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...

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...

Martensitic Stainless Steels...

Martensitic Stainless Steels Martensitic grades of stainless steel were developed in order to provide a group of stainless steels which are corrosion resistant and hardenable by heat treatment. Martensitic stainless steels are essentially Fe-Cr-C alloys and are similar to carbon or low alloy steels with a structure similar to the ferritic steels. However, due the addition of carbon, they can be hardened and strengthened by heat treatment, in a similar way to carbon steels. The main alloying elements are chromium (10.5 % to 18 %), molybdenum (0.2 % to 1 %), no nickel (except for two grades), and carbon (0.1 % to 1.2 %). Major grades in the family of martensitic group of stainless steels are given in Fig 1. Fig 1 Major grades of martensitic stainless steels History The characteristic body centered tetragonal martensitic microstructure was first observed by German microscopist Adolf Martens around 1890. In 1912, Elwood Haynes applied for a U.S. patent on a martensitic stainless steel alloy. This patent was not granted until 1919. Also in 1912, Harry Brearley of the Brown – Firth research laboratory in Sheffield, England, while seeking a corrosion resistant alloy for gun barrels, discovered and subsequently industrialized a martensitic stainless steel alloy. The discovery was announced two years later in a January 1915 newspaper article in The New York Times. Brearley applied for a US patent during 1915. Properties The structures of martensitic stainless steels are body centered tetragonal (bct) and they are classified as a hard ferro magnetic group. In the annealed condition, these steels have tensile yield strengths of around 275 N/sq mm and hence they can be machined, cold formed, or cold worked in this condition. These stainless steels have good ductility and toughness properties, which decrease as strength increases. Martensitic stainless steels can be moderately hardened by...

Tool Steels

Tool Steels The term tool steel is a generic description for those steels which have been developed specifically for tooling applications. These steels are used for making tools, punches and dies etc. Tools used for working steels and other metals must be stronger and harder than the steels or the materials they cut or form. Normally tool steels are known for their distinctive toughness, resistance to abrasion, their ability to hold a cutting edge, and/or their resistance to deformation at elevated temperatures (red hardness). Some of the operations that tool steels are used for include drawing, blanking, mould inserts, stamping, metal slitting, forming and embossing, although their use is not limited to just these areas. The metallurgical characteristics of various compositions of tool steels are extremely complex. There are hundreds of different types of tool steels available and each may have a specific composition and end use. Tool steels are mainly medium to high carbon steels with specific alloying elements added in different amounts to provide it special characteristics. The carbon in the tool steel is provided to help harden the steel to greater hardness for cutting and wear resistance while alloying elements are added to the tool steel for providing it greater toughness or strength. In some cases, alloying elements are added to retain the size and shape of the tool during its heat treat hardening operation or to make the hardening operation safer and to provide red hardness to it so that the tool retains its hardness and strength when it becomes extremely hot. Various alloying elements in addition to carbon are chromium (Cr), cobalt (Co), manganese (Mn), molybdenum (Mo), nickel (Ni), tungsten (W), and vanadium (V). The effect of the alloying elements on the properties of tool steels is as follows. Chromium –...