What is desulfurisation?

Sulfur is normally considered an undesirable impurity in steels, originating from the ores and coals used during its production. Even in metals produced by electric melting from scrap, the scrap is likely to contain sulfur from its original production. If left untreated, sulfur will adversely affect impact properties.

The standard steelmaking approach to negate the impact of sulfur content is to treat it with manganese, which combines with the sulfur to form non-metallic inclusions. As long as they are sufficiently small and well-distributed, these inclusions have less effect on impact properties. However, in stainless steels the presence of inclusion can encourage pitting corrosion where the inclusions happen to break the surface of the machined component.

Desulfurisation is the process to reduce sulfur content, thereby reducing the absolute content of sulfur and the need to manage inclusions. Typically, limestones high in CaO and MgO are added to the molten steel during or immediately after melting, which combine with any sulfur present to form a slag. As this slag is less dense than the molten metal, it will aggregate at the surface and can be poured off or removed.

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