Working Metal—The Process of Annealing

This process prevents metal from fracturing when working it

Cast iron in annealing oven in a foundry

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Annealing in metallurgy and materials science is a heat treatment that alters the physical properties (and sometimes chemical properties) of a material to increase its ductility (ability to be shaped without breaking) and reduce its hardness.

In annealing, atoms migrate in the crystal lattice and the number of dislocations decreases, leading to the change in ductility and hardness. This process makes it more workable. In scientific terms, annealing is used to bring a metal closer to its equilibrium state (where there are no stresses acting against each other in the metal).

Annealing Causes a Phase Change

In its heated, soft state, the uniform microstructure of metal will allow for excellent ductility and workability. In order to perform a full anneal in ferrous metals, the material must be heated above its upper critical temperature long enough to fully transform the microstructure to austenite (a higher temperature form of iron that can absorb more carbon).

The metal must then be slow-cooled, usually by allowing it to cool in the furnace, to allow maximum ferrite and pearlite phase transformation.

Annealing and Cold Working

Annealing is commonly used to soften metal for cold working, improve machinability, and enhance electrical conductivity. One of the main uses of annealing is to restore ductility in metal.

During cold working, the metal can become hardened to the extent that any more work will result in cracking. By annealing the metal beforehand, cold working can take place without any risk of fracturing. That's because annealing releases mechanical stresses produced during machining or grinding. 

The Annealing Process

Large ovens are used for the process of annealing. The inside of the oven must be large enough to allow air to circulate around the piece of metal. For large pieces, gas-fired conveyor furnaces are used while car-bottom furnaces are more practical for smaller pieces of metal. During the annealing process, the metal is heated to a specific temperature where recrystallization can occur.

At this stage, any defects caused by deforming the metal can be repaired. The metal is held at the temperature for a fixed period of time then cooled down to room temperature. The cooling process must be done very slowly to produce a refined microstructure.

This is done to maximize softness, usually by immersing the hot material in sand, ashes, or another substance with low heat conductivity. Alternatively, it can be done by switching off the oven and allowing the metal to cool with the furnace. 

Treating Brass, Silver, and Cooper

Other metals such as brass, silver, and copper may be fully annealed by the same process but may be quickly cooled, even water quenched, to finish the cycle. In these cases, the process is performed by heating the material (generally until glowing) for a while and then slowly letting it cool to room temperature in still air.

In this fashion, the metal is softened and prepared for further work, such as shaping, stamping, or forming. Other forms of annealing include process annealing, normalization, and stress relief annealing.