferrite, with little or no pearlite. Representative examples include AISI 1010 and AISI 1015 grades, widely used in the industry for their excellent weldability and predictable behavior in cold forming processes.

It should be noted that the term "sweet" has no gustatory connotation, but rather refers to the softness and smoothness of the material, in contrast to medium- or high-carbon steels, which can be heat-treated to achieve higher levels of hardness and strength.

The principle behind this treatment is simple but effective: a surface resistant to wear and abrasion is sought, while maintaining a more ductile and tough interior that can absorb impacts without fracturing. This technique is especially useful in mechanical components such as gears, shafts, or tools that require high surface strength without compromising internal flexibility.

A historical analogy that illustrates this concept well is the manufacture of katanas by Japanese masters. Although not exactly the same procedure, the approach is similar: combining layers of steel with different properties to achieve a blade with exceptional external hardness and superior internal flex capacity. This structural duality offered obvious mechanical advantages over European swords, which tended to be more homogeneous in composition.

Surface hardening can be achieved through techniques such as carburizing, nitriding, or carbonitriding, all of which aim to modify the chemical composition of the surface without significantly altering the core. The result is a piece optimized to withstand combined stresses, with a long lifespan and superior performance under demanding conditions.

The toughness of these steels is not a spontaneous property but the result of a well-adjusted chemical composition and appropriate heat treatment. Unlike low-carbon steels, medium-carbon steels can be effectively quenched and tempered, which allows the final alloy to be hardened without completely sacrificing its deformation capacity. Quenching generates martensite in the matrix, increasing wear resistance, while subsequent tempering modulates brittleness, improving the overall toughness of the material.

Representative examples of this category include AISI 1045 and AISI 1050, widely used in the mechanical and tool industries. In more demanding applications, AISI S2 —an alloy steel with superior impact properties— offers exceptional performance. These steels offer versatility, making them favorites for those seeking resistant, durable materials with excellent response to heat treatments.

Steels with a carbon content greater than 0.5% by mass belong to a category where heat treatment plays a decisive role. As the proportion of carbon increases, the alloy loses ductility and malleability, but significantly gains in hardness, stiffness, and wear resistance. This structural evolution allows for much higher modulus of elasticity than steels with lower carbon content, making them ideal materials for applications requiring dimensional stability and long-lasting mechanical strength.

Although their malleability decreases, these steels can be hot-formed, allowing for industrial processing without compromising their final properties. One of the most emblematic applications is the manufacture of railway tracks. These parts, subjected to extreme loads—such as freight trains weighing over 100 tons—must maintain their shape without deforming, resisting both weight and constant friction. Structural rigidity and surface hardness are essential to ensure the safety and durability of the railway system. Using a low-carbon steel, such as AISI 1010, in this type of infrastructure would be unfeasible: its excessive flexibility would cause progressive deformations that would compromise the integrity of the route and jeopardize operations. Therefore, medium- to high-carbon steels, heat-treated to optimize their microstructure, are essential in environments where resistance to mechanical stress and abrasion are fundamental requirements.

Precise knowledge of the types of steel and their associated treatments is not only a technical issue, but also a guarantee of functionality, safety, and efficiency in the design of critical components.

High carbon steels, with carbon contents exceeding 0.85% by mass, represent the pinnacle in terms of hardness and rigidity within the family of carbon steels. Their resistance to deformation is exceptional, and their load capacity—especially against compressive stresses—makes them ideal materials for applications demanding extreme hardness. Cutlery is, without a doubt, the most emblematic example, although they are also used in cutting tools, precision springs, and components subjected to intensive wear.

The high proportion of carbon allows for the formation of martensite through quenching, which imparts outstanding hardness to the steel. However, this hardness is accompanied by a significant loss of ductility and malleability, resulting in a brittle response to impact stresses if not properly controlled. Therefore, these steels require specific heat treatments that allow their mechanical properties to be adjusted according to the desired application.

There are multiple heat treatment techniques—at least five main variants—that can be applied to high carbon steels, not all of them aimed at hardening. In some cases, especially when the carbon content is between 0.8% and 0.9%, the goal is to increase rigidity through quenching and tempering, taking advantage of the presence of elements like nickel, which improves toughness without compromising hardness. In other cases, when the steel lacks alloying elements like manganese or nickel, the objective may be to soften the structure to facilitate machining, through processes such as annealing or normalizing.

Given that these steels are difficult to work in a hardened state, heat treatment is usually applied at the end of the manufacturing process, once the part has been formed. In situations where a balance between surface hardness and internal flexibility is required, hot treatments that modulate the microstructure without completely eliminating martensite are used. This approach is especially necessary in steels with carbon contents exceeding 0.9%, where the material's natural rigidity demands precise thermal adjustments to prevent brittle failures.

From 1% carbon by mass, steel reaches extreme hardness levels, especially if it has been quenched, but it also becomes too rigid for many practical applications. Therefore, heat treatments in this category not only seek to harden but also to optimize toughness and workability, achieving a functional balance between strength and flexibility.

Steels with a carbon content exceeding 0.85% by mass formally belong to the high carbon steel category, although due to their extreme mechanical properties and specialized applications, they can be called ultra-hard steels. These materials stand out for their high hardness, rigidity, and resistance to deformation, making them ideal candidates for manufacturing knives, swords, cutting tools, and components subjected to intensive wear. Although their use in modern weaponry has decreased, they remain fundamental in sectors where impact and abrasion resistance are prioritized.

AISI 1095 grade is one of the most representative of this class. Its composition includes between 0.95% and 1.1% carbon, along with small proportions of manganese (≈ 0.45%) and silicon (≈ 0.35%), in addition to traces of phosphorus and sulfur. This combination generates a highly hardened microstructure, with the ability to form martensite after appropriate heat treatment. Handling this steel in a hot state requires precision and force, as its rigidity is maintained even at elevated temperatures. In thin parts, an error during forging can cause fractures, both during hammering and subsequent cooling.

When manufactured correctly, ultra-hard steel offers superior hardness within the family of plain carbon steels, without the need for complex alloys. However, it is common to find additions of elements such as chromium and vanadium in controlled proportions—up to 1% for chromium and 0.40% for vanadium—with the aim of improving wear resistance, thermal stability, and edge durability in cutting applications. These alloys are frequently used in bearings, high-performance cutlery, and precision tools, where mechanical demands are maximal.

The difficulty of working, brittle behavior under certain conditions, and the need for specific heat treatments make ultra-hard steels complex materials, but irreplaceable in contexts where hardness and wear resistance are fundamental requirements.

Tempering is one of the most important thermal processes in steel metallurgy, designed to modify the properties obtained after quenching and bring the material to a more stable balance between hardness and toughness. Its principle is based on heating the quenched steel to temperatures below its critical transformation points, maintaining it in that range for a specific time to cause controlled changes in the microstructure. This procedure aims to relieve internal stresses generated by quenching, which if left untreated can lead to deformation, cracks, or brittle fractures. During quenching, the steel's crystalline structure transforms into martensite, a very hard and wear-resistant phase, but also extremely brittle due to its high carbon content trapped in a solid solution. Tempering allows some of that carbon to diffuse and form more stable structures like ferrite and cementite, arranged in configurations such as troostite or tempered sorbite, which provide the material with greater ductility and impact resistance.

The tempering temperature range varies according to the desired properties: low temperatures, near 150 °C, tend to preserve hardness and wear resistance, slightly reducing brittleness; medium temperatures, between 300 °C and 450 °C, balance hardness and toughness; and high temperatures, near 600 °C, maximize shock resistance and ductility, sacrificing some surface hardness. In all cases, the holding time and the subsequent cooling medium are key parameters, as inadequate control can reverse some of the benefits or induce undesirable structures. There are specific tempering modalities, such as stress-relief tempering for parts that have not been quenched but have undergone intensive machining, or multiple tempering, in which the cycle is repeated to homogenize the structure and ensure constant properties throughout the section.

From a practical point of view, tempering is indispensable in the manufacture of cutting tools, gears, springs, fasteners, and mechanical components that require optimal performance under real working conditions. Without this process, a quenched steel might achieve outstanding laboratory hardness but quickly fail in service due to its tendency to break suddenly under dynamic stresses or impacts. Additionally, tempering reduces susceptibility to stress corrosion cracking, a phenomenon where cracks propagate rapidly in the simultaneous presence of mechanical load and corrosive agents.

In modern industry, tempering efficiency is enhanced by controlled atmosphere furnaces that prevent surface decarburization and maintain constant temperatures with minimal tolerances. Even in artisanal operations, such as knife making, tempering is an art that requires experience to interpret the visible tempering colors on the surface—straw, purple, or blue hues—as indicators of the temperature reached and resulting properties. This tradition dates back to the first blacksmiths who, without knowing the scientific fundamentals, understood through trial and error that steel that was too hard could be as useless as soft steel.

In summary, tempering is the phase that gives quenched steel its true practical value, allowing the hardness achieved not to become a defect and the acquired toughness to enable it to withstand the rigors of the final application. It represents the balance between maximum strength and structural reliability, and it is the bridge that converts a technically hardened material into a truly functional and durable tool or component.

Age hardening of steel, also known as age hardening, is a heat treatment primarily applied to high-carbon steels. Its objective is to improve the toughness of the material without significantly compromising its hardness. This process involves maintaining the alloy at a moderately elevated temperature for a prolonged period, allowing a gradual redistribution of carbon atoms within the metallic matrix. Through this regrouping, the phases present in the steel are stabilized, reducing internal stresses and promoting the formation of more homogeneous and impact-resistant structures. In steels such as AISI 1095, this treatment can be especially useful after quenching, as it helps improve the mechanical behavior of parts subjected to dynamic stresses, such as knives, cutting tools, or precision components. Although not as common a process as tempering or normalizing, thermal aging represents an effective alternative for optimizing the performance of ultra-hard steels in demanding applications where the combination of hardness and toughness is essential. 

Water quenching is a fundamental technique in the heat treatment of high-carbon steels, especially when seeking to induce the formation of martensite, the hardest and most brittle phase of steel. This process involves heating the part to a critical temperature, where the austenitic structure stabilizes, and then rapidly submerging it in water. The abruptness of the cooling prevents carbon atoms from diffusing and forming ferrite or pearlite, forcing the crystalline structure to transform into martensite. This phase is characterized by a tetragonal distorted arrangement of iron, saturated with carbon, which confers exceptional hardness to the steel. However, this hardness is accompanied by high brittleness, making it common to apply subsequent treatments like tempering to improve toughness without losing strength. Water quenching is especially effective in steels with more than 0.7% carbon, such as AISI 1095, where martensite formation is more pronounced, allowing for cutting parts with great edge retention and wear resistance. 

Oil quenching is a variant of the tempering heat treatment that seeks to induce the formation of martensite in high-carbon steels, generally exceeding 0.7% carbon. Unlike water quenching, this method uses non-combustible mineral oil as the cooling medium, which provides a more controlled and uniform cooling rate. This moderation in temperature descent reduces the risk of internal cracks or deformations, especially in complex or delicately shaped parts. When the red-hot piece is submerged in oil, the iron atoms are forced to adopt a tetragonal distorted crystal structure, saturated with carbon, known as martensite. This phase confers outstanding hardness to the steel, ideal for applications requiring high wear resistance and edge retention. Oil quenching is considered a higher-quality procedure due to its ability to preserve the dimensional integrity of the part, minimize residual stresses, and optimize mechanical performance in steels like AISI 1095, widely used in cutlery, cutting tools, and components subjected to extreme stresses. 

Tempered steel reveals a fascinating complexity when observed at a microscopic level. Unlike other simpler alloys such as tin or copper, steel presents multiple microstructures coexisting in the same metallic matrix. This means that in a single piece, such as a demolition hammer or a structural partition, different phases that determine its mechanical properties can be found. The microstructures of steel are combinations of fundamental phases such as austenite, cementite, graphite, ferrite, and martensite, whose presence or absence depends directly on the heat treatment applied and the manufacturer's objectives.

In the case of quenched steel, two microstructures stand out for their relevance and frequency. The first is martensite, an extremely hard and brittle phase that forms when steel is rapidly cooled from its austenitic state, preventing carbon diffusion. This structure is characterized by a tetragonal distorted arrangement of iron, saturated with carbon, which provides exceptional hardness but requires subsequent treatments like tempering to improve its toughness. The second common microstructure is bainite, which can appear in less aggressive quenching or in processes like austempering. Bainite offers a balanced combination of hardness and ductility, with an acicular morphology that is easily distinguished under the microscope.

These structures not only define the mechanical behavior of steel but also allow its properties to be adjusted for specific applications, from cutting tools to heavy machinery components. The study of microstructures is essential to understand how steel responds to stresses, wear, and thermal conditions, consolidating its role as one of the most versatile and sophisticated alloys in modern engineering.

Spheroidite is a microstructure of steel distinguished by the spherical shape of its granules, composed mainly of undissolved cementite (Fe₃C). This configuration is obtained through a prolonged heat treatment, in which the steel is maintained at a constant temperature of approximately 700°C for about 30 hours. During this period, the iron carbides present in the metallic matrix begin to reorganize into microscopic spheres, which gives this structure its name. The process is completely automatic and does not require uncontrolled cooling; it is enough to let the piece cool naturally once heating is completed.

The formation of spheroidite can also be achieved at lower temperatures, such as 500°C, although in that case, the exposure time must be considerably extended, reaching 40 or even 50 hours. There is no linear relationship between temperature and time, as the kinetics of transformation depends on multiple factors, such as the chemical composition of the steel, the initial grain size, and the thermal history of the part. This microstructure is especially valued for its high ductility and easy machinability, making it an ideal option for cold forming processes or for parts that require good toughness without compromising structural strength.

Pearlite is a characteristic microstructure of carbon steels that forms as a result of the eutectoid transformation of steel, typically when it is slowly cooled from the austenitic phase. It is composed of alternating layers of ferrite (virtually pure iron) and cementite (iron carbide, Fe₃C), arranged in a lamellar pattern resembling a "sandwich" model. This configuration gives pearlite a balanced combination of hardness and ductility, making it a useful structure for applications requiring moderate mechanical strength and good workability.

The name "pearlite" comes from its appearance under an optical microscope, where the ferrite and cementite lamellae reflect light similarly to nacre (mother-of-pearl). This structure can be deliberately generated through controlled heat treatments or appear as a byproduct in non-accelerated cooling processes. Its presence is common in medium-carbon steels and can even be observed with the naked eye in traditionally forged materials, such as Damascus steel, where the pearlitic bands contribute to both the aesthetics and the mechanical performance of the piece.

Pearlite represents an intermediate point on the scale of hardness and toughness among steel microstructures, being less hard than martensite but more resistant than pure ferrite. Its formation and distribution within the metallic matrix are key to defining the steel's behavior against stresses, impacts, and forming processes.

Carbon is the key element that transforms iron into steel, significantly modifying its physical and mechanical properties. Its influence depends not only on the amount present in the alloy but also on how it is distributed within the metallic matrix. When carbon dissolves homogeneously in iron, a uniform mixture is obtained, ensuring consistency in the material's behavior throughout the entire piece, whether it's a bar, an axle, or a nut. This distribution directly affects the steel's response to heat treatments and mechanical stresses.

Functionally, carbon increases the hardness and rigidity of iron, which in its pure state is malleable and ductile. However, an excess of carbon can make the steel brittle, causing it to lose its ability to absorb impacts without breaking. For this reason, the carbon content must be kept within controlled ranges to achieve an optimal balance between hardness and toughness. In carbon steels, which do not contain alloying elements like chromium, molybdenum, or vanadium, this content typically ranges between 0.1% and 1.10%, depending on the desired application.

The AISI classification, widely used in America, Europe, and Asia, allows the percentage of carbon in steel to be identified by a numerical code. For example, an AISI 1010 steel contains approximately 0.10% carbon, while an AISI 1085 steel has 0.85%. Above 1% carbon, carbon steel loses practical utility due to its excessive brittleness, even after heat treatments such as quenching or tempering.

In addition to modifying mechanical properties, carbon also reduces the melting point of steel, which facilitates certain manufacturing processes, albeit at the cost of decreasing its corrosion resistance, which is already limited in iron. Therefore, understanding the role of carbon in steels is essential for designing materials that adequately meet the technical demands of each industrial application.