Diffusion annealing

          This process, also known as homogenizing annealing, is employed to remove any structural non-uniformity, like dendrites, columnar grains and chemical inhomogeneities are generally observed in the case of ingots, heavy plain carbon steels castings. These defects promote brittleness, and reduce ductility and toughness of steel.

Process:

• Steel is sufficiently above the upper critical temperature (say 1000-2000°C), and held at this temperature for prolonged time, usually 10-20 hours, followed by slow cooling.
• Segregated zones are eliminated, and a chemically homogeneous steel is obtained by this treatment as a result of diffusion.
• Heating to such a high temperature results in considerable coarsening of austenitic grains & heavy scale formation. The coarse austenite thus obtained further transforms to coarse pearlite on cooling, which is not a desirable structure as mechanical properties are impaired.
• The main aim of homogenising annealing is to make the composition uniform, i.e., to remove chemical heterogeneity.
• The impact energy and ductility of the steel increases as the homogenizing temperature increases and the hardness yield strength and tensile strength decrease with an increase in the homogenizing temperature.
• Homogenizing annealing has a few shortcomings as well. It results in:
• Grain coarsening of austenite, there by impairing the properties.
• Thick scales on the surface of steels.
• It is an expensive process.