Steel composition

13 April 2017 Author : 

Commercial steels are never simple iron-carbon alloys, but often contain other elements, including:

  • Elements intentionally added during the manufacturing process to improve the quality (manganese, silicon);
  • Elements always present as impurities that is not able to completely eliminate (Sulfur, phosphorus);
  • Non-metallic inclusions or gas atoms present as interstitial (hydrogen, nitrogen);
  • Elements deliberately added to enhance certain properties of the steels (chromium, nickel, molybdenum, vanadium, etc.).

Here are some of the properties that the alloying elements give to the material. Since the purpose of these additions will vary depending on the classes of materials, here are some general guidelines on various chemical elements.

Carbon (C) Increases edge retention and raises tensile strength. Increases hardness and improves resistance to wear and abrasion.

Chromium (Cr) Increases hardness, tensile strength and toughness. Provides resistance to wear and corrosion. Steels with more than 12% Cr is generally known as Stainless Steel. Increases hardenability as it reduces hardening critical speed.

Cobalt (Co) Increases strength and hardness, and permits quenching in higher temperatures. Intensifies the individual effects of other elements in more complex steels.

Niobium (Nb) or Columbium Improves strength and toughness. Provides corrosion resistance. Improves grain refinement and precipitation hardening.

Copper (Cu) Increases corrosion resistance.

Manganese (Mn) Increases hardenability, wear resistance and tensile strength. Deoxidizes and degasifies to remove oxygen from molten metal. In larger quantities, increases hardness and brittleness.

Molybdenum (Mo) Increases strength, hardness, hardenability and toughness. Improves machinability and resistance to corrosion. Helps to retain fine grain sizes.

Nickel (Ni) Adds strength and toughness

Nitrogen (N) Used in place of carbon, it offers unusual advantages in corrosion resistance.

Phosphorus (P) Improves strength, machinability and hardness. Creates brittleness in high concentration.

Silicon (Si) Improves the hardenability, the mechanical strength, hardness, wear resistance and stability to tempering. However, causes negative effects such as excessive fibrous or crystalline grain growth. It is used in combination with manganese to increase the flexibility of steel. Deoxidizes and degasifies to remove oxygen from molten metal.

Sulfur (S) Impurity coming from minerals used in steel production and therefore always present in final product. In commercial steel S is kept under 0.05%, in quality steel is usually less than 0.035%. The desulfurization of steel is normally achieved by treatment with manganese. In absence of such treatment, the sulfur has a tendency to segregate, causing a phenomenon of fragility to intergranular decohesion (hot-short). In certain cases it is used to re-sulfurate steel to give good characteristics of machinability.

Tungsten (W) Adds strength, toughness and improves hardenability.

Vanadium (V) Increases the fatigue limit and facilitates hot workability, Increases strength, wear resistance and increases toughness. Limits grain size.


Bibliography

Structure and properties of metallic materials - Alberto Cigada - Città studi Edizioni
Tecnologia meccanica - Carlo Gaccia Ed. Zanichelli
Zoom on mechanics - L.Chiara, C. Rizzardi Ed. Blugarini Firenze

 

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