Chromium in Steels

Chromium in Steels ¬†Chromium (Cr) (atomic number 24 and atomic weight 52.01) has density of 7.1 gm/cc. Melting point of Cr is 1850 deg C and boiling point is 2680 deg C. The phase diagram of the Fe-Cr binary system is at Fig 1. ¬†Cr has got a body centered cubic (bcc) crystal structure.   Fig 1 Fe-Cr phase diagram Around 85 % of the chromite (chrome ore) mined is used in metallurgical application, namely stainless steels, low alloy steels, high strength alloy steels, tool steels, some maraging steels (high strength alloy steels of the precipitation hardening type), and high performance alloys such as chromium-cobalt- tungsten (or molybdenum) alloys, nickel-chromium-manganese-niobium-tantalum (or titanium) alloys, nickel-chromium-molybdenum alloys, and cobalt-chromium alloys. Cr is the most versatile and widely used element in alloying of steel. It is a key component of stainless steels. Around 70 % of Cr used in steelmaking goes into the production of stainless steels. Consumption of Cr in the constructional alloy steels comes next. Most of the constructional alloy steels contain Cr less than 3 %. Tool steels, super alloys and other specialty steels, though have higher in Cr content account for lower consumption of Cr since these steels are produced in smaller quantities. Addition practice during steel making Cr in the steel comes either from Cr containing scrap or from ferrochrome (Fe- Cr) during the production of Cr alloyed steels. Fe-Cr used in steel making are commercially available in several grades . The main impurities in Fe-Cr are carbon (C) and silicon (Si). Low C grades are costlier than the high C grades. The widespread shift toward duplex refining practices such as the AOD, CLU, etc., for the production of stainless steels has resulted into the increased use of high carbon Fe-Cr. Low...