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STEELS

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2. General description

Steels form a family of Alloys characterized by the addition of a variable proportion of Carbon and iron. Other elements can complement the alloy to give steel a very wide range of mechanical and physical properties. The carbon content determines the hardness of the alloy. We speak of mild or extra-mild steels for low carbon content (-0.20 to 0.40%) and of hard or extra-hard steels for high carbon content (-0.40 to 0.70%). Although steel is an alloy, it is called unalloyed steel when it contains only iron and carbon.
A large number of elements can be added to iron and Carbon. Chromium, nickel, Tungsten, Titanium, Aluminium, vanadium, phosphorus or Silicon are the main ones. Some, depending on their content, will improve corrosion resistance, others will promote machinability or hardenability, etc. In watchmaking, they occupy a central place due to their excellent balance between mechanical resistance, elasticity, machinability and dimensional stability.

3. History

We can consider that there are three ferrous products: iron, cast iron and steel. Historically, the Iron Age begins (depending on the culture) around the year -1200. The Chinese made the first castings of cast iron around the year -500. They thus produced the first steels, iron being recarburized in contact with charcoal. In Antiquity, combustion gases were added to charcoal to recarburize iron and improve its hardness (particularly that of the cutting edge of tools and weapons).

The theorization of alloys began in the 18^th century, but it was not until the middle of the 19^th century for science to allow understanding of them and producing quality steels. In 1855, Henry Bessemer filed the patent for a low-cost Steel production process that would mark the industrial revolution. From the early 20^th century, the air converter of the Bessemer process was replaced by liquid gas (oxygen) converters that were much more precise, efficient and economical.

Materials science has continued to evolve since then. The nature and composition of Alloys, as well as Steel production processes, are constantly evolving to produce ever more suitable and high-performing Materials.

4. Types of Steel used in watchmaking

Carbon Steels

These are the traditional Steels used for:

• Axes (Balance axes, pivots)
• Springs (Barrel springs old)
• Horologist (watchmaker) tools

Characteristics:

• Very good Hardness after quenching
• Excellent finish possible (mirror Polishing)
• Sensitive to corrosion

Stainless Steels

Stainless steels contain at least 10.5% chromium, which forms a protective layer against corrosion.

Use:

• Watch cases
• Crowns, Screws, external parts components

Examples:

• 316L (horological standard)
• 904L (better corrosion resistance, more expensive)

Steels for springs

Used for balance springs (historically) and various springs.

Examples:

• Improved carbon steels
• Alloys of the Elinvar type (low thermal coefficient)

Special alloy steels

Steels enriched with alloying elements to improve certain properties:

• Chromium, molybdenum → wear resistance
• Nickel → toughness
• Silicon → elasticity

Used for:

• Wheel trains
• Pinions
• Components subject to fatigue

5. Watchmaking applications

Steel is used for many components:

• Shafts and pivots → strength + precision
• Screw → mechanical strength
• Springs → elasticity
• Watchmaking tools → hardness
• Cases → corrosion resistance (stainless steel)

6. Heat treatments and surface treatments

The properties of steels are strongly dependent on the treatments applied:

• Quenching → increased hardness
• Tempering → improved resilience
• Nitriding → high surface hardness
• Polishing → reduced friction

7. Advantages and limitations

Advantages

• Excellent mechanical resistance
• High rigidity (high modulus)
• Very good wear resistance after treatment
• Good suitability for Heat treatments
• Controlled machinability
• Versatility (wide range of Alloys)
• Controlled cost
• Possibility of very high quality finishes (mirror Polishing)

Limitations

• Sensitivity to magnetism
• Risk of corrosion (except stainless steel)
• Relatively high Mass
• Higher friction than some modern Materials