ALLOYING ELEMENTS AND THEIR ROLE

Boron is a chemical element that, although present in small quantities, can exert a significant influence on the properties of steel. Its controlled incorporation is primarily used as a hardener, similar in function to carbon, especially in steels designed for critical applications such as barriers in nuclear power plants. In these environments, the selection of materials with high structural strength and neutron absorption capacity is fundamental to ensure operational safety.

Iron, the main component of steel, possesses notable resistance to nuclear alteration, making it a suitable base for structures exposed to radiation. However, its neutron absorption capacity is limited. This is where boron becomes relevant: its atoms present a high effective cross-section for fast neutron capture, which helps reduce environmental reactivity and improve the control of nuclear processes. For this reason, boron-alloyed steels are used in shielding components, pressure vessels, and containment structures in nuclear facilities.

In conventional steels, boron is usually found as a residual impurity, generally in very low concentrations. In these cases, it is not considered harmful as long as its mass content does not exceed that of the carbon present in the alloy. An excess of boron can alter the microstructure of the steel, affecting its toughness and behavior during heat treatments. Therefore, its presence must be carefully controlled during manufacturing.

In addition to its use in the nuclear field, boron is also employed in high-strength steels for automotive applications, cutting tools, and components subjected to wear. In these cases, its main function is to improve the hardenability of the steel, allowing for more uniform martensitic structures in large cross-section parts.

In summary, boron is a strategic element in materials engineering, whose incorporation into steel can provide significant benefits in terms of mechanical strength and radiation behavior. However, its use requires precise control to avoid adverse effects on the microstructure and final properties of the material.

Magnesium, an alkaline-earth metal, has a poor affinity for Iron and only appears as an impurity. It is very difficult to find it in this industry unless the use of sacrificial salts like Magnesia for the reduction of iron-containing ore is considered. 

Aluminum is not an element commonly used as a primary alloying agent in steel manufacturing, except under very specific circumstances. Its most common function is as a deoxidizer, similar to silicon, although with certain differences that limit its widespread use.

During the steel manufacturing process, it is essential to eliminate dissolved oxygen in the liquid metal to prevent the formation of oxides that compromise the quality of the final product. Aluminum, thanks to its high chemical affinity for oxygen, can effectively fulfill this function. However, unlike silicon, aluminum does not provide significant improvements in the mechanical strength or hardenability of steel, which reduces its attractiveness as a structural alloy.

Furthermore, the excessive use of aluminum can generate compounds like alumina (Al₂O₃), which tend to form non-metallic inclusions that are difficult to remove and negatively affect the workability of the steel, especially in hot forming processes. For this reason, its addition is carried out in a controlled manner and in very small quantities.

In special applications, such as in killed steels, aluminum is used to achieve complete deoxidation, preventing the formation of gas bubbles during solidification. It can also be used in ferritic stainless steels to stabilize the structure against the formation of nitrides and carbides, although in these cases its presence is carefully balanced with other elements such as chromium and titanium.

In summary, although aluminum can play a useful role as a deoxidizer in steel metallurgy, its limitations in terms of mechanical properties and its tendency to form inclusions relegate it to a specific and technical use, rather than a structural or functional one.

Silicon is one of the most versatile elements in steel metallurgy, primarily used as a deoxidizer during manufacturing and as a modifier of mechanical properties in specific applications. Its controlled incorporation can improve the steel's flexibility, impact resistance, and elastic recovery capacity, which is especially useful in components subjected to dynamic stresses.

During steel production, silicon acts as an effective deoxidizer, removing dissolved oxygen from the liquid metal and reducing the formation of harmful oxides. This function is essential for obtaining a clean and homogeneous microstructure, especially in killed and high-quality steels.

The curious thing about silicon is that, in its pure state, it is an extremely brittle and fragile material, with a crystalline structure that suggests no flexibility whatsoever. However, when alloyed in small proportions (up to approximately 1.5%), it can significantly improve steel's ability to absorb and release kinetic energy without permanent deformation. This property makes it ideal for steels intended for impact, such as AISI S2, and for the manufacturing of springs and elastic components.

Furthermore, silicon contributes to increasing the electrical resistance of steel, making it a key element in electrical steels used in transformer cores and motors. In these cases, its presence can exceed 2%, although strict control of the composition is required to avoid excessive brittleness.

In summary, silicon is a seemingly contradictory element: fragile in its pure form, yet capable of imparting flexibility and dynamic resistance to steel when used correctly. Its role as a deoxidizer and as an enhancer of mechanical properties makes it an essential component in various industrial applications.

Phosphorus is an element that, in steel metallurgy, is not usually added deliberately due to its generally detrimental effects on the mechanical properties of iron. Its presence in steels is almost always residual, as an unavoidable impurity originating from the raw materials used in the manufacturing process.

From a structural point of view, phosphorus tends to increase the brittleness of steel, especially at low temperatures, a phenomenon known as phosphorus embrittlement. This effect results in a decrease in toughness and an increased susceptibility to cracking under dynamic stresses or impacts. Although its negative influence does not reach the level of sulfur—which forms highly detrimental sulfide inclusions—phosphorus is still considered an undesirable element in most structural steels.

However, there is an interesting exception: in steels intended for ornamental or decorative applications, phosphorus can be intentionally added in small amounts. In these cases, its ability to increase atmospheric corrosion resistance becomes an advantage. Furthermore, phosphorus can improve the steel's surface finish, facilitating processes such as polishing or engraving, making it useful in the manufacturing of architectural elements, railings, metal sculptures, and other products where aesthetics take precedence over mechanical strength.

In quantitative terms, acceptable limits for phosphorus in common steels are usually kept below 0.05%, although in ornamental steels it can reach up to 0.10% or more, depending on the design and function of the final product.

Sulfur is considered one of the most detrimental impurities in steel metallurgy. Its presence, even in minimal amounts, can seriously compromise the material's mechanical properties, especially its toughness and impact resistance at low temperatures. For this reason, in most structural and high-demand steels, its incorporation is avoided at all costs.

When sulfur is present in steel, it tends to form iron sulfide (FeS) inclusions, which are located at the grain boundaries and act as crack initiation points. These inclusions reduce the internal cohesion of the material and promote the appearance of cracks under mechanical or thermal stresses, a phenomenon known as sulfur embrittlement. This effect is particularly severe in cold conditions, where the steel can become brittle and fail catastrophically.

However, there is a notable exception: in steels intended for intensive machining processes, such as those used in the manufacturing of coins, screws, automotive components, or precision parts, sulfur is deliberately added in controlled proportions. In these cases, its presence improves the machinability of the steel by facilitating the formation of short chips and reducing tool wear. These steels, known as free-cutting steels, can contain up to 0.3% sulfur, always accompanied by manganese to form MnS inclusions, which are less detrimental than FeS.

In summary, sulfur represents a technical dilemma in steel manufacturing: an impurity that must be strictly controlled in structural applications, but which can become a useful tool in contexts where machinability is a priority. Its proper management is key to ensuring the performance and safety of the final product.

Another alkaline metal, barely used. It is a deoxidizer but usually appears as an impurity. It has no importance with respect to steels, as an alloying element. It is indispensable for the casting process, but its use ends there. 

Poorly soluble in Iron. Too expensive and impractical. Not used. 

Titanium is a metal with exceptional properties: high mechanical strength, low density, excellent corrosion resistance, and remarkable thermal stability. However, despite these virtues, its use in common steels is extremely limited. The reasons are both metallurgical and economic.

One of the main technical difficulties lies in the low solubility of its oxide (TiO₂) in molten iron. During the manufacturing process, titanium tends to form stable and refractory oxides that do not easily integrate into the metallic matrix, generating inclusions that can negatively affect the quality of the steel. Furthermore, titanium has a strong affinity for carbon, nitrogen, and oxygen, which can lead to the formation of hard but brittle carbides and nitrides if not properly controlled.

For these reasons, titanium is not employed in conventional carbon steels or general-purpose alloys. Its presence is reserved for special grades of stainless steel, where it fulfills very specific functions. In these steels, titanium acts as a stabilizer, preventing the formation of chromium carbides at grain boundaries and improving resistance to intergranular corrosion. Typical examples include stabilized stainless steels like AISI 321, used in high-temperature applications and aggressive environments.

Given that this chapter is not dedicated to stainless steels, titanium falls outside the scope of relevant elements for common steels. Nevertheless, its study remains important in specialized contexts, where its properties can make a critical difference in material performance.

Vanadium is one of the most valuable alloying elements in steel engineering, especially when seeking to improve the toughness, hardness, and durability of the material. Although it is added in very discrete proportions — as little as 0.5% can make a significant difference — its effects are notable even at lower concentrations, thanks to its ability to efficiently modify the microstructure of steel.

In many aspects, vanadium shares similarities with molybdenum, as both contribute to the formation of hard and stable carbides that reinforce the ferritic or martensitic matrix of the steel. These vanadium carbides (VC) act as barriers to dislocation, improving wear resistance, thermal stability, and the ability to maintain mechanical properties under extreme conditions.

Vanadium is not usually found in common or everyday steels, as its incorporation is reserved for applications requiring superior performance. It is frequent in tool steels, high-speed steels, and structural steels subjected to dynamic loads, impacts, or elevated temperatures. In these contexts, vanadium allows for a more refined microstructure, with smaller grains, which improves strength without sacrificing ductility.

Furthermore, its presence favors the hardenability of steel, allowing for more effective and uniform heat treatments. This makes it an essential component in the manufacturing of drill bits, milling cutters, industrial blades, molds, and heavy machinery components.

In summary, vanadium is a strategic element in advanced steel metallurgy. Its ability to form carbides, improve toughness, and increase wear resistance positions it as an indispensable resource in steels designed to face particularly demanding conditions.

Chromium is one of the most widely used alloying elements in steel metallurgy, surpassed only by manganese in terms of frequency. Its popularity is due to both its natural abundance and its excellent value for money, being considerably more economical than other transition metals like vanadium, but with equally notable effects on steel properties.

In small proportions, chromium acts as an effective hardener. It promotes the formation of martensite during heat treatments, which improves the mechanical strength and load-bearing capacity of the steel. Additionally, it forms chromium carbides (Cr₇C₃, Cr₂₃C₆), which are extremely hard and stable, allowing the alloy to accept a greater amount of carbon without compromising the internal structure. This expands the design margin for high-performance steels.

As the chromium content increases, the hardness of the steel increases, although its brittleness also increases, due to the material's tendency to adopt more crystalline and less ductile structures. In high-speed steels, chromium is used in high concentrations, typically between 4% and 6%, to improve wear resistance and maintain hardness at high temperatures, which is essential in cutting tools.

From a content of approximately 5–6% by mass, chromium begins to exert a significant effect on the corrosion resistance of steel. This is due to its ability to form a passive layer of chromium oxide (Cr₂O₃) on the surface, which protects the material against oxidation and chemical agents. When the chromium content reaches 10.5% by mass, the steel is technically considered stainless, although some standards and authors require a minimum of 12% to formally classify it as such.

In summary, chromium is a multifunctional element in steel engineering: it improves hardness, stabilizes the microstructure, allows for greater carbon solubility, and, in adequate concentrations, transforms steel into a corrosion-resistant material. Its versatility makes it an essential component in tool steels, machinery, structures exposed to aggressive environments, and high-demand applications.

Manganese is, after carbon, the most relevant element in the composition of non-stainless steels. Its presence is practically universal in this type of alloy, and its effects on the microstructure and mechanical behavior of steel are as broad as they are beneficial.

One of its primary functions is the refinement of grain size during solidification and heat treatments. By promoting a finer and more homogeneous structure, manganese improves the toughness of steel – that is, its capacity to absorb energy without fracturing. This property is essential in components subjected to impacts, vibrations, or cyclic loads.

Furthermore, manganese increases hardness and mechanical fatigue resistance, making it a key element in steels intended for machinery, tools, structures, and wear parts. Its hardening effect is potentiated when combined with other elements such as carbon, chromium, or vanadium.

One of its most valuable contributions is its ability to fix sulfur. Instead of allowing the formation of iron sulfide (FeS) inclusions, which are brittle and detrimental, manganese reacts with sulfur to form manganese sulfide (MnS), which is much more stable and less harmful. This reaction reduces the brittleness of steel and improves its workability, especially in hot forming processes.

For all these reasons, manganese can be considered a "purifier" of steel. It not only neutralizes impurities but also enhances the mechanical and metallurgical properties of the material. Its typical content in carbon steels ranges between 0.3% and 1.5%, although in special steels it can exceed 2%, depending on design requirements.

In summary, manganese is an indispensable element in the metallurgy of non-stainless steel. Its versatility, effectiveness, and low cost make it a fundamental pillar for obtaining strong, durable, and reliable alloys.

Cobalt is a transition element that, despite its outstanding properties, is barely used in the manufacturing of common steels. The reasons are multiple: its high cost, its relative scarcity in the Earth's crust, and the fact that many of its functions can be fulfilled by other more economical elements like chromium, molybdenum, or vanadium.

From a metallurgical standpoint, cobalt does not provide significant improvements in hardness, toughness, or wear resistance that justify its use in general-purpose steels. However, its exceptional behavior at elevated temperatures makes it a valuable resource in very specific applications. Cobalt is, in fact, the "champion of heat" among the metals of the first transition series: it maintains its crystal structure and mechanical properties even under extreme thermal conditions, where other metals begin to lose effectiveness.

For this reason, cobalt is employed in steels and superalloys designed for high-temperature environments, such as turbines, cutting tools subjected to intense friction, aerospace components, and energy generation systems. In these cases, its presence improves thermal stability, creep resistance, and hot hardness retention—critical characteristics for the material's performance and safety.

In high-speed steels, cobalt can appear in proportions of up to 5–12%, depending on the grade and application. Its inclusion allows the cutting edge to maintain its hardness even when the tool reaches temperatures exceeding 600 °C during machining.

In summary, cobalt is an element of limited but strategic use. Although it is not competitive in conventional steels for economic and metallurgical reasons, its ability to resist extreme heat makes it an essential component in steels and alloys for thermally demanding applications.

Nickel is, without a doubt, one of the most influential elements in improving the mechanical properties of steel. After chromium and manganese, it ranks as the third most important transition metal in steel metallurgy, especially in alloys destined for demanding environments where toughness and structural reliability are priorities.

Its main virtue is its ability to increase the toughness of steel—that is, its resistance to fracture under dynamic loads or impacts, without compromising hardness. This combination of properties is especially valuable in applications where a strong but not brittle material is required, such as in structures subjected to vibrations, cyclic stresses, or extreme temperature and pressure conditions.

Nickel also contributes to refining the microstructure, favoring the formation of a more homogeneous and stable matrix. It improves hardenability, reduces distortion during heat treatments, and increases fatigue resistance. Furthermore, it has a positive effect on corrosion resistance, especially in humid or saline environments, which makes it an essential component in steels for naval, aerospace, and petrochemical use.

Due to its high cost and limited availability, nickel is reserved for technically demanding applications, such as steels for pressure vessels, armor, cryogenic components, precision tools, and austenitic stainless steels. In the latter, nickel is responsible for stabilizing the austenitic phase, allowing for a non-magnetic, ductile, and highly corrosion-resistant structure.

In terms of composition, nickel-alloyed steels can contain from 1% to over 20%, depending on the type of alloy and the intended use. Even in moderate proportions, nickel's effect on steel quality is notable, making it an innegotiable element in designs where safety and performance are critical.

Copper is an element rarely used as a primary alloying agent in steel manufacturing, due to its low solubility in iron and metallurgical effects that can compromise material quality if not properly controlled. Unlike other transition metals such as nickel or chromium, copper does not form carbides nor does it significantly contribute to the hardness or hardenability of steel. 

However, in special circumstances, copper can provide specific benefits, especially regarding atmospheric corrosion resistance. In small proportions (typically between 0.2% and 0.5%), copper improves the formation of a stable surface patina that protects steel against oxidation, without the need for additional coatings. This property makes it useful in structural steels exposed to the outdoors, such as those used in architecture, bridges, or urban furniture. 

From a microstructural point of view, copper has the ability to reduce the ferrite-austenite transition temperature, which can facilitate certain heat treatments and improve the toughness of steel under specific conditions. Nevertheless, its effect is limited and usually does not justify its use in conventional steels, where other elements such as manganese, nickel, or vanadium offer more consistent and economical results. 

In high concentrations, copper can cause problems of hot shortness (hot brittleness), especially during rolling or forging processes, due to the formation of liquid phases at grain boundaries. For this reason, its use must be carefully measured and controlled. 

In summary, copper is a secondary alloying agent in steel metallurgy, with specific applications in weathering steels and in contexts where improving toughness without compromising corrosion resistance is sought. Although not a primary player in most alloys, its presence can make a difference in specific designs. 

Zinc is not directly alloyed with steel, but it is very important in the steel industry because steel parts are coated with it (galvanizing) to protect them from corrosion. 

It is rarely used in pure form. However, research has been conducted with Yttria (its most stable oxide). 

Zirconium is an element rarely used in the manufacturing of conventional steels. Its application is almost exclusively restricted to special stainless steels, where it fulfills very specific functions related to the stabilization of microstructure and the improvement of certain properties under extreme conditions. 

One of its main metallurgical characteristics is its ability to form zirconium carbides (ZrC), which are extremely hard and thermally stable. However, both the pure metal and its compounds exhibit low solubility in iron, which limits their effective integration into the steel matrix. This low solubility can lead to undesirable inclusions or segregations that affect the material's homogeneity, thus requiring very precise control during the alloying stage. 

In high-specialty stainless steels—such as those used in nuclear environments, aerospace, or in components subjected to intense radiation—zirconium can provide specific benefits. These include resistance to intergranular corrosion, stability against the formation of harmful precipitates, and compatibility with highly oxidizing environments. Nevertheless, its application remains marginal due to its high cost, metallurgical complexity, and the availability of other more efficient and economical elements to fulfill similar functions (such as titanium or niobium). 

In summary, zirconium is a niche alloying element, reserved for high-performance stainless steels where a very specific combination of thermal, chemical, and structural properties is required. Although its ability to form carbides is technically interesting, its low solubility in iron limits its use to highly specialized contexts. 

Niobium is a highly beneficial alloying element for steel, although its use is restricted for economic reasons. Its high price and limited availability mean that it is reserved exclusively for high-performance alloys, such as elite stainless steels and structural steels subjected to extreme conditions.

From a metallurgical perspective, niobium acts as a stable carbide former (NbC), which strengthens the steel matrix and improves its wear resistance, toughness, and thermal stability. These carbides also contribute to refining the grain size, resulting in a more homogeneous microstructure and a significant improvement in mechanical strength without sacrificing ductility.

In stainless steels, niobium plays a critical role as a stabilizer against intergranular corrosion by fixing carbon in the form of niobium carbides, thereby preventing the formation of chromium carbides at grain boundaries. This is especially important in applications where steel is exposed to high temperatures or aggressive environments, such as in the chemical, nuclear, or aerospace industries.

Furthermore, niobium improves creep resistance and high-temperature toughness, making it an essential component in steels for pressure vessels, turbines, reactors, and highly demanding structural components.

In summary, niobium is an elite alloying agent, reserved for steels where quality, durability, and strength are absolutely paramount. Although its cost limits its use in conventional steels, its technical impact makes it a strategic resource in advanced metallurgy.

Molybdenum holds a prominent place in modern steel metallurgy, being considered the fourth most important transition metal, after manganese, chromium, and nickel. Its relevance has grown significantly in recent decades, displacing tungsten in many applications, thanks to its excellent balance between technical properties and cost.

One of its main virtues is its ability to form molybdenum carbides (Mo₂C), which are extremely hard and stable. These strengthen the steel matrix and improve its wear resistance. This property makes it an effective hardener, especially in tool steels, high-speed steels, and components subjected to intense friction.

Molybdenum also increases toughness, allowing the steel to maintain its structural integrity even under dynamic loads or impacts. Unlike other hardening elements that can compromise ductility, molybdenum improves deformation resistance without sacrificing toughness, making it ideal for applications requiring a combination of strength and resilience.

Another key aspect is its resistance to structural weakening from sudden temperature changes. Molybdenum stabilizes the steel's microstructure against thermal creep and embrittlement induced by thermal cycling, making it an essential component in steels for pressure vessels, turbines, heat exchangers, and aerospace components.

In stainless steels, molybdenum improves pitting corrosion resistance and resistance to chlorinated environments, being indispensable in grades such as AISI 316, used in marine and aggressive chemical environments.

In summary, molybdenum is a strategic alloying agent that provides hardness, toughness, thermal stability, and chemical resistance, solidifying its position as a pillar in the manufacture of high-performance steels for critical applications.

Ruthenium (Ru), Rhodium (Rh), and Palladium (Pd) are elements belonging to the platinum group, known for their exceptional properties in highly demanding metallurgical contexts. Although their most common use is associated with luxury jewelry and elite industrial applications, their technical potential extends far beyond, especially concerning corrosion resistance and structural modification of steel.

Ruthenium, despite not forming carbides, is used as a hardening agent in specific alloys. Its incorporation significantly improves corrosion resistance, even in extreme environments, although it does not promote austenite formation. This characteristic makes it an ideal candidate for steels requiring high durability without compromising structural stability at elevated temperatures.

On the other hand, both rhodium and palladium do promote the formation of austenite in steel, which is especially useful in alloying processes where improved toughness and ductility of the material are sought. It has been observed that an approximate addition of 10% of either of these two elements is sufficient to induce a significant transformation in the steel's microstructure, favoring the austenitic phase and, with it, its mechanical properties.

Although experimental studies on these metals in the field of metallurgy are still limited, the available results offer promising perspectives. Their singular behavior, both in terms of reactivity and structural influence, positions them as key elements in the development of special steels for critical applications, where chemical resistance and mechanical stability are essential.

Post-transition metals such as cadmium (Cd), tin (Sn), lead (Pb), bismuth (Bi), and antimony (Sb) play a secondary but strategic role in materials engineering, especially in the treatment and modification of steel. Although their use is limited compared to other transition elements, their specific properties allow for the improvement of concrete aspects of steel's behavior in certain applications.

Cadmium and tin, like zinc (Zn), are primarily used as protective coatings through galvanization or plating processes. These coatings act as barriers against oxidation, extending the service life of steel in aggressive environments. Their ability to form thin, adherent films makes them effective options for structures exposed to humidity, salinity, or corrosive chemical agents.

On the other hand, lead, bismuth, and antimony share a common characteristic: their low solubility in the ferritic matrix of steel. This property, far from being a limitation, is exploited to improve the machinability of the material. When incorporated in small proportions, these elements generate soft inclusions that facilitate cutting, turning, and other mechanical operations, reducing tool wear and improving precision in manufacturing processes.

Despite their low participation in structural alloys, post-transition metals offer specific and effective solutions in contexts where surface protection or ease of machining are priorities. Their study and application remain relevant in the development of functional steels adapted to specific industrial needs.

Alkaline and alkaline-earth metals do not form eutectic solutions with Iron. 

Lanthanides, also known as rare earths, constitute a group of chemical elements that, while possessing unique physical and electronic properties, are hardly used in the manufacture of conventional steels. Their limited participation in the steel industry is due, in part, to their tendency to form intermetallic compounds with iron (Fe), which do not integrate effectively into the typical crystalline matrices of steel, nor do they contribute significantly to its mechanical or structural properties.

Despite this, some lanthanides have found specific applications outside the realm of structural steel. Neodymium (Nd), for example, is used in the manufacture of high-power permanent magnets, with the chemical formula Fe₁₄Nd₂B. This compound, known as NdFeB, is not a steel in the strict sense, but an inorganic material with exceptional magnetic properties, used in electric motors, hard drives, wind generators, and precision devices. Its crystalline structure and magnetic behavior place it in a completely different category from traditional steels, although its ferrous base tangentially links it to the metallurgical field.

In summary, lanthanides do not form part of any relevant family of industrial steels, and their presence in iron metallurgy is more anecdotal than structural. Nevertheless, their ability to form functional compounds with specific properties, such as magnetism, gives them technical value in highly specialized sectors.

Similar to Zirconium, Hafnium is used in superalloys, not in normal steel.

Its use in common steels is practically non-existent. The main reason lies in its geological scarcity and the high cost associated with its extraction and processing. These economic limitations make its incorporation into general-purpose alloys unfeasible, reserving its presence for highly specialized sectors such as aerospace, electronics, or the manufacture of surgical equipment, where its unique properties justify the investment.

Although tantalum can form stable carbides and contribute to the hardening of metallic matrices, its role in conventional steelmaking is more theoretical than practical. Consequently, it remains an elite resource, admired for its qualities but relegated by its inaccessibility.

Tungsten (W), also known as wolfram, holds a privileged position in steel metallurgy, considered the fifth most important metal in this field. Its affinity with iron (Fe) is not only excellent but practically legendary, and it has been the subject of study in multiple technical volumes due to the breadth and depth of its applications. Historically, its relevance solidified during the First World War and, especially, the Second World War, where its use in hardened alloys made it a strategic resource of the highest order.

The primary virtue of tungsten lies in its ability to form extremely hard carbides, which significantly increase both the hardness and toughness of steel. This structural transformation elevates the elastic modulus of the material, generating alloys of exceptional robustness and durability, practically “invincible” from a military perspective. No other non-precious or semi-precious metal—such as rhenium (Re), osmium (Os), or iridium (Ir)—offers a comparable benefit in terms of mechanical reinforcement of steel.

Steel alloyed with tungsten is characterized by its high density and hardness, making it ideal for elite applications, such as high-speed steel (HSS) cutting tools, ballistic components, and specialized armor. However, its main limitation is its poor ability to improve corrosion resistance, even when present in proportions as high as 18% by mass. In this aspect, chromium (Cr) and molybdenum (Mo) are more effective, which has led to a progressive substitution of tungsten in certain applications, not due to technical inferiority but for economic reasons. The assertion that molybdenum or molybdenum-cobalt combinations surpass tungsten is, in reality, a fallacy: the mechanical performance of tungsten remains superior, although its high cost has conditioned its use.

Another notable aspect of tungsten is its ease of alloying. As early as the 1920s, it was successfully incorporated into cast iron in oxide form, taking advantage of its chemical nobility to be directly reduced in the metallurgical process. This characteristic facilitated its early adoption in the industry, solidifying its role as a key element in the evolution of special steels.

Additionally, tungsten possesses relevant nuclear properties: its high and even atomic number 74, along with its considerable atomic mass, makes it an excellent material for radiation shielding, being used in reactors, radioactive waste containers, and nuclear protection devices.

Rhenium (Re) is, without a doubt, an "outstanding" metal due to its extraordinary physicochemical properties, although its application in steel metallurgy is rather limited for economic and availability reasons. Its geological rarity and high cost have kept it apart from conventional steels, reserving it for highly demanding technical contexts where every atom counts.

When incorporated into steel, rhenium provides benefits comparable to those of tungsten (W), especially concerning toughness and corrosion resistance. These improvements, though significant, do not decisively surpass those already offered by tungsten, which has generated some skepticism about its viability as a substitute. In fact, some historical sources suggest that during the Second World War, its use as a replacement for tungsten was explored by the German industry, although this hypothesis remains a subject of debate and is not supported by conclusive evidence.

From a structural point of view, rhenium presents a significant limitation: it does not form recognizable stable carbides within the steel matrix. This deficiency reduces its effectiveness as a hardening agent, placing it below other refractory metals such as molybdenum (Mo), tantalum (Ta), or tungsten itself, which do generate hard and resistant carbides. Therefore, although rhenium improves certain aspects of steel behavior, its impact on hardness is more discreet and less decisive.

In summary, rhenium represents an elite technical option, with valuable properties but at a cost that distances it from mass industrial production. Its role in steel metallurgy is more symbolic than structural, and its study remains relevant in the development of special alloys for aerospace, nuclear, and electronic applications, where every property counts and every gram is justified.

Osmium (Os), iridium (Ir), and platinum (Pt), like their counterparts ruthenium (Ru), rhodium (Rh), and palladium (Pd), belong to the select group of precious metals with exceptional physico-chemical properties. However, their participation in steel metallurgy is practically marginal, limited to highly specialized contexts where their qualities justify their high cost and scarce availability.

These elements are not part of any common industrial steel family, and their incorporation into ferrous matrices responds more to specific needs than to generalized trends. When used, they are part of superalloys designed to withstand extreme conditions of temperature, pressure, or chemical aggression, such as those found in aerospace turbines, nuclear reactors, or space probe components. Elite organizations like NASA have resorted to these alloys in missions where material reliability is critical and cannot be compromised for economic reasons.

The reason for their exclusivity does not lie in a lack of affinity with iron, but in the combination of factors that make them unviable for mass production: their geological rarity, their extraction complexity, and their prohibitive price. Despite this, their study remains relevant in the development of frontier materials, where every property—from oxidation resistance to structural stability at temperatures above 1,500 °C—can make the difference between success and technical failure.

Gold (Au), the quintessential symbol of nobility and chemical stability, lacks practical utility in the field of steel metallurgy from a mechanical standpoint. Its low hardness, poor structural resistance, and high cost exclude it from any functional application in ferrous alloys designed to withstand stress, impact, or extreme conditions. It does not form carbides nor does it contribute to the toughness, hardness, or corrosion resistance of steel, so its presence in this field is merely anecdotal.

However, some experimental alloys between iron (Fe) and gold have been developed with the aim of exploring new colorations in the 'king metal'. These combinations, far from seeking technical improvements, are oriented towards aesthetics and innovation in jewelry or decorative material design. In this context, iron can act as a chromatic modifier, allowing for unusual hues such as blue to be obtained through specific crystalline structures that alter the way light interacts with the metallic surface.

It should be noted that these Fe-Au alloys do not strictly correspond to steel, as they do not meet the structural or functional criteria that define this material. They are rather hybrid compositions for artistic or scientific purposes, whose interest lies in the exploration of new optical properties and not in mechanical performance.

Actinides, a group that includes elements such as thorium (Th), uranium (U), neptunium (Np), and plutonium (Pu), possess unique nuclear and physical properties, but their use in steel metallurgy is extremely limited and, in most cases, confined to experimental or strategic contexts. Thoria, in the form of thorium dioxide (ThO₂), has been used in certain special steel alloys designed to withstand extremely high temperatures, thanks to its thermal stability and oxidation resistance. However, these compositions are so rare that they are hardly found outside of elite scientific or industrial environments.

As for uranium, neptunium, and plutonium, their low solubility in iron prevents effective integration into the steel matrix, drastically reducing their conventional metallurgical interest. They do not form useful carbides nor do they improve relevant mechanical properties, so their presence in structural steels is practically nil. Nevertheless, uranium stands out for a singular property: its high density and pyrophoric nature, meaning it can spontaneously ignite upon impact with hard surfaces. This characteristic makes it a high-performance ballistic resource, used in specialized alloys to manufacture armor-piercing ammunition, especially in military applications where maximum penetration capability is required.

Tungsten (W), also used in this type of ammunition for its density and hardness, does not exhibit pyrophoric behavior, making it more stable but less reactive upon impact. Even so, its mechanical performance remains outstanding, and its use in elite armors and projectiles continues to be preferred in many cases for reasons of safety and control.

In conclusion, actinides are not part of the usual repertoire of steel metallurgy, but their study and specific application in extreme contexts—be it thermal, ballistic, or nuclear—maintain them as elements of strategic interest beyond their limited industrial viability.