Annealing in metallurgy and material science is a heating treatment that modifies the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness. In the annealing process, atoms migrate in the crystal lattice and the number of dislocations decreases, leading to a 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.
In its heated, soft state, the uniform micro-structure of metal will allow for excellent ductility and workability. In order to perform a full anneal in ferrous metals, the materials must be heated above their upper critical temperature long enough to fully transform the micro-structure to austenite. The metal must then be slow-cooled, normally by allowing it to cool in the furnace, so as to allow maximum ferrite and pearlite phase transformation.
The annealing process is commonly used to soften metal for cold working, improve ‘machinability’, and enhance electrical conductivity.
Annealing & Cold Working
One of the most common uses of annealing is to restore ductility. During cold working, the metal can become hardened to the extent that even more work will result in cracking. By annealing the metal beforehand, cold working can take place without any risk of cracking. 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 re-crystallization can occur.
At this stage, any defects caused by deformation of 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 micro-structure. This is done to maximize softness and is often done 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.