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...

The Iron-Carbon Phase Diagram Mar11

The Iron-Carbon Phase Diagram...

The Iron-Carbon Phase Diagram In their simplest form, steels are alloys of Iron (Fe) and Carbon (C). The study of the constitution and structure of iron and steel start with the iron-carbon phase diagram. It is also the basis understanding of the heat treatment of steels. The Iron Carbon diagram is shown in Fig. 1. Fig 1 Iron Carbon phase diagram The diagram shown in Fig 1 actually shows two diagrams i) the stable iron-graphite diagram (dashed lines) and the metastable Fe-Fe3C diagram. The stable condition usually takes a very long time to develop specially in the low temperature and low carbon range hence the metastable diagram is of more interest. Many of the basic features of this irpn carbon system also influence the behavior of alloy steels. For example, the phases available in the simple binary Fe-C system are also available in the alloy steels, but it is essential to examine the effects of the alloying elements on the formation and properties of these phases. The iron-carbon diagram provides a solid base on which to build the knowledge of both plain carbon and alloy steels. There are some important metallurgical phases and micro constituents in thr iron carbon system. At the low-carbon end is the ferrite (?-iron) and austenite (?-iron). Ferrite can at most dissolve 0.028 wt% C at 727 deg C and austenite (?-iron) can dissolve 2.11 wt% C at 1148 deg C. At the carbon-rich side there is cementite (Fe3C). Between the single-phase fields are found regions with mixtures of two phases, such as ferrite & cementite, austenite & cementite, and ferrite & austenite. At the highest temperatures, the liquid phase field can be found and below this are the two phase fields liquid & austenite, liquid & cementite, and liquid...