Argon Oxygen Decarburization (AOD)

The AOD process is carried out in a refractory-lined converter similar to the BOF but with two to six argon-oxygen tuyeres installed in the lower side wall. The tuyeres consist of two concentric steel tubes, with the inert gas flowing in the outer annulus and oxygen in the inner tube. The converter has tilting and emission-control equipment similar to that of the BOF; the lining is also basic, but it lasts only 50 to 100 heats because of the long refining time and the high temperature of more than 1,700° C (3,100° F) that is necessary for improving the chromium yield. Most shops have three converter shells and one trunnion ring at a blowing station, rotating them between operation, relining, and preheating.

When making austenitic stainless steel, the AOD converter is charged with liquid high-carbon chromium-nickel steel that has been melted in a regular EAF and may contain 1.5 percent carbon, 19 percent chromium, and 10 percent nickel. The blow starts with a high-oxygen gas mixture of, for instance, 80 percent oxygen and 20 percent argon, because there is still plenty of carbon in the steel with which oxygen prefers to combine. As the carbon level drops, the gas mixture is gradually changed into one rich in argon; this may end with a blowing gas of 20 percent oxygen and 80 percent argon. After a blowing time of about one hour, the final carbon content is on the order of 0.015 percent, and only about 2 percent chromium has been lost. The steel is then deoxidized by ferrochrome silicon and desulfurized with burnt lime. Argon is also blown during this end phase for better mixing and removal of hydrogen and nitrogen.

The tap-to-tap time is about two hours, and consumption of oxygen and argon is about 25 and 20 cubic metres, respectively, per ton of steel. To minimize cost, argon is sometimes replaced by nitrogen or compressed air at the beginning of the blow. AOD converters with capacities up to 160 tons are in operation.