The main element in ferrous metals is, naturally, iron, which only occurs in metallic form when bonded to other elements (unlike copper, gold and silver, which are found in nature already in metallic form).
In the following lines I will briefly look at the chemical elements most commonly used in iron-based alloys, above all in relation to the effects they have on the metal itself; for each element I will give its chemical symbol in brackets.
Carbon (C)
In nature it is present in millions of different compounds, some very soft (graphite) and others extremely hard (diamond); in ferrous alloys carbon is probably the most important element, present in almost every case but in differing percentages.
Its main function is to increase strength, hardness and brittleness, improve hardenability and reduce weldability.
Depending on its content, the metal can be classified into 3 main categories:
– low-carbon steel (up to 0.5% carbon), known in the trade as “soft iron”, very soft and difficult to harden.
– medium-carbon steel (from 0.5% to 2.1% carbon), which is steel proper in everyday terms, and can be hardened by specific heat treatments.
– high-carbon steel (from 2.1% carbon upwards), commonly called cast iron, which is extremely hard.
Chromium (Cr)
A silvery-white metal that is not very common in nature, where it is never found free but combined in various minerals, the most important of which is chromite. Its main function is to make the metal stainless, encouraging the formation of an extremely thin surface layer of oxide (invisible to the eye) that protects the layers beneath from further oxidation. It also aids heat treatment, improving the attainable hardness and the resistance to corrosion and wear, at the cost of greater brittleness.
Vanadium (V)
A silvery-grey metal, soft and ductile, used mainly in the production of high-quality metal alloys; usually present in small percentages (0.15-0.2%), it improves the response to heat treatment and considerably increases hardness, corrosion resistance and impact resistance.
Manganese (Mn)
This element, too, is mainly used in the production of metal alloys, which almost always contain a small percentage of it.
Similar in appearance to iron, hard and very brittle, it is difficult to melt because it oxidises so easily.
In metallurgy it performs the important task of deoxidising the metal and improving its ability to alloy; it also increases hardness and improves the response to heat treatment.
It may be present in percentages ranging from 0.5 to 2%. In percentages from 12% to 14% (Hadfield steels) it gives exceptional wear resistance.
Nickel (Ni)
Used by man for several millennia in alloys with iron or copper, it is an element present in some quantity on Earth and, according to some theories, it makes up (together with iron) much of the Earth’s core. Silvery-grey in colour and very ductile, it raises hardness and improves toughness, aids hardenability and reduces sensitivity to overheating. Combined with chromium, it improves resistance to heat and corrosion, as well as resilience.
Molybdenum (Mo)
Its name comes from Greek and means “like lead”, because of its silvery-grey colour; in nature it is found only in compounds corresponding to its oxidation states. 80% of molybdenum is used in the production of alloy steels, where it increases hardness, resistance to corrosion and abrasion, and toughness at high temperatures. In small percentages it increases hardenability and wear resistance.
Phosphorus (P)
Its name comes from Greek and means “light-bearer”, probably because when exposed to air it gives off a faint light (phosphorescence). In its natural state it can be found in various forms and colours; adding it improves resistance to atmospheric corrosion and to wear, and also improves machinability (the ease of cutting it into chips).
Tungsten or Wolfram (W)
A silvery-white metal, rare in nature, never found free but in compounds corresponding to its oxidation state. It is used above all in tool steels, where it improves hardness at high temperatures: even at 600° tools are able to keep their cutting edge intact.
Silicon (S)
Known to man as silica, a compound used to make glass, it is so abundant in nature that it is thought to be one of the most common elements in the entire universe, together with hydrogen, helium and oxygen. In iron and steel making it is used to improve mechanical properties, with the exception of toughness, and to reduce the ease of plastic working. It also improves hardenability.
Cobalt (Co)
Fairly widespread in Egypt and Mesopotamia from the first half of the 3rd millennium BC, it is steel-grey in colour, quite similar to iron but harder and less malleable. Used particularly in high-speed steels, it increases their hardness, toughness and resistance to oxidation at high temperatures.
Aluminium (Al)
Another of the most abundant metals in nature, where it is never found free but combined; it is silvery-white in colour and light in weight. Aluminium can be added together with chromium to improve resistance to oxidation at high temperatures. It also performs the important function of promoting the absorption of nitrogen during nitriding, a process applied to steels from which high hardness and fatigue resistance are required.
Lead (Pb)
A metal rarely found in nature in its native state, bluish-white in colour, very soft, malleable and heavy; it is used in many fields and is a fundamental element in many non-ferrous metal alloys.
Perhaps because of the relative ease with which it can be extracted from its ores, it was one of the first metallic elements to be known, probably to the Egyptians as early as 5000 BC; it was later used by the Phoenicians and the Chinese in 2000 BC, and finally became known to the Romans, who used it on a large scale, for example to make water pipes.
It is usually added in small percentages (0.15-0.30%) and its only function is to improve machinability; it adds no other properties.
Some classifications of steels
The properties of steels, understood as alloys of iron and carbon, can be considerably modified to achieve particular characteristics by adding the elements listed above (the most common ones): in this case we speak of alloy steels.
Depending on the number of elements present in the alloy and on their quantity, we can speak of low-alloy and high-alloy steels (generally when an alloying element exceeds a content of 5%).
Manganese steels, for example, are used to make parts that must withstand impacts (excavator buckets) or be difficult to work (sheet metal for safes). Nickel is very important, giving better hardenability, hardness, fatigue resistance and corrosion resistance. Similar properties are provided by chromium, which, when present in quantities above 10%, makes steel stainless. High-speed steels, which must be hard and resistant to breakage even at high temperatures, contain tungsten in percentages ranging from 10% to 18%; we speak of super high-speed steels if, in addition to large amounts of tungsten, chromium and vanadium are also added. Effects similar to those of adding tungsten and chromium are obtained with molybdenum.
Another family of steels with important applications is the one containing silicon, above all in electrical and electromechanical construction (motors, transformers, etc.).
The unified standards for classifying steels in Italy are set out in a UNI table that divides steels into two groups according to whether they are designated on the basis of their physical or their chemical properties, making it possible to represent the type and characteristics of each with a limited number of letters and digits.
Designation often refers to foreign standards, in particular American ones (SAE and AISI), which are based on the adoption of a series of characteristic numbers.
Bibliography
– Nuovo dizionario di merceologia e chimica applicata, Volume 1, by Vittorio Villavecchia, Gino Eigenmann, published by Hoepli.
– Saldatura per fusione, Volume 2, edited by Istituto italiano della saldatura, published by Hoepli.
– Enciclopedia della chimica, published by Garzanti.