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...

Quenched and Tempered Reinforcement bars Feb24

Quenched and Tempered Reinforcement bars...

        Quenched and Tempered Reinforcement bars  The Tempcore process for the manufacture of reinforcement bars has been developed in the seventies by Centre de Rechaerche Metallurgiques (CRM) Belgium in order to manufacture high yield strength weldable concrete reinforcement bars from mild steel billets. It is one of the most effective processes for the production of popularly known TMT (Thermo Mechanically Treated) reinforcement bars. Thermex from HSE Germany is the other process for the manufacture of Reinforcement bars.  Principle Both the processes are similar in nature and are based on heat treatment principle. These processes impart high strength to the bars using the technique of Thermo Mechanical Treatment as against Cold Twisting which is used for the manufacture of Torsteel reinforcement bars. Due to the heat treatment the micro structure of the bar consists of Tempered Martensite in the periphery and Ferrite Pearlite in the centre (Fig !). The strength to the bar is by Tempered Martensite layer while the ductility to the bar is incorporated by the Ferrite- Pearlite layer.   Fig. 1 Microstructure of Reinforcement Bars  The schematic diagram of the process on a Time-Temperature curve is given on Fig. 2.   Fig 2 Schematic Diagram on a Time – Temperature Curve  Process Heated steel billets are rolled in the rolling mill to the final size and shape of the reinforcement bar. At this stage the bar is processed by this process. The process is carried out in three successive stages as described below. The first stage begins as soon as the bar leaves the final mill stand. The bar is rapidly and energetically cooled (Quenched) with water through a short cooling installation, where it undergoes surface hardening (Martensite layer). At this stage the surface layer is converted into Martensite while the core remains Austenitic. The...