Alloying elements in steel
What steel can do is decided by what it is made of. Steel is produced from iron–carbon alloys together with other elements – the alloying additions. Their content has to exceed the minimum concentration below which the structure and properties of the steel do not change. The additions used most often are nickel, titanium, vanadium, chromium, silicon, molybdenum, tungsten, cobalt, aluminium, copper, niobium and manganese.
Alloying elements are introduced not only to give the steel particular properties, but also to improve hardenability, make heat treatment easier, increase resistance to corrosion and wear, and raise physical, physicochemical, technological and mechanical performance. Each addition changes the steel in its own way. Not all of them do so for the better, which is why a distinction is drawn between beneficial and harmful additions.
Molybdenum increases the corrosion resistance of steel. In austenitic acid-resistant steels its content is around 2.5%, but it can reach as much as 7%. The element raises strength and hardenability, reduces brittleness and improves creep resistance.
Copper has physical properties close to those of iron, but is far more resistant to corrosion. This addition is increasingly appreciated, particularly in the melting of new steel.
Nickel not only makes hardening easier and deeper, it also lowers the austenitic transformation temperature; dissolved in ferrite it strengthens the steel and increases impact strength. It is an important constituent of acid-resistant steels, because it ensures good weldability and the ability to be formed. Nickel is added at 0.5–4% for quenching and tempering, and above 8–10% for acid-resistant steel.
Chromium, like nickel, affects hardenability, increases strength and refines the grain. It is an addition often used in structural, heat-resistant, stainless and tool steels. In stainless steel it is chromium that makes the steel stainless in the first place. Its content ranges between 12% and 30%, depending on the grade.
Silicon is usually regarded as an unwanted addition; it affects brittleness, hardness, elasticity and strength. Its main use is as a constituent of spring steels. It also lowers impact resistance and raises resistance to high temperatures, so it is added to grades that work hot or come into contact with concentrated nitric and sulphuric acid.
Manganese improves resistance to impact and abrasion without changing ductility. It is the element that makes steel tougher.
Among the harmful additions, two deserve a mention above all: sulphur, which makes steel harder to forge, and phosphorus, which reduces strength and impact resistance while increasing hardness and causing cold shortness.
By the share of alloying elements, steels divide into:
– low-alloy – a single element below 2% (used mainly for products with modest mechanical requirements, such as sheet, wire or tube);
– medium-alloy – a single element below 8% (used for machine parts such as shafts, sleeves or gears);
– high-alloy – a single element above 8% (used for building components such as tubes and sections).
By application, steels divide into:
Tool steels– grades used to make cutting tools such as knives, drills and milling cutters, and forming tools such as dies and punches. Tool steels have to be hard and wear-resistant, because a cutting tool must hold its edge for a long time to do its job. That is why such steels contain carbon, chromium, molybdenum, vanadium or cobalt, all of which improve their properties.
Structural steels– grades for building structures: bridges, buildings, machinery and equipment. Their defining feature is high strength and resistance to stress, which keeps a structure stable and safe. Structural steels contain carbon, manganese, silicon, phosphorus and sulphur among others, and their properties are improved by hardening.
Steels with special properties are grades with exceptional characteristics that suit them to specialist uses. They include stainless steel, resistant to corrosion and used for surgical instruments, domestic appliances and in the food industry. Other examples are electrical steels, whose magnetic and electrical properties make them suitable for electric motors and transformers.
Steels with special properties also include the special steels distinguished by high corrosion resistance. By structure they can be classified as ferritic, martensitic, precipitation-hardened martensitic, austenitic and ferritic-austenitic. They can also be grouped by chemical composition – high-chromium, chromium-nickel and chromium-nickel-manganese. Such steels are used in the chemical, petrochemical and power industries, and anywhere else that demands high resistance to corrosion.
