converting furnace

A converting furnace is a vessel in which liquid pig iron is oxidized and refined into steel through the injection or blowing of air or oxygen. The furnace holds several tons of molten metal and subjects it to high temperatures, typically 1600 to 1700 degrees Celsius, while a stream of gas passes through the charge. This oxidation burns off excess carbon, silicon, manganese, and other impurities, transforming brittle cast iron into a material with the carbon content and properties needed for steel.

The most widely used design is the Bessemer converter, a pear-shaped vessel lined with refractory brick that tilts on a trunnion. Air is forced upward through a tuyere pipe in the bottom, creating violent turbulence in the molten metal. The oxidation reactions generate enormous heat, so no external fuel is required once the process begins. The Siemens-Martin open-hearth furnace, by contrast, is a fixed brick chamber heated by external flame and allows greater control over temperature and composition, making it better suited to scrap-based steelmaking. Modern electric arc furnaces and oxygen converters have largely replaced these older designs, but the converting furnace remains conceptually central to understanding steel production.

The conversion process lasts only 15 to 25 minutes in a Bessemer converter. The operator must judge the moment when oxidation is complete by watching the flame and color of the metal and listening to the sound of the blow. Stop too early and excess carbon remains; continue too long and the metal becomes too thin and brittle. Once the blow is complete, the furnace is tilted to pour the molten steel into ladles.

The name and its origins

The term 'converter' comes from its function: converting one form of iron into another. Henry Bessemer patented his pneumatic converter in 1855, revolutionizing steel production by eliminating the need for fuel-intensive puddling furnaces. The converter worked so well that it became synonymous with the entire steelmaking process for decades, and the term has remained in use even as furnace designs have evolved. In steelworks, older workers still refer to the basic oxygen furnace simply as 'the converter'.

Converting furnaces sit at a critical point in the iron supply chain. They consume molten pig iron from the blast furnace and produce the raw steel that must then be cast, rolled, and finished. Their design and operation directly affect steel quality, yield, and cost. Any delay in the converting shop becomes a bottleneck for the entire mill, so reliability and thermal control are paramount concerns for the operator and maintenance crew.