Note: Whether plain carbon or alloyed ones, all steels contain impurities like sulphur, phosphorus, hydrogen, nitrogen, oxygen, silicon and manganese, tramp elements like copper, tin, antimony, and non-metallic inclusions. These impurities are to be controlled during steelmaking

Effect of impurity elements on steel properties (some effects are given; details can be seen in the references given at the end of this lecture)

Carbon imparts strength to iron. It reduces ductility and impact strength. But presence of carbon allows heat treatment procedures.

Sulphur segregates during solidification (segregation coefficient is 0.02). Sulphur causes hot shortness due to formation of FeS formed during solidification of steel. Sulphide inclusions lower weldability and corrosion resistance. Presence of sulphur may also lead to development of tear and cracks on reheating the steel.

Phosphorus segregates during solidification (segregation coefficient is 0.02). Presence of phosphorus impairs plastic properties.

Silicon and manganese: Silicon reduces the drawing capacity of steel. Manganese is beneficial; it increases strength without affecting ductility and sharply reduces hot shortness.

Gases: Nitrogen impairs plastic properties and increases embrittlement at lower temperatures. Hydrogen causes defects such as flakes, fish-scale fracture.

Inclusions: Presence of inclusions at the grain boundary weakens intra-granular bonds. Inclusions also act as stress concentrators. Some type of inclusions is brittle.

Tramp elements: Tramp elements like copper, zinc, tin, antimony etc create problems during reheating of steels because their melting points are much lower than steel reheat temperature.