Any steel will contain some minute impurities when being made. Sulfur can be a particularly troublesome impurity, especially when there is a high manganese content present in the steel. The sulfur can form what is known as plastic inclusions, which are generally considered to be nonmetallics. The sulfur plastic inclusions are seen as (FeMn)S. The strength of the formation of these sulfides will determine the amount of manganese sulfides. The presence of the sulfides in the steel in significant quantities will begin to influence the mechanical properties of the steel, and it has the potential to reduce the fatigue strength, the transverse ductility and, to some extent, elongation.
Sulfur can also have a positive effect (when low manganese percentages are present) on the machining characteristics. This is now becoming a preferred method of making “free-machining steel.” The older method of assisting the machining characteristics of the steel was to add lead (Pb). The use of this element has now almost diminished for free-machining steel because of the toxicity of lead. In addition, the lead will begin to evaporate and leech out of the steel surface of carburizing steels when carburizing. The resulting effect is a surface porosity of the carburized component containing lead.