SECTION 3.7
The Function of Additives

If chlorine is present when the steel is freshly cut, an iron chloride film will result. Iron chloride is a relatively weak solid with a fairly high melting point. As such, it provides an excellent lubricant to keep the chips and the tool apart, thereby reducing chip-to-tool friction and increasing the shear angle.

Sulfur is also used and it, correspondingly, forms iron sulfides. Iron sulfide finds a better niche when the speeds, and resulting temperatures, are relatively higher. It seems that too much sulfur activity at low temperatures will have an abrasive effect on tool faces.

The effectiveness of these films is understood to be limited by their melting points. Iron chloride has a melting point of 1100°F and iron sulfide has a melting point around 1850°F. Both chlorine and sulfur react and function essentially the same way; however, chlorine is more reactive than sulfur and begins to combine with the substrate metal at a lower temperature, thus making it better for low speed operations.