44 Annealing
Annealing is the process for making material softer while producing uniform material properties. A material can be annealed by heating it to a specific temperature and then letting it slowly cool down inside an oven. This process is expensive because the oven is unusable during the cool down process.
In metallurgy, annealing is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for an appropriate amount of time and then cooling.
In annealing, atoms migrate in the crystal lattice and the number of dislocations decreases, leading to a change in ductility and hardness. As the material cools it recrystallizes. For many alloys, including carbon steel, the crystal grain size and phase composition, which ultimately determine the material properties, are dependent on the heating rate and cooling rate. Hot working or cold working after the annealing process alters the metal structure, so further heat treatments may be used to achieve the properties required. With knowledge of the composition and phase diagram, heat treatment can be used to adjust from harder and more brittle to softer and more ductile.
In the case of ferrous metals, such as steel, annealing is performed by heating the material (generally until glowing) for a while and then slowly letting it cool to room temperature in still air. Copper, silver and brass can be either cooled slowly in air, or quickly by quenching in water. In this fashion, the metal is softened and prepared for further work such as shaping, stamping, or forming.
Thought Experiment:
The lecturer mentions that annealing is used to soften metal that has been work hardened. Isolate the concept of work hardening for a minute as there are many reasons to anneal metal and not all of them have to do with relieving stresses induced by work hardening.
Videos
Watch this 5:52 video Annealing Metal by AlbertaUArts. (2016)
Watch this 8:09 video Detailed Explanation of Annealing Heat Treatment | Process, Furnace, Stages, Types, etc by James Sword Engineering (2024)
Process Annealing
Process annealing, also called intermediate annealing, subcritical annealing, or in-process annealing, is a heat treatment cycle that restores some of the ductility to a product being cold-worked so it can be cold-worked further without breaking.
The temperature range for process annealing ranges from 260 °C (500 °F) to 760 °C (1400 °F), depending on the alloy in question. This process is mainly suited for low-carbon steel. The material is heated up to a temperature just below the lower critical temperature of steel. Cold-worked steel normally tends to possess increased hardness and decreased ductility, making it difficult to work. Process annealing tends to improve these characteristics. This is mainly carried out on cold-rolle