Annealing of Steels

Annealing of Steels Annealing is a process of heat treatment that alters the properties of steel to increase its ductility and to make it more workable. It involves heating the steel to slightly above its critical temperature, soaking at that temperature for a time sufficient to allow the necessary changes to occur and then cooling at a predetermined rate (Fig 1) Fig 1 Heating range of different annealing processes There are two main reasons for annealing. The first is to soften the steel material and remove the stresses. The second is to homogenize the structure of the steel material. By the process of annealing the properties of steel material are enhanced to meet machinability requirements. Annealing process induces ductility, improves toughness, softens the steel, relieves internal stresses, refines the structure by making it homogeneous, and improves cold working properties. Annealing also prepares the steel for further heat treatment. Theory of annealing process Annealing occurs by the diffusion of atoms within the steel material, so that the steel material progresses towards its equilibrium state. Heat increases the rate of diffusion by providing the energy needed to break bonds. The movement of atoms has the effect of redistributing and destroying the dislocations in the steel material. This alteration in dislocations allows steel material to deform more easily, so increases its ductility.] The amount of process initiating Gibbs free energy in a deformed steel material is also reduced by the annealing process. This reduction of Gibbs free energy is termed also as stress relief. The relief of internal stresses is a thermodynamically spontaneous process. However, at room temperatures, it is a very slow process. The high temperature at which annealing occurs serve to accelerate this process. The reaction that facilitates returning the cold worked steel material to its stress free state has many reaction pathways, mostly involving the elimination of lattice vacancy gradients...