Harmful impurities in steels: negative effects and ways to combat them

Harmful impurities in steels are chemical elements and compounds that, present in an alloy in small amounts, have a substantial negative effect on its structure and physical-mechanical characteristics. These impurities can enter steel as a result of melting, forming, or raw-material processing.

Sulfur and phosphorus — harmful impurities in steels and alloys

The main harmful impurities are considered to be sulfur and phosphorus. It is they that significantly worsen material properties and make them unsuitable for use in some industries.

Sulfur reduces material ductility and toughness and raises its tendency to cracking (hot shortness) during hot working (rolling, forging, stamping). Its elevated content is allowed only in steels used to manufacture non-critical structures.

Phosphorus also reduces alloy ductility because the type of its crystal lattice, structure, and atom size differ substantially from iron, nickel, and other metals.

Among the negative effects of sulfur and phosphorus the following can be distinguished:

• elevated brittleness — sulfur and phosphorus promote formation of a brittle phase in steel or alloy, leading to reduced strength and impact toughness;
• reduced corrosion resistance — these chemical elements promote formation of compounds that accelerate corrosion processes, leading to rapid material destruction;
• worsened ductility — harmful impurities cause inhomogeneities in alloy structure, reducing material ductility and decreasing its ability to deform without cracking;
• reduced thermal stability — phosphorus and sulfur can cause formation of new crystalline phases, worsening the alloy’s ability to retain mechanical properties when working temperature rises;
• worsened weldability — elevated content of harmful impurities promotes formation of new undesirable compounds in the weld zone, leading to defects and cracks in that area.

Gaseous harmful impurities

Even a small amount of oxygen, hydrogen, or nitrogen can have a sharp negative effect on material properties. Elevated nitrogen or oxygen content reduces steel toughness and ductility and also leads to formation of non-metallic foreign inclusions that worsen alloy mechanical properties. Presence of hydrogen makes steel brittle and can lead to internal cracks called flakes.

Methods of combating harmful impurities

The main way to reduce the negative impact of harmful impurities is adding special alloying elements able to bind sulfur, phosphorus, and other substances, thereby reducing their content in the alloy and improving the material’s mechanical characteristics. Useful additions include manganese and silicon, which can effectively deoxidize steel and make it stronger.

Another method of combating harmful impurities is strict quality control of raw materials used. This helps avoid phosphorus, sulfur, and other elements entering alloys and keep materials’ physical-mechanical properties at the required level.

Harmful impurities in precision alloys

Precision alloys used in high-precision products require special attention to their chemical composition. Limiting sulfur and phosphorus content in such alloys is decisive for ensuring high accuracy and reliability of products.

Thanks to its own spectral laboratory, PZPS engineers can clearly control the quantity of harmful impurities and other chemical elements in products manufactured. This makes it possible to obtain steels and alloys with precise chemical composition and optimal physical-mechanical characteristics, ensuring reliability, strength, and durability of finished products.