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...

Ferritic Stainless Steels...

Ferritic Stainless Steels Ferritic stainless steels are high chromium (Cr), magnetic stainless steels that have low carbon (C) content. The chromium content of these stainless steels varies from 10.5 % to 29 % taking into account a very wide range of applications. Ferritic stainless steels can also contain other elements such as molybdenum (Mo), titanium (Ti), aluminum (Al), and niobium (Nb) etc. It is a cost saving material since most of the grades do not have expensive nickel (Ni) additions. Their market share has grown in the recent past and they represent already about 30 % of total global stainless steel production. In comparison to austenitic stainless steels, which have a face-centered cubic (fcc) grain structure, ferritic stainless steels are defined by a body-centered cubic (bcc) grain structure. In other words, the crystal structure of such steels is comprised of a cubic atom cell with an atom in the center. This crystal structure is the same as that of pure iron (? iron) at room temperature. Although the Schaeffler diagram (Fig. 1) is mainly used for welded structures, it is very useful to illustrate the different areas of stability of stainless steel microstructures. Fig 1 Schaeffler diagram The ferritic grades  Ferritic grades are usually classified into five groups (Fig 2) consisting of three families of standard grades and two of ‘special’ grades. By far the greatest current use of ferritic stainless steels, both in terms of tonnage and number of applications, is centered on the standard grades.    Fig 2 – Classification of ferritic stainless steel grades Group 1 – These have the lowest chromium content (10 % to 14 %, grades 409 and 410L) of all stainless steels and are ideal for slightly corrosive environments where localized rust is acceptable. These stainless steels have...

Stainless Steel Manufacturing Processes May04

Stainless Steel Manufacturing Processes...

Stainless Steel Manufacturing Processes Stainless steels contain from 10 % to 30 % chromium. These steels also contain varying amounts of nickel, molybdenum, copper, sulphur, titanium, and niobium etc. The majority of production of stainless steel was through the electric arc furnace (EAF) till around 1970. With the use of tonnage oxygen in steel production, the EAF stainless steel making practice changed. Oxygen gas could be used for improving the decarburization rate. This could be achieved by injecting high oxygen potential but it was accompanied by the adverse reaction of extensive oxidation of chromium to the slag. This necessitated a well defined reduction period in which ferro silicon was used to reduce the oxidized chromium from the slag. Production of stainless steel started by duplex process with the successful development of argon oxygen decarburization (AOD) converter process. Though duplex process with AOD converter is the prominent one, there are several duplex processes are being used today for making stainless steels. In these processes there is an EAF or similar furnace that melts down scrap, ferroalloys and other raw materials to produce the liquid steel. This liquid steel, which contains most of chromium and nickel as well as some other alloying elements, is the charge of the converters. The converters are used to achieve low carbon stainless steels. The versatility of the EAF-AOD duplex process led steelmakers to re-examine the use of different converters for melting of stainless steels. This led to the development of several other converters for duplex processes. The development work to make stainless steels using conventional BOF (basic oxygen furnace) had begun in the late 1950s and early 1960s. By the mid 1960s, some steelmakers were using existing BOF converters for a partial decarburization followed by decarburization in a ladle under...

Stainless steels

                         Stainless steels  Stainless steel is a family of alloys of iron that contains at least 10.5% Chromium and a maximum of 1.2 % carbon which is essential of ensuring formation of a self healing surface passive layer. This passive layer provides the corrosion resistance. These characteristics make stainless steels totally different from mild steels. The stainless steel was discovered between 1900 and 1915. In 1904, Leon Guillet discovered alloys with composition similar to steel grades 410, 420, 442, 446 and 440-C. In 1906 he also discovered an iron-nickel-chromium alloy which was similar to the 300 series of stainless steel. In 1909 Giesen researched on the chromium-nickel (austenitic 300 series) stainless steels. In Germany, in 1908, Monnartz & Borchers found that a relationship exists between a minimum level of chromium (10.5%) on corrosion resistance as well as the importance of low carbon content and the role of molybdenum in increasing corrosion resistance to chlorides.  Stainless steel production process Stainless steel is produced in an electric arc furnace where carbon electrodes contact recycled stainless scrap and various alloys of chromium, nickel and molybdenum etc. depending on the type of stainless steel. A current is passed through the electrode and the temperature increases to a point where the scrap and alloys melt. The liquid steel can also be produced in LD converter using hot metal as a major input material. The liquid steel from the electric arc furnace or LD converter is then transferred into an AOD (Argon Oxygen Decarbonization) converter, where the carbon levels are reduced and the final alloy additions are carried out to achieve the desired chemistry.  The liquid steel is either cast into ingots or continually cast into slabs or billets. The slabs or billets are either hot rolled or forged into...