Phosphorus and Sulfur are primarily considered residual impurities in ASTM A36, and their content is strictly limited to preserve ductility, toughness, and weldability.
Phosphorus (P): Maximum 0.040%. Phosphorus dissolves in the iron matrix and strengthens the steel. However, it causes a severe detrimental effect known as "cold shortness," meaning the steel becomes exceptionally brittle at room and low temperatures. High phosphorus can lead to catastrophic brittle fracture. The strict limit ensures adequate fracture toughness.
Sulfur (S): Maximum 0.050%. Sulfur combines with iron to form iron sulfide (FeS) inclusions. These inclusions have a low melting point and remain liquid during hot rolling, leading to "hot shortness"-the steel becomes brittle and prone to cracking during high-temperature working (welding, forging, hot rolling itself). Sulfur inclusions also reduce ductility and impact toughness in the final product. While sulfur can improve machinability (it helps break chips), in a structural steel like A36, the requirements for weldability and toughness outweigh this benefit.
The low limits for P and S are hallmarks of "quality" structural steel, distinguishing it from lower-grade steels where these limits might be higher.