In metallurgy, when an alloy has been subjected to rigorous cold working (plastic deformation), annealing is the process used to restore ductility to the metal. This is done by heating the material to an elevated temperature for a controlled length of time, followed by natural forced cooling, which brings it back to room temperature. Thus annealing metal eliminates the stresses created during mechanical working, and restores the metal to maximum softness.
The Three-Stage Process Defined
In a practical sense, a material cannot be rolled indefinitely. Since the goal of the annealing is the elimination of the stored energy (about 5% used for plastic deformation) that accumulates in a metal during cold working (plastic deformation), a cold-worked metal structure can pass through the following three stages during this process:
- Recovery: This initial phase “recovers” the physical properties of the metals such as: electrical conductivity, thermal expansion, and internal energy. These properties are sensitive to point defects, whereas strength properties are controlled by dislocation and are not affected at recovery temperatures.
Side note: This change in strength properties occurs because the reduction in point defect
releases stored energy causes dislocations to move to positions of lower energy in the strained
crystal lattice, forming dislocations of opposite signs to cancel each other.
During recovery, dislocations align themselves into walls of small cells consisting of stress-free interiors. Though no observable change in the optical microstructure occurs, this new, stress-free material is integral to start the next stage: Re-crystallization.
- Re-crystallization: This stage of the annealing process produces the metal properties most-desired by our industry. It’s the phase where the greatest change in hardness and other mechanical properties occurs. In order for re-crystallization to take place, nuclei of new, unstressed metal form and grow, consuming the strained matrix. As these new grains of stress-free material grow, they eventually meet each other and erase all evidence of previous cold work. Successful re-crystallization is readily detracted by metallographic methods and is evidenced by a decrease in hardness or strength, and an increase in ductility. When desiring a high quality, metallurgical product, it is imperative to stop the annealing process at this stage to avoid the production of large grains during the last step of the process: Grain Growth.
- Grain Growth: Included to present the complete annealing process, this stage is generally avoided when seeking a high-quality, metallurgical product due to the occurrence of smaller, less stable grains being consumed by larger, more stable grains. Excess time spent during this stage results in larger crystals which, when deformed, create a texture known as “orange peel”. This texture results in an uneven surface with decreased reflectivity that is difficult to polish to a lustrous finish. It is possible to retard the formation of large, coarse-grained material with minor additions of cobalt to a gold alloy, which can produce precipitated phases in grain boundaries, but that’s a battle not often won.
Achieving Complete Anneal
Though many of us may look at annealing as idle or even nonproductive time, since the product does not appear to get any closer to its final shape. This stance pales in comparison to the many benefits that can be gain