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...

Cold Rolling of Steels Oct08

Cold Rolling of Steels...

Cold Rolling of Steels  The primary purpose of cold rolling of steels is to reduce the thickness of the hot rolled steel strips (normally in the range of 1.5 mm to 5 mm) into thinner thicknesses (usually in the range of 0.12 mm to 2.5 mm) which cannot be normally achieved during hot rolling in a hot strip mill. Besides reduction in thickness cold rolling is done for improving the surface finish of steels, for improving the thickness tolerances, for offering a range of ‘tempers’, for improving the physical characteristics, and for preparing the strip for surface coating. Cold rolling makes the cold rolled sheets a much improved product. Cold rolled steel products offer good control of thickness, shape, width, surface finish, and other special quality features that compliment the need for highly engineered end user applications.  To meet the various end user requirements, cold rolled sheets are metallurgically designed to provide specific attributes such as high formability, deep drawability, high strength, high dent resistance, good magnetic properties, weldability, enamelability, and paintability etc. Cold rolling of hot rolled steel strips is done below the recrystallization temperature normally at room temperature. In cold rolling process, usually no heat is applied to the hot rolled strip before rolling.  However, frictional energy at the contact surfaces of the strip being rolled gets converted into heat. This heat may increase temperature of the strip being rolled in rapid adiabatic process to a level of 50 deg C to around 250 deg C. During cold rolling process the reduction in thickness is due to the plastic deformation which occurs by means of dislocation movement. Steel gets hardened because of the buildup of these dislocations. This increases strength and strain hardening upto 20 %. These dislocations reduce the ductility of the...

Normalizing Process for Steels...

Normalizing Process for Steels Normalizing process for steels is defined as heating the steel to austenite phase and cooling it in the air. It is carried out by heating the steel approximately 50 deg C above the upper critical temperature (AC? for hypoeutectoid steels or Acm in case of hypereutectoid steels, Fig 1) followed by cooling in air to room temperature, or at no greater than 1 bar pressure using nitrogen if the process is being  run in a vacuum furnace. Normalizing temperatures usually vary from 810 deg C to 930 deg C. After reaching the soaking temperature the steel is held at that temperature for soaking. The soaking time depends on the thickness of the work piece and the steel composition. Higher temperatures and longer soaking times are required for alloy steels and larger cross sections. Fig 1 Typical normalizing temperature range for steels In normalizing, steel is uniformly heated to a temperature which causes complete transformation to austenite. Steel is held at this temperature for sufficient time for the formation of homogenous structure throughout its mass. It is then allowed to cool in still air in a uniform manner. Air cooling results into faster cooling rate when compared with the furnace cooling rate. Thus, the cooling time in normalizing is drastically reduced as compared to annealing. Soaking periods for normalizing are usually one hour per 25 mm of thickness of the work piece but not less than 2 hours at the soaking temperature. The mass of the work piece can have a significant influence on the cooling rate and thus on the resulting microstructure. Thin work pieces cool faster and hence are harder after normalizing than the thicker work pieces. This is different than in the case of annealing where the hardness...