Si – Silicon (silicium) alloying addition to steel

Silicon – influence on the properties of spring steel, heat-resistant steel and others

The discovery of silicon and its importance in metallurgy

Silicon as a chemical element was discovered in 1823 by Jöns Jakob Berzelius, which initiated its later use in metallurgy. Although its role was initially underestimated, the dynamic development of the steel industry in the 19th century allowed for a more detailed understanding of silicon's influence on steel properties. A key turning point was the discovery that silicon improves strength and corrosion resistance, which enabled its widespread use in various industries.

Silicon – physicochemical properties

Chemical characterization

• Silicon is a semimetal and has a diamond crystal structure.
• Under normal conditions, it is resistant to oxygen thanks to the SiO₂ passivation layer.
• It does not react with water and most acids (exception: HF, where it forms volatile SiF₄).
• Reacts with bases to form silicates.
• An important alloying addition in steels – it improves hardenability, creep resistance, acts as a deoxidizer and stabilizer of the ferritic structure.
• In electronics, it is a basic semiconductor – the basis of microprocessors, photovoltaic cells and sensors.

Early use of silicon in metallurgy

In the first half of the 19th century, silicon began to be used primarily as a deoxidizer in metallurgical processes. Its ability to bind oxygen and form slags allowed the removal of undesirable inclusions from molten steel, improving its homogeneity, eliminating porosity, and increasing structural purity. Over time, it was recognized that higher silicon content affected the mechanical properties of steel, leading to the deliberate addition of this element as an alloying component.

Silicon exhibits varying solubility in iron structures. In γ-iron ( austenite ) , it dissolves up to 2% , but in the presence of 0,35% carbon, its solubility increases to 9%. In α-iron ( ferrite ) , silicon reaches a solubility of up to 18,5% , making it extremely important in steels subjected to high mechanical and thermal loads.

The influence of silicon on the properties of steel

Silicon significantly affects the microstructure and functional properties of steel. Its presence causes:

• Increased hardness and tensile strength – silicon reduces the number of defects in the steel structure and improves its homogeneity.
• Increased elasticity – important in spring and structural steels.
• Improved resistance to gas corrosion and oxidation – key in heat-resistant and acid-resistant steels.
• Reduction in impact strength – high silicon content can make steel more brittle, which requires control of its amount in the alloy composition.

Up to 0,4% silicon is considered a natural residue from the steel deoxidation process. However, at higher levels, it becomes a key alloying element, determining the performance parameters of various steel grades.

Silicon in spring steels

In spring steels, silicon is combined with chromium, vanadium, or manganese. This combination of elements improves fatigue resistance , which is crucial for components subjected to repeated dynamic loading. These steels are used in:

• Motorization – suspension springs, shock absorbers, stabilizers.
• Machine industry – elements exposed to cyclic loads.
• Aviation – aircraft components requiring high elasticity and resistance to cracking.

Silicon in heat-resistant steels

Thanks to its ability to form a stable oxide layer on the alloy's surface, silicon increases steel's resistance to oxidation at high temperatures. In heat-resistant steels, it is often used in combination with chromium, molybdenum, and aluminum , providing exceptional structural stability at temperatures ranging from 900 to 1100°C . These steels are used in applications such as:

• Boilers and steam turbines – components exposed to hot gases.
• Industrial furnaces – grates, fireboxes, thermal covers.
• Exhaust systems and engine valves – where resistance to oxidation and gaseous sulfur corrosion is important.

Silicon in specialized steels

In valve steels used in combustion engines, the silicon content reaches 3% , which improves resistance to high temperatures and the aggressive effects of exhaust gases.

Iron alloys with 12–18% silicon exhibit exceptional resistance to sulfuric and nitric acids , making them indispensable in the chemical industry. Such steels are used in:

• Tanks for storing aggressive chemicals.
• Industrial pipelines resistant to acid corrosion.
• Chemical apparatus, where high resistance to acidic environments is required.

Silicon and hardenability and plastic processing

The addition of silicon affects the hardenability of steel, especially when combined with carbide-forming elements such as manganese, chromium, or molybdenum . Steels with high silicon content are characterized by improved abrasion resistance and reduced energy losses during operational processes.

However, excessive silicon can impair cold working , limiting its use in structural steels to the 4–7% range . Above this value, the steel becomes difficult to form and machine.

Magnetic steels – a key application of silicon

One of the most important applications of silicon in steels is its role in electrical steels. Silicon in amounts of 1–3% significantly reduces hysteresis losses , improving the efficiency of transformers, generators, and electric motors.

Properties of silicon in magnetic steels:

• Increases electrical resistance, reducing eddy current losses.
• Improves magnetic permeability, which allows for better performance of transformer cores.
• Reduces magnetostriction, which reduces noise generated by electrical devices.

Thanks to these features, silicon steels are widely used in the energy industry and the production of electrical equipment.

Silicon, a versatile alloying element, plays a key role in the production of steel with diverse properties. Its influence on hardness, corrosion resistance, and temperature resistance allows the creation of specialized alloys, such as:

Spring steels – increased strength and elasticity.

50HS – Spring steel with increased silicon and manganese content, characterized by high elasticity and fatigue resistance. Used in the production of springs, suspension springs, and shock absorbers.

C67S – Carbon spring steel with approximately 0,67% C content, exhibiting good hardenability and high abrasion resistance. It is used in the production of springs, spring bands, and tools with increased hardness.

C75S – Spring steel with approximately 0,75% C content, characterized by high mechanical strength and good elasticity. Used in the production of technical springs, hand tools, and blades.

C85S – High-carbon steel (approx. 0,85% C) with high hardness after quenching and excellent wear resistance. Used for the production of knives, springs, blades, and tools requiring high mechanical strength.

C100S – A spring steel with a very high carbon content (approx. 1,00% C), providing the highest hardness and abrasion resistance. Used in the production of precision springs, tool blades, and components requiring extreme hardness.

Heat-resistant steels – resistance to oxidation at high temperatures.

H13JS – A heat-resistant ferritic steel with approximately 13% Cr and a silicon additive, which improves resistance to oxidation and aggressive gases. Used in heat exchangers, furnaces, and furnace components operating at high temperatures.

H18JS – Heat-resistant steel with approximately 18% Cr and a silicon additive, which increases the stability of the protective oxide layer. It exhibits high resistance to oxidation and gas corrosion, making it suitable for use in industrial furnaces, exhaust ducts, and chemical equipment.

H24JS – Heat-resistant steel with high chromium content (approx. 24% Cr) and a significant silicon content, which increases resistance to oxidizing and sulfurous atmospheres. It is used in high-temperature installations in the metallurgical and energy industries.

1.4828 – A heat-resistant austenitic steel (approx. 19% Cr, 11% Ni) with added silicon, which improves oxidation resistance up to 1000°C and structural stability at high temperatures. Used in exhaust systems, gas burners, and industrial furnace components.

1.4841 – Austenitic heat-resistant steel (approx. 25% Cr, 20% Ni) with the addition of silicon, which enhances resistance to oxidation and gas corrosion up to 1100°C. Used in turbines, industrial boilers, heating systems, and blast furnaces, where high stability under extreme temperature conditions is required.

Valve steels – resistance to high temperatures in combustion engines.

H9S2 – A heat-resistant ferritic steel with approximately 9% Cr and 2% Si, where silicon improves resistance to oxidation and gas corrosion at high temperatures. Used in furnace components, heat exchangers, and installations operating in high-temperature conditions.

H10S2M – A heat-resistant ferritic steel with approximately 10% Cr, 2% Si, and a molybdenum additive, which increases corrosion resistance in sulfurous atmospheres. Due to the presence of silicon, it exhibits high stability at temperatures up to 1000°C. Used in the energy and chemical industries.

Magnetic steels – optimization of energy losses in electrical devices.

Thanks to ongoing research into the effects of silicon on steel, we can expect further development of new, even more advanced metallurgical technologies.