STEELS

Steel is a family of iron-based alloys characterized by the addition of carbon in variable proportions. Additional alloying elements may be incorporated to provide a wide range of mechanical and physical properties. The carbon content primarily determines the hardness of the alloy: low-carbon steels (approx. 0.20–0.40%) are referred to as mild steels, while higher carbon contents (approx. 0.40–0.70%) result in hard steels.

Although steel is, by definition, an alloy, it is referred to as unalloyed steel when it contains only iron and carbon.

A wide variety of alloying elements may be added, including chromium, nickel, tungsten, titanium, aluminum, vanadium, phosphorus, and silicon. Depending on their concentration, these elements can enhance corrosion resistance, machinability, hardenability, and other key properties.

In watchmaking, steel occupies a central position due to its excellent balance of mechanical strength, elasticity, machinability, and dimensional stability.

History

Steel belongs to the broader family of ferrous materials, which includes iron, cast iron, and steel. The Iron Age began around 1200 BC, depending on the region. The earliest cast iron production appeared in China around 500 BC, where iron was carburized through contact with charcoal, leading to the first forms of steel.

In Antiquity, combustion gases were introduced into charcoal furnaces to further carburize iron and improve its hardness, particularly for tools and weapons.

The scientific understanding of alloys began in the 18th century, but it was not until the mid-19th century that steel production became controlled and industrialized. In 1855, Henry Bessemer patented a low-cost steelmaking process that played a major role in the Industrial Revolution. In the early 20th century, air-blown converters were replaced by oxygen-based processes, offering improved precision and efficiency.

Since then, materials science has continuously evolved, enabling the development of increasingly specialized and high-performance steels.

Main Properties

Steels used in watchmaking are characterized by the following properties:

• High Young’s modulus (~200–210 GPa) → ensures rigidity
• High mechanical strength → suitable for stressed components (shafts, springs)
• Excellent response to heat treatment → hardening, tempering, nitriding
• Variable machinability depending on composition
• Sensitivity to corrosion (except stainless steels)
• Magnetic behavior (except non-magnetic alloys)