Converter (metallurgy): reactors for turning pig iron into steel

Historical development

The first practical converter process was introduced in the mid‑19th century and revolutionized steel production by sharply reducing cost and increasing scale. The early Bessemer converter was a large, pear‑shaped vessel equipped with tuyeres—nozzles that injected air into molten metal—to burn off carbon and other impurities. Its rapid, exuberant reaction made mass steelmaking economically viable. In the mid‑20th century the industry moved to oxygen‑based converters, commonly called basic oxygen furnaces, which use nearly pure oxygen to achieve faster, cleaner decarburization and better control of chemistry. The basic oxygen approach was developed in the late 1940s in Austria and became the dominant large‑scale steelmaking route.

Design, parts and operation

Modern converters are large, refractory‑lined vessels mounted to tilt for charging and tapping. Typical elements and stages include:

• Charging: molten pig iron, often mixed with scrap metal, is poured into the vessel.
• Refractory lining: a basic lining (e.g., magnesia and lime bricks) protects the steel shell and helps control slag chemistry and phosphorus removal.
• Oxygen delivery: an oxygen lance injects a focused stream of oxygen into the bath to oxidize carbon and other impurities.
• Slag formation and refining: fluxes such as lime capture oxidized impurities into a separate slag phase for removal.
• Tapping and secondary treatment: refined liquid steel is poured out and further treated to adjust composition and temperature before casting.

Converters are engineered to withstand extreme thermal shock and corrosive slag. The vessel shell is typically made from structural steel steel and relies on periodic relining to maintain service life.

Chemistry and metallurgical mechanisms

The core reactions in a converter are oxidations: oxygen reacts with carbon to form carbon monoxide and carbon dioxide, and with silicon, manganese, and phosphorus to form oxides that either escape as gas or are incorporated into slag. The oxidation sequence, rate and temperature influence the final chemistry and quality of the steel. Scrap metal often contains rust (iron oxides) and other oxygen‑bearing compounds; these can contribute heat when they reduce in the molten bath, so scrap is used strategically alongside molten pig iron to manage energy and temperature. When air was used historically, dissolved nitrogen could enter the steel and impair certain properties; modern converters avoid this by using nearly pure oxygen supplied by an oxygen plant or oxygen cylinder system oxygen.

Practical uses, scale and variants

Converters—especially basic oxygen furnaces—are the primary route for producing large volumes of flat and long steel products because they convert molten iron to steel quickly and with predictable chemistry. Typical shop practice blends:
- hot metal from a blast furnace,
- various grades and amounts of scrap metal, and
- fluxes and alloying additions as needed.

Alternative steelmaking methods, such as electric arc furnaces, compete where scrap feedstocks or smaller, more flexible production are advantageous. Converters remain preferred where a steady supply of hot metal exists and very large heats are economical.

Notable distinctions and environmental aspects

Key distinctions among converters focus on the oxidant and lining chemistry. The historical Bessemer process used air and required an acidic lining; modern basic oxygen converters use oxygen and a basic lining to facilitate removal of phosphorus and sulfur into slag. Environmental concerns include emissions of carbon monoxide, carbon dioxide and particulates during blowing and tapping; these are controlled with hooding, gas cleaning, and dust capture systems. Advances in process control, gas recovery and material efficiency continue to reduce the environmental footprint of converter‑based steelmaking.

For further technical details and operational guidelines see overview references on ironmaking and steel refining processes: pig iron and carbon, basic oxygen history, and process summaries on scrap management and refractory practice scrap, oxygen, converter materials.