Duplex Stainless Steels...

Duplex Stainless Steels  Duplex stainless steels belong to the stainless steels family and are characterized by high chromium (Cr, 19 % to 32 %) and molybdenum (Mo, up to 6 %) and lower nickel (Ni) contents than austenitic stainless steels and identified by a dual phase micro structure. They have a well balanced two phase structure. The two phases in their microstructure consist of grains of ferritic and austenitic stainless steels. The microstructure contains roughly 50 % austenite and 50 % ferrite. Commercial duplex stainless steels may have 30 % to 70 % austenite and 70 % to 30 % ferrite. Fig 1 shows micro structure of duplex stainless steel.  Fig 1 Micro structure of duplex stainless steel  Duplex grades account for less than 3 % of global stainless steel production, however with a strong growth rate.  They are most commonly used when a combination of high mechanical strength and high corrosion resistance is required. The idea of duplex stainless steels dates back to the 1920s with the first cast being made at Avesta in Sweden in 1930. These first generation duplex stainless steels provided good performance characteristics but had limitations in the as welded condition. The heat affected zone (HAZ) had low toughness because of excessive ferrite and significantly lower corrosion resistance than that of the base metal. The second generation duplex stainless steels are defined by their nitrogen (N) alloying. This new commercial development began in the late 1970s. With this the duplex steels have begun to ‘take off’ in a significant way. This is mainly due to advances in steelmaking techniques (AOD process) particularly with respect to control of nitrogen (N) content. The interactions of the major alloying elements, particularly the Cr, Mo, N, and Ni, are quite complex. To achieve a...

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...