Desulfurisation is the process of reducing the sulfur content of steel during or immediately after melting. Lowering the sulfur reduces the need to manage the inclusions that sulfur would otherwise form.
Why sulfur is a problem
Sulfur is normally an undesirable impurity, coming from the ores and coals used in production. Even metals melted from scrap are likely to carry sulfur from the scrap's original production. Left untreated, sulfur adversely affects impact properties.
The standard response is to treat the steel with manganese, which combines with the sulfur to form non-metallic inclusions. As long as these are small and well distributed, they have less effect on impact properties. In stainless steels, though, inclusions can encourage pitting corrosion where they happen to break the surface of a machined component.
How desulfurisation works
Limestones high in CaO and MgO are added to the molten steel during or just after melting. They combine with any sulfur present to form a slag. Because the slag is less dense than the molten metal, it collects at the surface and can be poured off or removed.