STEEL TYPES ACCORDING TO THEIR COMPOSITION

Unlike alloys based on Nickel, Titanium, Lead, Cobalt, Tin (among others), the number of existing steel grades is so vast that a separate book could be written just mentioning half of them. Therefore, I will focus on the most commonly used ones and explain the reason for their selection in each specific application.

Before continuing, I would like to offer you a piece of advice that will surely serve you from now on, for the rest of your life, if you are truly interested in the subject I am about to discuss.

Throughout the extensive bibliographic history that has recorded so many steel grades with names in the form of codes combining letters with numbers, their abundance is such that it is very easy to get lost. For this reason, above all, you must follow this guideline: pay attention to the quantity of Carbon. In such a vast and complex world, a simple piece of advice like this will make things much easier for you. Even in steels where the percentages of third elements vary so much that they seem absolutely distinct from each other, the percentage of Carbon should serve as your guide. What I mean is that two steel grades where the % of Carbon is between 0.45 and 0.50 will have more in common with each other, even if one has 5% Chromium, than two that have the same amount of Chromium but a very different % of Carbon. This means that it is the % of Carbon, first and foremost, that places each grade in its own class. By this point in the book, you will know that there are three main families of steel, based on the % of Carbon.

Mild steels are the cheapest, but not because they have such low carbon content. Instead, it's because they do not require extensive heat treatments, except sometimes on the surface. The AISI 1010, AISI 1015, and AISI 1035 grades are particularly popular. They are used in applications where significant mechanical rigidity, hardness, or resistance to deformation from mechanical stress are not required. 

The most popular grades are AISI 1045 and AISI 1060, although there are others. These steels are heat treatable and are considered (with the exception of special steels) the toughest among the "standard" carbon grades. A 0.45% concentration of this element is more than sufficient to achieve optimal toughness for manufacturing items like hammers, chisels, anvils, and so forth. This is the author's preferred concentration. 

Beginning with AISI 1085, containing 0.85% carbon by mass, steel starts to become especially rigid and hard, offering excellent abrasion resistance. From this point, we enter the realm of steels for all types of cutlery. The AISI 1095 grade is the most affordable among those considered hard as frozen bread, with the added benefit of maintaining decent toughness. Beyond 1% carbon, steels typically begin to lose their toughness, which is why this percentage is usually not exceeded. It was (or rather, is) very popular for the manufacture of swords, war axe heads (though currently still used for civilian axes), spearheads, and so on. It has always been more expensive than lower carbon grades because it was (and still is) more difficult to work with. The Spanish expression “batirse el Cobre” (to beat copper), which in that culture signifies great sacrifice or strenuous effort, falls very short in comparison to the sweat (and sometimes tears) that must be shed when working these grades by hand, even when the steel is red hot and more malleable. The impressive thickness of blacksmiths' dominant arm/hand biceps is a testament to working with this grade of steel, which proves so stubborn during the hot hammering process. 

Technically, High-Speed Steels (HSS) are Carbon Steels, but they contain percentages of metals, mostly from the refractory group, that form internal Carbides which increase the alloy's hardness. You might know them by their English acronym, HSS, which stands for "High Speed Steel." The "high speed" doesn't refer to any kind of racing, but rather the short time it takes to perform the same work that would be more tedious with a High-Carbon Steel. They contain large amounts of already expensive metals such as Molybdenum, Tungsten, Chromium, Vanadium, and in special cases, small percentages of Nickel (to increase toughness), while always maintaining Manganese and Silicon content, which is more a product of the steel manufacturing itself than added components with a particular use.

Steel is mixed with Tungsten, Chromium, Molybdenum, et cetera (Group 6 of the Periodic Table is the most popular) to increase rigidity. These steel grades are used to manufacture drills, bits, drilling and cutting parts, abrasive tools, et cetera. A fact rarely mentioned is that these alloys have quite good corrosion resistance; it's no coincidence that mass metals like those already mentioned form a moderately corrosion-resistant oxide layer. Some High-Speed Steels, like T1, with 18% Tungsten, were once very popular due to the well-earned reputation of Tungsten's virtues as a hardener (with or without carbon involved) in steels. The problem is that Tungsten is very expensive, and it is currently used directly as a Carbide, with superior properties to HSS.

Cobalt is a metal widely used today for the manufacture of HSS Steels because it improves resistance to high temperatures and prevents the weakening of the alloy, which usually occurs when temperatures exceed 150ºC during action on a lathe or milling machine (a milling machine is a cutting machine). Cobalt, which is naturally heat-resistant, helps sustain Martensite under extreme conditions when iron by itself could not, even when accompanied by other typically used metals. However, I have seen lathe tips now sold as "Cobalt," no, excuse me, they are not "Cobalt." They contain Cobalt, which is different (sometimes not exceeding 10% by mass), but to make a sale, people are not explained what they are buying (even if it is a good product).

There are no steel grades more specialized than these, in the sense that they have been specifically tested to perform a specific function. Most have relatively simple chemical compositions, but they behave substantially differently than standard carbon steels.

As their name suggests, they were designed for the manufacture of all types of tools (cutting, impact, etc.). It's important to keep in mind that steels that cut well are generally not good for making pieces that can take a beating, while those that are tough may not be hard enough to perform their intended purpose. Again, the Americans, to the chagrin or pleasure of many, went to great lengths and did their best to establish a list of grades, each grouped by a capital letter (famous examples: A, S, and D).

They are designated by a capital letter followed by a single-digit number, for example, O1.

The capital letter refers to its use, so we have:

“A” for “Air Hardening”, “D” for steels with a high chromium and carbon content (similar to HSS), “F” for steels with a high tungsten and carbon content, “H” for “Hot Working”, that is, those used to manufacture parts subjected to high temperatures and containing tungsten, chromium or molybdenum, “L” for “Low Alloy”, that is, inexpensive steels with a small percentage of expensive elements, “M” for those rich in molybdenum, “O” for those hardened with oil (heat treatment), “P” for “Plastic Mold”, which are used to make molds for pouring plastic and contain nickel to improve the cleaning process, “S” for “Shock-Resistant” or shock resistant (my favorite – used in hammers, chisels, anvils) and Finally the “T” series which is rich in Wolfram (Tungsten).

These steels were created with the express purpose of achieving good performance in alloys that combined hardness, toughness, and high-temperature resistance using the minimum content of alloying elements, which in other cases are much more expensive due to their high mass percentage, as is the case with High-Speed Steels (HSS) or Tool Steels. Despite their name, Low Alloy Steels contain many metals such as Nickel, Chromium, Vanadium, Molybdenum, Tungsten, and in special cases, Niobium, among others. However, they are called "low alloy" because the quantities of these elements are very small. It might seem as if they were made from the scraps of other alloys, but the truth is that they are very interesting and useful. As you might have guessed, there isn't a single grade. 

Despite its name, Silver Steel is nothing more than a version of 1095 steel with altered values of Manganese, Silicon, Phosphorus, and Sulfur, where the latter has been reduced to a minimum to achieve better quality. The "Silver" in the name, comes, in theory, from the intense luster of the alloy. However, being English, it's not surprising that it was named this way, as some alloys like German Silver or Pewter with slight silver content (Tibetan Silver) also carry the word in their name, sometimes as an adjective, other times as a nominal noun, when they don't contain (or contain very little, as in the case of Tibetan Silver) this precious metal. 

These are the most popular among low-alloy steels, and their price is slightly higher than that of Carbon Steels, but with better properties regarding rigidity, hardness, and therefore, durability. They are also more resistant to high temperatures.

These steels contain approximately 1% Carbon, with AISI 52100 being one of the most popular grades. It is used in bearings (balls, cages, rollers), cutlery, etc. Although Chromium is famous for its role in Stainless Steel to improve corrosion resistance, in quantities below 5% and in conjunction with a high amount of Carbon, it forms its own Carbide, which, unlike Cementite, is more stable, harder, and resistant to high temperatures. Above 5%, Chromium is not usually used except directly in the case of Stainless Steels, where it jumps directly to a typical 12%, but this time as an anti-corrosive agent.

Most common Tool Steels fall into this category and vice versa; that is, Chromium-Vanadium steels can be said to belong to the AISI Tool Steels group, even if the manufacturer, especially if European or Asian, does not use the AISI code, but rather DIN, SAE, or any other. This is merely an asterisk resulting from zeal, more than anything, of the "theirs with theirs and ours with ours" type; AISI records are more concise, and therefore, I prefer them.

Chromium-Vanadium steels are similar to Chromium steels but with the addition of Vanadium, which improves their mechanical properties. The % of Vanadium is typically low (a typical grade contains around 0.35% Vanadium) and, like Chromium, forms its own Carbides, refines grain size, and improves the toughness and durability of the part. It is used to manufacture all kinds of tools, frequently for domestic use or those of low responsibility. They are more expensive than typical Chromium steels like AISI 52100 but less expensive than specialized steels or High-Speed Steels (HSS) because the Chromium-Vanadium content is low even when summing the quantities of both metals by mass. You will frequently find them with a small inscription that reads "CV Steel" or "CrV Steel", which, as you might have imagined from its acronyms, corresponds to "Chromium-Vanadium Steel".

Unfortunately, manufacturers do not usually specify the exact grade they have used to manufacture the part; they typically limit themselves to putting "CV" and that's it.

Excluding exotic alloys like those containing a lot of Rhenium, we are looking at the family of steels that are the densest, hardest, and most heat-resistant. Although relatively scarce today and increasingly less used in everyday applications, they are among the oldest and most exploited in the military field. It is important to note that many alloys first pass through the battlefield before finding civilian or "commercial" use.

This family of steels became very popular during the Second World War due to its association with German technology. I say German and not "Nazi" because many military personnel, engineers, and designers were acquitted or did not consider themselves part of the party, even though they participated in what, for the good of some and the ill of others, would be technological supremacy. According to my professors, this is one of the examples responsible for why metallurgy has a bad reputation or is associated with wars, among other things. It is not clear what the first use of Tungsten Steel was. We know it was used to manufacture high-penetration projectiles capable of piercing the hulls of Allied tanks and battleships that until then seemed inexorable; therefore, it can be said that one of the first uses, if not the first, was in the manufacture of ammunition. Legend has it that Tungsten Steel projectiles were reserved for the best shooters. This is logical, whether myth or legend, since Tungsten is very expensive and each projectile represented a loss. The reason for the effectiveness of these projectiles is due to the mixture of high density, dimensional stability (Lead projectiles would deform from the heat as soon as they left the cannon barrel), and hardness. However, the true material of preference for heavy artillery use was none other than Uranium alloyed with Zirconium and Yttrium. In this context, it was not used because it was radioactive, but because, in addition to being denser than Tungsten (and therefore much more so than Tungsten Steel), it had a greater penetration capacity. The pyrophoric nature of Uranium was another advantage, as it exploded on impact. Very lethal.

Tungsten was also used for structural steel alloys in frontline tanks designed to receive barrages of cannon fire, withstand engagement, and survive. The addition of Tungsten to the steel used until then was a kind of "German signature" in the sense that they were, if not the only ones, the ones who made the most use of the metal.

It should be clarified that the percentage of Tungsten in these steels rarely exceeded 20% by mass, mainly because that amount was already more than sufficient. Furthermore, the rule that is still maintained to this day was followed: "maximum quality with minimum content." If steel itself was already a precious commodity, imagine Tungsten, which is an extremely scarce metal compared to Iron.

In Spain, and specifically in Galicia, the metal became famous because it was bought "at gold price" by the Nazis during this period. The phrase "at gold price" is not literal, of course. The ore was sold, which was then processed in Germany. Portugal, which like Spain had remained neutral during the conflict, found itself in a difficult position as the British and Germans bid to buy Tungsten minerals (Portugal has the largest reserves in Western Europe).

Today, Tungsten is used less and less, and has been replaced by mixtures of Chromium and Molybdenum. Manufacturers excuse themselves by claiming these are superior, which is incorrect. I will say it as many times as necessary. In fact, Molybdenum and Chromium are used precisely to replace Tungsten, which has been earmarked for governmental reserves by current military superpowers because, unlike other more abundant metals, it is very scarce.

Nickel is one of the best companions to Iron, whether pure or as plain or Stainless Steel. No other element is more beneficial in terms of toughness. Nickel improves the steel's resistance to impact and mechanical stress, even at high temperatures. Unlike the vast majority of typical steel alloying elements, Nickel DOES NOT increase hardness, but in fact decreases it. Up to 5%, Nickel increases the toughness ("strength") of the alloy. Higher contents begin to have positive effects regarding corrosion resistance, but it is never as good as that achieved with Chromium, and Nickel itself is very expensive.

Normally, ultra-hard varieties of steel saturated with Tungsten, Chromium, Molybdenum, etcetera, carry a discrete percentage of Nickel to compensate for the brittleness fostered by these elements and the internal carbides they form within the matrix.