Nickel in Steels

Nickel in Steels  Nickel (Ni) (atomic number 28 and atomic weight 58.69) has density of 8.902 gm/cc. Melting point of Ni is 1455 deg C and boiling point is 2910 deg C. The phase diagram of the Fe-Ni binary system is at Fig 1. Ni has a face centered cubic (f.c.c.) crystal structure. It is ferromagnetic up to 353 deg C, its curie point.   Fig 1 Fe-Ni phase diagram Ni is an important and widely used constituent of alloy steels. It is best known as a solid solution strengthener, a mild hardenability agent and, most important, as a means of promoting high toughness, especially at low temperatures. Ni is an important ingredient in stainless steel, helping it to prevent rust, scratches and resist heat. Around 65 % of global Ni production goes into the production of stainless steel. Ni alloyed steels contain as little as fraction of a percent to almost 30 % Ni. As may be expected, properties of these alloy steels range from strengths similar to plain carbon steel to some of the strongest metallic materials known. On the lower side of the Ni percentage in the steels are the alloy and HSLA (high strength and low alloy) structural steels. Hot rolled steels with yield strengths of 345 MPa may contain 0.50 % to 2.00 % Ni for toughness and added corrosion resistance. Age hardening steels contain 1.3 % to 1.5 % Ni plus copper (Cu) and niobium (Nb). Quenched and tempered or normalized and tempered structural steels contain nickel (Ni) up to 2.25 %, as well as a variety of other constituents including chromium (Cr), molybdenum (Mo) or boron (B). Nickel bearing addition agents Ni bearing addition agents are ferro- nickel (Fe- Ni) ferroalloy, Ni containing steel scrap, Nickel oxide...

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 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...