What is steel made of? The short answer is: iron and carbon. The complete one opens up a whole world. Steel is a metal alloy composed of over 95% iron, but it's the remaining elements that determine its characteristics.
Understanding the composition of steel means understanding why an S355 behaves differently from a 42CrMo4, why an AISI 304 resists corrosion while a C40 doesn't, and how to interpret a designation in order to choose the grade most suited to your component.
By definition, steel is an iron-carbon alloy with a carbon content of between about 0.02% and 2.1% by weight. Below 0.02%, we speak of technically pure iron; above 2.1%, we enter the field of cast irons, characterised by very different properties and behaviours.
Carbon is the element that, more than any other, influences the properties of steel:
Iron constitutes the crystalline matrix: at room temperature it presents itself in the ferritic phase (body-centred cubic, ductile) or, after quenching, in the martensitic phase (hard and strong). The phase transformations of steel, described by the iron-carbon diagram, are the basis of all heat treatments.
Pure iron is a relatively soft metal (ultimate tensile strength ~250 MPa) and of little structural use. Steel is iron with a controlled addition of carbon, of between 0.02% and 2.1%, which significantly increases its strength, hardness and heat treatability. In industrial practice, the "iron" sold commercially is almost always a low-carbon mild steel.
In addition to carbon, numerous elements can be added to steel in controlled quantities to modify specific properties. Each performs a precise function.
Chromium is the element that distinguishes stainless steels from the others: with a content equal to or greater than 10.5%, the steel forms on its surface a passive film of chromium oxide that protects it from corrosion. In quenched and tempered steels, at lower percentages (0.5-2%), it improves hardenability and wear resistance.
Nickel stabilises the austenitic phase at room temperature, giving rise to the austenitic stainless steels of the AISI 300 series, characterised by excellent corrosion resistance, toughness at low temperatures and non-magnetism. In quenched and tempered steels, such as 39NiCrMo3, it improves toughness and hardenability.
Molybdenum increases resistance to pitting corrosion in stainless steels, representing the main difference between AISI 304 and 316. It also improves resistance to high temperatures and the hardenability of structural steels.
Present in almost all steels in percentages of between 0.3% and 1.5%, manganese improves hardenability, counters the negative effects of sulphur by forming manganese sulphides in place of the brittle iron sulphides, and increases mechanical strength without excessively reducing ductility.
Silicon is a deoxidiser used in foundries and an element that strengthens ferrite. At high percentages, of between 1% and 3%, it significantly increases the elastic limit and is therefore the basis of spring steels. It also improves resistance to oxidation at high temperatures.
Vanadium refines the grain and forms very hard carbides. Even in small quantities, between 0.1% and 0.3%, it significantly improves strength and toughness. It's present in tool steels and in many high-performance quenched and tempered steels.
Tungsten forms carbides stable at high temperatures, essential in high-speed steels for cutting tools. Cobalt helps maintain hot hardness. Together they characterise the HSS high-speed steels used in machining operations.
Nitrogen strengthens austenite and is used in duplex and superaustenitic stainless steels to increase mechanical strength and resistance to pitting corrosion, partly replacing nickel.
Generally considered impurities, sulphur and phosphorus are kept at very low levels in most steels. Sulphur can be added intentionally, in percentages of between 0.15% and 0.35%, in free-machining steels to improve their machinability, though accepting a reduction in ductility. Phosphorus is almost always undesirable because it makes steel more brittle.
Chemical composition is the first criterion used to classify steels. There are five main families.
They contain mainly iron and carbon, with small quantities of manganese, silicon and impurities. They are distinguished according to carbon content:
They contain from 0.25% to 0.50% carbon and moderate quantities of chromium, nickel and molybdenum, generally below 5% for each element. Designed to be quenched and tempered, they reach high strengths while maintaining good toughness. Among the most common are 42CrMo4, 39NiCrMo3 and 34CrNiMo6.
They contain at least 10.5% chromium and are divided into four subfamilies according to microstructure.
They are characterised by a high content of carbon and carbide-forming elements, such as chromium, tungsten, molybdenum and vanadium. Designed to resist wear, heat and deformation under extreme conditions, they include the HSS high-speed steels, those for cold-work operations, such as X210Cr12, and those for hot-work operations, such as X40CrMoV5-1.
They contain small additions of vanadium, niobium or titanium, generally below 0.1%, which refine the grain and improve strength and toughness even without heat treatment. They are used in welded structures, high-pressure pipelines and the automotive sector.
The designation describes the composition of the steel. Two systems coexist on the market.
European system (EN 10027): the code indicates the alloying elements and their content. Example: 42CrMo4 → 0.42% C, chromium as the main alloying element and molybdenum; the final number expresses the content according to the factors set out in the standard. X5CrNi18-10 (AISI 304) → 0.05% C, 18% Cr and 10% Ni.
American system (AISI/SAE/UNS): it uses numerical codes. The 1xxx series identify carbon steels, the 41xx the Cr-Mo steels and the 43xx the Ni-Cr-Mo. Stainless steels follow the three-digit AISI classification, such as 304, 316 and 430.
For the designer, the designation makes it possible to trace the technical data sheet of the grade: guaranteed composition, mechanical properties, applicable treatments, weldability and product standards.
Steel is the most machined material in the world by chip removal. In on-demand production through CNC machining, the most requested grades are:
The raw material is supplied in the form of a round bar, flat bar, plate or block, with an EN 10204 3.1 certificate attesting to its composition and mechanical properties. Weerg machines these steels on 5-axis CNC centres, transforming them into finished components with tight tolerances and a controlled surface finish.
Steel is composed of iron and carbon, but it's the alloying elements that define its true identity: chromium makes it stainless, nickel stabilises its austenitic structure, molybdenum protects it from pitting and vanadium refines its grain. For designers and technicians, knowing the composition means being able to read the designation, understand the material, choose the most suitable grade and communicate with suppliers and laboratories through a shared language.
In on-demand production, steel remains the protagonist of CNC machining: from structural S355 to quenched and tempered 42CrMo4, from AISI 304 for the food sector to AISI 316 for marine applications, every grade starts from certified raw material and becomes a finished component within a few days.
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