Defects in Thermo Mechanical Processing of Metals...

Defects in Thermo Mechanical Processing of Metals  Thermo mechanical processing of materials is a technique designed to improve the mechanical properties by controlling the hot-deformation process. This was originally designed to produce the required external shape of the product. Controlled rolling, controlled-cooling and direct-quenching are typical examples of thermo mechanical processing. Such processing saves energy in the manufacture of steel by minimizing or even eliminating the heat treatment after hot-deformation, thus increasing the productivity for high grade steels. It normally requires a change in alloy design and often reduces the productivity of the hot deformation process itself, but at the same time makes it possible to reduce the total amount of alloying additions and to improve weldability, while sometimes producing new and beneficial characteristics in the steel. Thermo mechanical processing is the sophisticated combination of well-defined deformation operations and well-defined heat treatment in a single production stage to control the microstructure of the material being formed. It produces materials with the desired external qualities (dimensions, shape and surface quality) and acceptable mechanical properties. The process is normally considered as the final stage in the production of steels. Thermo mechanical process defects are usually focused on individual forming technique. The defects generally range from mostly macroscopic ‘form and fracture’ related defects to defects related to strain localizations, as well as imperfections related to microstructure.  The defects in case of thermo mechanical processing have two possible origins namely (i) process related, and/or (ii) metallurgical. The first one is usually related fully to the practices of the thermo mechanical processes including the forming techniques and the heat treatment, while the metallurgical origin defects can range from the starting solidification structure to structural developments during thermo mechanical process. It is difficult to establish a clear demarcation between the...

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...

Metal Forming Processes...

Metal Forming Processes Metal forming processes consists of deformation processes in which a metal work piece (billet, bloom, or blank) is shaped by tools or dies. The design and control of such processes depend on the characteristics of the material of the work piece, the requirements of the finished product, the conditions at the interface of the tool and the work piece, the mechanics of plastic deformation (metal flow), and the equipment used. These factors influence the selection of geometry and material of the tool as well as processing conditions (examples are temperatures of die and work piece and lubrication). Since many of the metalworking operations are rather complex, models of various types, such as analytical, physical, or numerical models, are often used to design these processes. A brief historical view, a classification of metalworking processes and equipment, and a summary of some of the more recent developments in the field are described below. Historical view Metalworking technology is one of three major technologies used for the fabrication of the metal products. The other two are casting process and powder metallurgy (P/M) technology. It is possibly the oldest and most established of the three technologies. The earliest records of metalworking show that the simple hammering of gold (Au) and copper (Cu) was practiced in various regions of the Middle East around 8000 BCE. The forming of these metals was crude since the skill of refining by smelting was not known and since the ability to work the material was limited by impurities that remained after the metal had been separated from its ore. With the start of Cu smelting around 4000 BCE, a useful method became available for purifying metals through chemical reactions in the liquid state. Later, in the Cu age, it was...

Thermo Mechanical Control Processing in Rolling Mills Jul07

Thermo Mechanical Control Processing in Rolling Mills...

Thermo Mechanical Control Processing in Rolling Mills Thermo mechanical controlled processing (TMCP) is a technique designed to improve the mechanical properties of materials by controlling the hot-deformation process in a rolling mill. This was originally designed to produce the required external shape of the product. Controlled rolling, controlled-cooling and direct-quenching are typical examples of thermo mechanical controlled processing. Such processing saves energy in the manufacture of steel by minimizing or even eliminating the heat treatment after hot-deformation, thus increasing the productivity for high grade steels. It normally requires a change in alloy design and often reduces the productivity of the hot deformation process itself, but at the same time makes it possible to reduce the total amount of alloying additions and to improve weldability, whilst sometimes producing new and beneficial characteristics in the steel. TMCP process has several advantages that can help overcome issues related to the addition of major alloying elements and conventional heat treatments. TMCP steels with added micro alloys have been developed to manage the conflicting requirements of strength, toughness and weldability through grain refinement. TMCP effectively enables a reduction of the preheating temperature, thus lowering the rolling cost. As TMCP steels afford good weldability, they are highly valued in industries such as shipbuilding, offshore structures, pipelines and building construction. TMCP is the sophisticated combination of well-defined deformation operations and well-defined heat treatment in a single production stage to control the microstructure of the steel being rolled. TMCP produces steels with the desired external qualities (dimensions, shape and surface quality) and acceptable mechanical properties. TMCP is normally considered as the final stage in the production of steels. TMCP is generally associated with hot rolling operations in hot strip mills, plate mills and bar and rod mills. For example, in case of...

Cobalt in Steels

Cobalt in Steels  Cobalt (Co) (atomic number 27 and atomic weight 58.94) has density of 8.85 gm/cc. Melting point of Co is 1493 deg C and boiling point is 3100 deg C. At temperatures below 417 deg C cobalt exhibits a hexagonal close packed structure. Between 417  deg C and its melting point of 1493 deg C, Co has a face centered cubic (fcc) structure. Co is a magnetic metal with a curie temperature of 1121 deg C. The phase diagram of the Fe-Co binary system and is given at Fig 1. Fig 1 Fe-Co binary phase diagram Co is not a popular element which is commonly added to alloy steels. It does have some effects but these can also be achieved with other alloying elements such as molybdenum (Mo), and nickel (Ni) etc. at lower costs and mostly with better results. Due to this factor, Co does not find enough use in high tonnage low alloy steel production. However it does have some niche markets in steel. Co becomes highly radioactive when exposed to the intense radiation of nuclear reactors, and as a result, any stainless steel that is in nuclear service will have a restriction in the Co content which is  usually around 0.2 % maximum. Adding agents In the production of co bearing alloy steels, additions of Co during the steel making is made in the form of Co metal which is supplied to steel producers in the form of briquettes, granules, and broken electrolytic cathodes. Content of Co in these additive agents is usually in the range of 98 % to 99.9 %. Scrap of super alloys normally contains high percentage of Ni and hence is not used for the production of tool steels. However this scrap can be used...