Midrex Process for Direct Reduction of Iron Ore Apr09

Midrex Process for Direct Reduction of Iron Ore...

Midrex Process for Direct Reduction of Iron Ore Midrex is an ironmaking process, developed for the production of direct reduced iron (DRI). It is a gas-based shaft furnace process is a solid state reduction process which reduces iron ore pellets or lump ore into DRI without their melting using reducing gas generally formed from natural gas. The principle of the reduction process using reducing gas is shown in Fig 1. Fig 1 Principle of reduction process using reducing gas The history of the Midrex process goes back to 1966 when Donald Beggs of the Surface Combustion Corporation conceives the idea for the Midrex direct reduction process.  The original process was developed by the Midland-Ross Co., which later became Midrex Technologies, Inc. It is now a wholly owned subsidiary of Kobe Steel. A pilot plant was built in Toledo, Ohio in 1967. The first commercial plant, having a production capacity of 150,000 tons per year, was built in Portland, Oregon, in 1969. The genius of the Midrex process is its simplicity. Donald Beggs’ concept of combining stoichiometric natural gas reforming with shaft furnace direct reduction of iron ore was a breakthrough innovation which has stood the test of time. Since 1969, DRI production through Midrex process has crossed 500 million tons. Production from many of the Midrex plants exceeds their design capacity. Each year since 1987, DRI production through Midrex process is over 60 % of the total global production of DRI. The process was immature in 1978, when Kobe Steel began the construction of a plant with a production capacity of 400,000 tons/year in the State of Qatar. Kobe Steel significantly modified the design, exploiting the company’s technologies developed through blast furnace operation, and stabilized the then new process. On the other hand, Midrex...

Metallurgical Principles in the Heat Treatment of Steels Nov04

Metallurgical Principles in the Heat Treatment of Steels...

Metallurgical Principles in the Heat Treatment of Steels Heat treatment of steels is carried out for achieving the desired changes in the metallurgical structure properties of the steels. By heat treatment, steels undergo intense changes in the properties. Normally very stable steel structures are obtained when steel is heated to the high temperature austenitic state and then slowly cooled under near equilibrium conditions. This type of heat treatment, normally known as annealing or normalizing, produces a structure which has a low level of the residual stresses locked within the steel, and the structures can be predicted from the Fe (iron)- C (carbon) equilibrium diagram. However, the properties which are mostly required in the steels are high strength and hardness and these are generally accompanied by high levels of residual stresses. These are due to the metastable structures produced by non-equilibrium cooling or quenching from the austenitic state. Crystal structure and phases The crystal structure of pure Fe in the solid state is known to exist in two allotropic states. From the ambient temperature and up to 910 deg C, Fe possesses a body centered cubic (bcc) lattice and is called alpha-Fe.  At 910 deg C, alpha-Fe crystals turn into gamma-Fe crystals possessing a face-centered cubic (fcc) lattice. The gamma crystals retain stability up to temperature of 1400 deg C.  Above this temperature they again acquire a bcc lattice which is known as delta crystals. The delta crystals differ from alpha crystals only in the temperature region of their existence. Fe has two lattice constants namely (i) 0.286 nm for bcc lattices (alpha-Fe, delta-Fe), and (ii) 0.364 nm for fcc lattices (gamma- Fe). At low temperatures, alpha-Fe shows strong ferromagnetic characteristic. This disappears when it is heated to around 770 deg C, since the lattice...

Carburizing Process and Carburizing Steels...

Carburizing Process and Carburizing Steels  Carburizing is one of the most widely used surface hardening processes. It has been in use for a long time. The process involves diffusing carbon into a low carbon steel to form a high carbon steel surface. Carburizing process is also referred to as case hardening or case carburizing process. It is a heat treatment process that produces a surface which is resistant to wear, while maintaining toughness and strength of the core. The carburizing process has evolved with advancements in heat treatment techniques that have improved the hardness and durability of products like carbon steel wire springs and carbon steel forgings. Carburizing is a heat treatment process in which steel absorbs carbon liberated when the steel is heated between 850 deg C to 950 deg C in the presence of a carbon bearing material, such as charcoal or carbon monoxide, with the intent of making the steel harder. The heated steel at this temperature has austenitic structure which has got high solubility for carbon and which is a stable structure. Depending on the amount of time and temperature, the affected area can vary in carbon content. Longer carburizing times and higher temperatures typically increase the depth of carbon diffusion. When the steel is cooled rapidly by quenching, the higher carbon content on the outer surface becomes hard via the transformation from austenite to martensite while the core remains soft and tough as a ferritic and/or pearlitic microstructure. The typical carburizing process cycle including the quenching and tempering steps is shown in Fig 1. Fig 1 Typical carburizing cycle including the quenching and tempering step  Carburized steel consists of a composite material, where the carburized surface is hard but the unaffected core is softer and ductile. Compressive residual stresses are formed in...

Nitriding Process and Nitriding Steels...

Nitriding Process and Nitriding Steels  According to DIN EN 10052:1994-01, nitriding is defined as the thermo-chemical treatment of a work piece in order to enrich the surface layer with nitrogen. Carbo-nitriding involves enriching the surface layer with nitrogen and carbon. The nitriding process, which was first developed in the early 1900s, continues to play an important role in many industrial applications. It often is used in the manufacture of aircraft, bearings, automotive components, textile machinery, and turbine generation systems. It remains the simplest of the case hardening techniques. The basic of the nitriding process is that it does not require a phase change from ferrite to austenite, nor does it require a further change from austenite to martensite. In other words, the steel remains in the ferrite phase (or cementite, depending on alloy composition) during the complete procedure. This means that the molecular structure of the ferrite (bcc) does not change its configuration or grow into the face-centered cubic (fcc) lattice characteristic of austenite, as occurs in more conventional methods such as carburizing. Also, since only free cooling takes place, rather than rapid cooling or quenching, no subsequent transformation from austenite to martensite occurs. Again, there is no molecular size change and, more importantly, no dimensional change, only slight growth due to the volumetric change of the steel surface caused by the nitrogen diffusion. What can (and does) produce distortion are the induced surface stresses being released by the heat of the process, causing movement in the form of twisting and bending. The purpose of nitriding is to enrich the surface layer of a work piece with nitrogen in order to increase the hardness in the surface. The process of nitriding takes advantage of the low solubility of nitrogen in the ferritic crystal structure...